Strength Training for Diabetes and Insulin Resistance: Muscles as a Blood Sugar Buffer

A research group at the Norwegian School of Sport Sciences in Oslo summarized where humans keep their glucose reserves. The skeletal muscles hold about 500 grams of glycogen, the liver only about 100 grams. Under laboratory conditions with high insulin levels, 70 to 90 percent of the glucose taken up in healthy people was stored as glycogen in muscle.

What this means for you: when it comes to blood sugar, your muscle is not a supporting actor. It is the biggest customer.

Researchers at the University of California, Los Angeles analyzed data from 13,644 adults from a large US health survey. Muscle mass was estimated by bioimpedance and set in relation to body weight. Age, sex, ethnicity, overweight and abdominal fat were taken into account. Even so: for every 10 percent more relative muscle mass, the insulin resistance index HOMA-IR was 11 percent lower. Prediabetes or diabetes was 12 percent less common.

What this means for you: more muscle goes hand in hand with better regulated blood sugar. Whether the muscle is the cause, however, is something a snapshot like this cannot prove.

A team at Yale University compared young, lean people with insulin resistance with people of the same age without insulin resistance, matched for weight and physical activity. After two carbohydrate-rich meals, the insulin-resistant muscles built up about 60 percent less glycogen. At the same time, new fat production in the liver was more than twice as high. Blood triglycerides were 60 percent higher, and HDL was about 20 percent lower.

What this means for you: when muscle drops out as a store, the sugar ends up somewhere else. For example, as fat in the liver.

At Yale, twelve young, lean, insulin-resistant people ate a carbohydrate-rich meal twice: once at rest, once after 45 minutes of training on an elliptical trainer. After the training, their muscles built up more than three times as much glycogen. New fat production in the liver fell by about 30 percent, and the formation of new triglycerides in the liver by about 40 percent.

What this means for you: a single session was enough to redirect the sugar from the detour through the liver back into the muscle. The training here was endurance. The principle behind it, that emptied stores take up more, is described by the Oslo review for muscle work in general.

A team at the University of Copenhagen had ten men with type 2 diabetes and seven healthy men train only one leg for six weeks. Training took place three times a week for at most 30 minutes, and the other leg served as a comparison within the same body. In the trained leg, insulin-dependent glucose uptake increased, and the muscle contained more GLUT4 transporters, more insulin receptors and more of the storage enzyme glycogen synthase.

What this means for you: strength training can do more than build muscle. It can also change the machinery in the cell that brings sugar in and stores it. The authors see this as local adaptations mediated by the muscle work itself.

A team at Tufts University in Boston randomly assigned 62 older people with type 2 diabetes, 66 years old on average, to two groups. One group did supervised, progressively increased strength training three times a week for 16 weeks. Their HbA1c fell from 8.7 to 7.6 percent, and the glycogen stores in their muscles grew from 60.3 to 79.1 mmol of glucose per kilogram of muscle.

In the control group, HbA1c remained unchanged and the muscle stores shrank. In 72 percent of those who trained, the prescribed diabetes medication was reduced. What this means for you: the sponge can grow, and that can show up in your long-term blood sugar. Adjusting medication always remains the job of your doctor.

A Copenhagen team led by Pedersen describes skeletal muscle as a gland: muscle fibers release several hundred messengers, known as myokines. One of them, IL-6, appears to influence the liver, fat tissue and immune system and to be involved in the coordination between hormone-producing gut cells and the pancreas. Some of these connections come from animal experiments and have not yet been conclusively clarified in humans.

What this means for you: when you use your muscle, it sends signals to organs that help steer your blood sugar. Lack of exercise might change this conversation.

A team at the University of Ottawa divided 251 adults with type 2 diabetes into four groups: endurance, strength, both or no training. Training took place three times a week for 22 weeks. Compared with the control group, HbA1c fell by 0.51 percentage points with endurance and by 0.38 with strength. The combination lowered it by an additional 0.46 percentage points compared with endurance alone and by 0.59 compared with strength alone.

What this means for you: both forms of training can lower long-term blood sugar, and together most clearly. The authors themselves name one limitation: the combination group trained for longer overall.

At the Pennington Biomedical Research Center in Louisiana, 262 previously inactive people with type 2 diabetes did endurance training, strength training, both or no training at all for nine months. Only the combination lowered HbA1c compared with the control group in a statistically significant way, by 0.34 percentage points. Strength alone, at minus 0.16, and endurance alone, at minus 0.24 percentage points, showed no significant effect here. The strength group did, however, lose 1.4 kilograms of fat mass.

What this means for you: this large study, too, points more toward a both-and than an either-or.

At the University of Verona, 40 people with type 2 diabetes did either endurance or strength training for four months. HbA1c fell almost equally, by 0.40 and 0.35 percentage points. Insulin sensitivity, measured with the elaborate clamp method, also rose similarly in both groups, and abdominal fat on MRI decreased to a similar extent.

What this means for you: when it comes to insulin sensitivity, your muscle seems to understand both languages. The endurance group gained more fitness, the strength group more strength.

A team at Stanford University studied an often overlooked group: people with type 2 diabetes and normal body weight, a form that often goes hand in hand with low muscle mass. Over nine months, only strength training lowered HbA1c significantly, by 0.44 percentage points, and it performed better than endurance training. Only in the strength group did muscle mass in the arms and legs increase relative to fat mass. Exactly this predicted the improvement in HbA1c.

What this means for you: if you are lean and still have diabetes, missing muscle might be an important part of the story. The participants were predominantly of Asian descent, which limits how far the results can be generalized.

At the International Diabetes Institute in Melbourne, 36 overweight people with type 2 diabetes aged 60 to 80 lost a moderate amount of weight. Half of them also did intensive, progressively increased strength training. Both groups lost a similar amount of weight and fat. But only with strength training did fat-free mass increase, plus 0.5 instead of minus 0.4 kilograms. After six months, HbA1c fell by 1.2 instead of 0.4 percentage points.

What this means for you: same scale, different metabolism. What you keep while losing weight can help decide what happens to your blood sugar.

In the Health Professionals Follow-up Study, 32,002 men in the US were followed for 18 years, and 2,278 of them developed type 2 diabetes. Men who trained with weights for at least 150 minutes a week had a 34 percent lower risk, independent of endurance exercise. With at least 150 minutes of endurance exercise a week, the risk was 52 percent lower, and with strength and endurance combined, 59 percent lower.

What this means for you: strength training seems to make its own contribution, and the combination the largest. An observational study, however, shows associations, not proof of cause and effect.

In the Women's Health Study, 35,754 healthy women, 62.6 years old on average, were observed for about eleven years. Women who did any form of strength training developed type 2 diabetes 30 percent less often than women without strength training, even after taking other activities into account. Strength plus endurance was associated with an even lower risk than endurance alone.

What this means for you: even the step from no strength training at all to some might count.

Two researchers at the Icahn School of Medicine at Mount Sinai in New York summarized the data on the acute and long-term effects of training. Their conclusion: the improved insulin sensitivity is based mainly on a prolonged acute effect that can last up to 72 hours. Untrained people and people with lower insulin sensitivity seem to benefit from it in particular.

What this means for you: each session can open a window of up to about three days. Those with the most room for improvement can gain the most.

A team at the University of Sydney pooled 14 studies on regular training in type 2 diabetes. Insulin sensitivity was significantly improved compared with control groups, even 48 to 72 hours after the last session. Even in measurements taken more than 72 hours later, the benefit was still detectable.

What this means for you: regular training can build an effect that reaches beyond the individual window.