Exercise and type 2 diabetes

There is already evidence that physical activity is associated with reduced risk of type 2 diabetes. The following study looks at this association in more detail, examining whether the frequency and intensity of physical activity makes a difference to the risk of type 2 diabetes in women.

Both vigorous exercise (e.g. running) and moderate exercise (i.e. walking) reduce the risk of type 2 diabetes in women. The more exercise taken, the greater the risk reduction. Just over three hours a week of vigorous exercise reduces the risk by 46%. Just three hours a week brisk walking reduces the risk by 42%. In other words, a daily brisk walk of thirty minutes approximately halves the risk of type 2 diabetes in women.

Participants were 70,102 women, aged 40 to 65 years, from the US Nurses' Health Study. All women were free from diagnosed diabetes, cardiovascular disease and cancer at the start of the study in 1986. Information on physical activity was obtained by questionnaire in 1986 and updated in 1988 and 1992.

Physical activity was assessed by:

Vigorous exercise was defined as an activity requiring 6 METs or more (a 6-fold or greater increase above resting metabolic rate) e.g. running, bicycling. Walking was defined as a moderate form of exercise, requiring an energy expenditure of 2 to 4.5 METs, depending on pace. Results were categorised into quintiles of MET score (1 = lowest activity; 5 = greatest activity).

5 MET hours per week vigorous exercise is the equivalent of 45 minutes per week.

5 MET hours per week walking is the equivalent of 1.5 hours brisk walking per week.

Every two years, participants were asked whether they had been diagnosed with diabetes and if so, sent a further questionnaire on symptoms, diagnostic tests and hypoglycaemic therapy to confirm the diagnosis according to a set of criteria. Cases were confirmed by an endocrinologist using medical records, unaware of the information in the supplementary questionnaires.

There were 1,419 cases of type 2 diabetes during eight years of follow-up.

As physical activity increased, risk of diabetes decreased. Using the first quintile as a comparison (lowest activity), the reduced risk of diabetes across the other quintiles of MET hours per week (from least to most activity) was 23%, 25%, 38% and 46%. Table 1 shows the amount of exercise associated with these percentages, relative risks and 95% confidence intervals. Results were adjusted for age, smoking status, alcohol consumption, menopausal status, parental history of diabetes, history of hypertension and high cholesterol level. After further adjustment for body mass index the reduced risk across the quintiles was 16%, 13%, 23% and 26%.

Compared with sedentary women (less than 2 MET hours /less than 20 minutes per week), active women (more than 10.4 MET hours/more than 1.5 hours per week) had a reduced risk of 41% (relative risk 0.59, 95% confidence interval 0.46 to 0.75).

In women whose only exercise was walking (using the first quintile as a comparison) the reduced risk of diabetes across the other quintiles of MET walking scores was 9%, 27%, 31% and 42%. Table 2 shows the amount of exercise associated with these percentages, relative risks and 95% confidence intervals. Further adjustment for body mass index attenuated these risk reductions.

Walking pace was also associated with risk of diabetes, after adjusting for time spent walking per week. Compared with an easy walking pace, risk reductions were 28% for a normal pace and 59% for a brisk pace. After further adjustment for body mass index, the risk reduction was 41% for a brisk pace. Table 3 shows the relative risks and 95% confidence intervals.

This is a large study with impressive results. It provides yet another example of the large benefits to be gained from just a little exercise, and shows that walking is just as beneficial as vigorous activities.

It is not surprising to find attenuated risk reductions when adjustments are made for body mass index. Firstly, individuals who exercise are going to be leaner (and consequently have a reduced risk of diabetes). Secondly, adjusting for weight when individuals already weigh less may be overcorrecting the results.