Caballero B. (ed.) Encyclopaedia of Food Science, Food Technology and Nutrition. Ten-Volume Set

metal and fifth most abundant element (after oxygen,

carbon, hydrogen, and nitrogen) in the human body.

In both plants and animals, calcium plays key struc-

tural and metabolic roles.

0006 In plants, calcium forms salt bridges between pectin

molecules in the middle lamella, providing the ‘glue’

that holds the walls of adjacent cells together. Calcium

in solution regulates the activity of many enzymes in

plant cells, and protoplasmic calcium concentrations

are regulated by sequestration of calcium in vacuoles

where it is precipitated in an inert form such as

calcium oxalate.

0007 In animals, calcium in mineral form is plentiful as

the carbonate (CaCO

) in shell and coral and as

hydroxyapatite (Ca

(PO

)

(OH)) in bone and tooth

enamel. Calcium in solution acts an important intra-

cellular signal mechanism in animals, essential for

muscle contraction, blood clotting and many other

functions. (See Calcium: Physiology.)

Overview of Occurrence in Foods

0008 Roughly 70% of the calcium present in the food

supply in the USA is in the form of milk and dairy

products, with about 10% in fruits and vegetables,

5% in grain products, and the remainder in all other

foods. Similar proportions have been reported from

other Westernized countries. The relative importance

of these food sources to people’s calcium intakes

depends upon the relative amounts consumed. Even

so, the main food sources of calcium for most people

consuming omnivorous diets in Western countries

are milk and dairy products, typically accounting

for over 50% of total calcium intake. Although the

concentration of calcium in cereal products and vege-

tables is typically much lower than in dairy products,

the amount of these foods eaten means that foods of

plant origin do make a significant contribution to

total calcium intakes for most people (20–25% from

cereals in the USA and the UK, with about 14% from

bread due to calcium fortification of white flour). It is

commonly estimated that the total contribution of

nondairy foods eaten over the course of a day pro-

vides 200–300 mg of calcium to those consuming a

mixed diet, or about the equivalent of one glass of

milk. Other foods that contain considerable amounts

of calcium include tinned fish such as sardines or

salmon (largely due to the calcium in the bones and

skin), tofu (especially if it has been processed

with calcium) and certain seeds and nuts, but typical

eating habits indicate that these do not contribute

much to total calcium intake for most people. Vegans,

or others who do not consume milk and dairy prod-

ucts, may need to take particular care to ensure

adequate calcium intake from their diet or may

require a supplemental source of calcium, particularly

during periods of rapid growth. Lactovegetarians,

who do not avoid milk and dairy products, generally

have little difficulty in attaining adequate calcium

intakes.

Factors Influencing Calcium

Bioavailability

0009Knowledge of the calcium content of a food must be

tempered to some extent by understanding the dietary

and physiological factors that can influence the bio-

availability of calcium. Calcium bioavailability can

be operationally defined as the amount of calcium

that is available to be absorbed and used for normal

metabolic functions in the body. Physiological factors

influencing calcium bioavailability include rate of

growth, calcium status and age (See Calcium: Physi-

ology). A number of dietary factors can influence the

bioavailability of calcium, including the level of cal-

cium intake and its chemical form, whether calcium is

taken with food or as a supplement on an empty

stomach, and the presence of inhibitory or enhancing

compounds coingested with the calcium. Calcium is

more bioavailable when taken with food than on an

empty stomach, due at least in part to the longer

passage time through the GI tract. Oxalate and phy-

tate are well-known inhibitors of calcium uptake

through the formation of insoluble complexes within

the gut. With the exception of vitamin D, which is

involved in the active transport uptake of calcium

from the gut (See Calcium: Physiology; Cholecalcif-

erol: Physiology), potential enhancers of dietary cal-

cium absorption are generally less well defined than

the inhibitors, and may have only limited influence on

the intestinal absorption of calcium ingested with a

meal. Absorption from the gastrointestinal tract is not

the only consideration in bioavailability; dietary

sodium and protein (particularly proteins high in

sulfur amino acid content due to the physiological

acidification effect) influence the amount of calcium

excreted in urine and consequently not available for

essential functions in the body.

Sources of Calcium Intake

0010As alluded to previously, calcium is quite unevenly

distributed in the diet of humans. Some foods are

rich sources of calcium, whereas others contain

relatively little. With developments in food

processing and fortification policies and practices,

the increasing availability of calcium-fortified foods

and dietary supplements containing calcium salts is

leading to a wider range of rich dietary sources of

calcium.

766 CALCIUM/Properties and Determination

Foods of Animal Origin

0011 Milk and many dairy products are good sources of

bioavailable calcium (Table 2). Removal of the milk

fat fraction from fluid milk leads to slight increases

in the calcium concentration as one proceeds from

whole milk to skim milk. Fluid milk with added

modified milk ingredients is a relatively new entry

into selected marketplaces, containing some 20–

25% more calcium than regular milk due to the add-

ition of whey calcium concentrates. Naturally, foods

containing milk or milk products, whether ‘hidden’ in

the recipe as in cakes or other baked goods or more

obviously added as with cream or cheese sauces, will

also provide a fair amount of calcium.

0012 Soft cheeses and cream cheese generally contain

less calcium on a wet-weight basis due to their higher

water content. The calcium content of cheeses also

depends on the process used in preparation and

whether the calcium precipitates with the milk solids

or remains in the whey. Cheeses precipitated with

lactic acid, such as cottage cheese or cream cheese,

contain lower calcium levels in the curd, as most

of the calcium remains soluble in the acid whey.

Rennin coagulation (e.g., in production of cheddar

or mozzarella cheese) yields cheeses with a higher

calcium content as the curd is formed before signifi-

cant acidification takes place.

0013 Meat and fish are not typically rich sources of

calcium (Table 2). Mechanically deboned meats can

contain much higher levels of calcium due to the

abrasion of bone during the deboning process. Tinned

salmon provides higher levels of calcium than fresh

salmon fillet (Table 2), because the bones and skin

tend to be consumed along with the flesh. Similar

reasoning applies to small fish consumed whole,

such as sardines and anchovies. Calcium is generally

somewhat higher in crustaceans than in fin fish.

Foods of Plant Origin

0014Foods of plant origin are not generally very rich

sources of calcium, and some contain significant

levels of calcium absorption inhibitors such as phy-

tate or oxalate. However, due to the large amounts

consumed, this class of foods typically makes a fairly

significant contribution to total calcium intake.

Whole grains and seeds generally provide more cal-

cium than most fruits or vegetables. Fortification of

refined flours and breakfast cereals can significantly

increase the contribution of these foods to total cal-

cium intake. Similarly, the high calcium content of

baking powder and the variety of calcium-containing

food additives used (e.g., as dough conditioners or

yeast nutrients – see below) adds to the amount of

calcium in baked goods.

0015While spinach appears to contain a reasonable

amount of calcium (129 mg per 95-g serving,

see Table 3), the high oxalate content of spinach

renders much of the calcium insoluble and much less

tbl0002 Table 2 Calcium content of selected animal-source foods

Foodname Servingsize Weight (g) Calcium (mg) Concentration

(mgg

1

as is)

Milk and dairy products

Milk, whole, 3.3% M.F.

250 ml 258 308 1.19

Milk, partly skimmed, 2% M.F. 250 ml 258 314 1.22

Milk, skim 250 ml 259 319 1.23

Milk, skim, with added milk solids 100 143 1.43

Milk, condensed, sweetened, canned 300 ml 388 1100 2.84

Milk, evaporated, partly skimmed, canned, undiluted, 2% M.F. 250 ml 268 739 2.76

Yogurt, fruit bottom, 1–2% M.F. 175 g 175 214 1.22

Cheese, cheddar 4 slices 52 378 7.27

Cheese, mozzarella, shredded, 22.5% M.F. 125 ml 60 321 5.35

Cheese, cottage, 2% M.F. 125 ml 119 82 0.69

Meat, fish, and egg

Egg, poached 1 large 50 24 0.48

Beef, ground, lean, medium broiled 1 patty 80 9 0.11

Beef, mechanically deboned, raw 100 485 4.85

Pork chop, centre cut, lean, pan-fried 1 chop 69 16 0.23

Chicken, broiler, breast, roasted ½ breast 98 5 0.05

Salmon, Sockeye, baked or broiled ½ fillet 155 11 0.07

Salmon, Sockeye, canned, solidsþboneþliquid 125 ml 79 181 2.29

Sardine, Atlantic, canned in oil, drained, with bone (7.5 cm long) 4 sardines 48 183 3.81

Data from Health Canada (1999) Nutrient Value of Some Common Foods. Ottawa: Health Protection Branch, Health Canada, Canadian Government

Publishing, except where no serving size is specified, which data are from the Canadian Nutrient File, Health Canada, 1997 revision.

Milk fat.

CALCIUM/Properties and Determination 767

bioavailable, so that it has been estimated that over

15 servings of spinach would be required to obtain

the same level of absorbable calcium as one glass of

milk (Table 4). Turnip greens contain a similar level

of calcium to that found in spinach, but without the

oxalate, so that about 2 servings of turnip greens

provide a similar level of absorbable calcium to one

glass of milk (Table 4). However, these types of food

calcium absorption values are often determined under

laboratory conditions while feeding only the single

food. Calcium bioavailability from a given food

source can be substantially modified by other foods

eaten at the same time. For example, milk calcium

absorption has been shown to decline from 33 to

27% when coingested with spinach, whereas spinach

calcium absorption showed an increase from 3%

when fed alone to 11% when coingested with milk.

In a Western mixed diet, the overall calcium absorp-

tion by adults typically averages about 25–30%.

Processing Effects on Food Calcium

0016There are numerous food additive uses of calcium

salts that may add appreciable amounts of calcium

to some foods. The Food Chemicals Codex, fourth

edition (1996) lists over 30 calcium compounds used

as food additives or processing aids. Some of the

common functions of calcium-containing food addi-

tives (and examples of the calcium compounds used)

include: dough conditioners such as calcium carbon-

ate, calcium iodate, calcium lactate, calcium oxide,

calcium phosphate or calcium sulfate; pH adjusters

such as calcium hydroxide or calcium phosphate; anti-

oxidants such as calcium ascorbate; preservatives

such as calcium propionate, calcium disodium EDTA

tbl0003 Table 3 Calcium content of selected plant-based foods

Foodname Serving size Weight (g) Calcium (mg) Concentration (mg g

1

as is)

Vegetables and fruit

Beans, snap, frozen, boiled, drained 125 ml 71 35 0.49

Carrots, raw 1 medium 80 22 0.28

Peas, green, frozen, boiled, drained 125 ml 85 20 0.24

Spinach, boiled, drained 125 ml 95 129 1.36

Apple, raw, with skin (7 cm diameter) 1 apple 138 10 0.07

Banana, raw 1 medium 115 7 0.06

Oranges, raw 1 fruit 131 52 0.40

Grain and cereal products, nuts, and seeds

Wheat, flour, all purpose 250 ml 132 20 0.15

Bread, white, commercial 1 slice 25 27 1.08

Corn meal 125 ml 73 4 0.05

Flax seeds 125 ml 73 196 2.68

Soy flour, defatted 250 ml 106 255 2.41

Tofu, firm, prepared with magnesium chloride 1 piece 80 164 2.05

Tofu, firm, prepared with calcium sulfate 100 683 6.83

Tofu, regular, prepared with calcium sulfate 100 350 3.50

Almonds, dry-roasted, salt added 125 ml 73 206 2.82

Peanuts, dry-roasted, without shell 125 ml 78 42 0.54

Hazelnuts, chopped, dried 125 ml 61 119 1.95

Sesame crisp 4 pieces 35 247 7.06

Data from Health Canada (1999) NutrientValueofSomeCommonFoods. Ottawa: Health Protection Branch, Health Canada, Canadian Government

Publishing, except where no serving size is specified, which data are from the Canadian Nutrient File, Health Canada, 1997 revision.

tbl0004 Table 4 Bioavailability of calcium from various food sources

Food Serving size (g) Ca content (mg) Fractionalabsorption (%) Servingsequivalent to 240 mlmilk

Milk 240 300 32.1 1

Soy milk (unfortified) 120 5 31.0 60.4

Almonds, dry-roasted 28 80 21.2 5.7

Sesame seeds, no hulls 28 37 20.8 12.2

Broccoli 71 35 52.6 5.0

Brussel sprouts 78 19 63.8 8.0

Spinach 90 122 5.1 15.5

Turnip greens 72 99 51.6 1.9

Tofu, calcium set 126 258 31.0 1.2

Selected data from Weaver CM and Plawecki KL (1994) Dietary calcium: adequacy of a vegetarian diet. American Journal of Clinical Nutrition 59

(supplement): 1238S–1241S.

768 CALCIUM/Properties and Determination

or calcium sorbate; anticaking agents such as calcium

stearate, calcium phosphate or calcium silicate;

and thickeners such as calcium gluconate or calcium

alginate. The percentage composition of calcium in

these compounds ranges considerably, e.g., calcium

gluconate is 9% calcium, calcium carbonate is 40%

calcium, and calcium oxide is 71% calcium by

weight. Solubility also differs considerably between

compounds, with some such as calcium carbonate

being relatively insoluble at neutral pH, whereas

others like calcium acetate or calcium chloride are

highly soluble.

0017 Tofu set with calcium sulphate contains consider-

ably more calcium than tofu set with magnesium

chloride (Table 3). Firm tofu contains a higher con-

centration of calcium than does regular tofu due to

the lower water content (and consequently higher

solids). Fermentation (including leavening of bread)

or germination of selected plant foodstuffs can

increase the availability of calcium due to breakdown

of complexing compounds such as phytic acid. The

calcium content of meat can be increased by process-

ing or cooking in acidic solutions due to the dissol-

ution of calcium from bone. For example, pork spare

ribs or chicken cooked with vinegar contain more

calcium than the uncooked flesh.

0018 Tap water typically contains calcium and many

other elements, and may make some contribution to

total calcium intake particularly in hard water areas.

Water hardness is measured as milligrams of calcium

carbonate equivalents per liter, and hard water may

be over 300 mg of CaCO

per liter. Calcium carbon-

ate is 40% elemental calcium by weight. Thus, a 250-

ml glass of water may provide 30 mg of elemental

calcium in some areas. The hardness of water added

in processing may also influence the calcium content

of foods to some extent. Conversely, losses through

soaking and/or boiling may amount to 5–25% of the

calcium content of a variety of vegetables that are

commonly prepared in this way.

0019 Foods may have nutrients added for a variety of

reasons including restoration of processing losses, or

to provide similar nutrient levels in substitute foods

(e.g., soy ‘milk’ and other plant based beverages,

which may be fortified with calcium in some coun-

tries, as they are used as a substitute for dairy prod-

ucts in the diet of their consumers), for fortification

(which, in this context, refers to addition of calcium

with the intent of raising the calcium content of the

food for the consumer, e.g., enriched flour or infant

cereals), or for special purpose foods such as meal

replacements or formulated liquid diets (See Legisla-

tion: Codex). Regulations and practices concerning

food fortification may vary considerably between

countries.

Supplements

0020Nutritional supplements and antacid medications can

make a significant contribution to calcium intakes for

some people. Calcium supplements are available in a

number of forms, including carbonate, phosphate,

lactate, or citrate salts of calcium, among others. Cal-

cium carbonate is found in many over-the-counter

antacid preparations. Multivitamin and multimineral

supplements may also contain calcium, though fre-

quently in smaller amounts. Natural source supple-

ments include oyster shell and dolomite, though it

should be noted that concerns have been raised with

the potential for lead contamination of these forms. It

has been estimated that, among supplement users in

the USA (who represent approximately 20% of the

population), an average of 300 mg of Ca per day is

consumed from supplements. In addition, calcium-

containing antacids, supplying 200–400 mg of Ca

per tablet, are consumed by about 18% of USA adults.

Analysis of Calcium in Foods and

Biological Samples

0021There is no satisfactory routine biochemical method

for assessing calcium nutritional status (See Nutri-

tional Assessment: Biochemical Tests for Vitamins

and Minerals). Serum calcium concentration varies

very little, even across a wide range of dietary calcium

intakes, because of adaptive alterations by which the

endocrine system regulates the level of this mineral in

blood (See Calcium: Physiology). Deviations of cal-

cium concentration from this narrow range are med-

ically significant, and in this setting, the measurement

of total plasma or serum calcium or specific measure-

ment of the ionized calcium fraction is important.

Total calcium in plasma or urine can be measured

by atomic absorption spectrophotometry, either dir-

ectly on the diluted sample or following sample min-

eralization (‘ashing’–see below), or by colorimetric

assay (e.g., using arsenazo III or orthocresolphthalein

complexone) on a clinical autoanalyzer. Ionized cal-

cium in plasma (the fraction that is not bound to

proteins or low-molecular-weight ligands, and conse-

quently is most physiologically active) represents

about 45% of plasma calcium under normal

conditions and can be assessed using an appropriate

ion-sensitive electrode. Assessment of intracellular

calcium levels in the research laboratory can be ad-

dressed using fluorescent probes such as FURA 2 or

QUIN 2, among others. Calcium nutritional status is

more often assessed indirectly through balance stud-

ies or by measurement of bone mineral concentration

or bone mineral density, reflecting the major struc-

tural role played by this mineral nutrient.

CALCIUM/Properties and Determination 769

Sample Preparation

0022 Most foods and biological tissue samples can be pre-

pared for analysis of their calcium content either by

dry ashing (in a muffle furnace) or wet ashing (acid

digestion) techniques or some combination of the

two. Dry ashing techniques typically involve com-

bustion at elevated temperature (e.g., 500–550



until organic matter is fully destroyed, followed by

dissolution of the ash in a suitable acid for subsequent

analysis. Wet ashing techniques typically involve

destruction of organic matter by heating in concen-

trated nitric acid until a pale straw color is attained,

and may feature additional oxidation steps with

perchloric acid or hydrogen peroxide to clarify the

sample solution. In wet ashing procedures, particu-

larly for samples with a high calcium content such

as bone meal, sulfuric acid is generally to be avoided

due to the ready precipitation of calcium from

solution by sulfate, forming plaster of Paris (calcium

sulfate hemihydrate) or gypsum (calcium sulfate

dihydrate).

Determination of Calcium Content of Foods and

Tissues

0023 Determination of calcium content of foods and

tissues is commonly done by atomic absorption

spectrophotometry, though a number of alternative

methods exist including titrimetric methods using

EDTA or KMnO

, neutron activation analysis, or

inductively coupled plasma emission spectrometry.

Details of sample preparation and analytical proced-

ures for a variety of food sample types are published

by the Association of Official Analytical Chemists.

Atomic Absorption Spectrophotometry

0024 For atomic absorption spectrometric analysis of cal-

cium, 0.1–1.0% (w/v) lanthanum is included in the

analytical working solution as a matrix modifier to

reduce anion interferences due to phosphate or sul-

fate, which otherwise can form refractory complexes

and depress the absorption of light by atomic calcium.

Absorbance is measured at the 422.7-nm calcium

spectral line, following atomization in a reducing

air–acetylene flame, and compared with certified

analytical standard calibration solutions. A reducing

flame gives a higher sensitivity, though an oxidizing

flame may give a higher precision where this is crit-

ical. Typical analytical working ranges are obtained

up to 5 mg l

1

in the analytical working solution

when using a standard nebulizer assembly, and may

be approximately doubled with the use of a high

sensitivity nebulizer. (See Spectroscopy: Atomic Emis-

sion and Absorption.)

KMnO

Titrimetric Method for Calcium in Wheat

Flour (Method from the Association of Official

Analytical Chemists)

0025After dry ashing and dilution of the ash to a suitable

volume with demineralized water, bromocresol green

indicator is added along with enough 20% sodium

acetate solution to change the pH to 4.8–5.0 (blue).

The sample solution is covered and heated to boiling.

The calcium is precipitated by slow addition (1 drop

every 3–5 s) of 3% oxalic acid solution (w/v) until pH

is 4.4–4.6, as indicated by a distinct green shade. The

solution is then boiled for 1–2 min and allowed to

settle overnight. The supernate is filtered through

quantitative paper, Gooch, or fine fritted glass filter,

and the beaker and precipitate are washed with small

portions of ammonium hydroxide (1 þ50). A mix-

ture of 125 ml of water and 5 ml of sulfuric acid is

added to the precipitate with heating to 80–90



The solution is finally titrated at 70–90



C with

0.01 M KMnO

to a slight pink endpoint, 1 ml of

permanganate solution equating to 1 mg of calcium.

See also: Calcium: Physiology; Cholecalciferol:

Physiology; Nutritional Assessment: Biochemical Tests

for Vitamins and Minerals; Spectroscopy: Atomic

Emission and Absorption

Further Reading

Bender AE (1978) Food Processing and Nutrition. London:

Academic Press.

Flynn A and Cashman K (1999) Calcium. In: Hurrell R (ed.)

The Mineral Fortification of Foods, pp. 18–53. Leather-

head, UK: Leatherhead International.

Fraser D, Jones G, Kooh SW and Radde IC (1986) Calcium

and phosphate metabolism. In: Tietz NW (ed.) Textbook

of Clinical Chemistry, pp. 1317–1372. Philadelphia, PA:

W.B. Saunders Company.

Gue

guen L and Pointillart A (2000) The bioavailability of

dietary calcium. Journal of the American College of

Nutrition 19(2): 119S–136S.

Health Canada (1999) Nutrient Values of Some Common

Foods. Ottawa: Health Protection Branch in cooper-

ation with Health Promotion and Programs Branch,

Health Canada. Canadian Government Publishing.

Hibbins SG (1992) Calcium and calcium alloys. In: Kirk–

Othmer Encyclopedia of Chemical Technology, 4th edn,

vol. 4, pp. 777–786. New York: John Wiley.

Horowitz W (ed.) AOAC (2000) Official Methods of An-

alysis of AOAC International, 17th edn. Gaithersburg,

MD: The Association of Official Analytical Chemists.

Institute of Medicine (1996) Food Chemicals Codex, 4th

edn. Washington, DC: Committee on Food Chemicals

Codex, Food and Nutrition Board, Institute of Medi-

cine, US National Academy of Sciences. National

Academy Press.

770 CALCIUM/Properties and Determination

Institute of Medicine (1997) Dietary Reference Intakes for

Calcium, Phosphorus, Magnesium, Vitamin D, and

Fluoride. Washington, DC: Standing Committee on the

Scientific Evaluation of Dietary Reference Intakes, Food

and Nutrition Board, Institute of Medicine, US National

Academy of Sciences. National Academy Press.

Karmas E and Harris RS (eds) (1988) Nutritional Evalu-

ation of Food Processing, 3rd edn. New York: Van

Nostrand Reinhold Company.

Petersen RL and Freilich MB (1992) Calcium compounds

(survey). In: Kirk–Othmer Encyclopedia of Chemical

Technology, 4th edn, vol. 4, pp. 787–796. New York:

John Wiley.

Weaver CM and Plawecki KL (1994) Dietary calcium: ad-

equacy of a vegetarian diet. American Journal of Clinical

Nutrition 59(supplement): 1238S–1241S.

Physiology

MRL’Abbe

, Nutrition Research Division, Ottawa,

Ontario, Canada

Background

0001 Calcium (Ca, element number 20, atomic weight

40.08, oxidation state þ2) is the most abundant cation

in the human body and is a nutritionally essential min-

eral. More than 99% of body calcium is in the skeleton,

accounting for 1.5–2.0% of body weight (approxi-

mately 1.5 kg). Calcium has two main types of func-

tions: structural and metabolic. It is responsible for the

mechanical and structural functions of bone and teeth

as well as many metabolic functions involving numer-

ous regulatory enzymes that require calcium. The

physiological functions of calcium are so vital that

total serum calcium is maintained in a very narrow

range through a highly integrated system of hormones

(parathyroid hormone, calcitonin, and vitamin D). If

dietary sources of calcium are inadequate, bone re-

serves are mobilized; inadequate dietary calcium has

long been associated with osteoporosis – a disease

characterized by low bone mineral density.

Calcium Metabolism

Absorption

0002 Intestinal absorption of calcium proceeds by two

independent routes, a saturable, active absorption

pathway regulated almost exclusively by vitamin D

and a nonsaturable passive pathway that depends on

the concentration of calcium in the intestinal lumen.

Most calcium absorption occurs in the duodenum

and jejunum, but minor amounts can also be

absorbed in the colon. The active absorption pathway

is located primarily in the duodenum and upper

jejunum. Absorption by this pathway varies with

age, calcium and vitamin D status, and total calcium

intake. It constitutes the mechanism whereby the

body increases relative absorption during periods of

low dietary calcium intakes or low calcium status.

Most calcium is absorbed by the nonsaturable passive

route in adults with a mature skeleton, and active

transport is of relatively minor importance to the

total calcium absorbed. Thus, the major factor that

determines the amount of calcium absorbed is the

quantity ingested. Although the efficiency of calcium

absorption declines with increasing calcium dose,

there is evidence that the total amount absorbed

may continue to rise.

0003The absorption of calcium from food can vary

from 5 to 60%. It is influenced by calcium status,

vitamin D metabolites, pH of the gut lumen, and

dietary factors such as protein, phosphorus, sodium,

phytate, and oxalate (see sections on Bioavailability

Dietary Factors Affecting Calcium Balance, for more

details).

Bioavailability

0004Foods vary in both calcium content and bioavailabil-

ity. Together, these two factors determine how much

calcium a particular food source will provide per

serving. Information on the content and bioavailabil-

ity of calcium in various food products is given in

Table 1. Milk and milk products have a high calcium

content and bioavailability, and thus are major food

sources of calcium in Western diets. Many vegetables

have a higher fractional absorption (bioavailability)

of calcium but a lower content of calcium than is

found in milk and milk products. In addition, some

vegetables and plant products contain inhibitors of

calcium absorption, such as oxalate (e.g., spinach), or

phytate (e.g., wheat bran, legumes, nuts, oats, soy

beans, maize), which reduce the bioavailability of

calcium.

Balance

0005Calcium balance is controlled through an integrated

response to calcium-regulating hormones that affect

calcium transport at three primary sites of regulation:

intestine, bone, and kidney. The most important cal-

cium-regulating hormones are parathyroid hormone

(PTH), calcitonin and vitamin D, and the effects of

these are briefly described below. Glucocorticoids,

thyroid hormones, growth hormone, insulin, estro-

gen, and testosterone also affect bone turnover and

calcium metabolism.

CALCIUM/Physiology 771

0006 At equilibrium, calcium balance is maintained

through regulation of plasma calcium levels, with

the calcium influx/efflux occurring at several sites,

as illustrated in Figure 1. If dietary calcium intakes

are insufficient, or in the case of malabsorption

syndromes, calcium is mobilized from bone to main-

tain blood concentration.

Hormonal Regulation

0007Vitamin D is an important nutrient for regulating

plasma calcium concentration and bone metabolism.

The two most physiologically active forms are vita-

min D

(ergocalciferol), found mainly in yeast and

plant foods, and vitamin D

(cholecalciferol), synthe-

sized by skin and the most common form added to

foods. Vitamin D is hydroxylated first in the liver at

the 25-carbon position to form the 25-hydroxyvita-

min D (25(OH)D), which enters the circulation. Cir-

culating (25OH)D is a good measure of vitamin D

status. In the kidney, the 25(OH)D is further hydro-

xylated to form the dihydroxy compound (1,25

(OH)

D), which is the most active metabolite. The

production of 1,25(OH)

D in the kidney is tightly

regulated via PTH (see below). The 1,25(OH)

increases calcium absorption in the intestine and is

absolutely critical during growth as well as during

times of low calcium intake. In calcium deficiency,

1,25(OH)

D production is increased, causing en-

hanced intestinal absorption and renal reabsorption

of calcium, and increased bone resorption.

0008PTH is released in response to a decrease in extra-

cellular fluid calcium concentration. PTH directly

stimulates bone and renal tubular calcium resorption

and indirectly increases intestinal absorption through

tbl0001 Table 1 Content and bioavailability of calcium in various food products

Serving

size (g)

Calcium

content (mg)

Fractional

absorption (%)

Estimatedabsorbable

Ca/serving (mg)

Servings needed

to ¼1cupofmilk

Milk (1 cup) 240 300 32.1 96.3 1.0

Almonds, dry roasted (1 oz) 28 80 21.2 17.0 5.7

Beans, pinto 86 4.7 17.0 7.6 12.7

Beans, red 172 40.5 17.0 6.9 14.0

Beans, white 110 113 17.0 19.2 5.0

Broccoli 71 35 52.6 18.4 5.2

Brussel sprouts 78 19 63.8 12.1 8.0

Cabbage, Chinese (bok choi) 85 79 53.8 42.5 2.3

Cabbage, green 75 25 64.9 16.2 5.9

Cauliflower 62 17 68.6 11.7 8.2

Kale 65 47 58.8 27.6 3.5

Kohlrabi 82 20 67.0 13.4 7.2

Mustard green 72 64 57.8 37 2.6

Radish 50 14 74.4 10.4 9.2

Rutabaga 85 36 61.4 22.1 4.4

Sesame seeds no hulls (1 oz) 28 37 20.8 7.7 12.2

Soy beverage, not fortified 120 5 31.0 1.6 60.4

Spinach 90 122 5.1 6.2 15.5

Tofu, calcium set 126 258 31.0 80.0 1.2

Turnip greens 72 99 51.6 51.1 1.9

Watercress 17 20 67.0 13.4 7.2

Selected data from Weaver CM and Plawecki KL (1994) Dietary calcium: adequacy of a vegetarian diet. American Journal of Clinical Nutrition 59(5):

1238S–1241S.

Based on a half-cup serving size unless otherwise noted.

Skin

Plasma

10 mg/100 mL

(350 mg)

Gut

Feces

Kidneys

Urine

Bone

500

500 8460

180

8640

Diet

300 150

8051000

fig0001 Figure 1 Outline of calcium balance (mg per day) under equi-

librium conditions (input ¼output) in adult humans. *Assuming

equilibrium conditions and 30% absorption, 300 mg of the daily

dietary intake of 1000 mg are absorbed from the gut. Endogenous

calcium secretion returns 150 mg per day to the gut lumen. Using

the same assumption of 30% absorption, 45 mg per day of the

endogenously secreted calcium are reabsorbed. Net calcium

uptake is therefore 195 mg per day under these conditions,

which offsets the net loss of 195 mg per day through the skin

and in urine.

772 CALCIUM/Physiology

enhanced formation of 1,25(OH)

D in the kidney

from the monohydroxy form, 25(OH)D.

0009 Calcitonin is synthesized by the C cells of the thy-

roid in response to a rise in serum calcium; it inhibits

bone resorption and agents that have resorptive

effects on bone (PTH, vitamin D, prostaglandins,

and vitamin A).

Excretion

0010 The kidneys filter 8–10 g of Ca per day, with almost

all of it reabsorbed; only about 100–200 mg per day

is excreted in urine. Tubular reabsorption therefore

plays an important part in the conservation of cal-

cium, and a reduction in the number of functional

nephrons or in their capacity to reabsorb normally

can result in hypocalcemia.

0011 Under ideal conditions (i.e., in calcium balance),

excretion via the urine (and, to a lesser degree, via

sweat and feces) is matched by the influx into the

extracellular fluid compartment, usually through

intestinal absorption.

Dietary Factors Affecting Calcium

Balance

Vitamin D

0012 As mentioned above (see section Hormonal Regula-

tion), adequate vitamin D is critical to the absorption

of calcium D; yet, there is little or no natural vitamin

D in most foods that humans normally eat, except

some fish. Vitamin D is present in the flesh of

fatty fish such as sardines and herring, and in the

liver oils of other fish such as cod and tuna.

Meat such as poultry, pork, and beef contain small

amounts of vitamin D. In Canada and in many

other countries, vitamin D fortification of milk and/

or margarine provide the major sources of dietary

vitamin D.

0013 The other major source of vitamin D for humans is

synthesis by skin exposed to ultraviolet or sunlight

irradiation, and most people could obtain sufficient

vitamin D from this source. However, this vital

photosynthetic process can be affected by anything

that limits the amount of solar ultraviolet B photons

to reach the skin’s surface and penetrate into the

viable epidermis. In northern latitudes, the photosyn-

thetic production of vitamin D is negligible during

the winter months (e.g., from November to March,

or virtually half the year in much of Canada). The

elderly also have a reduced capacity for vitamin D

synthesis, and deficiency is most prevalent among

the elderly, particularly among institutionalized or

house-bound individuals. Factors such as pollution

in the air and melanin of the skin can affect the

cutaneous synthesis of vitamin D. In addition,

sunscreens block the absorption of the sunlight spec-

trum that helps the cutaneous synthesis of vitamin D,

and there has been a marked increase in the use of

sunblocks in recent years in many countries. Thus,

providing a reliable, safe, and adequate source of

vitamin D in the diet is essential for optimum calcium

absorption.

Phosphorus

0014There is some controversy concerning the effects of

high phosphorus on calcium balance. In the past,

considerable emphasis was placed on the Ca:P ratio

of the diet, particularly in infant nutrition. In the most

recent review of calcium and phosphorus require-

ments done by the National Academy of Sciences,

this concept was considered to have no demonstrable

relevance in adults, although the report stated: ‘The

concept still has some utility under conditions of

rapid growth (in which a large share of the ingested

nutrients is converted into tissue mass).’ The ratio of

calcium to phosphorus by itself does not take into

account the differing bioavailabilities of the two

nutrients, but estimates of optimal Ca:P ratios have

frequently been based on the calcium and phosphorus

needs of bone building. It was estimated that an

absorbed Ca:P ratio sufficient to support the sum of

bony and soft tissue growth would be approximately

1.3:1, and correcting for the relative net absorptions,

this would be met by Ca:P intakes in the range of

about 2:1.

0015Currently, in the USA, the intake of phosphorus

is high relative to calcium. Furthermore, an individ-

ual’s actual phosphorus intake may be higher than

estimated due to inaccurate nutrient composition

tables for phosphorus values, coupled with underre-

porting of processed foods. In recent years, the

usage of phosphate salts as food additives has in-

creased by nearly 20%. Currently, there are more

than 45 phosphorus-containing compounds ap-

proved for use in food processing as nutrients, dietary

supplements, or for functional purposes such as to

preserve moisture or color, and as emulsifiers. Cola

beverages and root beer beverages can be a significant

source of phosphorus, containing about 70 mg of

phosphorus per serving. One study has suggested

that high intakes of phosphoric acid-containing cola

beverages were associated with slight reductions in

serum calcium in Mexican children, but it is unclear

to what extent the effect was due to the acid load of

the cola beverages, low calcium intakes, or the phos-

phorus itself.

0016Because phosphorus absorption is not strictly regu-

lated, a high-phosphorus diet will directly increase

the level of serum phosphate, which in turn lowers

CALCIUM/Physiology 773

serum calcium, a signal for PTH release. Current

research describing diets in terms of the Ca:P ratio

has suggested that diets with low ratios (i.e., high

phosphate intakes where Ca:P equals 0.25:1) cause

serum calcium to drop, thereby increasing PTH secre-

tion, and inducing secondary hyperparathyroidism.

This increase in PTH is often thought to be harmful

to bone. Other researchers, however, have shown that

calcium and phosphorus have no effect on each other

in the healthy adult population. For example, balance

studies in human adults with Ca:P ratios ranging

from 0.08:1 to 2.40:1 had no effect on either calcium

balance or calcium absorption.

Protein

0017 North Americans consume considerably more cal-

cium than people in developing countries; yet, the

incidence of osteoporosis is much higher in North

America. Recommended intakes for calcium in de-

veloped countries is much higher than for countries

that consume a plant-based diet. One possible reason

for this is the high amount of processed foods and

animal protein found in Western diets that are not

present in diets from primarily cereal-based food

economies. With regard to processed foods, phos-

phorus has been discussed above, and salt and

caffeine will be discussed below. Animal proteins

contain a high proportion of sulfur amino acids, and

the oxidation of these sulfur amino acids to inorganic

sulfate generates endogenous acid, which in turn

increases urinary excretion of acid and calcium. In-

creasing urinary calcium excretion will result in bone

loss when there is insufficient capacity to replace the

loss by absorbing more dietary calcium. High-pro-

tein-induced hypercalciuria is a misnomer, in that

simply comparing high protein with low protein

does not show hypercalciuria unless phosphorus is

controlled or the amount of protein added is

exceedingly high, or there is a change in the type of

protein from one low in sulfur amino acids to a

protein that is high in sulfur amino acids. In general,

for every gram of total protein consumed, urinary

calcium increases by approximately 1 mg.

Salt

0018 High intakes of sodium chloride (NaCl) result in

increased renal calcium loss, which promotes nega-

tive calcium balance. Urine calcium level increases by

approximately 26 mg for every gram of salt ingested.

Habitual excessive sodium chloride intakes have been

suggested as a factor in promoting bone loss. How-

ever, given the small amount of calcium loss for every

gram of salt consumed, the overall impact from diet-

ary salt on calcium metabolism is likely minimal

unless excessive amounts are consumed.

Caffeine

0019Oral doses of caffeine increase the urinary excretion

of calcium for at least 3 h after consumption. Calcium

balance studies have been performed in which

caffeine was shown to have no effect when dietary

calcium levels were adequate. The amount of caffeine

in a small cup of coffee could raise urinary calcium

excretion by approximately 4 mg, an effect that is less

than that for salt. However, for persons with a low

calcium intake, caffeine ingestion can be a significant

predictor of low bone mineral density. Caffeine is

found in only a small number of food products (e.g.,

colas, coffee, tea); its consumption would be unlikely

to have a negative impact on calcium status, as urin-

ary losses would be more than compensated for by a

teaspoon of milk.

Methods for Assessment of Calcium

Status

0020Serum concentration of calcium varies very little,

even across a wide range of dietary calcium intakes,

because of the adaptive alterations by which the

endocrine system regulates the level of this mineral

in blood. Therefore, there is no satisfactory routine

biochemical method for assessing nutritional calcium

status. If serum calcium levels are found to be outside

the normal range, it is usually due to underlying

disease conditions rather than nutritional problems.

Calcium is found in several forms in serum: ionized,

protein bound, and other minor blood components,

with the ionized form of calcium being the function-

ally regulated form. Numerous biological markers or

techniques have been used to assess calcium status.

These can be classified into several groups: serum

calcium or ionic calcium, urinary excretion of cal-

cium, bone mineral density measurements or other

techniques to measure the degree of bone loss, quan-

tification of markers of bone resorption or bone re-

modelling, and finally calcium balance or absorption

studies. Information on each of these measures is

given in the following section.

Serum Calcium

0021Serum calcium concentration cannot be used as an

index of calcium status, as it is homoeostatically con-

trolled. Calcium concentration is serum varies very

little, despite large changes in dietary calcium,

because of the adaptive alterations by which the

endocrine system regulates this mineral (see section

Hormonal Regulation). Calcium in serum is distrib-

uted in the following forms: ionized 47.5%, protein

bound 46%, Ca citrate 1.7%, CaHPO

1.6%, and

unidentified complexes 3.2%.

774 CALCIUM/Physiology

Serum-ionized Ca Concentration

0022 This test measures ‘physiologically active’ calcium

and thus better reflects calcium metabolism than

total calcium values. It is useful in determining cal-

cium changes in patients with altered proteins (e.g.,

chronic renal failure, nephrotic syndrome, hyperpara-

thyroidism, malabsorption, multiple myeloma) but is

not well correlated with dietary intakes of calcium.

Urinary Calcium Excretion

0023 Urinary excretion of calcium can be elevated

following high intakes of dietary calcium, although

urinary excretion of calcium accounts for only 0.2%

of the calcium filtered by the kidneys, because 99.8%

of filtered calcium is reabsorbed. Many conditions

can be associated with hypercalciuria, including

metabolic disturbances such as hyperparathyroidism,

bone metastases, osteoporosis (especially after

immobilization), Cushings syndrome, and vitamin D

intoxication. High dietary sodium or high dietary

protein can also promote increased excretion of

calcium. Hypercalciuria usually occurs prior to

hypercalcemia.

Bone Mineral Density

0024 Bone mineral density (BMD) is used as an index of

body calcium stores, and techniques such as radiog-

raphy, single or dual energy X-ray absorptiometry,

and computerized tomography have been developed

to measure BMD. The standard technique for diag-

nosing the presence or absence and severity of osteo-

porosis is based on BMD and a comparison with

reference ranges established for normal BMD in

young adults (see section Osteoporosis).

Markers of Bone Resorption

0025 Blood biochemical markers of bone resorption rates

tend to be less sensitive and specific than those for

bone remodeling and include: urinary hydroxy-

proline, a measure of total collagen turnover, and

pyridinoline and deoxypyridinoline, which are 3-

OH-pyridinium derivative cross-links more specific

to bone collagen. These measures indicate the degree

by which bone reserves of calcium are being depleted.

Markers of Bone Formation

0026 Markers of bone formation include serum osteocalcin

(bone gla-protein), bone alkaline phosphatase, and

urinary nephrogenous cyclic AMP.

Calcium Balance

0027 Calcium status has been assessed indirectly by assess-

ing calcium balance data, as increased retention is

usually seen when status is poor. Techniques used

include stable isotope or radiolabeled absorption

and calcium retention measures to estimate calcium

balance.

Function

0028Calcium has two main types of functions: structural

and metabolic. It is responsible for the mechanical

and structural functions of bone and teeth as well as

many metabolic functions involving numerous soft

tissue regulatory enzymes that require calcium. These

include such processes as neuromuscular transmis-

sion of chemical and electrical stimuli, enzyme acti-

vation, membrane transport of inorganic ions, muscle

contraction, hormone and cellular secretions, signal

transduction, blood clotting, and reproductive

functions such as sperm motility and fertilization of

the ovum. Calcium regulates release and storage of

neurotransmitters and hormones, the uptake and

binding of amino acids, absorption of vitamin B

and gastrin secretion. The activities of many enzymes

are affected by calcium, either acting directly (e.g.,

glyceraldehyde phosphate dehydrogenase, pyruvate

dehydrogenase, a-ketoglutarate dehydrogenase) or

indirectly through its activation of calmodulin (e.g.,

adenylate cyclase, cyclic nucleotide phosphodiester-

ase, Ca, Mg-ATPase, myosin light chain kinase,

phosphorylase b kinase, NAD kinase, ornithine dec-

arboxylase). Calcium therefore plays a role in normal

cardiac function, renal function, respiration, blood

coagulation, cell and membrane capillary permeabil-

ity, cyclic nucleotide metabolism, glycogen metabol-

ism, microtubule and microfilament function, and

cell division.

0029The physiological functions of calcium are so vital

to survival that total serum calcium is maintained in

a very narrow range (2.2–2.5 mmol l

1

). Calcium in

bone is in constant exchange with calcium in plasma.

When calcium absorbed from the diet is insufficient

to balance obligatory fecal and urinary losses, it is

drawn from bone to maintain plasma levels of ionic

calcium within this tightly controlled range.

Calcium Requirements

0030Recommended intakes for calcium for North Amer-

ica were recently released by the Standing Committee

on the Scientific Evaluation of Dietary Reference

Intakes (DRI), US National Academy of Sciences,

Food and Nutrition Board, Institute of Medicine,

under a program jointly commissioned by federal

government departments in the USA and Canada.

These revised standards encompass both recom-

mended dietary allowances (RDA) and tolerable

upper intake levels for nutrients. It should be noted

CALCIUM/Physiology 775

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