6
Nutrition and Metabolism of Lipids
Bruce A Griffi n and Stephen C Cunnane
Key messages- Lipids are organic compounds composed of a carbon skeleton
with hydrogen and oxygen substitutions. The most abundant
lipids are sterols or esters of fatty acids with various alcohols such
as glycerol and cholesterol. - Fatty acids are the densest dietary source of energy, but lipids
also have important structural roles in membranes. The processes
controlling the synthesis, modifi cation, and degradation of fatty
acids contribute to the fatty acid profi le of membrane and storage
lipids. - By enhancing the taste of cooked foods, some dietary lipids are
potentially signifi cant risk factors for obesity and other chronic,
degenerative diseases that infl uence human morbidity and
mortality.
© 2009 BA Griffi n and SC Cunnane.
- Dietary lipids (fats) are emulsifi ed, lipolyzed (hydrolyzed), and
solubilized in the upper small gut before they are absorbed in
the ileum, entering enterocytes with the help of fatty acid-binding
proteins. - Lipids are precursors to hormones such as steroids and
eicosanoids, and dietary lipids are carriers for fat-soluble
vitamins. - Lipids are transported in the blood circulation as lipoprotein
particles: the chylomicrons, very low-density, low-density, and
high-density lipoproteins. - Some polyunsaturated fatty acids are vitamin like because they
cannot be synthesized de novo (linoleate, α-linolenate).
6.1 Introduction: the history of lipids in
human nutrition
The term “lipid” was introduced by Bloor in 1943, by
which time the existence of cholesterol had been
known for nearly 200 years and individual fats for 130
years. Cholesterol was named “cholesterine” (Greek
for bile-solid) by Chevreul in 1816, although he did
not discover it. Cholesterol’s association with aortic
plaques dates at least to Vogel’s work in 1843. Chevreul
isolated a mixture of 16- to 18-carbon saturated fatty
acids in 1813 that was called margarine because he
thought it was a single 17-carbon fatty acid, marga-
rate. The mixed triacylglycerol (TAG) of palmitate
(16:0) and stearate (18:0) was also called margarine,
whereas the triglyceride of oleate, stearate, and palmi-
tate became known as oleomargarine. Phospholipids
were discovered by Thudicum, who isolated and
named sphingosine in 1884 and also lecithin (phos-
phatidylcholine) and kephalin (phosphatidylethanol-
amine). The difference in polarity across phospholip-
ids is a key attribute of these molecules and was
termed “amphipathic” by Hartley in 1936 and
renamed “amphiphilic” by Winsor in 1948.
The fi rst understanding of how fat was absorbed
emerged in 1879 when Munk studied fat emulsions
and showed that lymph contained TAG after a fatty
meal, and even after a meal not containing TAG. In
1905, Knoop deduced that fatty acid β-oxidation
probably occurred by stepwise removal of two carbons
from the fatty acid. The probable role of two carbon
units as building blocks in the synthesis of fatty acids
was recognized by Raper in 1907, but it took until the
1940s for Schoenheimer, Rittenberg, Bloch, and others
to confi rm this, using tracers such as deuterated water
and carbon-13. The late 1940s was a seminal period
in our understanding of how fatty acid oxidation
occurs. Green and colleagues discovered that ketones
were fatty acid oxidation products, and Lehninger
demonstrated the role of mitochondria as the cellular
site of fatty acid oxidation. Microsomal desaturases
were shown to introduce an unsaturated bond into
long-chain fatty acids by Bloomfi eld and Bloch in
1960.