Agriculture Reference
In-Depth Information
AdIPosIty to essentIAlIty
Lipids are substances that are greasy to the touch, insoluble in water, and soluble in
ether and include fats, oils, waxes, fatty acids, cholesterol, and related compounds.
An understanding of the physiological significance of dietary lipids was late arriv-
ing, but early observers noted that well-fed animals or humans accumulated body
fat deposits that disappeared rapidly as a consequence of starvation or hard work. In
ancient Egypt, oils were used as cosmetics and as medicines, and one report stated
that a mixture of fats from gazelle, serpents, crocodiles, and hippopotami was good
for growing hair. Soap-making by boiling tallow (beef fat) with alkali was an ancient
art, and boiling cattle hooves in water produced a floating liquid, called neat's-foot oil,
that was a useful lubricant because it did not solidify, even at freezing temperatures.
In Paris, F. C. H. Vogel conducted studies with lard (hog fat) and human fat and
noted in 1805 that when they were broken down by distillation, the product was acidic.
Fatty acids were described by M. E. Chevreul, a professor at Lycée Charlemagne,
in an 1823 treatise on the chemistry of fats; also noted was that glycerol separates
from fats when they are converted into soaps. Glycerol was originally discovered in
1783 by C. W. Scheele, a Swedish chemist, and named glycerin (after the Greek word
glykeros
, for sweet). Marcellin Berthelot established the chemical nature of a solid
fat (comparable to that found in tallow) in 1854 by heating glycerol with stearic acid
(a fatty acid) in a closed vessel to synthesize tristearin (three stearic acid molecules
combined with glycerol, known generically as a triacylglycerol).
The late eighteenth and the nineteenth century was an extremely active period for
lipid chemists. Oxidative rancidity was described, and the chemical nature of waxes,
phospholipids, cholesterol, and some of the saturated and unsaturated (containing one
or more double bond) fatty acids was explored. One of the more interesting controver-
sies during this period concerned whether body fat originated only from fat in the diet
or whether dietary carbohydrate could be converted to body fat. Ultimately, it was
established that body fat can have its origins in dietary fat, carbohydrate, or protein.
It was not until 1930 that George and Mildred Burr, at the University of Minnesota,
discovered the essential nature of a fatty acid. When linoleic acid was missing from
the diet, specific deficiency signs appeared, indicating that, for this lipid, carbohy-
drate was no substitute. For the cat (and perhaps most carnivores), it was shown in
1975 that arachidonic acid is also a dietary essential because it cannot be synthe-
sized from linoleic acid in the cat's tissues, even though such synthesis is possible in
humans. Evidence for the essentiality of other fatty acids was equivocal until 1967,
when α-linolenic acid was shown to be required by rainbow trout and in 1982 was
shown to be required by humans.
lIPId functIons
e
n e r g y
r
e s e r v e
As humans grow, mature, and age, characteristic changes in body composition occur.
These changes are influenced by gender, heredity, stature, and of course, food sup-
ply in relation to need. One of these changes involves deposition of fat in adipocytes
