LOS ANGELES –
“Loss of lean body mass and function with aging decreases the amount of metabolically active tissue, which can lead to insulin resistance,”, said at the World Congress on Insulin Resistance, Diabetes and Cardiovascular Disease. “Insulin resistance reduces muscle protein anabolism and accelerates sarcopenia, perpetuating a vicious cycle.”
Sarcopenia, the involuntary loss of muscle mass and function that occurs with aging, is an ICD-10 codable condition that can be diagnosed by measuring muscle strength and quality, said Dr. Volpi, director of the Sealy Center on Aging at the University of Texas Medical Branch at Galveston. In the Health, Aging and Body Composition Study (Health ABC), researchers followed 2,292 relatively healthy adults aged 70-79 years for an average of 4.9 years (). The researchers used isokinetic dynamometry to measure knee extension strength, isometric dynamometry to measure grip strength, CT scan to measure thigh muscle area, and dual X-ray absorptiometry to determine leg and arm lean soft-tissue mass. “Those individuals who started with the highest levels of muscle strength had the greatest survival, while those who had the lowest levels of muscle strength died earlier,” said Dr. Volpi, who was not affiliated with the study. “That was true for both men and women.”
More recently, researchers conducted a pooled analysis of nine cohort studies involving 34,485 community-dwelling older individuals who were tested with gait speed and followed for 6-21 years (). They found that a higher gait speed was associated with higher survival at 5 and 10 years (P less than .001). “Muscle mass also appears to be associated in part with mortality and survival, although the association is not as strong as measures of strength and gait speed,” Dr. Volpi said.
Data from the 2009 Korea National Health and Nutrition Examination Survey of 1,537 participants, aged 65 years and older, found that sarcopenia is independently associated with cardiovascular disease (PLoS One. 2013 Mar 22.Most of the risk factors for cardiovascular disease – such as age, waist circumference, body mass index, fasting plasma glucose, and total cholesterol – showed significant negative correlations with the ratio between appendicular skeletal muscle mass and body weight. Multiple logistic regression analysis demonstrated that sarcopenia was associated with cardiovascular disease, independent of other well-documented risk factors, renal function, and medications (odds ratio, 1.77; P = .025).
In addition, data from the British Regional Heart Study, which followed 4,252 older men for a mean of 11.3 years, found an association of sarcopenia and adiposity with cardiovascular mortality and all-cause mortality (). Specifically, all-cause mortality risk was significantly greater in men in the sarcopenic and obese groups (HRs, 1.41 and 1.21, respectively), compared with those in the optimal reference group, with the highest risk in sarcopenic obese individuals (HR, 1.72) after adjustment for lifestyle characteristics.
“Diabetes also accelerates loss of lean body mass in older adults,” added Dr. Volpi. “Data from the Health ABC study showed that individuals who did not have diabetes at the beginning of the 6-year observation period ... lost the least amount of muscle, compared with those who had undiagnosed or already diagnosed diabetes.”
The precise way in which sarcopenia is linked to metabolic disease remains elusive, she continued, but current evidence suggests that sarcopenia is characterized by a reduction in the protein synthetic response to metabolic stimulation by amino acids, exercise, and insulin in skeletal muscle. “This reduction in the anabolic response to protein synthesis is called anabolic resistance of aging, and it is mediated by reduced acute activation of mTORC1 [mTOR complex 1] signaling,” Dr. Volpi said. “There’s another step upstream of the mTORC1, in which the amino acids and insulin have to cross the blood-muscle barrier. Amino acids need to be transported into the muscle actively, like glucose. That is an important unexplored area that may contribute to sarcopenia.”
Dr. Volpi went on to note that endothelial dysfunction underlies muscle anabolic resistance and cardiovascular risk and is likely to be a fundamental cause of both problems. Recent studies have shown that increased levels of physical activity improve endothelial function, enhance insulin sensitivity and anabolic sensitivity to nutrients, and reduce cardiovascular risk.
For example, in a cohort of 45 nonfrail older adults with a mean age of 72 years, Dr. Volpi and colleagues carried out a phase 1, double-blind, placebo-controlled, randomized clinical trial to determine if chronic essential amino acid supplementation, aerobic exercise training, or a combination of the two interventions could improve muscle mass and function by stimulating muscle protein synthesis over the course of 24 weeks (). “We found that exercise supervised three times per week on a treadmill for 6 months improved physical function in both groups randomized to exercise,” Dr. Volpi said. “Disappointingly, there was no change in total lean mass with any of the interventions. There was a decrease in fat mass with exercise alone, and no change with exercise and amino acids. [Of note is that] the individuals who were randomized to the amino acids plus exercise group had a significant increase in leg strength, whereas the others did not.”
Preliminary findings from ongoing work by Dr. Volpi and colleagues suggest that, in diabetes, muscle protein synthesis and blood flow really “are not different in response to insulin in healthy older adults and diabetic older adults because they don’t change at all. However, we did find alterations in amino acid trafficking in diabetes. We found that older individuals with type 2 diabetes had a reduction of amino acid transport and a higher intracellular amino acid concentration, compared with age-matched, healthier individuals. The intracellular amino acid clearance improved in the healthy, nondiabetic older adults with hyperinsulinemia, whereas it did not change in diabetic older adults. As a result, the net muscle protein balance improved a little in the nondiabetic patients, but did not change in the diabetic patients.”
The researchers are evaluating older patients with type 2 diabetes to see whether there are alterations in vascular reactivity and protein synthesis and whether those can be overcome by resistance-exercise training. “Preliminary results show that flow-mediated dilation can actually increase in an older diabetic patient with resistance exercise training three times a week for 3 months,” she said. “Exercise can improve both endothelial dysfunction and sarcopenia and therefore improve physical function and reduce cardiovascular risk.”
Dr. Volpi reported having no relevant disclosures.