Differences between estimated caloric requirements and self-reported caloric intake in the women's health initiative

Abstract

Purpose

To compare energy intake derived from a food frequency questionnaire (FFQ) with estimated energy expenditure in postmenopausal women participating in a large clinical study.

Methods

A total of 161,856 women aged 50 to 79 years enrolled in the Women's Health Initiative (WHI) Observational Study (OS) or Clinical Trial (CT) [including the Diet Modification (DM) component] completed the WHI FFQ, from which energy intake (FFQEI) was derived. Population-adjusted total energy expenditure (PATEE) was calculated according to the Harris-Benedict equation weighted by caloric intakes derived from the National Health and Nutrition Examination Survey. Stepwise regression was used to examine the influence of independent variables (e.g., demographic, anthropometric) on FFQEI-PATEE. Race, region, and education were forced into the model; other variables were retained if they increased model explanatory ability by more than 1%.

Results

On average, FFQEI was approximately 25% lower than PATEE. Regression results (intercept = −799 kcal/d) indicated that body mass index (b = −23 kcal/day/kg·m⁻²); age (b = 15 kcal/day/year of age); and study arm (relative to women in the OS, for DM women b = 169 kcal/d, indicating better agreement with PATEE) increased model partial R² > .01. Results for CT women not eligible for DM were similar to those of women in the OS (b = 14 kcal/d). There also were apparent differences by race (b = −152 kcal/d in Blacks) and education (b = −67 kcal/d in women with<high school).

Conclusion

This large, carefully studied population confirms previous observations regarding underestimates in self-reported caloric intake relative to estimates of metabolic need in younger women, and those with higher weight, with less education, and in Blacks. These differences, along with effects related to intervention assignment, underline the need for additional research to enhance understanding of errors in dietary measurement.

Introduction

Food frequency questionnaires (FFQ) are commonly used to assess diet in large-scale epidemiologic and clinical studies 1., 2.. Because they rely on limited food lists, estimates of total caloric (energy) intake (EI) derived from FFQs generally fall short of estimated metabolic requirements 3., 4.. In comparing EI estimates from FFQs to other methods including diaries or recalls, previous studies have identified larger underestimates of EI in women 4., 5., 6., among the obese 3., 7., 8., and in Blacks (9).

The Women's Health Initiative (WHI), a multifaceted research program examining factors associated with disease risk in postmenopausal women, presents an opportunity to examine differences between EI estimated by an FFQ and reasonable estimates of metabolic need based on height, weight, and age, in a large sample of post-menopausal women. In addition to differences in age, regional, and racial/ethnic representation, the trial has a unique characteristic related to diet. Women were eligible for the Diet Modification (DM) component of the Clinical Trial (CT) if they had baseline fat intakes ⩾32% of calories (energy) from fat (CF), as estimated by the WHI FFQ. This screening was meant to exclude women already consuming low-fat diets, thereby ensuring a substantial difference in dietary fat intake between the control and low-fat intervention groups. In contrast, women in the Hormone Replacement Therapy (HRT) component of the CT or the Observational Study (OS) had no dietary fat intake eligibility requirement.

The purposes of this study are to compare estimates of EI obtained from the WHI FFQ (FFQEI) with estimates of total energy expenditure (TEE) based on height, weight, and age (1), and to examine associations between observed differences and demographic and other background data. Under conditions of energy balance, TEE is a de facto measure of EI, an assumption that underlies the comparison between energy consumption and expenditure. In general, body mass is extremely well regulated over moderate to long periods. This is illustrated by the fact that a 0.5-kg gain in body mass per year, approximately the US average for adults (10), is equivalent to a net difference between expenditure and intake of about 0.050 MJ (12 kcal) per day.