There is a strong link between diabetes and fitness. Many studies have shown that people with type 2 diabetes lose more muscle mass and strength over time than people with normal blood sugars. This is thought to be a major reason why diabetes is associated with functional limitation, impaired mobility, and loss of independence. Studies have also shown that combining aerobic and resistance training can not only improve blood sugars in people who have diabetes, but can also prevent diabetes from developing.
For these reasons, scientists are very interested in the relationship between diabetes and fitness, teasing out the differences between muscle strength and cardiorespiratory fitness.
In a 2019 study published in the journal Mayo Clinic Proceedings, researchers looked at 4,681 adults, measured their muscle strength and cardiovascular fitness, and followed them over about eight years. Both upper and lower body muscle strength were measured using bench and leg presses at increasing loads, and participants were scored as having low, medium, or high strength based on the maximum weight lifted per kilogram of body weight.
They found that those with medium strength had a 32% reduced risk of developing diabetes than those with low strength. This is all fine and good and consistent with prior research. However, they did not see that those with high strength had any further reduction of diabetes risk. As a matter of fact, there was no association at all.
How could this be?
The authors focus largely on the also very important cardiorespiratory fitness factor. They point out that those participants with medium strength also tended to have good cardiorespiratory fitness, with good correlation between the two. However, in the low and high strength groups, it was a bit of a mix, with some people in the low strength group having high cardiorespiratory fitness, and vice versa. They point out that there may be added benefit to having both good muscle strength and good cardiorespiratory fitness, not just good muscle strength alone.
But another consideration is how things like strength and cardiorespiratory fitness are measured. It's important to note that just about every study looking at muscle strength uses a different method than this study. Hand grip strength is very common, for example. One large 2018 study of 8,208 Korean adults found that stronger hand grip strength was significantly associated with lower fasting blood sugars, HbA1c levels, and fasting insulin levels (all markers of prediabetes and diabetes). It's possible that hand grip is somehow a superior method of measuring strength than bench and leg press, or vice versa.
Maybe cardiorespiratory fitness is the more important factor after all?
This has been found to be particularly important in diabetes prevention. One large 2018 study out of Japan looked specifically at cardiorespiratory fitness (as measured by oxygen uptake while exercising on a cycle ergometer) in 7,804 men, and followed them over about 20 years, checking several times to see if anyone developed diabetes. They found that higher cardiorespiratory fitness was significantly associated with lower risk of developing diabetes at all follow-up periods. This is a pretty powerful association, though it would be good to do this study in women and in other ethnic groups.
Let's look at the big picture
Being in good overall shape, meaning having both decent muscle strength and cardiorespiratory fitness, is just good for you. Both can very likely lower your risk of developing diabetes, and even if you have diabetes, being fit can improve your blood sugars.
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Association of Muscular Strength and Incidence of Type 2 Diabetes. Mayo Clinic Proceedings, March 11, 2019.
Accelerated Loss of Skeletal Muscle Strength in Older Adults with Type 2 Diabetes. Diabetes Care, June 2007.
Patients With Type 2 Diabetes Show a Greater Decline in Muscle Mass, Muscle Strength, and Functional Capacity With Aging. Journal of the American Medical Directors Association, August 2013.
Muscle dysfunction in type 2 diabetes: a major threat to patient's mobility and independence. Acta Diabetologica, December 2016.
Effects of Aerobic and Resistance Training on Hemoglobin A1c Levels in Patients with Type 2 Diabetes. JAMA, November 24, 2010.
Association between muscle strength and type 2 diabetes mellitus in adults in Korea. Medicine, June 2018.
Long-term Impact of Cardiorespiratory Fitness on Type 2 Diabetes Incidence: A Cohort Study of Japanese Men. Journal of Epidemiology, May 5, 2018.