Older adults historically are at risk of protein intake below the Recommended Dietary Allowance (RDA) for healthy adults (0.8 g/kg/day). Older adults fail to ingest the highest acceptable macronutrient distribution for protein of 35% of energy intake. One study of adults over the age of 50 noted that 27% to 41% of women and 15% to 38% of men consumed less than the RDA for protein.20
Myostatin Regulation. Myostatin is a key protein in regulating muscle growth; when myostatin is inhibited, muscle hypertrophy occurs.53 Myostatin and its effects on muscle hypertrophy is a novel topic in the scientific community. While animal studies have been enlightening, there has been only one study of myostatin regulation in humans published to date.54 The potential regulation of myostatin to ameliorate the adverse effects of muscle mass and strength loss not only seen in sarcopenia and other muscle wasting pathologies is appealing. There are various proposed methods of inhibiting myostatin in muscle which would aid muscle hypertrophy. Three methods being evaluated are through direct genetic deletion of myostatin, administration of follistatin, and administration of anti‐myostatin antibodies.
In mice, deletion of the myostatin gene resulted muscle mass increases through hypertrophy and hyperplasia, with as much as a 2.5‐fold increase in individual muscle mass compared to control mice.53 Another study targeted follistatin, a protein that binds to and antagonizes myostatin, thus, diminishing myostatin’s role in controlling muscle size.55 In mice over expressing follistatin a similar increase (3‐fold) in muscle mass was seen compared to control mice.55 Another method used to inhibit myostatin is through anti‐myostatin antibodies; injection of anti‐myostatin antibodies in mice demonstrated a 20% increase in muscle mass and strength increases over a 4‐week period.56
Increased myostatin levels in human muscle would potentially explain part of the fall in muscle mass associated with aging. Of two studies looking at myostatin mRNA in aged human muscle, one found an increase in myostatin mRNA while the other found no difference in myostatin mRNA compared to young controls.57,58 One human study manipulating myostatin has been conducted using a recombinant human antibody on patients with muscular dystrophy.54 The results of this study suggest that this approach may be a promising treatment to stimulate muscle growth in muscular dystrophy.54 Other myostatin inhibitors are being investigated for sarcopenia and other muscle‐wasting disorders such as cachexia by pharmaceutical companies.
This peptide has been shown to stimulate the release of a hormone called adrenocoticotropic hormone (ACTH) from the pituitary gland. Intravenous urocortin II has been shown to prevent muscle atrophy from being in a cast or taking certain medications; it has also been shown to cause muscle growth in healthy rats. But its use for building muscle mass in humans has not been studied and is not recommended.
Hormone Replacement Therapy (HRT)
When a woman’s production of hormones is diminished at menopause, hormone replacement therapy has been shown to increase lean body mass, reduce abdominal fat short-term, and prevent bone loss. However, in recent years there has been controversy surrounding the use of HRT due to increased risk of certain cancers and other serious health problems among HRT users.
Beta-hydroxy-beta-methylbutyrate (HMB) is the active metabolite of leucine. Studies have also been conducted with HMB to evaluate its use in muscle protein synthesis. Many of the studies have been conducted in young healthy individuals for use to enhance athletic performance. Results of these studies are conflicting, but HMB appears to be more effective in people who are not previously trained versus those individuals who are well trained.
Vitamin D levels decline with age and cutaneous vitamin D levels are up to four times lower in older compared with younger individuals. It is known that vitamin D plays an important role in bone and muscle metabolism. Several mechanisms have been suggested for the role of vitamin D in muscle function. Vitamin D binding to the vitamin D receptor found in skeletal muscle promotes muscle protein synthesis and enhances calcium uptake across the cell membrane. Low vitamin D levels result in atrophy predominantly of the type 2 (fast twitch) muscle fibers in common with sarcopenia. Low levels of vitamin D have been found to be associated with an increase in sarcopenia. A myopathy has been reported in severe vitamin D deficiency. In older people low vitamin D levels may produce functional problems including proximal muscle weakness, difficulty rising from a chair, difficulties in ascending stairs, and axial balance problems.