In a cross-sectional study, every 10% increase in the ratio of skeletal muscle mass to total body weight was associated with an 11% reduction in risk of insulin resistance and a 12% drop in risk of transitional, prediabetes, or overt diabetes, Preethi Srikanthan, MD, of the University of California Los Angeles, and colleagues reported online in the Journal of Clinical Endocrinology and Metabolism.
The findings point to the importance of gauging muscle mass, in addition to other established risk factors such as body mass index (BMI) and waist circumference, when assessing a patient's metabolic health, the researchers said.
The results may also have implications for the role of muscle-building exercises in preventing metabolic dysfunction, Preethi and colleagues wrote.
It's known that very low muscle mass (sarcopenia) is a risk factor for insulin resistance, but it's unclear whether increasing muscle mass outside of the sarcopenic range can boost insulin sensitivity or protect against diabetes.
So to determine whether increases in muscle mass are associated with improved glucose regulation, the researchers looked at data on 13,644 patients from the National Health and Nutrition Examination Survey (NHANES) III, conducted from 1988 to 1994.
Patients had data on homeostasis model assessment of insulin resistance (HOMA-IR); glycated hemoglobin (HbA1c); prevalence of transitional, prediabetes, or overt diabetes (PMD); and prevalence of overt diabetes mellitus. These four factors served as the study outcomes.
Muscle mass was assessed via bioelectrical impedance, which measures opposition to the flow of an electric current through body tissues, determining total body water to estimate body composition.
The researchers found that all four of the outcomes declined across quartiles from lowest to highest skeletal muscle index, or the ratio of skeletal muscle to body weight.
The smallest effect size was seen for HbA1c, with a 5.8% relative mean reduction between the highest and lowest quartiles.
On the other hand, the most striking effect was in diabetes prevalence, with a relative reduction of 63%. Prevalence was 14.5% in the lowest quartile compared with only 5.3% in the highest, the researchers reported.
After adjusting for confounders including age, ethnicity, sex, and obesity, the relationships persisted for insulin resistance and prevalence of transitional, prediabetes, and overt diabetes.
Specifically, each 10% increase in skeletal muscle index was associated with 11% relative reduction in HOMA-IR and a 12% relative reduction in the combined diabetes endpoint.
After excluding patients with diabetes, these relationships were strengthened, Preethi and colleagues wrote. For every 10% increase in muscle mass ratio, there was a 14% reduction in HOMA-IR and a 23% reduction in combined diabetes prevalence.
They explained that the weaker associations when diabetic patients were included were likely due to the effects of diabetes on muscle mass and on pancreatic beta-cell mass.
The researchers concluded that the relationship between muscle mass and insulin resistance was not limited to sarcopenia, as "increases in muscle mass above even average levels were associated with additional protection against insulin resistance and prediabetes."
The study was limited by its cross-sectional nature, and by its use of bioelectrical impedance alone to estimate muscle mass. Also, there was no differentiation between type 1 and type 2 diabetes in the original survey, they said.
As well, patients with high muscle mass tend to have low fat mass, so any of the associations may be due to adipose tissue, they cautioned, although they attempted to control for this.
Despite these findings, prior prospective studies of short-term strength training programs in overweight and obese patients have been unclear in terms of their effects on metabolic abnormalities, they said.
They called for more work to determine the proper duration of exercise interventions needed in order to improve insulin sensitivity and glucose metabolism, and ultimately to have an effect on diabetes incidence.
Primary source: Journal of Clinical Endocrinology and Metabolism
Source reference:
Srikanthan P, et al "Relative muscle mass is inversely associated with insulin resistance and prediabetes: Findings from the third NHANES" J Clin Endocrinol Metab 2011.