Lipids - Adequate Food for All - page 93

occur in some of the remaining, unhydrogenated double bonds, resulting in trans
configurations that are energetically more stable. However, these trans fatty acids
tend to increase blood cholesterol, decrease high-density (good) lipoprotein (HDL)
cholesterol, and raise low-density (bad) lipoprotein (LDL) cholesterol. Conjugated
trans fatty acids (e.g., vaccenic acid and conjugated linoleic acid) are natural compo-
nents of foods derived from ruminants, such as cattle, that may have antiatherogenic
and anti-inflammatory effects and may protect against diabetes and certain cancers.
e s s e n t i a L f a t t y a C i D s
linoleic Acid
Fatty acids in dietary fat that cannot be synthesized in the body from other precur-
sors and that have specific metabolic roles are considered essential . Linoleic acid
is a polyunsaturated fatty acid required for integrity of the skin, water retention in
tissues, normal growth, and optimal reproduction. As noted, this fatty acid is 18
carbon atoms long with two double bonds (18:2). It is also a member of the ω-6 fatty
acid series based on the position of the first double bond, counting from the methyl,
or omega (ω), end of the carbon chain. Sometimes, the letter n is substituted for the
omega symbol, so linoleic acid may be variously designated as a member of the ω-6
or n -6 fatty acid series.
α -linolenic Acid
α-Linolenic acid (18:3) is a polyunsaturated fatty acid required for normal brain
development in the fetus and that may influence postnatal vision, behavior, and cog-
nition. Essentiality is derived largely from its role as a starting point for synthesis of
the metabolically irreplaceable eicosapentaenoic (20:5) and docosahexaenoic (22:6)
fatty acids. It may be important that at least some of the last two forms be present
preformed in the diet of preterm infants. To the extent that these fatty acids are avail-
able preformed, the need for α-linolenic acid is diminished. These fatty acids are
members of the ω-3 (or n -3) fatty acid series. Eicosapentaenoic and docosahexaenoic
acids are also reported to reduce the risk of cardiovascular disease by reducing blood
cholesterol and promoting favorable shifts in ratios of LDL cholesterol and HDL
cholesterol in blood plasma.
f o o D s o u r C e s
Foods vary widely in fat concentration and proportions of fatty acids within that fat.
Animal fats tend to have higher proportions of saturated or monounsaturated fatty
acids than do vegetable fats, although polyunsaturated concentrations in animal fats
can be increased by diet. Corn, safflower, soybean, and sunflower oils are high in
linoleic acid (50% or more of total fat), but only soybean oil has a significant amount
(7%) of α-linolenic acid. Flaxseed and canola oils have significant concentrations of
α-linolenic acid but are relatively low in linoleic acid. Fish (particularly fatty fish)
and shellfish are generally good sources of eicosapentaenoic and docosahexaenoic
fatty acids.