The results of this study suggest relatively small shifts in patterns of food consumption over the past 3 decades. These results are contrary to our expectation of a population-wide increase in frequency of eating as a result of an increase in the “grazing” type of food consumption behaviors. Instead, our results suggest an increase in the quantity and energy density of foods over the past 3 decades. It is not surprising, therefore, that the quantity of food and the energy intake per eating episode were higher in later surveys relative to earlier ones.
To our knowledge, no published reports are available of secular trends in overall eating frequency or energy density of self-reported American diets. In our study, little change in amounts (in g) reported or energy density was apparent from 1971 to 1980; increases in amounts or energy density first appeared in the NHANES III (1988–1994), and a further increase occurred in the NHANES 1999–2002. These trends roughly parallel the trends in increasing prevalence of adiposity first noted in NHANES III and subsequent surveys relative to the NHANES I and II (1
). Although, given these trajectories, it is tempting to speculate that the amount (in g) and energy density of foods may have contributed to the trend for increasing body weight (), methodologic limitations (discussion to follow) temper the possible conclusions. Our finding of a positive trend for amounts of foods and beverages reported is consistent with other reports of an increase in portion sizes of foods reported in US surveys (9
). Rolls et al (38
) have shown that higher portion sizes increased energy intake irrespective of body weight in both men and women. Furthermore, the effects of portion size and energy density on energy intake were additive (39
FIGURE 1 Secular trends (1971–2002) in adjusted mean (±SE) energy density of foods and nutritive beverages, and amount of all foods and beverages, and the prevalence of obesity (BMI ≥ 30) in the US population. Data are from National Health (more ...)
Our results suggest that changes in food selections of women over the past 3 decades were somewhat worse than those of men. For example, the increase in total amount of foods and beverages from NHANES I to NHANES 1999–2002 was similar in men and women (≈14% increase); however, the corresponding increase in mean energy intake was 8% in men and 18% in women. This may reflect a greater increase in energy density of foods selected by women. In all surveys, the energy density of snacks reported by women was higher than for men. In the NHANES I and II, the energy density of breakfast and evening foods was lower in women than in men; however, in later surveys, the sex differences in energy density of breakfast and evening foods disappeared, because the slope of energy density was steeper in women. These results suggest a putative reason for the sex disparity in the rate of increase in adiposity. [The prevalence of obesity and the percentage increase in prevalence of obesity is slightly higher among women than among men (1
The decline in the number of adult Americans who report breakfast over the span of the 4 surveys in our study is in accord with a previous report that used 1 d of dietary data from the Continuing Survey of Food Intakes by Individuals, 1965–1991 (40
). The finding of a decline in the number of Americans who reported breakfast should be viewed with concern given that breakfast consumption has been reported as a characteristic of successful weight-loss maintainers in the National Weight Control Registry (41
). Also, breakfast intake is generally believed to be a positive predictor of adequacy of micronutrient intake, although the results vary with foods selected (42
In our study, the percentage of men (not women) mentioning a snack decreased from 91% in the NHANES I to 86% in the NHANES 1999–2002. Other reports on trends in snacking patterns were limited to children (17
) and young adults (18
) and are not directly comparable to our study. Nicklas et al (44
) reported that the number of eating and snacking episodes reported in a 24-h recall by children in the Bogalusa Heart Study declined from 1973 to 1994. In 2 reports from the Continuing Survey of Food Intakes by Individuals (1977–1996), the investigators concluded that the prevalence of snacking had increased among children aged 2–18 y and young adults aged 19–29 y (17
). Those studies estimated prevalence of snacking in a different reference period based on 3 d of dietary data that included a mixture of recalls and records and used a slightly different definition of snacking, and the prevalence rates were not adjusted for differences (if any) in characteristics of respondents in these surveys.
We urge cautious interpretation of our results because of changes in the methods used to collect the 24-h dietary recall in the NHANES over the course of the 4 surveys (19
). Because the NCHS did not conduct any bridging studies to determine the systematic effect of changes in dietary methods on reporting of meals, snacks, or food and nutrient intakes, the confounding of time effect with the method effect remains a possibility. The multiple pass methods used for obtaining dietary recalls in later surveys may be expected to improve the recall of all possible eating and snacking episodes. However, the eating episodes in earlier surveys may be underreported; in which case, we may expect a positive secular trend in estimates of reporting of eating episodes (or other variables obtained from the dietary recall). As is evident from and , this was not the case. Notably, the observed shifts in these food consumption variables were relatively minor and rarely in the expected direction. For example, the mention of breakfast on the recall day declined consistently in both men and women, and the number of snacking episodes either declined (men) or remained unchanged (women) from 1971 to 2002. Nevertheless, the increase in reported quantity of foods, energy intake, and energy density in the NHANES III coincides with changes in dietary methods and is in the expected direction with improved recalls from the use of multiple-pass methods (45
). Therefore, the results for these variables should be interpreted with due consideration for possibility of confounding. Clearly, the importance of bridging studies to allow understanding of these effects cannot be overstressed.
We also note that the recalls obtained in the NHANES I and II were limited to weekdays, whereas weekend days were included in later surveys (19
). Because food consumption and selection behaviors on weekends may differ from weekdays, we included day of the recall as a covariate in regression models used to obtain the estimates presented in –. As mentioned in Subjects and Methods, the eating occasions we considered as breakfast or snack were labeled differently in the NHANES I and II than in later surveys. The extent to which our results reflect these differences is not known. Finally, the survey nutrient database used for estimating energy and nutrient intake has changed over the period of the 4 surveys (46
). The database on nutrient composition of foods has expanded, and values of some nutrients may have changed because of improved analytic technology and food-sampling methods (46
). However, energy content of foods (variable used in this study) is not among the attributes that have changed in the database.
Low-energy reporting has been noted in the NHANES II and the NHANES III and is more likely to occur in association with higher body weight and a low level of education (29
). Low-energy reporting may attenuate the possible association of dietary variables and outcomes such as body weight examined in this study. In our evaluation of the association of food consumption patterns with obesity, we also examined these associations after adjustment for energy reporting status. The number of eating episodes, breakfast reporting, snacking, and, not surprisingly, the amount (in g) of food and its energy density predicted higher energy intake in all surveys. However, after adjustment for low-energy reporting status, only the amount (in g) of foods and beverages and the energy density of foods and nutritive beverages consistently predicted a higher BMI in both sexes.
In conclusion, our results do not support large increases in eating frequency, snacking, or evening eating by the American population over the past 3 decades. The quantity of foods and their energy density increased beginning with the NHANES III and may be implicated in contributing to higher energy intake and weight gain. However, these results coincide with changes in dietary methods in NHANES III and warrant cautious interpretation.