Teenagers who are out of shape regardless of body weight are at higher risk for developing Type 2 diabetes later in life, a new study has revealed. A poor level of cardiorespiratory fitness and muscle strength was found to be associated with long-term risk of diabetes even among teenagers with normal body mass index (BMI).

Led by Dr. Casey Crump, researchers used data which recorded the health of more than 1 million 18-year olds who enlisted in the military in Sweden from 1969 to 1997. The research team followed the conscripts until 2012. All of the participants had no history of diabetes.

In the follow-up, the research team identified which of the men were diagnosed with type 2 diabetes based on national hospital and outpatient registries. Out of the 1 million men, 2 percent or 34,000 were diagnosed with diabetes. It lasted into middle age for most of them. Half of the men were diagnosed with diabetes after turning 46 years old.

The study found that those who were least fit when they were 18-year-old were thrice more likely to develop diabetes than those with better aerobic strength and capacity. Men with normal and healthy BMI were not immune as well.

Crump said the risk for diabetes is increased even more by the combination of low muscular and aerobic fitness, rather than the sum of the individual factors. Additionally, genetics and activity level were important factors of physical fitness, but activity level was a modifiable factor.

Peter T. Katzmarzyk, who wrote the accompanying editorial for the study, said the findings indicate that fitness traits were vital for predicting risks for future diabetes at any body weight. He said it should not be ignored.

However, Katzmarzyk said every study uses a different definition of "fitness." He said it is not possible to come up with a single measure that can define fitness level, given that there is variation across ages and between genders.

Meanwhile, Crump said further research should be done in order to measure physical fitness, diet and BMI at other points in life to find out age windows most susceptible to diabetes. The research should also be expanded to women and other populations.

With that, the research team suggested ways to stay healthy and fit, and potentially avoid developing diabetes.

"Young people should maintain regular exercise and both aerobic and muscular fitness, and avoid barriers to this such as screen time," said Crump.

Guidelines recommend that young people should allot 60 minutes of exercise daily, but only half of children and teenagers in the United States do so.

The findings of the study are featured in the journal Annals of Internal Medicine.

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