Long-term Fitness Reduces Risk of Diabetes, Prediabetes

Liam Davenport

May 18, 2016

Higher levels of cardiorespiratory fitness maintained or increased from early adulthood to middle age significantly reduce an individual's risk of developing prediabetes and diabetes, a new analysis of data from a long-term prospective study suggests.

Crucially, Lisa S Chow, MD, from the division of diabetes, endocrinology, and metabolism, University of Minnesota, Minneapolis, and colleagues found that the association between incremental increases and objective physical fitness and prediabetes and type 2 diabetes risk held even after taking into account factors such as body mass index (BMI).

Dr Chow told Medscape Medical News that, although the results were "modest on an individual level," when they are considered on a population level, they "really emphasize the importance of fitness and that improving fitness through physical activity can reduce prediabetes and diabetes."

The research was published online in Diabetologia on May 16.

Measuring Cardiorespiratory Fitness Over a 20-Year Period

Dr Chow and colleagues say that a number of previous studies have shown people who maintain or increase their cardiorespiratory fitness through adulthood have a lower risk of developing diabetes, abnormal metabolic measures, cardiovascular disease, and cardiovascular mortality than those whose fitness declines. However, these previous studies are limited for several reasons, including use of a largely male population, measurement of fitness over a limited duration (5–7 years) or measurement of fitness at varying intervals prospectively.

To measure the impact of continued cardiorespiratory fitness in adulthood on the development of diabetes and prediabetes, the team studied 4373 black and white participants from the Coronary Artery Risk Development in Young Adults (CARDIA) study over a longer period of 20 years.

The participants, who were recruited in 1985–1986 when they were aged 18 to 30 years, completed baseline treadmill exercise testing to determine cardiorespiratory fitness. This was followed by repeat testing in year 7 of the study, when they were aged 25 to 37 years, and again at year 20, when they were aged 38 to 50 years.

Dr Chow explained that cardiorespiratory fitness is an "objective way to measure the fitness of somebody." However, the relationship is "not exactly" a 1:1 ratio. She noted that "fitness can also have a significant genetic component…and everybody's response to exercise is a little different."

She added: "It is well documented, however, that the more physical activity you do, the more your fitness increases." Indeed, in the current analysis, cardiorespiratory fitness was significantly associated with self-reported physical activity at all three time points (P < .01).

Prediabetes/diabetes status was determined at baseline and at years 7, 10, 15, 20, and 25. Prediabetes was defined as a fasting glucose of 5.6 to 6.9 mmol/L, a 2-hour oral glucose tolerance test of 7.8 to 11.1 mmol/L, or an HbA1c level of 5.7% to 6.4%. Diabetes was defined as a fasting glucose ≥7 mmol/L, use of medications for diabetes treatment, a 2-hour glucose tolerance test ≥11.1 mmol/L, or HbA1c ≥6.5%.

By year 25, 1941 (44.5%) participants developed prediabetes and 505 (11.5%) developed type 2 diabetes. Participants who developed prediabetes or diabetes by year 25 were more likely to be older, of black race, and male than those who did not and were more likely to take blood-pressure medication, smoke, and have a higher BMI, among other factors.

Taking into account age, race, sex, center, and time-varying BMI at all time points, the researchers found that cardiorespiratory fitness was associated with a reduced risk of developing prediabetes or diabetes, at a hazard ratio per metabolic equivalent (MET) increase of 0.99898 (P < .01).

Expanding the model to include smoking, energy intake, alcohol intake, education, systolic blood pressure, blood-pressure medication, and LDL and HDL cholesterol levels reduced the hazard ratio further to 0.99872 per MET increase (P < .01).

Or, to put it another way, an 8% to 11% higher fitness level on treadmill exercise testing reduced the risk for developing prediabetes or diabetes by 0.1%. This is equivalent to either vigorous physical activity for 30 minutes daily 5 days per week or moderate physical activity for 40 minutes daily 5 days per week.

"These findings emphasize the importance of fitness in reducing the health burden of prediabetes/diabetes," Dr Chow and colleagues reiterate.

How Does Exercise Reduce Diabetes Risk?

The researchers put forward a number of hypotheses as to how continued fitness may reduce the risk of developing prediabetes and diabetes, including the effect of exercise on visceral fat, the anti-inflammatory effect of exercise, and its impact on insulin sensitivity.

However, Dr Chow believes that there may be more than one mechanism at play.

She said: "I think it's all of them, and I think for different people it might be different, in the sense that some people when they exercise tend to lose more fat, whereas other people when they exercise tend to have a better cardiac response."

"I think there is a real need to personalize fitness to individuals, but, of course, that's beyond the scope of this study."

In terms of future research, Dr Chow said the US National Institutes of Health is "very interested in how people will respond to a given exercise intervention" in an attempt to see how different people respond to the "same dose of exercise."

The CARDIA study is supported by contracts from the National Heart, Lung, and Blood Institute (NHLBI), the Intramural Research Program of the National Institute on Aging (NIA), and an intra-agency agreement between NIA and NHLBI. The authors declare that they have no relevant financial relationships.

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Diabetologia. Published online May 16, 2016. Abstract


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