Introduction to Human Nutrition

Nutrition and Metabolism of Lipids 87

In 1929, Mildred and George Burr discovered that
the absence of fat in a diet otherwise believed to
contain all essential nutrients impaired growth and
caused hair loss and scaling of the skin of rats. This
led to the isolation of the two primary “essential”
polyunsaturated fatty acids, linoleate (18:2n-6) and
α-linolenate (18:3n-3). The prostaglandins are a sub-
class of eicosanoids that were discovered in the early
1930s by Von Euler, who mistakenly believed that they
originated from the prostate gland. The link between
the eicosanoids and polyunsaturates, principally
arachidonate, was established in the 1960s.

6.2 Terminology of dietary fats

Lipids

Like other organic compounds, all lipids are com-
posed of a carbon skeleton with hydrogen and oxygen
substitutions. Nitrogen, sulfur, and phosphorus are
also present in some lipids. Water insolubility is a key
but not absolute characteristic distinguishing most
lipids from proteins and carbohydrates. There are
some exceptions to this general rule, since short- to
medium-chain fatty acids, soaps, and some complex
lipids are soluble in water. Hence, solubility in a “lipid
solvent” such as ether, chloroform, benzene, or acetone
is a common but circular defi nition of lipids.
There are four categories of lipids, as classifi ed by
Bloor: simple, compound (complex), derived, and
miscellaneous (Table 6.1). Simple lipids are esters of
fatty acids with various alcohols such as glycerol or
cholesterol. They include triacylglycerols (TAG =
neutral fats and oils), waxes, cholesteryl esters, and
vitamin A and D esters. Compound lipids are esters
of fatty acids in combination with both alcohols and
other groups. They include phospholipids, glycolip-
ids, cerebrosides, sulfolipids, lipoproteins, and lipo-
polysaccharides. Derived lipids are hydrolysis products
of simple or compound lipids, including fatty acids,
monoacylglycerols and diacylglycerols, straight-chain
and ring-containing alcohols, sterols, and steroids.
Miscellaneous lipids include some wax lipids, carot-
enoids, squalene, and vitamins E and K.

Saturated and unsaturated fatty acids

The main components of dietary fat or lipids are fatty
acids varying in length from one to more than 30
carbons. They are carboxylic acids with the structure

RCOOH, where R is hydrogen in formic acid, CH 3 in

acetic acid, or else a chain of one to over 30 CH 2

groups terminated by a CH 3 group. The various

names for individual fatty acids (common, offi cial)

and their abbreviations are complicated, and the use

of one or other form is somewhat arbitrary. The

basic rule for the abbreviations is that there are three

parts: number of carbons, number of double bonds,

and position of the fi rst double bond. Thus,

the common dietary saturated fatty acid palmitate is

16:0 because it has 16 carbons and no double

bonds. The common dietary polyunsaturated fatty

acid linoleate is 18:2n-6 because it has 18 carbons, two

double bonds, and the fi rst double bond is at the sixth

carbon from the methyl-terminal (n-6). Beyond six

carbons in length, most fatty acids have an even

number of carbons (Table 6.2). Older fatty acid ter-

minology referring to saturated or unsaturated

carbons in lipids that still occasionally appears

includes: aliphatic (a saturated carbon), olefi nic (an

unsaturated carbon), allylic (a saturated carbon adja-

cent to an unsaturated carbon), and doubly allylic

carbon (a saturated carbon situated between two

unsaturated carbons).

Table 6.1 Classifi cation of lipids

Simple lipids (fatty acids

esterifi ed with alcohols)

Fats (fatty acids esterifi ed with

glycerol)

Waxes (true waxes, sterol esters,

vitamin A and D esters)

Complex lipids (fatty acids

esterifi ed with alcohols plus

other groups)

Phospholipids (contain

phosphoric acid and, usually,

a nitrogenous base)

Glycolipids (lipids containing a

carbohydrate and nitrogen but

no phosphate and no glycerol)

Sulfolipids (lipids containing a

sulfur group)

Lipoproteins (lipids attached to

plasma or other proteins)

Lipopolysaccharides (lipids

attached to polysaccharides)

Derived lipids (obtained by

hydrolysis of simple or

complex lipids)

Fatty acids (saturated,

monounsaturated, or

polyunsaturated)

Monoacylglycerols and

diacylglycerols

Alcohols (include sterols,

steroids, vitamin D, vitamin A)

Miscellaneous lipids Straight-chain hydrocarbons

Carotenoids

Squalene

Vitamins E and K