Fat intake of the average North American diet represents 38% of total calories consumed (8, 9). Over 95% of the total fat intake is composed of TG; the remainder is in the form of PL, free Fa, CH, and plant sterols. Total dietary TG in the North American diet is about 100 to 150 g per day. In addition to dietary intake, lipids enter the gastrointestinal tract by release from mucosal cells, biliary expulsion into the lumen, and bacterial action.
In almost no other instance can food choice influence nutrient composition as much as in the case of fats. As dietary TG vary widely in their FA composition, so does
FA consumption (T§bIe...4J..., see also more detailed tables of marine and nonmarine sources [Appendix Table I.V-A-1.9-A and Table I.Y-A:.19.-B.]). Large differences exist in the FA composition of oils from both plant and animal sources. Short-chain fatty acids (SCFA) (4 carbons) and medium-chain fatty acids (MCFA) (6 to 12 carbons) are found in vegetable oils and dairy fat, whereas fish oils and certain plants contain FA of the n-3 family. Long-chain n-3 FA can be found in a few terrestrial plants and range-fed nonruminant game animals. MUFA are found in plant oils, although meat fats also contain moderate amounts. As a rule, n-6 PUFA are found in vegetable fats and not in meat products, except in the case of C20:4n-6. Plant-derived oils vary widely in composition, largely because of genetic and environmental factors. In the case of animal fats, the composition of the feed also affects the final FA composition.
Intake of trans FA in the North American diet has not been firmly established, but it appears to range from 2 to 7% of the total energy intake ( 7, 10). Amounts of trans FA in the diet have remained relatively constant over past decades, partly because the rise in vegetable fat consumption has been counterbalanced by a decline in the trans FA content of many foods made with vegetable fat (6). Methodological limitations in measuring the various isomeric forms of dietary trans fA contribute to the imprecision in our knowledge of consumption levels.
The dietary contribution of CH varies significantly across foods. Typically, 250 to 700 mg of CH is consumed each day in the North American diet, with the larger proportion esterified to FA. Reduction of dietary CH levels can be readily achieved by excluding animal fats from the diet.
North American diets typically contain about 250 mg/day of plant sterols, with vegetarian diets containing much larger amounts ( 11). Most plant sterols are found as b-sitosterol, campesterol, and stigmasterol.
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