Nutritionally important carbohydrates in foods occur as monosaccharides, disaccharides, and polysaccharides. Quantitatively important monosaccharides are glucose and fructose; disaccharides include sucrose (glucose plus fructose) and lactose (galactose plus glucose). Dietary starches, which include amylose (unbranched glucose chains) and amylopectin (branched glucose chains), yield free glucose after digestion. The polysaccharide glycogen, also composed of branched glucose chains, is found exclusively in animal products at very low levels. Besides digestible saccharides, dietary carbohydrate also includes various forms of undigestible fiber, which has little nutritional value but may influence the digestion, absorption, or metabolism of ingested foods.

The substitution for sucrose, the principal form of sugar in the Western diet, with other sugars and sugar alcohols has been suggested to be of advantage when the reduction of
dietary glucose (diabetes) or energy (obesity) is a goal. Although fructose produces a smaller rise in plasma glucose than isocaloric amounts of sucrose, its use in a diet for diabetes is limited because of its adverse effects on total and low-density-lipoprotein cholesterol levels.3 The energy value of fructose is identical to that of sucrose (4 kcal/g), thereby providing no benefit as a means of reducing energy intake in obesity. The sugar alcohols sorbitol, mannitol, and xylitol also do not appear to offer any significant metabolic advantages in the treatment of diabetes.3 Their energy values range from 1.6 to 2.6 kcal/g, but their utility in weight reduction diets is limited by a laxative effect when consumed in large quantities and their reduced sweetening ability when compared with sucrose.4

Nonnutritive sugar substitutes approved by the U.S. Food and Drug Administration in the United States include aspartame, saccharin, acesulfame K, and sucralose. Aspartame is a dipeptide of aspartic acid and the methyl ester of phenylalanine, which is ˜200 times as sweet as sucrose.4 If used alone as a sweetener, it would contribute negligible calories. However, because it is sold with a dextrose and corn syrup filler, it contributes approximately one-eighth as many kilocalories as does sucrose. Saccharin is truly a nonnutritive artificial sweetener, with no caloric value and no effects on plasma glucose.4 Although a ban on saccharin use was proposed by the U.S. Food and Drug Administration in 1977 because of evidence of carcinogenicity, the U.S. Congress subsequently reversed the ban in response to public pressure.4 Acesulfame K (5,6-dimethyl-1,2,3-oxathiazin-4[3H]-one, 2,2-dioxide) is 200 times sweeter than sugar and provides no calories. Sucralose, synthesized by substituting three chlorine atoms for three hydroxyl groups on sucrose, is 600 times sweeter than sucrose and also yields no calories. When sold as a tabletop sweetener, sucralose contains 2 kcal per teaspoon as a result of added fillers.

Dietary lipid consists almost entirely (98% to 99%) of triglycerides, with the remainder being monoglycerides, diglycerides, free fatty acids, phospholipids, and cholesterol. Humans are capable of synthesizing all the fatty acids needed for endogenous triglyceride synthesis from carbohydrate and protein precursors, except for linoleic acid and linolenic acid.5,6 As the parent compounds of the omega-6 and omega-3 families of fatty acids, respectively, these two essential fatty acids are converted to polyunsaturated 20-carbon eicosanoids, which are the precursors of prostaglandins, thromboxanes, and leukotrienes.7 DRIs have not yet been formally established for linoleic acid and linolenic acid. However, clinical and biochemical symptoms of linoleic acid deficiency can be prevented by its inclusion in amounts approximating 1% to 2% of total kilocalories in the diets of adults. Based on similar criteria, the requirement for linolenic and other omega-3 fatty acids has been proposed to be 10% to 25% of the linoleic acid requirement.1 Typical American diets meet these preliminary recommendations.

Food composition tables are available that specify not only the carbohydrate, protein, and lipid content of foods, but also the amounts of fiber, cholesterol, and individual amino acids and fatty acids.8,9 The use of this information in dietary analysis has become more common as it has been compiled into computer databases. Currently available databases vary considerably in the number of food items included and the exact values used in computing nutrient intake, so care must be taken in choosing a software package.10,11 Computer databases have made it possible to convert information rapidly from standardized dietary histories into reasonably accurate, comprehensive descriptions of total nutrient intake. With this type of analysis, it has been possible to document changes over time in food consumption patterns in the United States. For example, the average daily percentage of kcal from carbohydrates has been shown to increase from 42.6% in 1977–1978 to 48.9% in 1989–1991. Over the same period, the average daily percentage of kcal from fat decreased from 40.5% to 34.8%, and the total intake decreased slightly from 1826 to 1774 kcal per day.12