Is the association between dietary fat intake and insulin resistance modified by physical activity?
The rising prevalence of type 2 diabetes, a condition associated with insulin resistance, is commonly attributed to changes in dietary patterns and physical activity levels in susceptible populations. However, few studies have described the independent effects of dietary intake and physical activity on the degree of insulin sensitivity within populations or examined the possibility of interactions between dietary factors and physical activity. This study was undertaken to describe the relationship between the quantity and pattern of dietary fat intake on fasting insulin levels (a marker of insulin sensitivity) and to investigate whether the association was modified by physical activity. A cross-sectional study of 815 nondiabetic men and women (30 to 71 years) recruited from a population-based sampling frame was undertaken. Diet was characterized using a semiquantitative food frequency questionnaire. Physical activity level (PAL), the ratio of total energy expenditure to basal metabolic rate, was estimated using individually calibrated heart rate monitoring, a method previously shown to be an objective and valid method for assessing total energy expenditure. In a linear regression model adjusted for total energy intake, total fat intake bordered on a significant association with fasting insulin (b = 0.000081; P =.058), and the polyunsaturated to saturated fat ratio (P:S ratio) of the diet was negatively associated with fasting insulin concentration (b = -0.37, P <.001). A negative association was observed between the PAL and fasting insulin (b = -0.12, P =.025). The association of the P:S ratio and PAL with fasting insulin remained significant when adjusted for each other and for total fat, total energy intake, body mass index (BMI), waist-to-hip ratio (WHR), age, sex, family history of diabetes, smoking status, and alcohol intake (P:S ratio, b = -0.24, P =.003; PAL, b = -0.13; P =.007). The association with total fat intake was no longer significant in this multivariate model (b = 6.7 x 10(-6); P =.858). There was no evidence for an interaction between total dietary fat intake and PAL (b = -0.000048; P =.243) or between the P:S ratio and PAL (b = -0.013; P =.949). These data demonstrate an independent association between the P:S ratio of the diet, the overall level of physical activity, and the fasting insulin concentration, a marker of insulin sensitivity. There was no evidence that the association between dietary fat intake and insulin resistance was modified by physical activity. The findings provide further support for efforts to promote increases in overall physical activity and modifications in the pattern of dietary fat intake in the whole population.