Featured article in the Winter 2016 Issue of Nutrition Close-Up; written by Christian Wright, Doctoral Student, Purdue University
“Bigger is better” for most Americans when it comes to skeletal muscle. One only needs to look at the latest cover of health magazines for headlines such as “build muscle fast” and “supersize your legs” to know this to be true. After all, a larger muscle is generally a stronger muscle, which is important for overall health. In fact, muscular strength is often used as a surrogate for one’s physical health as it is independently associated with not only mobility, but also the development of chronic disease.
Maintaining muscular strength is particularly important in older adults, where age-related decreases in muscular strength greatly reduces quality of life and increases risk of all-cause mortality.1 However, while a larger muscle may appear stronger and suggest optimal health, recent evidence suggests a disconnect from this long-standing relationship due to a progressive mismatch between loss of muscle mass and loss of muscular strength.
Data indicates that decreases in muscular strength occur long before reductions in muscle mass,2 illustrating that muscle quality, not muscle quantity, predicts overall muscular strength. Muscle quality, often calculated as simply a ratio of muscle strength to muscle mass, describes the amount of force a muscle produces relative to its size. This ratio of muscle force to size and its influence on overall physical health is highly regulated by the composition of the muscle. Changes in muscle fiber concentrations and increases in muscle fat are common examples of how alterations in muscle composition can degrade overall muscle quality.3 These alterations of muscle composition represent the disconnect between muscle size and strength and are considered important clinical targets for preserving muscle quality. Changes in muscle fat concentrations have recently gained critical attention, as they are not only associated with decreases in physical function, but also the development of several metabolic and inflammatory disorders.4,5
As the two images in Figure 1 illustrate, increases in muscle fat cause a “marbling” of muscle, where fat is accumulated between the individual muscle bundles, muscle fibers, and even within individual muscle cells. This ectopic fat, which is fat stored in a non-traditional site of fat storage (i.e. visceral fat), is broadly defined as intermuscular adipose tissue (IMAT) and refers to any fat stored underneath the deep fascia of the muscle. IMAT accumulation was long considered an unavoidable consequence of aging, but has recently been shown to be largely a product of illness, disuse, or inactivity. The exact mechanism behind IMAT accumulation is still unknown, but evidence supports the increased exposure of muscle to non-esterified fatty acids and/or pro-inflammatory cytokines as the two major contributing factors. These impair beta-oxidation, leading to fat accumulation, and potentially influence early muscle cells to become fat cells. As IMAT begins to accumulate, insulin signaling is impaired and mitochondrial function deteriorates. The resulting aftermath is an increased risk for physical impairments, type 2 diabetes, chronic inflammation, and hyperlipidemia.6
Weight loss and increased physical activity have long shown to decrease IMAT accumulation and improve muscle quality, which can be further facilitated by a higher protein diet. Given that IMAT accumulation is largely considered the product of illness, disuse, or inactivity, common interventions include exercise training and/or diet-induced weight loss. Though the combination of vigorous exercise training and diet-induced weight loss appears to elicit the greatest response, data suggest that intentional weight loss is the indispensable component to successfully decrease IMAT concentrations.6
However, composition of this diet-induced weight loss program also plays a pivotal role in improving overall muscle quality. A higher protein (1.2 g/kg BW/d) diet versus a normal protein (0.8 g/kg BW /d) weight loss regimen shows greater reductions in fat mass and a preservation of lean mass, strength, and physical performance.7,8 A practical and cost-effective approach to increasing intakes of dietary protein is the consumption protein-rich foods, which not only provide dietary protein but also a wide variety of distinct amino acids and functional nutrients. For example, increasing low-fat dairy consumption (3-4 servings/d) not only provides dietary protein but vitamin D and calcium. And in addition to being the most bioavailable source of dietary protein, whole eggs provide antioxidants like lutein and zeaxanthin, which may help regulate inflammation.
Alterations in muscle quality, such as increases in IMAT, not only impair muscle function but increase risk of metabolic disease. Weight loss, exercise training, and increased intakes of dietary protein provide viable and important clinical strategies for improving muscle quality.