How Do Fitness Level and Body Weight Status Predict the Risk of Developing Type 2 Diabetes in Women?

Over the past 40 years, there has been a sharp increase in the worldwide prevalence of type 2 diabetes in women. While some of the increase is likely due to the increased prevalence of obesity in recent decades, an additional important risk factor for type 2 diabetes is physical inactivity. One potential problem with previous studies regarding inactivity and type 2 diabetes is the fact that physical activity was typically estimated by using questionnaires. Sometimes people are not always honest when they are asked how physically active they are, or they may not remember accurately. As a result, physical activity questionnaires tend to yield somewhat inaccurate data.

As opposed to questionnaires, which estimate physical activity, a maximal treadmill exercise test provides an objective measurement of cardiorespiratory fitness; which is defined as the ability of the body to use oxygen at the cell level. In order to examine the contributions of body weight status and fitness level in determining the future risk of type 2 diabetes, researchers followed 6249 healthy women with an average age of 44 years who underwent a comprehensive examination in the Cooper Clinic. The women were divided into categories of low, moderate, or high fitness based on their treadmill test results and age category. During an average follow-up period of 17 years, 143 of these women developed type 2 diabetes. Using the low fit women as a reference group, moderate and high fit women were 14% and 39% less likely to develop type 2 diabetes, respectively. Women were also placed into 3 categories of body weight status based on their body mass index (BMI). When compared to normal weight women, overweight and obese women were 2.3 and 3.7 times more likely to develop type 2 diabetes, respectively.

In perhaps the most important analysis, the researchers then placed the women into 4 groups of fitness combined with fatness; these categories were:

1. Group 1. Fit-normal weight (moderate or high fit with normal BMI)
2. Group 2. Unfit-normal weight (low fit with normal BMI)
3. Group 3. Fit-overweight (moderate or high fit with overweight or obese BMI)
4. Group 4. Unfit-overweight (low fit with overweight or obese BMI)

The results are shown below in the Figure:

In the Figure, we see that among normal weight women (categories 1 and 2), both fit and unfit groups were at a very similar low risk for developing diabetes. In the overweight women (categories 3 and 4), fit-overweight women and unfit-overweight women were 1.8 and 2.6 times more likely to develop type 2 diabetes compared to fit-normal weight women. Importantly, risk within the fit-overweight women was significantly lower than that of the unfit-overweight women.

The researchers concluded that both low fitness and overweight are strong and independent predictors of type 2 diabetes in women. Among overweight women, some of the increased risk can be lessened by obtaining a moderate to high level of fitness. Thus, all women are encouraged to meet the Physical Activity Guideless for Americans, and to obtain a reasonable body weight.

Reference

Sui, X., et al. (2008). A prospective study of cardiorespiratory fitness and risk of type 2 diabetes in women. Diabetes Care, 31(3):550-555.

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