Muscle Strength: A Key Sign in Type 2 Diabetes Prevention?

This allows adults to pursue the exercise that they prefer, but it also raises questions, such as which form of exercise is the most beneficial for cardiometabolic health.

A previous review by Artero and colleagues, published in the November-December 2012 issue of the Journal of Cardiopulmonary Rehabilitation and Prevention, compared the cardiometabolic effects of aerobic exercise and resistance training. In general, aerobic exercise has a greater effect on reducing total body fat and abdominal adiposity compared with resistance training. Aerobic exercise is also more likely than resistance training to reduce body weight and resting blood pressure. However, resistance training improves muscular power to a greater degree than resistance training.

Both resistance training and aerobic exercise have been associated with lower levels of basal insulin and improved insulin sensitivity. The current study analyzes how muscular strength affects the risk for incident type 2 diabetes over time.

Study Synopsis and Perspective

Individuals at risk of developing type 2 diabetes could benefit from moderately increasing their body strength, say US researchers who found that, conversely, the risk reduction was entirely eliminated in people with the highest levels of strength.

The researchers looked at more than 4600 participants from a prospective health outcomes study who lacked type 2 diabetes at baseline.

After adjusting for multiple factors, including cardiorespiratory fitness, the authors found that moderate muscular strength was associated with a significant 32% reduction in the incidence of type 2 diabetes vs the lowest strength level.

The research, by Yuehan Wang, BMed, from the Department of Kinesiology, Iowa State University, Ames, and colleagues, showed, however, that the strongest individuals, despite being fitter and less likely to have hypertension or impaired fasting glucose compared with those in the lower-strength groups, had no reduction in type 2 diabetes risk.

The study was published online March 11 in the Mayo Clinic Proceedings.

Second author Duck-chul Lee, PhD, also from Iowa State University, said in a press release from the institution that the results are encouraging, as they show that even small improvements in muscle strength may help prevent type 2 diabetes. However, he noted that it is hard to recommend an optimal level of resistance exercise because of the lack of standardized measurements of muscle strength.

"Naturally, people will want to know how often to lift weights or how much muscle mass they need, but it's not that simple," he explained. "More work is needed to determine the proper dose of resistance exercise, which may vary for different health outcomes and populations."

Coauthor Angelique G. Brellenthin, PhD, also from Iowa State University, said that increasing muscle strength offers several health benefits, even if "you're not necessarily going to see the results of resistance training on your bathroom scale."

She also emphasized that getting started with resistance exercise does not require expensive equipment or a gym membership.

"We want to encourage small amounts of resistance training, and it doesn't need to be complicated," she said.

"You can get a good resistance workout with squats, planks, or lunges. Then, as you build strength, you can consider adding free weights or weight machines."

Muscular Strength Increasingly Recognized in Disease Prevention

The researchers point out that much of the literature around the association between physical activity and type 2 diabetes risk has focused primarily on aerobic exercise and cardiorespiratory fitness.

However, the role of muscular strength in chronic disease risk is increasingly being recognized, with studies suggesting it is linked to reduced cardiovascular risk factors, as well as lower rates of cardiovascular morbidity and mortality.

To examine its effect on incident type 2 diabetes, the team gathered data from the prospective Aerobics Center Longitudinal Study, in which men and women underwent medical examinations between 1981 and 2006.

At baseline, participants had comprehensive examinations that included muscular strength and maximal treadmill exercise testing alongside physical examinations, blood analyses, and a medical history questionnaire.

In total, 4681 men and women in the Aerobics Center Longitudinal Study who did not have diabetes at baseline or a history of myocardial infarction, stroke, or cancer were included in the analysis.

The average age of participants was 43.3 years, and 88.4% were male. Mean body mass index was 25.1 kg/m², and 21.9% had a history of hypertension.

In addition, 11.8% were current smokers and 24.2% were classified as heavy alcohol drinkers.

In terms of muscular strength, which was calculated using 1-repetition maximum values for upper and lower body strength, 1560 individuals were classified in the lower third, 1559 in the middle third, and 1562 in the upper third.

Participants in the upper third of muscular strength were significantly more likely than those in the 2 lower thirds to meet aerobic physical activity guidelines, and had significantly higher cardiorespiratory fitness values (P<.001).

They were also significantly less likely than those in the lower thirds to have a history of hypertension (P<.001) and impaired fasting glucose (P=.02). They also were less likely to be heavy alcohol drinkers (P=.01), and had a significantly lower mean body mass index (P<.001).

5% of Participants Developed Diabetes

During a mean follow-up of 8.3 years, 229 (4.9%) participants developed type 2 diabetes.

Accounting for potential confounders and estimated cardiorespiratory fitness, the team found that compared with participants in the lower third of muscular strength, those in the middle third had a significantly reduced risk for type 2 diabetes, at an adjusted hazard ratio of 0.68 (P=.02).

However, being in the upper third of muscular strength was not associated with a reduction in the incidence of type 2 diabetes vs the lower third, at an adjusted hazard ratio of 1.07 (P=.68).

The results were similar when participants were stratified by low and high estimated cardiorespiratory fitness and when looking at lifestyle factors such as smoking, drinking, and physical activity, as well as health conditions such as overweight, hypertension, and hypercholesterolemia.

The team writes: "These results indicate that very high levels of [resistance exercise] training may not be necessary to obtain the considerable health benefits on [type 2 diabetes] prevention. More observational studies with larger sample size and randomized controlled trials on the dose-response relationship of [resistance exercise] and muscular strength with [type 2 diabetes] and related chronic diseases...among diverse populations are clearly warranted."

The study was supported by grants from the National Institutes of Health. One coauthor has received unrestricted research grants from The Coca-Cola Company that were not used to support this work. The other authors have disclosed no relevant financial relationships.

Mayo Clin Proc. Published online March 11, 2019

Clinical implications

• In general, aerobic exercise has a greater effect on reducing total body fat and abdominal adiposity compared with resistance training. It is also more likely than resistance training to reduce body weight and resting blood pressure. Both resistance training and aerobic exercise have been associated with lower levels of basal insulin and improved insulin sensitivity.
• In the current study, moderate but not high muscle strength was associated with a lower risk for incident type 2 diabetes among adults.
• Implications for the Healthcare Team: The healthcare team should encourage resistance training at moderate levels among interested adults to promote better health outcomes, including a lower risk for type 2 diabetes.