Nutritional stimuli (amino acids and insulin) along with physical activity are the major acute physiological regulators of muscle protein turnover and are responsible for its diurnal oscillations and overall muscle protein net balance [39
]. In the present study, we examined how aging in men and women affects the rate of muscle protein synthesis during basal, postabsorptive conditions and during a hyperaminoacidemic-hyperinsulinemic-euglycemic clamp. We discovered that both old men and old women exhibit anabolic resistance to nutritional stimuli; in addition, the basal rate of muscle protein synthesis is increased in old women compared with young women and young and old men. Aging therefore affects muscle protein synthesis differently in men and women, and men and women need to be considered separately when evaluating the effect of aging on muscle protein synthesis.
Only a limited number of studies have been conducted to investigate potential sexual dimorphism in muscle protein turnover to date. Most of them were limited to young men and women only and found, as we did in the present study, no differences in muscle protein turnover between them [7
]. In another study we conducted, we found major differences in muscle protein synthesis rates between old men and women [12
]; however, the fact that subjects in this study were obese has been criticized as a major confounding variable. Only one study so far evaluated the effect of both sex and age on muscle protein turnover [13
] and found no age by sex interaction in the basal rate of muscle protein synthesis. However, this study [13
] was limited to basal, postabsorptive conditions only and included only old men with hypogonadism and old women with low serum dehydroepiandrosterone concentration, which may have confounded the results. Hypoandrogenemia is associated with a reduced lean body mass [14
] and treatment with testosterone increases the muscle protein synthesis rate [15
]. In the present study, plasma androgen concentrations were within the normal range for all subjects.
Consistent with the results from many earlier studies which focused on the effect of aging but included only men or both men and women without analyzing them separately [30
], we found no difference in the basal, postabsorptive muscle protein FSR in healthy young and old men and a blunted anabolic response to nutritional stimuli in old compared with young subjects (both men and women). The basal rate of muscle protein synthesis, however, was greater in our old compared with our young women and young and old men. This is consistent with what we have previously observed in obese older adults [12
] but contradicts the only other study we are aware of that specifically evaluated the effect of aging on muscle protein synthesis in women [48
]. Chevalier et al. [48
] report no difference between young and old women in the basal rate of muscle protein synthesis and the muscle protein synthesis rate during a simulated fed state. The reason(s) for the discrepancy are not entirely clear but could be related to the amount of amino acids provided. Chevalier et al. [48
] raised the plasma leucine concentration during the clamp to ~250% above basal values, which is equivalent to the peak increase after a maximally stimulating dose of protein (30
] or amino acids [40
]. The clamp in our study, on the other hand, was designed to achieve plasma insulin and amino acid concentrations equivalent to those seen after ingestion of ~22
g of casein or soy protein or consumption of a 2300–3300
kJ mixed nutrient meal containing ~26
g protein and ~70 to 90
g carbohydrates [50
]; plasma leucine concentration during the clamp in our study was therefore increased to only
40% above basal values. If this is indeed the main reason for the difference in results between our study and the one by Chevalier et al. [48
], it would suggest that old women may require a large amount of protein to respond adequately to nutritional stimuli. Old men, on the other hand, seem to be unable to benefit from the consumption of more protein/amino acids [40
]. It has also been proposed that differences in the availability of leucine per se, which is thought to be a major regulator of muscle protein synthesis [32
], might be a key factor responsible for whether or not there is anabolic resistance in older adults [53
]. However, plasma leucine concentration in our study was not different between young and old men and not different between young and old women and can therefore not help explain the anabolic resistance of muscle in older adults. Moreover, plasma leucine concentration was ~20% lower in women (both young and old) than in men, which is consistent with earlier work from our own group [12
] and by others [54
] but again does not help explain the differences in muscle protein synthesis rates between groups in our study. The reason(s) for the difference in plasma leucine concentration is currently unclear.
The mechanism(s) responsible for the faster basal muscle protein FSR in our old women and why aging affects the basal muscle protein synthesis rate differently in men and women is unclear. One potential explanation may be that the menopause-induced decline in estradiol and progesterone concentrations leads to an increase in the basal rate of muscle protein synthesis. In rodents, surgically-induced menopause (ovariectomy) increases the rate of muscle protein synthesis and replacement of either estrogen or progesterone prevents this effect [55
]. Whether the age-related declines in estrogens and/or progesterone similarly affect muscle protein turnover in women is not known; accordingly, it is also not known whether estrogens and/or progesterone are potentially important regulators of muscle protein metabolism in men.
In line with the muscle protein synthesis data in the present study, we have recently reported no difference in anabolic signaling between young men and young women [11
] and several other investigators have reported a blunted nutrient-induced increase in mTOR signaling in old compared with young subjects [40
]. In the present study, we therefore chose to focus on factors that could potentially help explain the greater basal rate of muscle protein synthesis in old women compared with young women and young and old men and possibly provide some insight into the anabolic resistance of older adults. To this end, we evaluated the muscle mRNA expression of the myogenic regulatory factor myoD, and the plasma concentration and muscle mRNA expression of the muscle growth inhibitor myostatin and follistatin. Myostatin is a muscle growth inhibitor which is produced primarily in skeletal muscle cells, circulates in the blood and acts on muscle tissue by blocking genes induced during differentiation (e.g., myoD [21
]) and by inhibiting the anabolic signaling cascade and muscle protein synthesis [22
]. Follistatin is ubiquitously expressed, circulates in the blood and binds to and thereby inhibits myostatin [27
]. We found that plasma follistatin concentration was, paradoxically, greater in old than young subjects but not different in men and women whereas plasma myostatin concentration and muscle myoD, myostatin and follistatin mRNA expressions were not different in men and women and not affected by aging. These findings are generally consistent with those reported by others [56
] and do not match the differences in muscle protein turnover between young and old men and women. We recognize that our muscle mRNA expression data may provide only limited information; however, in pilot experiments, we were not able to identify antibodies specific for myoD and myostatin that passed rigorous quality control criteria. The exact mechanism(s) of myostatin action (e.g., via plasma or locally within muscle or both) are not entirely clear and few studies have compared muscle and plasma myostatin concentrations. Nevertheless, those that did, show good qualitative agreement between the plasma myostatin concentration and myostatin protein expression in human muscle [60
The higher basal muscle protein FSR in old women is not inconsistent with a reduced muscle mass in old compared with young women because muscle mass is determined by the net balance between muscle protein synthesis and muscle protein breakdown. In fact, very high muscle protein synthesis rates are often observed in extremely catabolic conditions such as major burns because both muscle protein synthesis and muscle protein breakdown rates are upregulated but the increase in muscle protein breakdown exceeds the increase in muscle protein synthesis resulting in net muscle protein loss despite an increase in muscle protein synthesis [62
]. Our data therefore suggest that accelerated muscle protein breakdown may be a major contributor to the age-associated loss of muscle mass in older women.
We measured the global/mixed muscle protein synthesis rate and it is therefore possible, but unlikely, that our results are not applicable to myofibrillar proteins, which account for the bulk of muscle proteins. During basal, postabsorptive conditions at rest, there is very good correlation between the mixed and the myofibrillar protein FSR in both young and older subjects [63
]. Furthermore, the increases in myofibrillar, sarcoplasmic and mitochondrial protein synthesis rates in response to hyperaminoacidemia/hyperinsulinemia mirror each other [40
In summary, we report that healthy aging is associated with an increase in the basal rate of muscle protein synthesis in women and resistance to the anabolic effect of nutritional stimuli in both men and women. These findings indicate that there is sexual dimorphism in the age-related changes in muscle protein synthesis and the metabolic processes responsible for the age-related decline in muscle mass.