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transmembrane conductance regulator’ acting as a

chloride channel on the apical membrane of the bil-

iary epithelial cells. Cl



secretion stimulates a Cl



bicarbonate exchanger, resulting in bicarbonate se-

cretion and bicarbonate driven choleresis (Figure 6).

Bile Acid Synthesis and Enterohepatic

Circulation

Bile Acid Synthesis

0021 Biliary bile acids are derived from two sources,

de-novo hepatic synthesis and hepatic recycling via

the enterohepatic circulation. The former mechanism

compensates for a daily fecal loss of about 200–

600 mg. The liver is the unique site for bile acid

synthesis, and cholesterol is the obligatory precursor.

Bile acid synthesis involves both nuclear and side-

chain transformation of cholesterol molecules,

and about 25 intermediate compounds have been

isolated along this pathway. A ‘neutral’ and an

‘acidic’ bile acid biosynthesis pathway have been

identified, and the end products of the ‘neutral’ syn-

thetic pathway in man are two bile acids, cholic and

chenodeoxycholic acid, conventionally called ‘pri-

mary’ bile acids (Figure 1). 7-a-hydroxylation of

cholesterol, a process catalyzed by microsomal

7-a-hydroxylase, is the rate-limiting step in ‘neutral’

bile acid synthesis. The first step in the ‘acidic’ path-

way is catalyzed by sterol 27-hydroxylase, a mito-

chondrial P

450

enzyme, and chenodeoxycholic acid

is the predominant end product of this pathway. The

contribution of the ‘acidic’ pathway to the total syn-

thesis is unclear, but it can be as large as 50% of total

synthesis. Bile acids resulting from subsequent bacter-

ial modification of the two primary bile acids in the

enterohepatic circulation are termed ‘secondary’ bile

acids (see below). Bile acids resulting from hepatic

sulfation of lithocholic acid and hydrogenation of

7-oxo-lithocholic acid are termed ‘tertiary’ bile acids

(Figure 1).

0022It is generally agreed that bile acids returning to the

liver via the enterohepatic circulation regulate their

own synthesis by a negative-feedback mechanism.

Bile acid synthesis may increase as much as 20-fold

during interruption of the enterohepatic circulation in

the bile fistula rat, and an opposite effect is achieved

by increasing bile acid return to the liver. Inhibition of

both cholic and chenodeoxycholic acid synthesis ac-

companies intraduodenal or intravenous infusion of

either bile acid in the bile fistula rat. Individual bile

acids appear to exhibit a specific inhibitory capacity

for synthesis, which is less marked for the more

hydrophilic bile acid. This inhibitory effect of hydro-

phobic bile acids involves transcriptional downregu-

lation of cholesterol 7-a-hydroxylase. By contrast,

increased supply of cholesterol to the cell turns on

the 7-a-hydroxylase gene, thus enhancing cholesterol

elimination via bile acid synthesis.

mdr2

PC-TP

Cytosol

Cholesterol Bile salt

Canaliculus

Mixed micelle

Bile salt (micelles)

ATP

bsep

fig0005 Figure 5 Biliary canalicular events involved in lipid secretion.

PC-TP ¼ phosphatidylcholine transporter; bsep, bile salt export

pump; mdr 2, multidrug resistance 2. From Hofmann (1999) The

continuing importance of bile acids in liver and intestinal dis-

ease. Archives of Internal Medicine 159: 2647–2658, with permis-

sion.

CFTR

HCO

−

Secretin

fig0006Figure 6 Mechanism for HCO



-dependent choleresis in the

biliary epithelium. CFTR, cystic fibrosis conductance regulator.

BILE 475

Enterohepatic Circulation and Gallbladder Storage

of Bile

0023 The movement of bile acid molecules is mainly re-

stricted to the biliary tree, intestinal lumen, portal

blood and liver, topographically referred to as

the enterohepatic circulation. The driving force for

the enterohepatic circulation is provided by two

chemical pumps and two mechanical pumps.

The chemical pumps are provided by the active trans-

port processes underlying ileal absorption and

hepatic uptake, and the mechanical pumps are gall-

bladder-emptying and small intestinal transit. (See

Liver: Enterohepatic Circulation.) Gallbladder

motor function is the major factor affecting delivery

of bile into the intestine, thus influencing bile acid

recycling frequency in the enterohepatic circulation.

As a result of gallbladder storage and emptying func-

tions, duodenal bile acid output typically varies from

5 mmol kg

1

in the interdigestive period to

25 mmol kg

1

postprandially. Low duodenal bile

acid output during the interdigestive period is accom-

panied by diversion of about 90% of hepatic bile

into the gallbladder. Gallbladder filling alternates

with gallbladder emptying occurring synchronously

with the periodic aboral progression of the intestinal

interdigestive migrating motor complexes. This alter-

nation of filling and emptying also occurs postpran-

dially, and this ‘bellows’ effect may play a role in

mixing of gallbladder content during the whole

24 h. The net effect of these alternating episodes of

filling and emptying is that the gallbladder stores only

about 50% of the hepatic bile that enters it during

overnight fasting. Given a capacity of 20–40 ml, the

gallbladder would be filled very soon during fasting

beyond its storage capacity if it were not for its

marked capacity for active NaCl and NaHCO

,and

passive water reabsorption.

Functions of Bile Acids

Bile

0024 Despite the very low solubility of both phospholipid

and cholesterol in water, bile is an optically clear

solution that does not contain a macroscopically de-

tectable solid phase in health. At physiological bile

salt–lecithin–cholesterol molar ratios (approximately

75%–15%–5%), cholesterol monomers are solubil-

ized in large mixed micelles having a mixed disk

structure (Figure 2). This structure envisages bile

acid covering the external surface of the micelles,

but also dispersed inside the micellar core together

with phospholipid.

0025 The amount of cholesterol in a given bile

sample may exceed the solubilization capacity of the

micellar mechanism, as determined from in-vitro

studies on model bile solutions (supersaturated bile).

Hepatic bile is normally supersaturated with choles-

terol at low bile acid secretion rates (< 10–15 mmol

1

), and gallbladder bile is often supersatur-

ated in healthy subjects. Cholesterol monohydrate

crystals are not found in these supersaturated biles

when analyzed immediately following collection.

However, if these supersaturated biles are stored

under appropriate laboratory conditions, phase sep-

aration and cholesterol crystal formation occur

within days or weeks (nucleation time). These obser-

vations indicate that bile behaves as a metastable

system, and a nonmicellar mechanism of cholesterol

transport must be involved to account for this pro-

longed metastability. This mechanism involves the

formation of spherical unilamellar vesicles composed

mainly of cholesterol and lecithin (ratio ranging be-

tween 1:1 and 5:2, Figure 2). The proportion of chol-

esterol carried in vesicles relative to that carried in

mixed micelles is about 50% in hepatic bile and 40%

in gallbladder bile. This proportion is not constant,

and the vesicular system prevails at low bile acid

secretion rates. The degree of metastability of super-

saturated human bile, as assessed by ex-vivo nucle-

ation time, is greater than that predictable from the

nucleation time of model bile systems. A protein

component of bile, probably a lipid-containing bile

protein, may act as a ‘stabilizer’ of supersaturated

normal biles.

0026Phase transition occurs during passage of bile

down the biliary tree and during storage in the gall-

bladder (Figure 7). Cholesterol/phospholipid vesicles

are probably excreted by the biliary canaliculus as

such by a mechanism independent of that involved

in bile acid monomer secretion (Figure 5). As soon

as the bile acid concentration reaches the CMC

within the biliary canaliculus, vesicular lipids are dis-

solved in mixed micelles. Supersaturated mixed mi-

celles in hepatic bile rapidly form stable cholesterol/

lecithin vesicles, probably unsaturated with choles-

terol. On entry into the gallbladder, this lipid fraction

may be slowly redissolved into mixed micelles unsat-

urated with cholesterol. This latter effect occurs as a

result of the increased total lipid concentration in

concentrated gallbladder bile, and this is known to

enhance the micellar capacity for cholesterol. In

supersaturated gallbladder bile, cholesterol-rich ves-

icles may remain stable as a result of protein–vesicle

interaction.

Small Intestine

0027According to the classical theory of Hofmann and

Borgstrom, normal fat digestion can be divided into

two closely related events: a chemical event – lipolysis;

476 BILE

and a physical event – acqueous solubilization. Pan-

creatic lipolysis of dietary fat is a prerequisite for its

micellar solubilization, and micellar solubilization is

a prerequisite for its absorption. Bile acids play a key

role in both these events.

0028 From a physicochemical point of view, fat droplets

consist of long-chain triacylglycerol stabilized mainly

by phospholipids in natural food, and by other emul-

sifiers (arabic gum, mono- or diacylglycerols) in pro-

cessed food. Dietary triacylglycerol is hydrolyzed by

pancreatic lipase (lipolysis) to monoacylglycerol and

fatty acid. In the absence of bile acids, access of lipase

to lipid droplets is prevented by the emulsifiers sur-

rounding the oil droplets. Bile acids have the ability to

wash these off the oil droplets, thus giving access to

lipase. Bile acids would also wash off the lipase, a

phenomenon prevented by colipase. This coenzyme

binds to lipase and bile acid, thus fixing the lipase

molecule in contact with the underlying triacylgly-

cerol.

0029 Following digestion, the products of lipolysis are

kept in solution by the natural detergents, bile acids,

mainly in form of mixed bile acid–phospholipid–fatty

acid–monoacylglycerol micelles. Unilamellar vesicles

consisting of the products of lipolysis have been ob-

served under the microscope to ‘bud off’ the lipid

emulsion surface following lipase digestion. These

vesicles are rapidly dissolved by biliary micelles

freshly delivered by bile, so that during intestinal fat

digestion, saturated mixed-disk micelles coexist with

vesicles having the same lipid composition (Figure 7).

Both micelles and vesicles contribute to transport of

solubilized lipids across the intestinal unstirred water

layer lining the gut lumen. It is not known whether

lipid absorption involves monomeric absorption

or vesicle fusion with the plasma membrane of the

enterocyte.

Colon

0030Two hundred to 500 mg of bile acids escape the

enterohepatic circulation and enter the colon daily.

In the human colon, bile acids are exposed to obligate

anaerobes bacteria capable of producing hundreds of

metabolic products of bile acid. The two main bio-

transformations of bile acids in the colon are bacterial

deconjugation and 7-a-dehydroxylation. As a conse-

quence of the former biotransformation, conjugated

bile acids are totally absent from the colon. As a result

of 7-a-dehydroxylation of the primary bile acids,

cholic and chenodeoxycholic acid, the majority of

fecal bile acids consist of deoxycholic acid (about

50%) and lithocholic acid (33%) (Figure 1). Since

these two bile acids are reabsorbed and resecreted in

bile along with the two primary bile acids, they are

termed ‘secondary’ bile acids.

Bila

ers of fatt

acid and mono

ceride

Mixed micelles (and monomers)

+ Bile acid

Vesicles of PC and cholesterol

fig0007 Figure 7 Conversion of phosphatidylcholine (PC)–cholesterol bilayer to mixed micelles in bile, and of fatty acid–monoacyglycerol

lamellae to mixed micelles during fat digestion in the intestinal lumen. From Hofmann (1999) The continuing importance of bile acids in

liver and intestinal disease. Archives of Internal Medicine 159: 2647–2658, with permission.

BILE 477

0031 Most of the bile acids precipitate in feces, since the

slightly acidic pH of feces is lower than the pK

value

for unconjugated bile acids. The bile acid concentra-

tion in fecal water is about 0.5 mmol l

1

, and a small

proportion of these bile acids are reabsorbed from the

colonic mucosa by passive nonionic diffusion and

reenter the enterohepatic circulation. If the value of

1.5 mmol l

1

for bile acid concentration in fecal water

is exceeded, bile acids exert a cathartic effect by in-

hibiting water absorption and, at higher concentra-

tions, by promoting colonic electrolytes and water

secretion. The cathartic effect of bile acids depends

on the bile acid structure, and it is at a maximum for

a-dihydroxylated bile acids.

See also: Cholesterol: Properties and Determination;

Absorption, Function, and Metabolism; Factors

Determining Blood Cholesterol Levels; Role of

Cholesterol in Heart Disease; Colon: Structure and

Function; Gallbladder; Liver: Enterohepatic Circulation;

Phospholipids: Properties and Occurrence;

Determination; Physiology

Further Reading

Bahar RJ and Stolz A (1999) Bile acid transport. In: Cooper

AD (guest ed.) Bile salts: metabolic, pathologic, and

therapeutic considerations. Gastroenterology Clinics of

North America 28(1): 27–58.

Borgstrom B, Barrowman JA and Lindstrom M (1985)

Roles of bile acids in intestinal lipid digestion and

absorption. In: Daniellsson H and Sjovall J (eds) Sterols

and Bile Acids, pp. 405–425. Amsterdam: Elsevier

Science.

Carey MC and Duane WC (1994) Enterohepatic circula-

tion. In: Arias IM, Boyer JL, Fausto N et al. (eds) The

Liver: Biology and Pathobiology, 3rd edn, pp. 769–

786.New York: Raven Press.

Elferink RPJ and Groen AK (1999) The mechanism of

biliary lipid secretion and its defects. In: Cooper AD

(guest ed.) Bile salts: metabolic, pathologic, and thera-

peutic considerations. Gastroenterology Clinics of

North America 28(1): 59–74.

Erlinger S (1994) Bile flow. In: Arias IM, Boyer JL, Fausto N

et al. (eds) The Liver: Biology and Pathobiology, 3rd

edn., pp. 769–786. New York: Raven Press.

Hofmann AF (1994) Bile acids. In: Arias IM, Boyer JL,

Fausto N et al. (eds) The Liver: Biology and Pathobiol-

ogy, 3rd edn, pp. 677–718. New York: Raven Press.

Hofmann AF (1999) The continuing importance of bile

acids in liver and intestinal disease. Archives of Internal

Medicine 159: 2647–2658.

Lanzini A and Northfield TC (1991) Biliary lipid secretion

in man. Review. European Journal of Clinical Investi-

gation 21: 259–272.

Muller M and Jansen PLM (1997) Molecular aspects of

hepatobiliary transport. American Journal of Physiology

272: G1285–G1303.

Vlahcevic ZR, Pandak WM and Stravitz RT (1999) Regu-

lation of bile acid biosynthesis. In: Cooper AD (guest

ed.) Bile salts: metabolic, pathologic, and therapeutic

considerations. Gastroenterology Clinics of North

America 28(1): 1–26.

BIOAVAILABILITY OF NUTRIENTS

S J Fairweather-Tait and S Southon, Institute of

Food Research, Norwich, UK

Concept and Definition of Bioavailability

0001 Bioavailability (biological availability) is a term used

to describe the proportion of a nutrient in food that is

utilized for normal body functions. Although the

overall concept is simple, it is very difficult to describe

the bioavailability of most nutrients in quantitative

terms. Therefore, to facilitate its measurement and

interpretation of the data, the bioavailability of nutri-

ents can be subdivided into its three constituent

phases: (1) availability in the intestinal lumen for

absorption; (2) absorption and/or retention in the

body; and (3) utilization. The reasons for studying

bioavailability are to evaluate the nutritional quality

of foods and diets and to provide data for establishing

dietary requirements for nutrients.

0002The amount of any nutrient that is available for

absorption depends upon its release from the food

macro- and microstructure during digestion, chem-

ical and physical interactions with other food com-

ponents during digestion, the form in which the

nutrient is presented to the absorptive cells of the

intestine, and regulatory mechanisms in the intestinal

mucosa that reflect the individual’s physiological

need for the nutrient. These factors contribute to the

(often) highly variable proportion of absorbable

nutrient which is taken up and used by the body and

may explain the generally poor association between

total amounts of micronutrients consumed and

indices of micronutrient status.

0003Low absorbability of some minerals and vitamins

may be the underlying cause of certain nutritional

inadequacies. If intakes are well in excess of

478 BIOAVAILABILITY OF NUTRIENTS

physiological requirements but bioavailability is a

limiting factor, nutritional deficiency disorders may

occur, as for example with iron.

Determinants of Bioavailability

0004 There are many factors, both dietary and physio-

logical, that influence nutrient bioavailability.

Examples include: (1) the physical form of the nutri-

ent within the food structure and the ease with which

the nutrient can be released from that structure; (2)

the chemical form of the nutrient in a foodstuff and

its solubility in the lumen; (3) the presence of proteo-

lytic enzyme inhibitors (commonly associated with

legumes such as soybeans) which reduce the body’s

ability to digest protein; and (4) the presence of

enzymes such as thiaminase which partially hydro-

lyzes thiamin and makes it less biologically active.

0005 Diet-related factors include:

0006 Food structure

0007 Physicochemical form of the nutrient

0008 Enhancers of absorption, e.g., ascorbate (for iron),

some organic acids, sugars, amino acids, bulk lipid

(for fat-soluble vitamins), and specific fatty acids

0009 Inhibitors (primarily of inorganic micronutrient

absorption), e.g., phosphates (especially phytate),

polyphenols (including tannins), and oxalate

0010 Competition for transport proteins or absorption

sites, e.g., between metals.

0011 Physiological factors include:

0012 Gastric acidity

0013 Intestinal secretions

0014 Gut motility

0015 Luminal redox state

0016 Body status (e.g., tissue levels, nutrient stores)

0017 Short-term homeostatic mechanisms mediated

through the mucosal absorptive cells

0018 Anabolic demands (e.g., growth in infancy and

childhood, pregnancy, and lactation)

0019 Endocrine effects

0020 Infection and stress

0021 Genetic polymorphisms and inborn errors of

metabolism

0022 Gut microflora.

Certain food constituents have the ability to bind

nutrients, thereby rendering them more or less

absorbable. For nutrients that are transported into

the mucosal cell by means of a carrier-mediated path-

way, the degree of binding is an important determin-

ant of bioavailability. If the chelating compound has

a higher affinity for the nutrient than the specific

carrier molecule, then the net effect is a reduction in

bioavailability. Conversely, a weak chelator may act

as an absorption promoter by holding the nutrient in

a suitably soluble form ready to be taken up into the

intestinal mucosal cells. Binding compounds that

impair vitamin bioavailability include the protein

avidin in egg white, which binds biotin, making it

biologically unavailable.

0023Competitive inhibitors of nutrient metabolism

make up another category of dietary factors affecting

bioavailability. It has been suggested that minerals

with similar chemical properties may compete for

common binding sites or carriers. Transition metals

such as iron, zinc, and copper are typical examples of

competitive inhibitors. This will only take place at

high levels of intake when the sum of ionic species

present at the site of absorption exceed the critical

threshold relating to the absorption kinetics of the

minerals in question. (See Copper: Physiology; Iron:

Physiology; Zinc: Physiology.)

0024The bioavailability of lipid-soluble nutrients

(including carotenoids, tocopherols, and other fat-

soluble vitamins) is markedly influenced by their

physicochemical availability for incorporation into

mixed micelles during digestion. The release of lipid-

soluble components is thought to occur primarily

upon ingestion and initial digestion within the stom-

ach. The factors which may influence their release

are: localization within the food matrix; physical

break-up of the food; chemical/enzymatic breakdown

of the food during ingestion and initial digestion; and

the presence of a suitable lipid phase in the form of

emulsion droplets or free lipid.

0025The absorbability of a nutrient also depends upon

certain physiological factors, such as the composition

and volume of gastric and intestinal secretions. The

absorption and utilization clearly depend upon a

number of host-related variables. Most of these are

key participants in the body’s homeostatic regulatory

mechanisms such as nutritional status, developmental

state, gastric and intestinal secretions, mucosal cell

regulation, and gut microflora.

Methods of Determining Bioavailability

0026Various techniques have been employed to assess the

bioavailability of nutrients. The best estimate is prob-

ably obtained by combining the results from several

different methods. By and large, the methods chosen

depend on the resources available since detailed

human studies on nutrient bioavailability use special-

ized techniques, which can be very expensive.

In Vitro

Techniques

0027These range from simple measures of chemical solu-

bility and fractional dialysability following simulated

BIOAVAILABILITY OF NUTRIENTS 479

digestion to measures of uptake and transport by

mucosal tissue from experimental animals. The sub-

stance of unknown bioavailability is incubated with a

variety of intestinal preparations, including intestinal

rings, everted loops (sacs), isolated mucosal cells from

cell culture, and vesicles. Uptake of the nutrient into

the tissue is determined and equated with absorbabil-

ity. Perfused loops, with an intact blood supply, can

be used to measure the transport of the nutrient into

the body. More recently, Caco-2 cell systems have

been used for iron transport studies and these have

generated good predictive data for iron bioavail-

ability. This cell system may be useful for predicting

the bioavailability of other micronutrients. Measure-

ments in vitro of the kinetics and extent of parti-

tioning of the lipid-soluble nutrients into oil and

micellar phases also provide a means of determin-

ing the amount of these nutrients available for

absorption.

0028 Generally speaking, in vitro techniques are re-

producible and can be used for studying dietary

factors that affect nutrient bioavailability. They are

relatively simple and inexpensive, but their relevance

to utilization in humans requires validation. Results

obtained from in vitro studies are only an indication

of the proportion of a nutrient that is available for

absorption, and cannot be used to predict utilization

in vivo as this is dependent on many host-related

factors.

Balance Studies

Fecal Metabolic Balance Approach

0029 This relies on measurement of the difference between

input and excretion of the nutrient and is used to

attempt to quantify amount absorbed. Analysis of

intake and fecal content of the nutrient over the bal-

ance period, or after an acute dose, must be extremely

accurate because relatively small differences need to

be detected. The diet must be carefully controlled

during and sometimes for periods before and after

the dose. Fecal markers need to be employed to deter-

mine the start and end of the balance period and to

insure complete collection, and it must be assumed

that feces are the only significant excretory mechan-

ism, and that none of the unabsorbed nutrient has

undergone biotransformation (due to digestive pro-

cesses or microbial action), or otherwise been lost. It

is also essential to consider enterohepatic circulation

because the nutrient may have been absorbed, re-

excreted in the bile, and then lost to feces. Similarly,

enteric recycling can occur within the gut, involving

absorption, excretion, and reabsorption of nutrient

over several cycles. In both cases, the nutrient enters

the body pool where it will have an impact on the

metabolic kinetics and would therefore be considered

to be bioavailable.

0030It is likely that for many nutrients these assump-

tions cannot be made because of their susceptibility to

microbial and oxidative degradation in the gut, so

that the unabsorbed portion will not be quantitatively

recovered from the feces. Nevertheless, much of our

information on the extent of absorption of certain

nutrients is based on either acute or chronic fecal

mass balance methods which, not surprisingly, show

great variability in results.

The Gastrointestinal Lavage Technique

0031This approach has been used in an attempt to over-

come some of the problems inherent in the ‘simple’

fecal mass balance approach. In the gastrointestinal

lavage technique the entire gastrointestinal tract is

washed out by consuming a large volume of Colyte

containing polyethylene glycol and electrolyte salts.

Washout is complete with the production of clear

rectal effluent and the volunteers then consume the

test meal and are permitted only water or ‘diet’ soft

drinks for the next 24 h. All the effluent is collected

and pooled with the effluent collected on the

following day when another dose of Colyte is given

to wash out the remainder of the test meal. The

compound recovered in the stool is subtracted from

that fed to obtain an absorption figure. The advan-

tage of this approach is that the residence time of the

compound in the gastrointestinal tract (particularly

the large bowel, where fermentation occurs) is

reduced and is standardized. The disadvantages are

that the method is time-consuming, may give an

underestimation of absorption if Colyte causes

excessive transit rates and, again, the method

depends upon there being no degradation or loss of

unabsorbed compound.

The Ileostomy Mass Balance

0032In individuals who have undergone ileostomy the

colon has been surgically removed and the terminal

ileum brought to a stoma on the abdominal wall.

Ingested food passes through the stomach and ileum

in around 6 h as it would in the intact individual. The

digesta (ileal effluent) can be collected at regular

intervals (over about 12 h) from a test meal given in

the morning after an overnight fast. During this time

the volunteers may consume meals and beverages

which are free of the nutrient being studied. Using

this approach, the unabsorbed compound can be re-

covered from the ileal effluent in real time without the

delay of the colon and rectum, or the confounding

influence of the colonic microflora.

480 BIOAVAILABILITY OF NUTRIENTS

Bioassays

0033 The response of experimental animals or humans to

graded levels of the nutrient in the diet can be used to

assess bioavailability. The response criteria employed

should specifically reflect the utilization of the nutri-

ent of interest, either by direct measurement of the

nutrient or its metabolites in tissues, blood, or urine,

or by employing functional measurements, such as

enzyme levels.

Plasma Response: Acute Doses

0034 Measurement of changes in plasma, serum, or whole

blood concentration following acute or chronic dosing

is currently the most well-used approach for estimat-

ing absorption and clearance. However, changes in

blood concentration, particularly following a single

acute dose of dietary amounts of a nutrient, can be

difficult to detect and the results from such studies are

those most frequently misunderstood and misinter-

preted. The plasma response approach depends

upon frequent blood sampling after an acute dose of

an isolated compound, or after a single meal. Changes

in concentration of the compound of interest are

measured and plotted against time to produce a re-

sponse curve. The area under the curve (AUC) is then

calculated and used to determine the extent of ab-

sorption.

0035 At its simplest, the AUC is dependent upon the rate

at which the newly absorbed nutrient enters the blood

and the rate of disposal to other body compartments,

both of which are occurring at the same time.

Changes in the characteristics of the AUC within a

study may therefore be due to differences in absorp-

tion, or differences in kinetics of disposal, or both.

There may also be a problem of reexportation of the

compound between the plasma and other compart-

ments, so that the AUC now consists of three com-

ponents: newly absorbed compound entering the

plasma; disposal of the compound to other tissues

and possibly urinary excretion; and reexportation of

the compound from a tissue (or tissues) into the

plasma. In isolation, and without a mechanistic

understanding of the processes of absorption and

disposal, the AUC cannot usually provide quantita-

tive data on the extent of absorption. However, if

sufficient time points are obtained, and if metabolic

modeling techniques are employed, it is possible to

calculate both the rate and extent of absorption. This

approach is particularly useful for hydrophilic nutri-

ents present in the aqueous plasma phase. For lipo-

philic nutrients quantifying amounts absorbed is

more complex because the whole plasma response is

a multicomponent response involving the transfer of

the nutrient (and/or its metabolites) to, and between,

carriers in the blood which each have their own ab-

sorption and clearance kinetics. A better approach for

lipophilic nutrients is to examine that blood fraction

which contains the newly absorbed nutrient, e.g. the

triglyceride-rich lipoprotein (primarily chylomicron)

fraction.

Plasma Response: Chronic Doses

0036Chronic dosing with foods or isolated nutrients needs

to be carried out until the plasma concentration

reaches a plateau, being careful not to ‘flood’ the

system, e.g., not to exceed the renal threshold. Abso-

lute absorption cannot be determined by chronic

dosing (only relative absorption) and differences

between individuals in disposal rate to the tissues

can be a confounder when interpreting changes in

plasma concentration. As with acute plasma response

AUC, chronic response cannot be used to compare

different nutrients whose absorption and clearance

kinetics are not known.

Isotopes

0037Many of the problems of lack of measurement speci-

ficity can be overcome by using radio- or stable iso-

topes. Foods can be labeled with isotopes, and the

fate of the labeled nutrient following ingestion moni-

tored in the body. The method used to label the food

is an important consideration. Extrinsic labeling, as

used for heme and nonheme iron, is the simpler tech-

nique. The isotope is mixed into the food just before

consumption and is assumed to exchange fully with

the nutrient in the food. However, intrinsic labeling,

in which the isotope is incorporated into the foodstuff

whilst it is growing, is necessary for some minerals

(e.g., selenium) and most vitamins.

0038Isotopes can be used to measure the absorption,

and sometimes the utilization, of nutrients in the

body from different foods and under different dietary

and physiological conditions. A number of isotope

techniques can be used and alternative approaches

or refinements of existing methods are currently

under development, particularly for stable isotopes.

These include whole-body counting (for gamma-

emitting isotopes), fecal and urinary monitoring,

double isotope techniques (urinary and plasma meas-

urements), and plasma appearance/disappearance

kinetics (similar to methods used for measuring the

bioavailability of drugs). Selection of technique

depends on the nutrient under study, the questions

to be answered, and the experimental conditions.

0039The use of radioisotopes for human studies is re-

stricted by ethical considerations. The hazards asso-

ciated with exposure to ionizing radiation preclude

the use of radioisotopes in infants, children, and

BIOAVAILABILITY OF NUTRIENTS 481

pregnant women. Since these groups are most vulner-

able to nutritional deficiency, they are the subject

groups for whom nutrient bioavailability is of par-

ticular interest. Thus alternative methods to study

bioavailability have been sought using stable iso-

topes, which are safe and ethically acceptable. The

main disadvantages of stable as compared to radio-

isotopes stem from the larger quantities that have

to be employed, usually far exceeding the tracer

amounts required for radioisotope studies, and the

difficulties associated with their measurement. (See

Immunoassays: Radioimmunoassay and Enzyme

Immunoassay.)

Minerals

0040 For many nutrients the bioavailable fraction is high

(i.e., 90% or more), but for a number of inorganic

elements it is low and variable. Examples of ionic

species that have a low (< 25%), medium (25–75%),

and high (> 75%) absorption are:

0041 Low: Fe, Mn, Cr, Ni, V

0042 Medium: Ca, Mg, Zn, Cu, Se, MO, CO, PO

.0043 High: Na, K, Cl, I, F.

Iron

0044 Iron deficiency is a worldwide problem, yet iron is

one of the most abundant elements in the earth’s

crust. This paradox can be explained in terms of

bioavailability. The main factors that influence the

bioavailability of iron from the diet are the amounts

of heme and nonheme iron, the presence and

amounts of dietary factors influencing iron absorb-

ability, and the iron status of the individual. (See Iron:

Physiology.)

0045 Heme and nonheme iron are absorbed by two sep-

arate pathways: the heme complex is absorbed intact

into the mucosal cell, whereas nonheme iron is bound

to a transport protein. The absorption of nonheme

iron is very variable and generally much less than

heme iron. Absorption of heme iron is relatively con-

stant (approximately 20–30%), and only marginally

affected by dietary factors that enhance or inhibit

nonheme iron absorption. These include meat, ascor-

bic, and certain other organic acids, all of which

increase iron bioavailability; tannins, phytate, and

calcium decrease bioavailability. (See Ascorbic Acid:

Physiology; Phytic Acid: Nutritional Impact; Tannins

and Polyphenols.)

0046 Physiological variables have a profound influence

on iron bioavailability, notably body iron status. Pre-

vious dietary iron intake will also affect the bioavail-

ability of subsequent iron, probably mediated via

mucosal cell regulation. Iron is one nutrient for

which it is possible to estimate true bioavailability

because absorbed iron is utilized for hemoglobin pro-

duction. Therefore the incorporation of isotopically

labeled food iron into red blood cell hemoglobin can

be measured to assess dietary iron bioavailability.

Another method involves measuring the rate of hemo-

globin regeneration in deficient subjects in response

to different dietary sources of iron compared with a

reference substance (e.g., ferrous sulfate).

Zinc

0047A number of dietary factors affect zinc bioavailabil-

ity. Phytate is probably the most important zinc

antagonist, especially in the presence of calcium, as

it forms a chelate with zinc which is unavailable

for absorption. Other cations with similar physico-

chemical properties (copper and cadmium) or mutual

affinity for carrier protein (iron) also reduce zinc

bioavailability. Some proteins have been shown to

improve zinc bioavailability, but the mechanisms for

the effect are not yet clear. (See Cadmium: Toxicol-

ogy; Copper: Physiology; Zinc: Physiology.)

0048Zinc homeostasis in the body is controlled by alter-

ations in efficiency of absorption coupled with

changes in the quantities of endogenous zinc secreted

into the intestine. Thus, body zinc status plays an

important role in determining dietary zinc bioavail-

ability.

Calcium

0049Calcium absorption is vitamin D-dependent; there-

fore bioavailability depends upon vitamin D intake

and status. The efficiency of absorption is related to

physiological requirements for calcium and is dose-

dependent. Dietary inhibitors of calcium absorption

include substances that form complexes in the intes-

tine, such as phytate. Protein and sodium may also

modify calcium bioavailability in that high levels

increase urinary calcium excretion. Although this is

accompanied by an increase in intestinal absorption,

the net result may be a reduction in the proportion of

dietary calcium utilized by the body, i.e., lower bio-

availability. Lactose, on the other hand, promotes

calcium absorption. Other potential enhancers of cal-

cium absorption are currently under investigation

with regard to the development of functional foods

for the prevention of osteoporosis, such as caseino-

phosphopeptides and nondigestible oligosaccharides.

(See Calcium: Physiology; Cholecalciferol: Physi-

ology; Protein: Interactions and Reactions Involved

in Food Processing.)

482 BIOAVAILABILITY OF NUTRIENTS

Vitamins

0050 A number of vitamins are not fully bioavailable to the

human body. The more important ones are folate,

vitamin B

, niacin, vitamin A, carotenoids, and vita-

min E. (See Vitamins: Overview.)

Folate

0051 Folate exists in nature as a range of vitamers based

upon the parent compound folic acid (pteroyl mono-

glutamic acid). Vitamers differ in the extent of the

reduction of the pteroyl group, the presence of one-

carbon substituents, and the number of glutamyl

residues. The polyglutamyl form of folate makes up

approximately 80% of dietary folate and must be

cleaved by folate conjugase to the monoglutamate

form for absorption.

0052 There appears to be little or no difference in the

extent of absorption of the various monoglutamyl

forms, although stability in the gastrointestinal tract

and in vivo retention may differ.

0053 Numerous dietary and physiological factors influ-

ence the deconjugation and absorption of folate, and

its subsequent utilization in the body. Dietary factors

that may reduce folate bioavailability include conju-

gase inhibitors in foods, as found for example in

pulses and yeast extracts, and dietary fiber such as

wheat bran. Physiological factors that may reduce

folate bioavailability include intraluminal pH

(maximal absorption occurs at pH 6.3), and certain

nutrient deficiencies, including zinc, vitamin B

, and

vitamin B

deficiency. Although food processing and

storage can cause losses of folates, it is conceivable

that the action of endogenous conjugases during food

preparation may increase the bioavailability of natur-

ally occurring polyglutamyl forms. (See Cobalamins:

Physiology; Folic Acid: Physiology.)

Vitamin B

0054 Vitamin B

(3-hydroxyl-5-hydroxymethyl-2-methyl-

pyridine) exists in foods as either the free or phos-

phorylated form of pyridoxine, pyridoxamine, and

pyridoxal. Plant foods also contain pyridoxine glyco-

sides (forming 5–50% of the total vitamin B

con-

tent), and it has been suggested that the observed

lower bioavailability in plant than animal foods is

associated with this form of vitamin B

.(See Vitamin

: Properties and Determination.)

Niacin

0055 Niacin deficiency is associated with diets containing

high levels of maize because the niacin is chemically

bound to carbohydrate by ester linkages and unavail-

able for absorption. However, alkali treatment, as

used by certain cultural groups when processing

foods in the traditional way, renders the niacin more

bioavailable. Other grains, such as sorghum, wheat,

barley, and rice, contain niacin in chemically bound

forms. (See Niacin: Physiology.)

Vitamin A, Carotenoids, Vitamin E

0056These vitamins and provitamins need to be dissolved

and carried in lipid and lipid plus bile salt systems

(micelles) in order to be absorbed at the enterocyte

brush border. This mass transfer from bulk aqueous

food to lipid and micellar phases is a complex process

that is hindered by the presence of food structure.

Digestion products of dietary proteins (peptides)

may play a role in stabilizing micelles because of

their ability to act as surface active agents. Efficient

absorption of retinol is dependent on the presence of

normal mature enterocytes and is compromised in

inflammatory bowel disease, protein-calorie malnu-

trition, zinc deficiency, and alcohol abuse. (See Ret-

inol: Physiology.)

0057In addition to the level and type of fat, a number of

other factors influence the bioavailability of carote-

noids from foods. The major controlling factors are

food structure and the physical form of the carote-

noids within the food matrix (e.g., crystalline or lipo-

protein complexed). The absorption of carotenoids

from uncooked foods can be very low but cooking

can enhance absorption by increasing the ease with

which carotenoids are released from the food struc-

ture. Chopping and other types of food preparation

leading to particle size reduction also improve ab-

sorbability. In vitro results indicate that gastric pH

is an important physiological determinant of caroten-

oid availability. (See Carotenoids: Physiology.)

0058There are at least eight compounds of plant origin

that have vitamin E activity, but a-tocopherol ac-

counts for almost all of the vitamin E activity in

foods of animal origin. This isomer has by far the

highest biological activity of all the natural isomers.

Under normal dietary conditions about 20–80% of

ingested vitamin E is absorbed, depending on dose

and lipid content of the meal. Efficiency of absorption

decreases when large amounts of tocopherols are

consumed, or when individuals suffer from disorders

associated with fat malabsorption. It is reported that

high intakes of pectin, wheat bran, alcohol, and poly-

unsaturated fatty acids also reduce vitamin E absorp-

tion. Dietary constituents such as vitamin A, iron,

selenium, and zinc may affect vitamin E utilization.

(See Tocopherols: Physiology.)

BIOAVAILABILITY OF NUTRIENTS 483

Nutritional Implications of Bioavailability

0059 The ultimate goal in nutritional science is to under-

stand the interactions between diet and health. This

requires a detailed knowledge of nutrient bioavail-

ability, without which it is impossible to relate dietary

intakes to indices of physiological function used to

assess health. Estimates of dietary requirements

cannot be made without the appropriate bioavailabil-

ity factors. Where diets are marginal or inadequate in

one or more micronutrient it is often possible to

improve the quality of the diet by increasing the bio-

availability of the nutrients in question, with conse-

quent improvements in the nutritional status of

people consuming it. Future research should utilize

recently developed techniques to quantify the differ-

ent phases of bioavailability, differentiating where

possible between dietary and physiological factors.

The ultimate goal is to combine all the information

to form a coherent picture of bioavailability of nutri-

ents for different foods and diets.

See also: Ascorbic Acid: Physiology; Carotenoids:

Physiology; Cholecalciferol: Physiology; Cobalamins:

Physiology; Folic Acid: Physiology; Immunoassays:

Principles; Radioimmunoassay and Enzyme

Immunoassay; Niacin: Physiology; Phytic Acid:

Nutritional Impact; Protein: Interactions and Reactions

Involved in Food Processing; Retinol: Physiology;

Tannins and Polyphenols; Tocopherols: Physiology;

Vitamins: Overview; Vitamin B

: Physiology