Physical Activity in Individuals at Risk for Diabetes: Diabetes Prevention Program.

The Diabetes Prevention Program (DPP) was a randomized clinical trial of diabetes prevention in 3234 overweight individuals aged more than or equal to 25 yr with elevated fasting glucose levels and impaired glucose tolerance. The study was designed to determine whether a diet and exercise lifestyle intervention or treatment with metformin would delay the onset of diabetes in a heterogeneous group of individuals from 27 centers across the United States. Of the participants enrolled in the DPP, 45% were from ethnic or racial groups that suffer disproportionately from type 2 diabetes, including African Americans, Hispanic Americans, Asian Americans and Pacific Islanders, and American Indians. Participants were aged 25 yr and older, providing diversity of individuals by age as well.

Self-reported levels of leisure physical activity were assessed at baseline in the entire DPP cohort using physical activity questionnaires with three different time frames. One of these questionnaires, the Third National Health and Nutrition Examination Survey (NHANES III), was also administered to a national U.S. sample, a subgroup of which had impaired glucose tolerance. The time frame of these three activity assessment tools varied from the past week to the past year.

Given the success of the lifestyle arm of the DPP, it would be valuable to compare the DPP cohort with a national sample to better understand the generalizability of the DPP results. In addition, because (a) the physical activity questionnaires obtained at baseline in the DPP cohort assessed activity over different time frames and (b) the DPP population was highly diverse in regard to age, gender, race, and geographic location, it would be interesting to compare questionnaire results.

DISCUSSION

Many of the current findings regarding the physical activity levels of the DPP participants were anticipated and consistent with previous literature. Among the DPP participants, men reported participating in more leisure activity and, therefore, were less likely to report being inactive when compared with women. This was a consistent finding across three national U.S. surveys (National Health Interview Survey (NHIS) 1991, NHANES III 1988-1991, and the Behavioral Risk Factor Surveillance System 1992), where the ratio for the prevalence of physical inactivity for women compared with men ranged from 1.2 to 1.7 across these three surveys. That men reported more leisure activity and less inactivity than women was also seen in the subgroup of the NHANES III cohort with known type 2 diabetes. Physical inactivity measurement in the NHANES III was based on reported participation in leisure-time physical activity alone and did not include participation in nonleisure activity such as housework or occupational activity.

Based on the average number of hours of reported leisure activity per week averaged over the past year, the most active age group in the DPP cohort was aged more than or equal to 60 yr. This trend held, for the most part, across all sex and ethnic groups. This finding is inconsistent with the general notion that physical activity decreases with age in adults. Closer examination of national data collected around the time of the DPP, however, showed that the percent of adults, particularly men, who reported regular, sustained leisure physical activity (five or more times per week for 30 or more minutes per occasion) was somewhat higher in the aged 65- to 74-yr group compared with the aged 30- to 44- or 45- to 64-yr groups. For example, the percent of men and women reporting participation in regular, sustained physical activity in the 1992 NHIS was 24.1, 24.2, and 29.2% in men and 20.4, 20.6, and 21.3% in women for the three age groups (30-44, 45-64, and 65-74 yr). Likewise, the percent of men and women reporting participation in regular, sustained leisure physical activity in the 1992 BRFSS was 17.4, 18.9, and 26.8% in men and 18.5, 19.4, and 19.0% in women for the same three age groups.

An increase in leisure activity levels with retirement had also been seen in individuals from the Atherosclerosis Risk in Communities Study (7). These data suggest that a substantial number of older adults of retirement age spend a significant amount of their time engaging in measurable levels of leisure physical activity. An additional explanation may be that the older DPP participants may have been more physically active or lifestyle conscious throughout life, and thus may have attained a later age before becoming IGT and thus eligible for the DPP.

Three different activity questionnaires that varied by time frame were used to assess baseline physical activity levels in the DPP. The advantage of assessing activity using a survey with a short time frame, such as prior week, is that the estimate is less likely to suffer from recall bias. In contrast, assessment over a longer time period such as 1 yr is more likely to reflect "usual" behavior because activity levels can vary with season, poor health condition, or unexpected time pressure.

Comparing the leisure physical activity results from the three measures collected at baseline, it is not surprising that the MAQ and the LOPAR were not strongly related, at least in part because the latter assesses past-week activity, whereas the MAQ asks about physical activity over the past year. Previous studies in both adolescents and adults found stronger relationships between past-year physical activity assessment and the average of four past-weeks recalls collected over the same year than when just one past-week recall was used. It is possible that LOPAR repeated throughout the year would be more strongly related to the past-year MAQ than one time point alone.

Another consistent finding in the present study was that the leisure sections of the MAQ and the NHANES III questionnaires were more strongly related to each other than either one was to LOPAR. Part of the explanation for this finding is the specific components of physical activity that these three questionnaires assess. Both the MAQ and the NHANES III questionnaire assess the frequency of popular moderate- to high-intensity leisure activities, whereas LOPAR attempts to quantify the entire spectrum of intensities, including activities of daily living, by asking about the number of hours spent in sleep and in very light, light, moderate, and heavy leisure activities during the previous week. This difference in the components of physical activity assessed by these questionnaires resembles the comparison of food frequencies to that of food recalls in the dietary assessment literature and may explain, at least in part, the small correlations observed between the measures.

Biological variables of obesity (waist, hip, BMI, and WHR) and glucose tolerance (insulin, proinsulin, glucose, and HbA1) were examined in relation to the physical activity estimates obtained by these three questionnaires as criterion measures to validate the questionnaires. In general, the NHANES III and the MAQ were significantly related to each other and to most of these biological variables, with the exception of glucose and WHR. In contrast, the LOPAR was found to be less consistently related to any of these biological measures, suggesting that its use may be more appropriate in lower-functioning individuals for whom it was initially designed or in a less diverse population.

The most interesting finding was that the level of reported physical inactivity in the DPP cohort was less than that reported in the NHANES III subgroup with impaired glucose tolerance. This finding was evident across most age, BMI, and racial or ethnicity groups. Although the small sample size of the NHANES III subgroup may suggest a chance reason for this finding, another explanation might be the different sample selection in the two studies. The NHANES III survey is a cross-sectional observational study of a random sample of the U.S. population. In contrast, the DPP cohort represents a volunteer sample of individuals committed to a long-term intervention study.

In addition, one of the inclusion criteria for the DPP was that you had to be physically able to walk a quarter mile in 10 min, whereas such criteria for NHANES III did not exist. Considering the range of options described above for the finding that reported physical inactivity in the DPP was less than that reported in the NHANES III, the most likely explanation is that the DPP participants were a healthier and possibly more motivated group of individuals because they agreed to participate in this clinical trial and they met the DPP eligibility criteria. If this is true, these findings have implications for the translation of the DPP findings. On one hand, if the DPP cohort was healthier and somewhat more motivated than individuals in the general U.S. population with impaired glucose tolerance, then it could be more difficult to implement the DPP healthy lifestyle program in the latter. On the other hand, the impact of the DPP lifestyle intervention might be greater in those who are less active at baseline.

One of the main focuses in recent public health research is the effort to translate the lifestyle intervention portion of successful efficacy trials such as the DPP program into the community. Thus, the suggestion that those individuals in the general community in the United States with prediabetes would be less motivated than the DPP volunteer cohort is an important finding. Coupled with a smaller budget, less personnel, and less training and support, the likely addition of less motivated community participants adds to the challenge of these future translation efforts. In lieu of these difficulties, we need to adjust our expectations accordingly with regard to the potential impact of these community efforts.

In summary, baseline leisure activity in the DPP cohort suggests that men and women in the DPP trial may have been more active than the small subgroup of individuals from the NHANES III cohort who had impaired glucose tolerance. This may possibly be a "healthy volunteer effect"; if so, this would have implications for the future translation of the DPP lifestyle intervention in other populations. Finally, the weak relationship between leisure physical activity levels obtained with the MAQ and the LOPAR is consistent with the fact that they encompass both different time frames and different components of leisure physical activity.

Conclusion:

If the DPP participants were more active than a national sample of individuals with IGT, this would have implications for translation when using the DPP lifestyle intervention in less active or less motivated populations. Finally, the weak relationship between activity levels obtained with MAQ and LOPAR may result from the fact that they encompass different time frames and different components of leisure activity.