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and has been grown for centuries. A single domin-

ant gene controls the naked trait but the degree

of nakedness is affected by modifying genes and en-

vironmental factors. Naked oat is high in energy,

protein, and nutrition. The metabolizable energy

content of the groat is comparable to that of corn,

which makes naked oat suitable for animal feed

rations.

0006 Oat-breeding efforts continue to focus on the pro-

duction aspects of the crop, usually through hybrid-

ization, such as selecting for improved yield, winter

hardiness, lodging and disease resistance, plant

maturity, hull–groat relationships, seed size, test

weight, plant height, and protein content. Breeders

are characterizing the germplasm of the thousands of

accessions of oat in attempts to enhance specific traits

such as protein, oil, beta-glucans, and other nutri-

tional qualities. Genetic maps are being developed

to identify molecular markers that are linked to

quality and disease resistance traits. The Genetic

Resources Information Network (GRIN) database

of the US Department of Agriculture disseminates

information on oat accessions to breeders for improv-

ing the overall quality of oat. Future breeding object-

ives will likely focus more on oat as a food, as

research reports tend to characterize the positive

nutritional and physiological properties of oat and

oat products in human diets. Future demands by

fig0001 Figure 1 (see color plate 111) Oat panicle of cultivated Avena

sativa.

fig0002Figure 2 Whole kernels of the wild oat species, Avena fatua.

Note the characteristic long awns that are attached to two of the

kernels.

fig0003Figure 3 (see color plate 112) Oat panicle of Avena spp. From a

genetic cross between wild and hull-less oats. The oat derivative

can be used for decorative purposes.

4214 OATS

food processors will encourage breeders to develop

cultivars with improved milling qualities. Oat culti-

vars are likely to be adapted to different levels of

protein, starch, lipids, and gums, and to changing

seed size and shape for easier processing. Oat con-

tains protein of superior quality, highly digestible

carbohydrates, and an abundant source of non-

digestible fiber. As the functional properties of the

biochemical components become more evident,

breeders will be encouraged to evaluate other geno-

types for their agronomic, nutritional, and quality

traits.

Anatomy of Oat Kernel

0007 The compound inflorescence of an oat plant, referred

to as a panicle (Figures 1, 3 and 4), is a continuation

of the stem, and terminates in a single spikelet. Devel-

opment of spikelets involves the formation of several

florets, of which primary and secondary kernels

develop to maturity (Figure 5). Occasionally tertiary

kernels develop, but are smaller than the primary and

secondary kernels. The hull of an oat kernel is nor-

mally attached loosely to the caryopsis. The groat is

that portion of the oat kernel after removal of the hull

(Figure 6). The major anatomical features of the oat

kernel are the hull and groat (Figure 7). The hull

contains two tissue layers, a lemma and palea. The

outer portion of the groat contains the bran layers,

which consist of the pericarp, seed coat, and aleurone

cells. The aleurone layer is developmentally and

genetically part of the endosperm but adheres to the

outer bran layers upon separation of bran from the

endosperm during milling. The aleurone is usually a

monolayer of cells that represent the thickest part of

the bran fraction. Cells in the aleurone produce and

secrete enzymes that degrade storage material in the

endosperm. The embryo, also part of the groat, is

made up of the scutellum and embryonic axis. The

scutellum is located between the endosperm and

embryo. Scutellum secretes enzymes during germin-

ation and is involved in food transfer from the endo-

sperm to the embryo.

0008The proportional size of the hull and groat affects

processing and impacts the economic value of oat.

Oat kernels of uniform size and high groat percent-

ages mill more efficiently and yield higher quantities

of bran and endosperm than nonuniform and small

kernels. Groat percentages range from 65% to 85%

of the oat kernel and weigh from 10 to 40 mg in

cultivated species. Groat weight of the primary kernel

is higher than the groat weight of either secondary or

tertiary kernels. Cultivated oat kernels are generally

larger and have less hull content than kernels from

wild oat species. Environmental growing conditions

affect groat percentages. Thin kernels with high hull

tbl00 01 Table 1 Six-yearaveragearea,yield,andproductionofoat(1994–99)

CountryAreaharvested

(10

ha)

Percentofworld

totalareaharvested

Averageyield(tha

1

)Averageproduction(10

t)Percentofworld

totalproduction

Argentina2451.51.463681.2

Australia9415.81.5014164.8

Belarus3242.02.076722.3

Brazil1941.21.142160.7

Canada14829.12.47366212.4

China4162.62.018332.8

Finland3632.23.2011573.9

France1410.94.366142.1

Germany3081.94.8614865.0

Italy1460.92.363451.2

Kazakhstan 389 2.4 0.86 335 1.1

Poland 598 3.7 2.49 1485 5.0

Romania 246 1.5 1.55 380 1.3

Russian Federation 6637 40.7 1.17 7838 26.6

Spain 382 2.3 1.36 522 1.8

Sweden 310 1.9 3.66 1132 3.8

Turkey 154 0.9 1.78 276 0.9

Ukraine 550 3.4 1.78 989 3.4

UK 103 0.6 5.81 594 2.0

USA 1190 7.3 2.08 2476 8.4

% of world total 92.7 90.9

World total 16317 1.81 29465

ValuesfromFAOStatisticalDatabase,FoodandAgricultureOrganizationoftheUnitedNations(http://apps.fao.org/).Includedarecountrieswhere

average area harvested was greater than 100  10

ha.

OATS 4215

content and low test weight generally result from dry

conditions after heading.

0009 Oat hulls are cellulosic and fibrous materials that

are high in crude fiber and low in caloric value. Hulls

decrease bulk density during transportation, but

function to keep the groat clean for processing pur-

poses and to protect the groat from mechanical

damage during thrashing. Hulls are removed during

milling before processing groats into flour or rolled

oats. Oat dehulling is accomplished by feeding whole

kernels into the center of a high-speed rotor. The

kernels are dehulled by the centrifugal force and

impact against an abrasive rubber ring on the inside

housing of the dehulling machine. The hulls are fur-

ther separated from the groats by air aspiration based

on differences in density. The efficiency of the dehul-

ling process is affected by the quality of oat. Light-

weight kernels that are thin or short, and double

kernels in which the hull of the primary kernel

envelops the secondary kernel, do not dehull as easily

as larger and higher test-weight kernels. Whole oat is

generally graded for size during the cleaning oper-

ation by separating thin and short kernels from larger

kernels before dehulling.

0010The major fractions of the oat groat are the endo-

sperm and bran layers. Their relative proportions

are affected by both genotype and environment.

Approximate proportions of kernel fractions from

cultivated species have been estimated as follows:

hull, 24–32%; groat, 65–85%; embryonic axis,

1–2%; bran layers, 27–41% with a total thickness of

less than 0.1 mm; and starchy endosperm, 56–68%.

The bran contains higher concentrations of vitamins,

protein, lipid, minerals, and fiber while the endosperm

contains higher concentrations of digestible carbohy-

drates. The ratio of bran to endosperm has an effect on

the total chemical composition of the groat.

Biochemical and Nutritional Quality

Protein

0011Protein quantity varies within a single cultivar by

3–4% depending on environmental growing condi-

tions and location. The approximate protein concen-

tration in individual fractions of cultivated oat species

is: groat, 12–25%; embryonic axis, 25–40%; scutel-

lum, 24–32%, bran, 18–32% and starchy endo-

sperm, 9–17%. Oat hulls contain less than 2%

protein in cultivated species, but at higher concen-

trations in wild oat species. The embryonic axis and

scutellum contain higher protein concentrations than

other kernel fractions, but together make up only

about 3% of the groat size. Thus, most of the protein

is actually located in the bran and endosperm. The

bran contains about twice the protein concentration

as the endosperm, but only about half of the total

groat protein because of the difference in relative size

of bran and endosperm. Protein content in specific

oat fractions varies among genotypes. This is most

evident when comparing cultivated species with wild

species, which normally have thicker hulls and

smaller groats.

0012Oat groats contain protein of high quality. The

amino acids of oat are important for their human

nutritional quality; of special interest are the levels

of essential amino acids, which the body cannot syn-

thesize. Lysine, the first limiting amino acid in oat,

averages about 4.2% of the groat protein, which

is higher than other cereals but slightly below the

fig0004 Figure 4 (see color plate 113) Oat panicle of naked (hull-less)

Avena nuda.

4216 OATS

recommended Food and Agriculture Organization/

World Health Organization reference standard of

5.5%. The embryo contains the greatest concentra-

tion of lysine. Oat ranks above other cereals in lysine

content, including rice, because of the high protein

concentration in the groat. Other essential amino acid

concentrations associated within the groat are:

methionine, 2.5%; valine, 6.4%; isoleucine, 3.9%;

leucine, 7.4%; phenylalanine, 5.3%; and tryptophan,

1.7%.

Lipids

0013 Oat contains relatively high lipid content compared

with other cereal grains. Thus, oat serves as a

unique source of energy as a livestock feed. Lipid

concentration is genetically controlled and may vary

from approximately 2% to 12% in cultivated species.

Estimated percentages of free lipids in oat fractions

are: hull, 2%; endosperm, 5.2%; aleurone and bran,

6.4%; scutellum, 20.4%; and embryonic axis,

10.6%. The embryonic axis and scutellum together

contain the highest concentration of lipids in the oat

kernel, but because of its relative size compared with

other oat fractions, lipid quantity is low. The endo-

sperm contains low lipid concentration, but contains

over 50% of the lipids in the groat. The aleurone

layer is very rich in lipid content and constitutes the

major source of bran lipids.

0014Oat lipids are nutritionally important because of

the high concentration of polyunsaturated fatty acids,

especially linoleic acid. Linoleic acid, an essential

fatty acid, is utilized in the synthesis of prosta-

glandins, which function to regulate smooth muscles

like the heart. The approximate percentages of fatty

acids in oat lipids are: myristic, 0.4–4.9%; palmitic,

15.6–25.8%; stearic, 0.8–3.9%; oleic, 25.8–47.5%;

linoleic, 31.3–46.2%; and linolenic, 0.9–3.7%.

0015Oat and oat product quality may decrease upon

storage because of possible chemical changes in the

polyunsaturated fatty acids. Whole oat or undam-

aged groats that are stored below room temperature

and at low moisture levels (less than 10%) show little

change in quality. Conversely, damaged oat, groats,

or ground oat that are stored for extended periods of

time above room temperature and at high moisture

have increased levels of enzymatic (lipase) activity.

Lipase activity causes the release of free fatty acids

from triacylglycerols, which are the major abundant

class of oat lipids. Subsequently, unbound polyunsat-

urated free fatty acids such as linoleic acid undergo

oxidation by action of lipoxygenase activity, leading

to product rancidity. Lipase activity is highest in the

bran because of its association with the aleurone

layer, but can be minimized by thermally treating

whole oat or groats during processing.

Carbohydrates

0016Starch is the major carbohydrate in oat and is found

primarily in the endosperm. The amylose and amylo-

pectin starch components are present in a ratio of

about 1:3, respectively, which is similar to that of

wheat and corn. High amylopectin, or waxy oat,

and high amylose oat have not been reported in oat

genotypes.

0017Oat contains an abundant source of nondigestible

polysaccharides, which are carbohydrates that make

up dietary fiber. The nondigestible polysaccharides

are classified as either cellulosic or noncellulosic

carbohydrates. Cellulose is a linear polymer of beta-

1,4-linked glucose units, unlike the alpha-1,4-linked

fig0005 Figure 5 Kernels of cultivated white oat, Avena sativa (left), and

a derivative of a wild Avena sp. (right). The primary (large) and

secondary (small) kernels are attached.

fig0006 Figure 6 (see color plate 114) Kernels of cultivated white oat,

Avena sativa (left), hull-less oat, Avena nuda (center), and a de-

rivative of a wild Avena sp. (right). Whole oat kernels are pictured

on the top row and the corresponding groats on the bottom row.

OATS 4217

Nucellus

Phytin

crystal

Nucleus

Protein

body

Cell wall

Protein

body

Starch

Pericarp

Seed coat

Aleurone layer

Subaleurone

layer

Protein

bodies

Cell wall

Inner (starchy)

endosperm

Scutellum Endosperm

Nucleus

Phytin

crystal

Protein

bodies

Depleted

layer

Crease

Endosperm

Aleurone

layer

Germ

(c)

(b)

(a)

fig0007 Figure 7 Major structural features of the oat kernel. Oat kernel on the left has been split longitudinally to reveal the approximate size

and location of the major tissues. At the lower right is a cross-section view of the groat. (a), (b), and (c) are higher magnifications of

portions of the bran, starchy endosperm, and germ. (Reproduced from Webster FH (ed.) (1986) Oats: Chemistry and Technology. St Paul,

MN: American Association of Cereal Chemists, with permission.

4218 OATS

glucose units that make up starch molecules. Noncel-

lulosic polysaccharides include hemicelluloses, pectic

substances, and gums and mucilage. Oat gum con-

sists primarily of beta-glucans and pentosans. Beta-

glucans are defined chemically as linear molecules of

beta-1,3-and beta-1,4-linked d-glucopyranose units

that are associated with cell wall structural compon-

ents in both the bran and endosperm. Beta-glucans

may be soluble or insoluble in water and contribute to

the high viscosity of cooked rolled oats. Approxi-

mately 75% of oat beta-glucans are water-soluble.

Pentosans are l-arabinofuranose residues linked as

single-unit side chains to a backbone of beta-1,4-d-

xylopyranose residues. Pentosans are present mostly

in oat hulls and have been used in the commercial

production of furfural. Upon acid hydrolysis of oat

fiber, the polysaccharides yield sugars and sugar

derivatives of d-xylose, l-arabinose, d-galactose,

d-glucose, d-glucuronic acid, and 4-O-methyl-d-

glucuronic acid.

0018 Fiber-containing foods, upon ingestion, have a pro-

found effect on food utilization from mouth to anus.

Fiber directly affects gastric emptying time, rate

of nutrient absorption from the intestine, fecal bulk,

and frequency of bowel movements. Fiber indirectly

affects pancreatic hormone secretions, hepatic glu-

cose, and lipid metabolism. Under certain conditions,

water-soluble beta-glucans have been reported to

function in the small intestine as physiologically

active, cholesterol-lowering ingredients. The hypo-

cholesterolemic effects of beta-glucans tend to reduce

the potential risk of ischemic heart disease. Oat hulls

have little or no beta-glucan and are essentially water-

insoluble fiber, which acts as fecal bulk in the large

intestine. Total beta-glucan content in groats can vary

from less than 2% to greater than 6%. Higher con-

centrations of beta-glucans are found in oat bran,

which is obtained by separating endosperm and

bran fractions by sieving. Beta-glucan content in oat

bran is often higher than 7%.

Utilization of Oats

0019 Oat is grown primarily for use as a forage crop and

feed grain. Pliny, who reported on the human con-

sumption of oat, stated in his Natural History that the

Germanic tribes of the first century made ‘their por-

ridge of nothing else.’ Oat later gained wide accept-

ance in Ireland and Scotland, where a variety of

porridges were made. During the nineteenth century

in America, oatmeal was sold almost exclusively in

pharmacies and was suggested as a food for the

infirm. In the late nineteenth century, Ferdinand

Schumacher, a German immigrant, helped pioneer

oatmeal into the largest selling cereal in the USA.

His innovations included large-scale domestic milling,

movement of oatmeal from the pharmacy to the

grocery, and the development of packaging, brand

names, and promotional literature.

0020Oat and oat products are used extensively in a

variety of commercial food products. Thermal pro-

cessing of oat enhances a unique flavor and aroma

that contribute to utilization in cereals and baked-

food products. Whole groats and steel-cut (sectioned)

groats are processed into flakes. The traditional

rolled oats made from whole groats are thick flakes

requiring long cooking times, whereas flakes made

from steel-cut groats are thinner and require shorter

cooking times. Steam applied to sectioned groats pro-

duces variations of quick and instant flakes, with

cooking times of less than 3–5 min. Oat flour is util-

ized as an ingredient in a variety of food products,

and contains antioxidant activity in products likely to

undergo fat oxidation during storage. Oat bran,

obtained by sieving coarsely ground groats, is utilized

as an ingredient in a variety of hot and cold cereals,

and in baked products as breads, cookies, and

muffins. The high viscosity of oatmeal makes it useful

as a thickener in soups, gravies, and sauces, and a

meat extender in meat loaves and patties. Processed

oat hulls are used in the production of low-calorie,

high-fiber baked products. Studies have shown that

oat can be included in diets of celiac patients. The

prolamin storage proteins, which cause the chronic

condition in the intestine of celiac patients, are

denatured by thermally processing the groats. In

Sweden, Mill Milk Oat Drink was developed as a

supplement for people who lack the ability to produce

the lactase enzyme for digesting lactose found in dairy

milk. Nu-trim, a thermomechanically sheared prod-

uct from oat fiber, was developed for its high soluble

beta-glucan content and used as a fat substitute in

certain low-calorie products.

0021Oat has numerous industrial applications and

potential for other uses. Oat starch has adhesive

properties as a glue extender. Oatmeal has been used

for cosmetic purposes to absorb dirt and sebaceous

secretions from the skin. Oat hulls have cariostatic

properties to prevent dental caries. Oat hulls have

been used in the production of furfural and other

derivatives as components in petroleum extractions,

resins, plasticizers, insecticides, and pharmaceuticals;

they may be used as a brewery filter aid; a component

for making linoleum; and a component for making

antiskid tire treads.

See also: Carbohydrates: Classification and Properties;

Cereals: Contribution to the Diet; Dietary Importance;

Fats: Classification; Protein: Sources of Food-grade

Protein

OATS 4219

Further Reading

Coffman FA (1977) Oat History, Identification and Classi-

fication. Technical bulletin 1516. Washington, DC: US

Department of Agriculture, ARS.

Webster FH (ed.) (1986) Oats: Chemistry and Technology.

St Paul, Minnesota: American Association of Cereal

Chemists.

Welch RW (ed.) (1995) The Oat Crop: Production and

Utilization. London, England: Chapman and Hall.

Youngs VL and Forsberg RA (eds) (1987) Oat. In: Nutri-

tional Quality of Cereal Grains: Genetic and Agronomic

Improvement. Agronomy monograph no. 28. Madison,

Wisconsin: American Society of Agronomy.

Youngs VL, Peterson DM and Brown CM (1982) Oats. In:

Pomeranz Y (ed.) Advances in Cereal Science and Tech-

nology, vol. 5, pp. 49–105. St Paul, Minnesota: Ameri-

can Association of Cereal Chemists.

OBESITY

Contents

Etiology and Diagnosis

Fat Distribution

Treatment

Epidemiology

Etiology and Diagnosis

H Pande, Johns Hopkins Bayview Medical Center,

Baltimore, MD, USA

L J Cheskin, Johns Hopkins University, Baltimore,

MD, USA

Background

0001 The prevalence of obesity world-wide is increasing

rapidly. The WHO has designated obesity as a

major health problem throughout the world. In

1995, there were an estimated 200 million adults

classified as overweight and obese world-wide. By

2000, the number of overweight and obese adults

had increased to over 300 million. The obesity

epidemic is not restricted to industrialized soci-

eties. In developing countries, it is estimated that

over 115 million people suffer from obesity-related

problems.

0002 In the USA, the incidence of obesity among adults

increased by 50% from 1976 to 1994. The most

recent data demonstrate a prevalence of overweight

(BMI 25þ) or obesity (BMI 30þ) of 64% and of

obesity (BMI 30þ) of 30.5%. Nearly 80% of some

groups, e.g. middle-aged black women, are over-

weight or obese.

0003In most countries of Western Europe, the preva-

lence of obesity in adults is 15–25%. In the UK, over

20% of women and 17% of men are obese. Obesity is

also increasing in prevalence in Latin America and

the Caribbean. In South-east Asia, the Middle East,

China, Japan, and the Pacific region, a marked rise in

obesity is being seen in all populations and is now

being recognized as a major public-health problem.

0004Obesity is clearly associated with increased mor-

bidity and mortality. It is estimated to cause between

280 000 and 325 000 deaths per year in the USA. In

1995, the direct obesity-related health care costs in

the USA exceeded $50 billion – nearly 6% of the

entire national expenditure for health care.

Etiology

0005A simple and straightforward definition of obesity is

an excess of body fat. However, obesity is more than

just an accumulation of excessive amount of fat and

its cosmetic consequences. It is a chronic disease with

serious health consequences, is a major cause of

morbidity and mortality worldwide, and is now con-

sidered the most common disorder of nutrition in

developed countries.

0006The etiology of obesity is complex, multifactorial,

and incompletely understood, despite significant ad-

vances in recent decades. It is thought to result from a

4220 OBESITY/Etiology and Diagnosis

complex interaction between genetic, environmental,

metabolic, dietary, and behavioral factors.

0007 The pathogenesis of obesity can be considered the

sum of those biological factors, which increase the

predisposition towards expansion of adipose tissue

mass together with consequences of adaptation to

an environment that promotes and possibly rewards

increased food intake and decreased physical

activity.

0008 When daily energy intake is equal to daily energy

expenditure, a state of energy balance exists, and the

body weight stays constant. When energy intake

exceeds energy expenditure, a state of positive energy

balance exists, and body weight increases. Over-

weight and obesity are usually the result of months

and years of positive energy balance, resulting in

enlargement of fat cells.

Genetic Factors

0009 It has long been appreciated that obesity runs in

families. Familial studies confirm that BMI is highly

correlated among first-degree relatives.

0010 Twin studies have shown a similarity in BMI be-

tween twins, which is stronger in monozygotic than in

dizygotic twins. This correlation was found to persist

even when the twins were raised separately.

0011 Adoption studies have shown a significant correl-

ation between the BMI of biological parents and

adoptees, but not between the BMI of adoptive

parents and adoptees. These studies suggest that the

heritability of obesity may be as low as 10% (adop-

tion studies) to as high as 80% (twin studies).

0012 Some of the most compelling evidence for the gen-

etic basis of obesity comes from the discovery of five

mutations that cause spontaneous obesity and dia-

betes in mice. The Agouti (Ay), obese (ob), fat,

diabetes (db) and tubby (tub) genes have all been

cloned. In 1994, the discovery of the ob gene and its

product leptin in ob/ob mice was a significant ad-

vancement in our understanding of the genetics of

body-weight regulation. Since then, substantial pro-

gress has been made in characterizing genes related to

human obesity. Genes responsible for previously

existing mouse obesity mutations, Ay, db, ob, fat,

tub, have been identified and have led to an under-

standing of the physiological pathways underlying

each mutation.

0013 Leptin, for example, is a 16-kDa hormone pro-

duced by adipose tissue, which acts by binding to

receptors in the hypothalamus, which in turn alters

the expression of several neuropeptides that regulate

neuroendocrine function, appetite, and energy ex-

penditure. A decrease in body fat leads to a decreased

level of this hormone, which in turn stimulates food

intake. In addition, decreased leptin levels activate a

hormonal response that is characteristic of the starved

state, including reductions in the resting metabolic

rate. Increased body fat is associated with increased

levels of leptin, which act to reduce food intake. By

this mechanism, weight is maintained within a rela-

tively narrow range physiologically.

0014Mutation of the leptin gene, leading to deficiency

of leptin, results in the development of hyperpha-

gia, hypogonadotrophic hypogonadism, and severe

obesity with childhood onset (functionally equivalent

to the ob mouse model). So far, only two kindreds

with defects in leptin have been reported. In humans,

mutations have also been identified in the leptin

receptor in the hypothalamus (equivalent of the

db mouse model), in the melanocortin 4 receptor

(MC4R) and proopiomelanocortin (POMC), which

is a precursor for the natural ligand of MC4R (Agouti

model). Mutation in the prohormone and neuropep-

tide processing enzyme carboxypeptidase E in the fat

mouse model and its equivalent in human peptidase

PC1 have also been described.

0015Although monogenic mouse models have played an

important role in our understanding of the biology of

weight control, these mutations are rare and do not

explain the common genetic predisposition to obesity

seen in most human populations. They help us in

understanding why, within a relatively homogenous

environment, some individuals are lean and others

are obese. However, only a small fraction of human

obesity is due to a deficiency of leptin; instead,

human obesity is associated with increased plasma

concentrations of leptin. Thus, human obesity is char-

acterized by a central and/or peripheral resistance or

decreased responsiveness to leptin. The biological

effects of the daily peripheral administration of

recombinant human leptin in obese humans are cur-

rently being determined in clinical trials but are not

expected to be dramatic, considering the nonrespon-

siveness to the satiety effects of high physiologic levels

of leptin in obese individuals.

0016So far, more than 30 syndromes with a Mendelian

pattern of inheritance associated with obesity have

been identified. Most of these are pleiotropic syn-

dromes in which obesity is one of the several features.

0017Prader–Willi syndrome (PWS) is the most common

and best-characterized human obesity syndrome with

an estimated prevalence of 1:25 000 and autosomal

dominant pattern of inheritance. In addition to

obesity, it is characterized by hypotonia at birth,

small hands and feet, hyperphagia that usually

develops between 12 and 18 months of age, mental

retardation, short stature, and hypogonadism. Al-

though familial inheritance of PWS is sometimes

described, the vast majority of cases are sporadic. It

has been established that PWS is most often caused by

OBESITY/Etiology and Diagnosis 4221

a deletion of the paternal 15q11.2–12-chromosome

segment.

0018 A second, less common disorder, Bardet–Biedl

syndrome, which was previously incorrectly named

Laurence–Moon–Bardet–Biedl syndrome, is an auto-

somal recessive disorder. It shares many characteris-

tics with PWS, including an increased risk of obesity.

Studies of affected families have identified several

different chromosomal loci (chromosomes 16, 11, 3,

and 5) responsible for the syndrome, suggesting that

it is a heterogeneous condition, with obesity being

one of the common features.

0019 The more common forms of obesity are, however,

polygenic, and do not display a Mendelian pattern

of inheritance. It is a quantitative phenotype that

is probably influenced by numerous susceptibility

genes, accounting for variations in energy require-

ments, energy utilization and storage, and the meta-

bolic characteristics of the muscles, as well as being

strongly influenced by environmental factors.

0020 The strong environmental pressure to consume

calories in excess of the energy expended exceeds

the capacity for homeostatic adaptation in genetically

predisposed persons, leading to an energy imbalance

favoring fat storage. Genetic determinants of inter-

individual variation in obesity and related phenotypes

are likely to be multiple and interacting, with most

single variants producing only a modest effect.

Genome wide linkage studies in obese families and

in different ethnic groups have led to the estab-

lishment of the ‘human obesity gene map,’ which

contains entries for more than 120 genes and 16

chromosomal regions in which published studies

indicate a possible relationship to excess adiposity

or related phenotypes.

0021 Landmark discoveries of leptin, uncoupling pro-

teins, and neuropeptides involved in body-weight

regulation have highlighted the importance of mo-

lecular genetic factors in determining an individual’s

susceptibility to obesity. These factors alone, how-

ever, cannot explain the rapidly rising prevalence of

obesity in the past few decades and the current

obesity epidemic, as our genes have not changed sub-

stantially over this period. It is believed that our genes

simply permit us to become obese; the environment,

however, may determine whether or not we do,

indeed, become obese.

Metabolic Rate

0022 Metabolic rate is the function of genetic, medical

and voluntarily modifiable factors. The total daily

energy expenditure (TDEE) is the sum of resting

metabolic rate (RMR), the thermic effect of food

(TEF), and the energy expenditure in physical activity

(EEact). EEact can be divided into in activities of

daily living and energy expended in additional phys-

ical activity.

0023TDEE has been found to correlate with body

weight. The contribution of TEF and RMR to TDEE

is on average 10 and 60%, respectively. The available

evidence suggests that there is no association, or at

best a very small association, between RMR and TEF

and a tendency to gain weight. Activity-related energy

expenditure contributes to 30% of the TDEE, is the

most variable component, and may be the component

that predisposes to obesity and can be most readily

modified during efforts in weight control. Recent

evidence suggests that incidental physical activity,

such as large muscle contractile activity, which is,

unfortunately largely involuntary, is a significant

predictor of TDEE.

0024Decreases in TDEE can lead to obesity only if

energy intake is not reduced to match the new level

of energy expenditure. Humans have an effective

homeostatic system with the ability to finely match

the energy intake to energy expenditure to avoid

positive or negative energy balance and changes in

body weight. The effectiveness of this system is illus-

trated by the fact that adults on average consume

1 million calories every year, and a persistent mis-

match of even 1% (about 10 000 calories with energy

intake more than expenditure) would lead to an ac-

cumulation of fat and an increase in body weight of

about 30 pounds in a decade. However, usually the

body weight of an adult human increases only slightly

over the course of a decade, which means that the

homeostatic system operates with a remarkable

degree of precision.

Endocrine Causes

0025Endocrine disorders can upset the precise physiologic

regulation of metabolic rate and intake normally

operating to maintain a stable body weight. These

disorders include hypothyroidism, growth-hormone

deficiency, Cushing’s syndrome, insulinoma, polycys-

tic ovary disease, and primary empty sella syndrome:

all can lead to increased body fat. With treatment,

patients who are deficient, for example, in either

testosterone or growth hormone show a reduction

in visceral adiposity when their hormone levels are

normalized.

0026Studies have shown that patients with nonendo-

crine precipitants of visceral obesity have increased

cortisol and insulin secretion combined with low

production of sex steroids, such as testosterone in

men, and a low rate of secretion of growth hormone.

However, these altered endocrine functions seem

secondary to the obese state and at least partially

4222 OBESITY/Etiology and Diagnosis

responsible for the development of complications

related to the obesity. Weight reduction in general

is followed by normalization of endocrine function.

Environmental Factors

0027 Although genetic and metabolic-endocrine factors

may explain some of the variability between individ-

uals within a given environment, changes in the

environment must be responsible for the increase in

the prevalence of overweight and obesity in the popu-

lation in the past few decades. The rapid increase in

obesity that occurred from the 1970s through to the

late 1990s suggests that the environment has changed

to one that is now strikingly obesity-promoting.

0028 An increase in body weight would occur only when

environmental factors overwhelm the ability of the

regulatory process to adjust energy intake to energy

expenditure. Several changes in the environment have

contributed directly to the inability to match energy

intake to energy expenditure.

0029 The availability and consumption of energy-dense,

high-fat foods is one such factor. Numerous studies in

laboratory animals and humans indicate that high-fat

diets increase the risk of overeating and development

of obesity, and reducing dietary fat results in reduced

total caloric intake by reducing the probability of

overeating. Moreover, the appetite-suppressant effect

of leptin may be overridden by access to high-calorie

foods. Body-fat storage also occurs at a greater rate

when excess energy comes from fat than when it

comes from carbohydrate or protein.

0030 It is becoming clearer that, unlike total energy bal-

ance, fat balance is not as closely regulated. A positive

energy balance most often results from disruptions in

fat balance. It is known that the presence of excess

dietary fat does not acutely increase the rate of oxida-

tion of fat for energy. Also, unlike carbohydrates and

protein, the capacity for fat storage in humans is

virtually unlimited. Excess calories in the form of fat

are readily stored as triglyceride in adipose tissue,

with a very high efficiency.

0031 However even with a high-fat diet, most, but not

all, individuals will become obese. The ultimate

amount of weight gained will depend upon the

genetic makeup and nongenetic factors like voluntary

physical activity.

0032 Another environmental factor is the consumption

of more total calories in food as a result of an essen-

tially unlimited supply of convenient, high-energy-

density, relatively inexpensive, highly palatable foods,

along with an increase in the portion sizes. This

increases the likelihood of overeating.

0033 The fast-food and restaurant industry has played a

major role in this by using enticing food advertise-

ments, serving large portions of high-calorie foods,

and encouraging social eating behaviors. Studies have

demonstrated that many of the highly palatable food

items that have been introduced into the market by

the food companies are optimized to elicit a highly

positive reinforcing response, ensuring their con-

tinued ingestion and incorporation into our menu of

familiar foods.

0034The relationship between dietary fat consumption

and obesity, though, is not universally accepted. The

evidence for this association includes the close correl-

ation across nations between the percentage of energy

in the diet derived from fat and the prevalence of

obesity in that country, as well as laboratory studies

in animals and humans, which demonstrate that con-

sumption of a high-fat diet ad libitum tends to cause

excessive weight gain. However, the USDA nation-

wide food consumption surveys show that the aver-

age fat intake, and the average total calorie intake had

decreased between 1977 and 1988. Similar surveys in

UK from 1970 and 1990 and studies from France

have shown that fat consumption has decreased, yet

paradoxically, the prevalence of obesity in USA and

other countries has increased.

0035By contrast, self-reported food intake surveys

(National Health and Nutrition Examination Survey)

suggests that while dietary fat intake in absolute terms

has remained relatively constant over the period,

since the total energy intake has increased, the

proportional intake from fat has declined. It would

therefore be misleading to conclude that dietary fat

intake is not likely to play a role in the increase

in prevalence of obesity. Obesity prevalence might

have risen even faster, if not for the apparent diet-

ary-fat restraint and availability of reduced-calorie

food products.

Physical Activity

0036It is well recognized and appreciated that increased

energy intake and decreased physical activity, inde-

pendent of genetics, would inevitably promote the

development of obesity in individuals.

0037Various studies have shown that there is, indeed, an

inverse relationship between physical activity and

obesity. Cross-sectional, population, and cohort stud-

ies have shown that a low level of physical activity

predisposes to obesity, while a high level is protective.

In addition, normal-weight, postobese women who

report being nonexercisers gained more than twice as

much weight over 4 years of follow-up as regular

exercisers.

0038Our increasingly inactive lifestyle may play an im-

portant and perhaps dominating role in the increasing

prevalence of obesity. There are data to support that

OBESITY/Etiology and Diagnosis 4223