Our analyses reveal consistent evidence that primary eating time is inversely related to BMI. Other time diary research has found that Americans' time spent in primary eating activities has declined by an average of 11 minutes per day for women and 23 minutes per day for men between 1975 and 2006 [49
]. Taken together with the findings of this earlier study, the current research suggests that the rise in BMI over the past 30+ years may be associated, in part, with changes in Americans' time spent in primary eating activities. Specifically, based on our instrumental variables model, we estimate that an 11-minute decline per day in women's primary eating time may have translated into a .73 increase in BMI for women. Likewise, a 23 minute per day decline in primary eating time over this historical period would translate into 1.70 increase in BMI for men.
While time spent in primary eating activities has declined, trend analyses of time diary data show that secondary eating and drinking time has risen from an average of 20 minutes per day for women in 1975 to 80 minutes per day in 2006-07. Similarly, men's secondary eating and drinking time has risen from an average of 25 minutes per day to 70 minutes per day over that same historical period [49
]. Surprisingly, in the instrumental variables model, secondary eating time is associated with a significantly lower BMI for both men (p < .05) and women (p < .10). But, secondary drinking time is associated with higher men's and women's BMIs (p < .10). Our descriptive statistics reveal that secondary drinking time makes up approximately three-quarters of all time spent in secondary eating and drinking activities. Past studies have found a positive relationship between secondary eating and drinking time and BMI for women [18
] while others [25
] find little evidence of secondary eating/drinking effects on BMI. Ours is the first to parse out secondary eating and drinking time. As such, it sheds some light on the mixed findings in the literature, pointing the finger to increases in secondary drinking time (rather than secondary eating time) as a possible contributing factor to rising BMIs.
The social-psychological literature would suggest that less monitoring of caloric intake should occur when eating and drinking occur a secondary activities and thus time spent in these activities should be associated with higher BMI [20
]. We find this is true with respect to secondary drinking but not secondary eating. We do not have a ready explanation for the inverse relationship between secondary eating time and BMI. Given the limited and very mixed evidence regarding any possible linkage between time spent in secondary eating activities and BMI, further research on this point is sorely needed.
Findings regarding the role that food preparation time plays in BMI are intriguing. For women, the more time spent in food preparation and clean-up, the lower their BMIs. Presumably, more time spent in food preparation and clean-up is associated with using more primary foods and fewer prepared foods when cooking. It may also be associated with smaller serving sizes relative to those found in prepared meals. Since 83 percent of women but only 39 percent of the men identify themselves as the primary meal preparer in their households, it is not surprising that we do not observe the same relationship for the men. It would be interesting to investigate whether more time spent preparing meals by women translate into lower BMIs for other members of their households as well. Unfortunately, this question cannot be addressed with the ATUS data as only one member of each household in the sample provides time diary and BMI information.
Taken together, our findings regarding primary eating time, secondary drinking time, and time spent in food preparation and clean-up (by women) reinforce nutritional educators' emphasis on preparing meals and setting aside time where eating is one's primary focus. The role of secondary eating in healthy eating behaviours remains an open question, however.
While we did not find support for a link between physical activity and BMI, we found strong support for a link between physical inactivity
- as measured by television/video viewing time - and BMI. This finding is consistent with past research [8
] and with public health programs that encourage individuals to spend less time watching television/videos and more time being physically active [50
While our 24-hour diary may be too short to capture typical time spent being physically active each day, this is not true for television/video viewing time which is sufficiently prevalent to be adequately measured with a single, 24-hour diary. Indeed, it may be that television/viewing time is a more general marker for a sedentary lifestyle that could be used in place of the more infrequent physical activity time when analyzing 24-hour time diary data.
Our reduced form model estimates provide some insights regarding the role that changing prices, opportunity costs, and nonwage income may be playing in the rising overweight/obesity epidemic. Clearly, these economic factors matter. In the case of opportunity costs, we show that an increase in the hourly opportunity cost of time is associated with a significantly lower BMI for both women and men. It suggests that the recent economic recession, which precipitated a decline in workers' opportunity costs, may lead to more weight gain for Americans. And, this may be especially true for newly unemployed individuals who are drawing down on their savings that historically was a source of interest (i.e., nonwage) income. Indeed, it would appear that rising wage rates are not just good for the economy. They may also be good for Americans' weight management.
Finally, grocery prices are inversely related to BMI for both males and females. This is consistent with past research that has linked the historical drop in prices for energy-rich, processed foods to rising BMI in the United States [51
]. It also suggests a dilemma for policy makers. Lower food prices may increase food access, but at the same time they may also be serving to fuel greater caloric intake.
Our study results must be tempered with a couple of caveats. First, our proxies for biological differences in BMI are often statistically significant and there are clear sex-specific interactions with time use that merit further exploration. Although sample size limitations prevent us from exploring age and race/ethnicity time-use interactions, such research could provide valuable insights about the correlates of healthy body weight.
Second, our analysis presents a cautionary tale regarding the use of "small window" measures of physical activity time as it relates to BMI. Only about one-third of the women and two-fifths of the men in our ATUS sample report doing any 10-minute spells of physical activity that generate 3.3 METs or more during the 24-hour time period (See Table ). Recall that we do not find evidence of an inverse relationship between time spent in physical activity and BMI. This is counter to a number of past studies [4
] but not surprising given that our estimates of physical activity time are likely biased toward zero. The Centers for Disease Control recommends that adults age 18-64 spend 150 minutes per week
engaged in moderate intensity aerobic activity, or that they spend 75 minutes per week
in vigorous aerobic activity [40
]. Thus, even those who do follow these recommendations might not have been exercising on the randomly chosen diary day. Although it would be costly, future time-diary data gathering efforts should consider expanding the number of time diaries gathered for each respondent and/or asking additional questions about the usual time the respondent spends each week in certain infrequent, but potentially important activities.