Introduction to Human Nutrition

92 Introduction to Human Nutrition

skeletal muscle, etc.) of lean, undomesticated animals.
By contrast, in domesticated animals, TAGs or non-
membrane lipids present in subcutaneous and intra-
muscular adipose tissue deposits are the dominant
form of lipid on a weight basis. This is because domes-
tication usually involves rearing animals with minimal
exercise and on higher energy intakes, leading to more
subcutaneous and visceral TAG obtained through
both fat synthesis and deposition of dietary fat.
Animal meat lipids are the main dietary source of
arachidonate (20:4n-6), although it can also be
obtained from tropical marine fi sh.
Lipoproteins are the main form of lipid in the
blood (see Section 6.5). Like lipoproteins, milk lipids
also occur as globules consisting of a combination of
a mainly TAG core surrounded by a membrane con-
taining proteins, cholesterol, and phospholipids.
Phospholipids and cholesterol constitute the main
lipids of undomesticated edible fi sh, which usually
have low amounts of TAG or stored body fat. As in
domesticated animals, it is likely that subcutaneous
and intramuscular fat deposits of TAG will increase
in commercially farmed fi sh. Cold-water marine fi sh
are the main dietary source of the long-chain n-3
(omega-3) polyunsaturates eicosapentaenoate (20:5n-
3), and docosahexaenoate (22:6n-3), but the former

is also found in several types of edible seaweed. Tropi-

cal fi sh generally have higher arachidonate than do

cold-water fi sh.

Partial hydrogenation is a common feature of

unsaturated fatty acids in processed foods. Complete

hydrogenation makes fats very hard and is more

expensive than partial hydrogenation. Depending on

the applications and the source of the original oil or

fat, partial hydrogenation is an economical way to

control the properties of fats or oils used in food

production. Dietary diacylglycerols and monoacyl-

glycerols are used by the food industry for emulsifi ca-

tion of water- and oil-based components in foods

such as ice cream and mayonnaise.

6.4 Digestion, absorption, and transport

of dietary fat

The average daily intake of fat in a Western diet ranges

between 50 and 100 g and provides between 35% and

40% of total energy. It consists mainly of TAG, which

forms the principal component of visible oils and fats,

and minor quantities of phospholipids and choles-

terol esters (CEs). The physical properties of dietary

fat, such as their hardness at room temperature

(melting point) and subsequent metabolic properties

once in the body, are determined by the number of

double bonds in their constituent fatty acids (degree

of saturation or unsaturation) and length of the fatty

acid carbon chain (see Tables 6.2 and 6.3). As men-

tioned in Section 6.2, fats that are solid at room tem-

perature tend to consist of long-chain saturated fats

(>14 carbons, no double bonds), whereas oils consist

of long-chain unsaturated fats with several double

bonds. It has become conventional to refer to dietary

fats as “lipids” once they have been absorbed into the

body via the small intestine, although it is not incor-

rect to refer to dietary fat as “dietary lipid.”

Reception, emulsifi cation, lipolysis,

solubilization, and absorption

The digestion of dietary fat takes place in three phases,

known as the gastric, duodenal, and ilial phases. These

involve crude emulsifi cation in the stomach, lipolytic

breakdown by lipases and solubilization with bile salts

in the duodenum and, fi nally, absorption into the epi-

thelial cells or enterocytes lining the walls of the small

intestine or ileum. Digestion may actually be initiated

in the mouth under the infl uence of a lingual lipase

Table 6.3 Common food sources of lipids

Cholesterol Eggs, shellfi sh, organ meats
Phytosterols Soya products, olive oil
Short-chain fatty acids
(1–6 carbons)

Milk fat

Medium-chain fatty
acids (8–14
carbons)

Milk fat, coconut fat

Long-chain fatty acids
(16–20 carbons)

Saturates: animal fat, shortening, butter,

palm oil, peanuts

Monounsaturates: olive, canola oils

Linoleate: sunfl ower, saffl ower, corn oils,

soyabean

α-Linolenate: fl axseed oil, canola,

soyabean oil, walnuts

γ-Linolenate: evening primrose oil, borage

oil, blackcurrant seed oil

Stearidonate: blackcurrant seed oil

Arachidonate: lean meat and organ lipids

Eicosapentaenoate: marine cold-water

fi sh, shellfi sh, some seaweeds

Docosahexaenoate: marine cold-water

fi sh, shellfi sh

Trans fatty acids: partially hydrogenated

fats and oils