Introduction to Human Nutrition

Nutrition and Metabolism of Lipids 91

occupied by more unsaturated fatty acids. At inter-
faces between non-miscible polar and non-polar sol-
vents, phospholipids also form a fi lm or monolayer.

Sterols

The main sterol of importance in human nutrition is
cholesterol. It has multiple roles including being:

● a vital component of biological membranes
● a precursor to bile salts used in fat digestion
● a precursor to steroid hormones.

Sterols are secondary alcohols belonging to the poly-
isoprenoids or terpinoids (terpenes), which have a
common precursor, isopentenyl diphosphate. Other
members of the terpinoids include squalene, carot-
enoids, and dolichols. Bacteria appear to be the only
life forms not containing cholesterol. Sterols have
a common cyclopentano(a)perhydrophenanthrene
skeleton with different substitutions giving rise to the
multiple sterols and steroids.

6.3 Lipids as components of the diet

Food or dietary sources of lipids are listed in Table

6.3. Cholesterol is found only in animal lipids, while

a variety of other phytosterols occur in plants.

Soyabeans, leafy plants, and lean animal meat are rich

in dietary phospholipids. Animal fat and plant oils

from seeds or nuts are rich in TAG.

The leafy and fruit components of plants contain

phospholipids and sterols, whereas seeds contain tri-

glycerides. With rare exceptions such as fl axseed

(linseed), edible green leaves are proportionally much

richer in α-linolenate than are seeds. Seed oils are

usually rich in either linoleate or oleate. Common

plant sterols include β-sitosterol, β-sitostanol, and

campesterol. Foods enriched with esters of plant

sterols are used widely to lower blood cholesterol via

the inhibition of cholesterol absorption in the gut.

Phospholipids and cholesterol constitute the

majority of lipids in tissues (gut, kidney, brain,

Trans-membrane

protein/receptor

Ion channel

Carbohydrate unit

Saturated

phospholipid

Unsaturated

phospholipid

Cholesterol

Phospholipid head groups

(asymmetrically distributed)

Outer surface

Inner surface

Figure 6.3 Simplifi ed schematic view of a membrane bilayer. The main components are proteins, free cholesterol, phospholipids, and carbohy-
drates. There are many different proteins with a myriad of shapes, membrane distribution, and functions, of which three are illustrated. Membrane
phospholipids principally help to create the bilayer. They have four types of “head groups” (choline, ethanolamine, serine, and inositol) that are
located at or near the membrane’s two surfaces. The two fatty acids in phospholipids are mixtures of 16- to 22-carbon saturates, monounsaturates,
and polyunsaturates in all combinations, with those rich in unsaturated fatty acids occupying more space; hence, their trapezoid shape compared
with the narrower, rectangular shape of the more saturated phospholipids. Free cholesterol represents 30–40% of the lipid in most membranes.
The many different carbohydrates are on the membrane’s surfaces and are bound to lipids and/or proteins in the membrane.