only partially oxidized, resulting in the excretion of urea and other nitrogenous products. Oxidation of 1 g of dietary carbohydrate and 1 g of dietary protein (which is oxidized to urea) each yield approximately 4 kcal, whereas oxidation of 1 g of alcohol yields approximately 7 kcal and oxidation of 1 g of dietary fat yields approximately 9 kcal. The energy cost of storing dietary fats as triglycerides is lower than that of converting protein or carbohydrates into fat. Donato and Hegsted (1985) have suggested that in growing animals, dietary fat can be stored as body fat with little energy expenditure and, therefore, that dietary fat stored as adipose tissue fat still yields approximately 9 kcal per gram. In contrast, energy is required to store dietary carbohydrates as body fat and 4 kcal per gram of dietary carbohydrate yields only approximately 3.27 kcal when stored as fat and subsequently oxidized for energy. Therefore, the ratio of the energy required to store dietary fat as body fat relative to the energy cost to store dietary carbohydrates or protein as body fat may be close to 9 to 3.27. That is, the conversion of fats in food to body fat (triglycerides) is more efficient than the conversion of carbohydrates or protein in food to body fat.
Food disappearance data indicate that energy available in the food supply had not changed from 1909 to 1982, averaging 3,500 kcal/day per person during 1909-1913 and 3,600 kcal/day per person in 1985 (see Table 3-3). However, these amounts are considerably higher than actual intakes reported by individuals (see below). The percentage of total calories available from carbohydrates decreased from 57% during 1909-1913 to 46% in 1985, whereas the proportion from fats increased from 32% during 1909-1913 to 43% in 1985 (see Figure 3-3).
Caloric intake by men and women has been reported in several recent surveys (Beaton et al., 1979; Braitman et al., 1985; Goor et al., 1985; USDA, 1987). Using the 24-hour recall method, Beaton et al. (1979) found that the mean daily energy intake was 2,639 kcal/day for men and 1,793 kcal/day for women. Using data from the National Center for Health Statistics (NCHS), which were based on 24-hour recall, Braitman et al. (1985) reported intakes of 2,359 kcal/day for men and 1,639 kcal/day for women.
Data from the Nationwide Food Consumption Survey (NFCS) can be used to compare energy intakes in 1965 and in 1977. Data from a single 24-hour recall plus 2-day records indicate that the energy intakes of males 9 to 64 years of age in 1977 were 10 to 17% lower than in 1965. Females ages 23 to 50 consumed 8.5 to 9% fewer calories in 1977 than in 1965 (USDA, 1984). The highest intakes were found among younger people. In 1965, males ages 15 to 18 consumed 3,008 kcal/ day compared to 2,698 kcal/day in 1977. 'The highest intake for females in 1965 was 2,146 kcal/ day among 12- to 14-year-old girls. This declined to 1,903 kcal/day by 1977. With advancing age, the average caloric intake declines for men and women (Goor et al., 1985). Among 20-year-old men, the intake reported between 1972 and 1978 in the Lipid Research Clinics Prevalence Study (LRC, 1980) ranged from 2,800 to 3,500 kcal/day, but for 59-year-old men, this range was 1,900 to 2,600 kcal/day. Intakes for 20-year-old women during the same period ranged from 2,000 to 2,200 kcal/day. Intakes had fallen to a range of 1,500 to 1,700 kcal/day by age 59. These data suggest that energy intake has declined for both sexes by approximately 10% during the past 10 to 20 years.
Caloric intake is influenced by many variables, including age, sex, environmental temperature, energy expenditure, pregnancy, hormonal status, and dieting behaviors. Figure 6-1 shows the effect of age on caloric intake among Americans based on three studies: the NFCS (USDA, 1984), the Health and Nutrition Examination Survey (HANES I), and studies of the Lipid Research Clinics Prevalence Study (Braitman et al., 1985; Goor et al., 1985). For both sexes, caloric intake peaked in the second decade of life and declined thereafter. At all ages, males had higher total caloric intakes than females and higher intakes for all three classes of macronutrientsfats, carbohydrates, and protein.
Caloric intake is also modestly affected by environmental temperature: at high and low ambient temperature, energy intake increases. Energy expenditure also affects caloric intake. For example, long-distance runners have high caloric intakes. More moderate levels of physical activity may increase food intake by lean but not by obese subjects. In controlled metabolic ward studies, Woo et al. (1985) compared the effect of three levels of physical expenditure on food intake by