The benefits of strength training in normal-weight diabetes
One in five people with type 2 diabetes have a normal or low body weight, with reduced muscle mass relative to their fat mass. A new study reported in Diabetologia shows that they have more to gain from strength training than weight loss when it comes to glycaemic control. Dr Susan Aldridge reports.
While the majority of people with type 2 diabetes are overweight or obese, around 20% have a healthy weight with a BMI of 25 or less. This is known as normal-weight type 2 diabetes – it is now recognised as being a particular phenotype and it’s more common among Asians and older people. Normal-weight type 2 diabetes is associated with sarcopenia or loss of muscle mass, and research has suggested that this feature mediates the elevated mortality risk seen in this phenotype compared with diabetes with overweight.
Exercise is always recommended for people with type 2 diabetes. Guidelines are similar to those for the general population: three to five days per week of moderate-to-vigorous aerobic activity to reach a minimum duration of 150 min per week, plus two to three sessions of strength training. Trials comparing the impact of aerobic versus strength exercise on HbA1c have mostly been carried out in people with type 2 diabetes who also have overweight or obesity. For instance, the Diabetes Aerobic and Resistance Exercise (DARE) study and the Health Benefits of Aerobic and Resistance Training in individuals with type 2 diabetes (HART-D) study both found a combination of aerobic and strength training to be superior to either modality on its own in lowering HbA1c.
Individuals with obesity have both increased fat mass and increased lean muscle mass, while those with normal-weight type 2 diabetes are more likely to have a different body composition – namely, sarcopenia, especially related to their fat mass, which is known as relative sarcopenia. This suggests that the most effective exercise training for those with normal-weight type 2 diabetes may not be the same as for those who have overweight and obesity. That is why Yukari Kobayashi at Stanford University and colleagues set up the Strength Training Regimen for Normal Weight Diabetics (STRONG-D) study, which looks at the effects of strength training alone, aerobic training alone and a combination of the two upon glycaemic control in normal-weight people with diabetes. They hypothesised that these individuals might respond better to strength training than aerobic training, given their phenotype. The study looked at changes in body composition and muscle strength from these interventions and how these impacted HbA1c.
Focus on strength training
The 186 participants, of whom 83% were Asian, were assigned to either strength training only (ST), aerobic training (AER) or a combination of the two (COMB), which they did for three days a week for nine months. Strength training consisted of two sets of upper-body exercises (bench press, seated row, shoulder press and pull-down), three sets of leg exercises (leg press, extension and flexion) and abdominal crunches and back extensions. They worked up, increasing weight, to eight to 12 repetitions in a set.
The aerobic group worked on a treadmill or exercise bike to 50-80% of their metabolic equivalent of task, which is energy expenditure related to their weight. Combination was two strength-training sessions and a slightly reduced amount of aerobic training. The primary outcome was change in HbA1c and secondary outcomes were changes in body composition and muscle strength at nine months.
Mean HbA1c was 59.6 mmol/mol at the start of the trial and 131 participants actually completed it with data for analysis. At the end, there was a significant mean decrease in HbA1c in the ST group of 0.44%, compared with non-significant decreases of 0.35% in the COMB group and 0.24% in the AER group. Therefore, strength training alone was better than aerobic training or a combination of the two in reducing HbA1c levels in normal-weight individuals and combination training had an intermediate effect. Strength training also increased appendicular (arms and legs) lean mass relative to fat mass and was an independent predictor of a reduction in HbA1c.
This was the first clinical trial of exercise in normal-weight individuals but there was no significant difference in the AER or COMB group. Of course, the findings need to be confirmed in further, larger studies, but there is no reason not to apply a recommendation of strength training immediately to people with normal-weight diabetes, the authors say.
In the STRONG-D study, only the ST group showed a significant reduction in HbA1c, which suggests a potentially unique benefit of strength training in normal-weight individuals with type 2 diabetes. In comparison with the strength-training groups in the HART-D and DARE studies, the ST group achieved a higher absolute mean reduction in HbA1c. The participants in STRONG-D had a lower fat mass and much lower lean mass than the participants in the other two studies. Given that 80% of the insulin-mediated glucose uptake occurs in skeletal muscle or lean mass, it may be important to look at increasing lean mass for improving glycaemic control in this population.
Focus on body composition
An important finding of STRONG-D is that body composition – increase in lean mass, loss of fat mass – was independently associated with a reduction in HbA1c, while an increase in lean mass or decrease in fat mass alone was not. This adds to the growing body of evidence that estimates of muscle mass adjusted for fat mass show stronger associations with metabolic abnormalities than lean mass alone.
Strength training led to increased muscle mass relative to decreased fat mass and it is this that seems to be more beneficial for lowering HbA1c in individuals with normal-weight HbA1c. In contrast, individuals with overweight or obesity can lower HbA1c by lowering fat mass. At present, there isn’t enough data to support body composition as a central target for exercise training in type 2 diabetes. However, these new findings, along with previous studies that have shown a relationship between body composition and cardiovascular mortality, show the benefits of strength training in the normal-weight diabetes population.
Furthermore, weight loss is well established as being associated with a reduction in HbA1c in people with overweight or obesity and type 2 diabetes. In the STRONG-D study, significant weight loss was found only in the AER group and there was no relationship between weight loss and reduction in HbA1c. Thus, the most effective exercise regimen for overweight and obese individuals with type 2 diabetes may not necessarily be applicable to normal-weight individuals with type 2 diabetes.
The findings of STRONG-D make an important contribution to exercise recommendations for lean individuals with type 2 diabetes. They could also inform personalised exercise recommendations for different diabetes phenotypes. In the current clinical guidelines for people with type 2 diabetes, there are no recommended strength-training regimens, so the authors used the strength-training programme from the HART-D study.
In conclusion, the STRONG-D study shows that strength training alone was effective and superior to aerobic training alone for reducing HbA1c levels in individuals with normal-weight type 2 diabetes. Individuals with normal-weight type 2 diabetes present with relative sarcopenia and achieving increased muscle mass relative to decreased fat mass via strength training plays an important role in glycaemic control in this population. The findings of this study could help refine physical activity recommendations in type 2 diabetes by weight status.
To read this paper, go to: Kobayashi Y, Long J, Dan S, Johannsen NM, Talamoa R, Raghuram A, Chung S, Kent K, Basina M, Lamendola C, Haddad F, Leonard Mb, Church TS, Palaniappan L. Strength training is more effective than aerobic exercise for improving glycaemic control and body composition in people with normal-weight type 2 diabetes: a randomised controlled trial.
Diabetologia 26 July 2023. https://doi.org/10.1007/s00125-023-05958-9
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Any opinions expressed in this article are the responsibility of the EASD e-Learning Programme Director, Dr Eleanor D Kennedy.