This research examines trends in the propensity of obesity and overweight
(PO&O) in the United States from 1976 to 2001. An age-period-cohort
The objective was to isolate time periods and determine which time periods
are associated with a detrimental impact on Americans’ weight.
- The life course trajectory of weight gain
- The years in which Americans displayed the greatest degree of
- Whether trends differ by birth cohort.
The analyses use data from the National Health Interview Survey (NHIS), a
nationally representative annual health survey conducted by the National
Center for Health Statistics. The NHIS data are adjusted for the misreporting
of weight and height. The analyses use demographic methods,
specifically age-period-cohort analysis, and ordered logit models.
The ordered logit models show that the probability of being obese versus overweight,
and overweight versus normal weight has increased in every 5-year
period, after controlling for a host of factors. Between 1976 and 2001, the
probability of the adult population being obese has tripled. The probabilities of
obesity for women are now nearing those of men. Of the race/sex groups
studied, Black and Hispanic females possess the highest probabilities of being
obese, with Black women having a higher probability. Hispanic men have the
highest probability of being obese. When the data are disaggregated by year,
the results show the youngest age groups are increasing their likelihood of
being obese at the fastest rates.
Findings indicate that for every age group and for every birth cohort, current
group average Body Mass Indexes (BMIs) are larger than the previous period’s
BMI. Altogether, the adult population in the United States has increased its
BMI every 5-year period between 1976 and 2001. Further, the growth in
PO&O seems to be accelerating over time. Taking into account the characteristics
of the population does not mitigate the effect of period on Americans’
increased weight gain. One limitation of the data is that the sample contained
adults only. It is not known how many individuals entered the sample already
overweight or obese.
Because every birth cohort and age group showed an increase in BMI, explanations
that focus on specific subpopulations were not supported by the data.
Initially, it was hypothesized that a particular time period, age group, or cohort
could be isolated as an indicator of BMI increase. If that had been the case,
one could target policy interventions and design appropriate policy interventions
to mitigate the situation. The findings demonstrate that this was not the
case and that period effects dominate: the PO&O increases with every time
period, without exception.
This study provides some important insights into the age-period-cohort effects
of PO&O. Future research might concentrate on the aspects of American
culture that result in eating behavior that yields PO&O, how the American
lifestyle might be adapted to expend more energy, and how public policies
might arrest the PO&O trend in the United States.