Trend Study

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To examine secular trends in food consumption behaviors to understand their possible contribution to increasing energy intakes and adiposity in the American population.

All non-pregnant, non-lactating respondents aged 25-74 years with a reliable, self-reported 24-hour dietary recall and measured height and weight were included in the analytic sample.

Those over 74 years of age.

Recruitment

Recruitment for National Health and Nutrition Examination Surveys (NHANES) was published elsewhere. 

Design

Trend Study using dietary data from four consecutive NHANES to examine trends in frequency of eating episodes, meal and snack consumption, quality of food consumed and the energy density of foods.

Blinding used 

Not used 

Intervention

Not applicable 

Statistical Analysis

• Linear or logistic multiple regression models were used to assess secular trends in food consumption behaviors examined in this study
• All primary results were presented stratified by sex
• Output from linear or logistic multiple regression models was used to calculate the adjusted means or proportions (predictive margins) of food consumption variables (with SEs) for each sex and survey group
• Mexicans Americans and other Hispanics were grouped with whites in the NHANES 1999-2002
• This allowed for the categorization of race for all surveys as white, black and other
• In tests for trend across surveys, the four surveys from 1971 to 2002 were modeled as an ordinal independent variable
• The association of food consumption variables with energy intake and odds of obesity (BMI ≥30) were examined by using sex-specific linear and logistic regression models, respectively 
• In an attempt to understand whether low-energy reporting modified the associations of food consumption behaviors with the risk of obesity, the results were also presented adjusted for the ratio of reported energy intake to calculated energy requirement for basal energy expenditure (BEE)
• Data from four surveys were combined for these analyses and the data from the four different surveys were treated as independent variables from different populations for purposes of variance estimation
• The data was weighted in the analysis by using the National Center for Health Statistics (NCHS) assigned survey-specific sample weights so as to produce estimates that represented each population.

Timing of Measurements

1971-2002 

Dependent Variables

Obesity

Independent Variables

• Frequency of eating episodes
• Breakfast consumption
• Snack intake
• Evening eating
• Dietary energy density.

Control Variables

• Sex
• Race
• Age group
• Education
• Smoking status
• No leisure-time physical activity
• Self-reported chronic disease
• Energy intake: Basal energy expenditure ratio (EI:BEE). 

• Initial N: 39,094 included in the analysis, 51.4±0.3% women
• Attrition (final N): 39,094
• Age:
  • 25-39 years; 38±0.5%
  • 40-59 years; 42.3±0.4%
  • 60-74 years; 19.7±0.4%
• Ethnicity:
  • White; 86.6±0.6%
  • Black; 10.4±0.5%
  • Other; 2.9±0.3%
• Other relevant demographics:
  • Education
    • <12 years; 27.2±0.5%
    • 12 years; 32.7±0.5%
    • >12 years; 40.0±0.7%
  • Smoking Status:
    • Never smoked; 43.3±0.5
    • Former smoker; 25.1±0.4
    • Current smoker; 31.4±0.4
• Anthropometrics:
  • No leisure-time physical activity; 33.1±0.6
  • Self-reported chronic disease; 28.2±0.4
  • EI:BEE<1.2; 46.1±0.4
• Location: United States.

 

Key Findings

• The distribution of all characteristics examined varied across surveys (P≤0.002, chi-square test of independence for all variables)  
• The proportion of the population with >12 years of education and never smoked increased and those reporting <12 years of education, current smoker status or a ratio of energy intake to BEE <1.2 decreased from NHANES I to NHANES 1999-2002
• Over the three-decade span of the four surveys, the number of eating episodes reported in the 24-hour recall increased slightly in women (from 4.9 in NHANES 1971-1975 to 5.04 in NHANES 1999-2002) (P for trend =0.0002) but was unchanged in men
• Across surveys, the reported amount (in grams) of all foods and beverages, total energy intake and the amount (in grams) of food and energy per eating episode increased in both men and women (P for tend <0.0001)
• Americans reporting breakfast declined from 89% in NHANES I to 82% in NHANES 1999-2002 (P for trend <0.0001); however, the mean percentage of 24-hour energy intake from breakfast declined only in men
• The percentage of energy from evening food intake declined slightly in women but was unchanged in men
• Among men, the percentage reporting a snack and the number of snacking episodes decreased from 1971-75 to 1999-2002 (P for trend <0.0001); these snack behaviors were unchanged in women 
• However, the percentage of daily energy from snacks remained unchanged in men and increased slightly in women (P for trend =0.0007)
• Among snack reporters, although the amount (in grams) of foods and beverages reported per snacking episode did not changes from 1971 to 2002, the amount of energy consumed per snacking episode increased in both men and women (P for tend <0.0001)
• The energy density (in kcal per grams) of all foods and beverages reported in the recall increased in women but declined slightly in men
• The energy density (in kcal per grams) of foods and nutritive beverages increased over the period of the four surveys in both men and women (P for trend 0.0001)
• The energy density of all foods and beverages reported as snack, breakfast or evening meal increased across surveys (P for trend <0.0001)
• The sex differences in mean energy density of foods and beverages reported for breakfast or evening were present in earlier surveys but not in 1999-2002 (sex by survey interaction (P<0.05)
• The number of eating and snack episodes, mentioned of breakfast or snack and the amount (in grams) of foods and beverages and their energy density were significant independent predictors of higher energy intake in both men and women [P<0.0001 for all variables, except mention of breakfast in women (P=0.02)] 
• The interaction of sex and each food consumption variable (except mention of breakfast) for predicting obesity was not significant (P>0.05)
• The inverse association of the number of eating and snacking episodes with likelihood of obesity was not significant after adjustment for low-energy reporting in both sexes
• With adjustment for low-energy reporting status, the reported amount (in grams) of foods and beverages predicted obesity in all surveys combined (P≤0.001)
• For all surveys combined, the energy density of foods and nutritive beverages was a positive correlate of obesity irrespective of energy reporting status
• The association of obesity with the amount (in grams) of food intake and energy density of foods and nutritive beverages-adjusted for low-energy reporting status was also significant in each individual survey (expect energy density in NHANES I).

The results of this study suggest relatively small shifts in patterns of food consumption over the past three decades. These results are contrary to the 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, the results suggest an increase in the quantity and energy density of foods over the past three decades. It is not surprising, therefore, that the quantity of food and energy intake per eating episode were higher in later surveys relative to earlier ones.

In conclusion, the results do not support large increases in eating frequency, snacking or evening eating by the American population over the past three decades. The quantity of foods and their energy density increased beginning with 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.

Government:

National Institute of Health (NIH) and the intermural research program of the Department of Health and Human Services, NIH, National Cancer Institute

The authors note the following limitations:

• There were changes in the methods used to collect the 24-hour dietary recall in the NHANES over the course of the four surveys. 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, a positive secular trend in estimates of reporting of eating episodes was expected.
• This was not the case as the shifts in these food consumption variables were relatively minor and rarely in the expected directions
• Nevertheless, the increase in the 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. Therefore, the results for these variables should be interpreted with due consideration for possibility of confounding. The importance of bridging studies to allow understanding of these effects cannot be overstressed.
• It was also noted that the recalls obtained in the NHANES I and II were limited to weekdays, whereas weekend days were included in later surveys. Because food consumption and selective behaviors on weekends may differ from weekdays, day of the recall was included as a covariate in regression models used to obtain the estimates presented in the data.
• The eating occasions considered as breakfast or snack were labeled differently in the NHANES I and II than in later surveys. The extent to which the results reflect these differences was not known.
• The survey nutrient database used for estimating energy and nutrient intake as changed over the period of the four surveys. 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. However, energy content of the foods was not among the attributes that had changed in the database.
• Low energy reporting may have attenuated the possible association of dietary variables and outcomes such as body weight examined in the study. In the evaluation of the association of food consumption patterns with obesity, the associations were examined after adjustment for energy reporting status. The number of eating episodes, breakfast reporting, snacking and the amount of food and its energy density predicted higher energy intake in all surveys. However, after adjustment for low-energy reporting status, only the amount of foods and beverages and the energy density of foods and nutritive beverages consistently predicted a higher BMI in both sexes.