New data on over 1.5 million male conscripts in Sweden reveals that 18-year-olds with low aerobic capacity and low muscle strength are more than three times likelier to develop type 2 diabetes over the following decades than their fitter counterparts, even after taking into account body mass index (BMI).
The current findings are supported by several interventional studies and, taken together, point to a clear public-health message: "The enhancement of cardiorespiratory and muscular fitness through habitual physical activity in all persons should be recommended as a front-line therapy to address the public-health burden of type 2 diabetes mellitus," he states.
Low Muscle Mass, Low Aerobic Capacity Triple T2D RiskTo examine the relationship between physical fitness in early life and later type 2 diabetes risk, the researchers studied a national registry of 1,534,425 18-year-old males conscripted into the Swedish military between 1969 and 1997.
The conscripts, who had no severe chronic medical conditions, including diabetes, or documented disabilities, underwent a 2-day standardized physical and psychological examination on entry to the military, from which data on baseline aerobic capacity and muscle strength was extracted.
The Swedish Hospital Registry and Swedish Outpatient Registry were then used to identify all cases of type 2 diabetes diagnosed between 1997 and 2012, and these were then linked to the physical fitness records at conscription.
The mean follow-up period was 25.7 years, and the mean age at the end of follow-up was 45.9 years. During 39.4 million person-years of follow-up, 34,008 (2.2%) conscripts were diagnosed with type 2 diabetes, at a mean age of 44.7 years.
The absolute difference in cumulative incidence of type 2 diabetes between the lowest and highest tertiles of both aerobic capacity and strength was 0.22% at 20 years of follow-up, 0.76% at 30 years, and 3.97% at 40 years.
After adjustment for year of conscription examination, BMI, family history of diabetes, education, and neighborhood socioeconomic status, the team found that muscle strength and aerobic capacity were both independently associated with the risk of developing type 2 diabetes.
Specifically, among individuals with high aerobic capacity, low - as opposed to high - muscle strength was associated with a hazard ratio of developing type 2 diabetes of 1.58 (P < .001).
In contrast, among those with high muscle strength, low aerobic capacity, compared with high, was associated with a hazard ratio of type 2 diabetes of 1.90 (P < .001).
And those who were deficient in both departments, with low muscle strength and low aerobic capacity, were more than three times likelier to develop type 2 diabetes than those who scored high on both counts (hazard ratio, 3.07; P < .001), with a positive additive interaction identified between the two variables (P < .001).
These associations were seen even among men with normal body mass index.
Looking Upstream: Concerns Over Fitness of Normal-Weight YouthDr Crump pointed out the current study is unique because previous studies on the impact of physical fitness on diabetes risk have focused on mid-adulthood, "simply because that is...the easiest population to study and there are the most data looking at fitness in people in mid-adulthood."
And although the current investigation focused on young men, Dr Crump expects that physical fitness in adolescence and young adulthood would have much the same relationship with later type 2 diabetes risk in women; indeed, other studies, although not as detailed, have indicated as much, he said.
"I think the main contribution of this study is to look further upstream earlier in life and late adolescence...and then look at both aerobic and muscular fitness in relation to the long-term risk of diabetes over most of their adult life."
As such, the findings underline current concerns over levels of physical activity among young people.
"There have been other studies that have shown that technology changes and the increase in screen time among young people, whether TV or video games or other screen time, has really reduced physical activity, and as a result that has at least contributed to the obesity epidemic, as well as lower physical fitness," Dr Crump said.
In his editorial, Dr Katzmarzyk says this increased risk of type 2 diabetes among people who are of normal weight but not physically fit could easily be missed.
"Given the strong association between BMI and risk for type 2 diabetes mellitus, normal-weight persons may not receive appropriate lifestyle counseling if they are sedentary or unfit because of their lower perceived risk," he writes.
Indeed, say Dr Crump and colleagues, "Poor physical fitness levels during late adolescence may be a modifiable risk factor for type 2 diabetes later in life."
Future Studies: Look at Fitness Over a Range of Age WindowsIn future studies, Dr Crump would like to be able to examine the relationship between physical fitness and diabetes at different time points.
"Although these...data are quite amazing in terms of their level of detail...we have measures of physical fitness only at one [time] point, at age 18, when the military conscription exam in Sweden was done," he said.
It would be very helpful to be able to measure fitness and examine the susceptibility to diabetes in relation to fitness "at different age windows," he observed.
"I think that would be a very important next direction, as well as including women, whenever that's possible."
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