Tipping the Scales: Couch Potato Kids & Diabetes in Adulthood

It’s not unusual when research proves the benefits of what parents have been telling kids for generations: “Go outside and play!” Mom was right: sedentary kids have a higher risk for getting diabetes in adulthood, according to a new study.

According to the Endocrine Society’s 2016 Endocrine Facts and Figures Report, the prevalence of diabetes neared 10% of the total U.S. population, affecting almost 29.1 million individuals, in 2012. Of this number, type 2 diabetes mellitus (T2DM), characterized by chronic hyperglycemia due to insulin resistance, reduced insulin secretion, and increased hepatic glucose production, accounts for 90% – 95% of all cases. Although the parallel prevalence of obesity has well-documented associations with T2DM incidence, and obesity or overweight often accompanies a sedentary lifestyle, a relationship between T2DM in adulthood and physical fitness in youth has been difficult to establish, given the challenges of follow-up.

Taking Fitness into Account

In a new study, however, researchers from Mount Sinai in New York and Lund University in Sweden, which has a national healthcare system allowing health records to be tracked over a span of decades, compared aerobic capacity and muscle strength at age 18 years with adult diagnosis of T2DM (up to age 62 years). Published in Annals of Internal Medicine, “Physical Fitness Among Swedish Military Conscripts and Long-term Risk of Type 2 Diabetes Mellitus: A Cohort Study,” led by Casey Crump, MD, PhD, at the Icahn School of Medicine at Mount Sinai, is the first study of its kind “to examine potential interactions between aerobic and muscular fitness” as well as development of T2DM “even after adjusting for other characteristics such as body mass index (BMI), family history, and socioeconomic factors,” Crump says.

“Young people should maintain regular exercise that enhances both aerobic and muscular fitness and avoid barriers to this such as screen time.” — Casey Crump, MD, PhD, Icahn School of Medicine, Mount Sinai Hospital, New York

Data from 1,534,425 Swedish men from 1969 to 1997 without prior T2DM revealed that lack of cardiorespiratory capacity and muscle strength, conferred a three-fold higher risk of developing T2DM, resulting in 34,008 (2.2%) T2DM outpatient and inpatient diagnoses at a mean age of 46.8 years from 1987 to 2012 among the cohort. The measurements of aerobic and muscular capacity were taken during a two-day military standardized conscription examination with well-validated tests. A stationary bike was used to obtain cardiorespiratory values, and muscle strength was determined from isometric dynamometer tests of maximal knee extension, elbow flexion, and hand grip. “Not only were both low aerobic and muscular fitness linked with a higher long-term risk of diabetes, but this was true even among those with normal BMI,” Crump says. “This suggests that both aerobic and muscular fitness have important long-term health effects, even among people with normal weight. Also, we found that low aerobic and muscular fitness have a synergistic effect on the risk of diabetes (i.e., their combined effect exceeded the sum of their separate effects).”

The researchers posit that better glycemic control in a state of physical fitness, not just maintaining normal weight, helps ward off diabetes. Aerobic activity induces fatty acid oxidation and appropriate response to insulin, while strength straining may contribute to response to insulin by promoting type 2 muscle fiber growth, “fast-twitch” fibers that facilitate glycolysis.

These findings are important both for filling a former gap in the research literature that until now focused on adult (mostly aerobic) activity levels but also for spotlighting a population in need of fitness counseling that may have been thus far neglected. Although normal-weight individuals may once have been presumed to be at lower risk for T2DM, given the relationship between development of the disease with obesity, being out of shape might obviate this presumed lower risk. While genetics is also a major determinant of physical fitness, activity level is the most important modifiable factor, explained Crump.

Fortunately, with the inverse association between activity level and T2DM, its flip is also true — that increased activity level decreases risk for T2DM (as well as any number of other conditions). “Prevention of T2DM should begin early in life and include both aerobic fitness and muscular strength, even among persons with normal BMI,” Crump says. “These findings can help inform earlier and more effective lifestyle interventions to reduce the ongoing epidemic of T2DM.”

Physical Activity in Youth is Key

As study after study has shown, physical activity enhances health in myriad ways, yet less than half of U.S. children and youth meet recommended activity guidelines. “Young people should maintain regular exercise that enhances both aerobic and muscular fitness and avoid barriers to this such as screen time,” Crump says. In “Physical Activity Guidelines for Americans,” the U.S. Department of Health and Human Services issued recommendations for children and youth to include at least 60 minutes of exercise daily, most of which should be cardiorespiratory/aerobic and of moderate or vigorous intensity (with vigorous-intensity physical activity at least three days a week). In addition, this group should incorporate both muscle-strengthening and bone-strengthening activities on at least three days of the week. Furthermore, these activities should be age-appropriate, enjoyable, and varied to promote sustainability and to establish an active lifestyle as a lifelong habit.

These recommendations might seem quite high, but for good reason, as Crump explains: “Exercise requirements are higher for youth because it’s a period of rapid growth, and more exercise is needed in order for muscles, bones, and the cardiovascular system to grow and develop properly.” According to “Physical Activity in U.S. Youth Aged 12–15 Years, 2012,” from the Centers for Disease Control and Prevention, only 24.8% of youth fully meet these guidelines, 54.8% meet some of them, and 7.6% did not meet them at all. “Specific goals for physical fitness will depend on many factors including age and gender,” Crump says. “The Presidential Youth Fitness Program offers a physical assessment tool that educators can use to track children’s and youth’s progress toward healthy levels of fitness.”

Although it was limited to men, the sheer size of this study and its number of participants gives its findings particular weight. “Additional studies are still needed in women and other populations,” Crump says, “but evidence from other studies suggests that the main findings are likely to be similar in these other populations. Other studies are also needed that measure physical fitness as well as diet and BMI at other time points across the lifespan to examine age windows of susceptibility to these factors in relation to diabetes.”

“Exercise requirements are higher for youth because it’s a period of rapid growth, and more exercise is needed in order for muscles, bones, and the cardiovascular system to grow and develop properly.” — Casey Crump, MD, PhD, Icahn School of Medicine, Mount Sinai Hospital, New York

Another caveat with this study is that follow-up measurements of physical fitness and BMI later in adulthood were not available. Again, however, plenty of evidence from other studies suggests that even individuals who were sedentary as youths can decrease risk for T2D or delay its development by adopting an active lifestyle in adulthood.

In the meantime, this study’s implications are crystal clear: Kids need all the opportunities for physical activity they can get to be healthy now and to best maintain health in adulthood. T2DM is undeniably a complex, elusive disease, but increasing physical activity is one component we can control to reduce risk.

Horvath is a freelance writer based in Baltimore, Md. She wrote about the effects of weight and race on puberty in boys in the June issue.