Obesity Epidemiology

DIETARY ASSESSMENT METHODS 103such as age, sex, and BMI, are often adjusted for in the analyses. Total energy intake isseldom used as an exposure or outcome variable because it is difficult to measure andinterpret. In free-living populations, between-person variations in total energy intakeare primarily determined by individual differences in physical activity, body size, andmetabolic efficiency; energy balance over a period of time is primarily reflected in bodyweight change. After controlling for total or lean body mass, the variation in total energyintake appears to be primarily determined by physical activity levels. The substantialcontribution of physical activity to between-person variations in total energy intake mayexplain a positive association between total energy intake and physical activity levelsobserved in some epidemiologic studies. 92 Although there are individual differences inmetabolic efficiency, it is infeasible to measure them in epidemiologic studies.The best way to deal with underreporting in analyses is unclear. Some studies excludeunderreporters from the data set. This approach not only reduces power, but may alsointroduce selection bias because subjects with high BMI levels are more likely to beexcluded. 72 Nonetheless, this approach can be used in sensitivity or secondary analyses.For example, in the analysis of the relationship between dietary fat and CHD, we calculatedthe ratio of reported caloric intake to predicted caloric intake for each participantusing their age and weight. Excluding women with the greatest likelihood for underreporting(the lowest quintile of the ratio) did not change the associations. 90 It is possiblethat simultaneous adjustment for BMI and total energy in the overall analyses may havealready taken care of potential biases caused by underreporting.These analyses demonstrate the importance of measuring and adjusting for totalenergy intake in epidemiologic studies. Adjustment for total energy intake in data analyseshas several conceptual and practical advantages. 14 First, control for total energyin epidemiologic studies mimics isocaloric substitution of one macronutrient (e.g., fat)for another (e.g., carbohydrates) in controlled experimental studies. In most situations,dietary composition rather than absolute intake is the primarily interest in nutritionalepidemiologic studies. Absolute increase or decrease in nutrient or food intakes can oftenlead to changes in total energy intake. Unless physical activity levels are also changed,the changes in food intake will, theoretically, lead to weight gain or loss that make it difficultto interpret the nutrient-disease association. Second, because measurement errorsfor energy and nutrient intakes are correlated, they tend to cancel each other in energyadjustednutrients. The correlated errors between nutrients and energy typically resultfrom overreporting or underreporting of specific foods. In many validation studies, thecorrelation coefficients between nutrient intakes calculated by the FFQs and referencemethods improve after adjusting for energy intake, 1 which can be largely attributed toreduced measurement errors. Third, energy adjustment also removes “extraneous variation”that results from the differences in energy requirements among individuals of differentbody sizes and physical activity levels. It should be noted that because energyadjustment leads to a reduction in between-person variations in nutrient intakes, it cansometimes reduce the correlations between energy-adjusted nutrient intakes calculatedby the FFQ and diet records.Another reason to adjust for total energy intake is to control for confounding in caseswhere total energy is associated with disease risk, and a spurious association betweennutrient intake and disease may occur because of confounding by total energy intake.Using an energy-adjusted nutrient instead of absolute intake should eliminate such confoundingbecause this variable is, by definition, not correlated with total energy intake.The most commonly used method to adjust for total energy is to calculate nutrient density(i.e., percentage of calories contributed by a macronutrient). Public health recommendationsare generally expressed in these units. For nonenergy contributing nutrients, density