Metabolic activity of muscle and impact on energy expenditure

Muscle plays a central role in whole-body protein metabolism and provides a reservoir of amino acids that are precursors for the synthesis of proteins as well as for hepatic gluconeogenesis, which ensures that plasma glucose concentrations are maintained⁽ Reference Wolfe ^{38 )}.

The consequences of the loss of muscle mass on energy expenditure occur because it is more metabolically active than adipose tissue and is responsible for about 30 % of energy expenditure⁽ Reference Nair ^{2 ,} Reference Wolfe ^{38 )}. The energy released as a result of muscle protein synthesis may range from ∼2029·24 kJ/d in a young man with a high proportion to muscle to ∼502.08 kJ/d in an active elderly woman⁽ Reference Nair ^{2 )}. These differences in energy expenditure, resulting from protein turnover, could lead to substantial differences in energy balance (providing food intake and physical activity remained constant)⁽ Reference Nair ^{2 )}. Wolfe calculated that the equivalent of a 10-kg difference in lean mass would lead to differences in energy expenditure that equate to a difference of 418·4 kJ/d⁽ Reference Wolfe ^{38 )} and that over the course of a year this would lead to a potential gain of approximately 4·7 kg fat mass (assuming 1 kg fat stores 32216·8 kJ)⁽ Reference Wolfe ^{38 )}. Therefore, the loss of muscle mass could contribute to the onset of obesity via reductions in energy expenditure.

Blood glucose control and insulin resistance

Skeletal muscle accounts for ∼75 % of whole-body insulin-stimulated glucose uptake and therefore differences in the insulin resistance of muscle can affect glucose metabolism. Loss of total muscle mass also potentially influences insulin sensitivity and glucose metabolism⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 )}. Additionally, whole-body glucose uptake and muscle glucose transport are positively associated with type I fibres⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 )}. Obese people tend to have fewer type I fibres and a higher percentage of type II fibres than lean subjects⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 )}, and a recent comparison of different measures of muscle mass indicated that relative muscle mass relates to insulin resistance; greater muscle mass is associated with better insulin resistance⁽ Reference Bijlsma, Meskers and van Heemst ^{40 )}. Sarcopenia, independent of obesity, has also been associated with increased insulin resistance, in the US National Health and Nutrition Examination Survey study⁽ Reference Srikanthan, Hevener and Karlamangla ^{41 )}.

Regulation of extracellular potassium

Skeletal muscle is the main intracellular store of K⁺ and maintenance of extracellular concentrations of K are achieved through kidney excretion of K^{+ (} Reference McDonough, Thompson and Youn ^{42 )}. Excretion matches to intakes and to the K⁺ stored in skeletal muscle⁽ Reference McDonough, Thompson and Youn ^{42 )}. The Na⁺–K⁺–ATPase enzyme (also known as the sodium pump/Na⁺–K⁺ pump) is located in the sarcolemma and t tubules and maintains the ion gradients that are important for muscle function and activity⁽ Reference Clausen ^{43 ,} Reference Haddy, Vanhoutte and Feletou ^{44 )}. The pump is responsible for K⁺ reuptake in skeletal muscle and maintains ion distribution by via trans-membrane gradients for Na⁺ and K⁺, and so muscle membrane potential and muscle excitability⁽ Reference Kristensen and Juel ^{45 )}.

With the decrease in lean body mass and increase in body fat associated with ageing there is a decrease in total body water⁽ Reference Miller ^{46 ,} Reference Schlanger, Bailey and Sands ^{47 )} that occurs through reductions in the intracellular fluid compartments. Total body water reduces with age from 60 % of body weight in younger men to 54 % in men over the age of 65 years, and from 52 to 40 % in women⁽ Reference Miller ^{46 ,} Reference Norris, Lundy and Shock ^{48 ,} Reference Watson, Watson and Batt ^{49 )}. This change in total body water plays a role in the development of both hyponatraemia and hypernatraemia, which are more prevalent in older people⁽ Reference Miller ^{46 ,} Reference Schlanger, Bailey and Sands ^{47 ,} Reference Luckey and Parsa ^{50 )}. Also in a study of the muscle content of K⁺, Na⁺, Mg²⁺ and Ca²⁺, those with potassium depletion had higher concentrations of Na⁺ and Ca²⁺ in muscle and lower levels of Mg²⁺ than their replete counterparts⁽ Reference Schlanger, Bailey and Sands ^{47 ,} Reference Tavichakorntrakool, Prasongwattana and Sriboonlue ^{51 )}.

Muscle, bone density and osteoporosis

Muscle mass is positively associated with bone density, potentially due to its direct mechanical forces on bones, and so may be important in preventing falls and therefore, fractures⁽ Reference Bogl, Latvala and Kaprio ^{52 –} Reference Lu, Nayeem and Anderson ^{60 )}. In addition to its role in maintaining balance, muscle mass may also act as a protective barrier to reduce the impact of falls⁽ Reference Bogl, Latvala and Kaprio ^{52 –} Reference Szulc, Beck and Marchand ^{56 )}. Recently, lower muscle strength as well as muscle mass have been associated with poor cortical and trabecular bone microarchitecture⁽ Reference Szulc, Blaizot and Boutroy ^{58 )}.

Sarcopenia

Sarcopenia has recently been defined as the age-related loss of muscle mass and strength associated with ageing⁽ Reference Cruz-Jentoft, Baeyens and Bauer ^{13 ,} Reference Cooper, Dere and Evans ^{61 )}. Sarcopenia and loss of muscle mass is also associated with increased rates of mortality in older age and to functional impairment, physical disability and frailty⁽ Reference Janssen, Heymsfield and Ross ^{62 –} Reference Broadwin, Goodman-Gruen and Slymen ^{65 )}.

Estimated rates of prevalence of sarcopenia range from 9 to 18 % in those over the age of 65 years to up to 50 % in those over 80 years old⁽ Reference Fielding, Vellas and Evans ^{3 ,} Reference Sayer ^{66 ,} Reference Abellan van Kan ^{67 )}. The current costs of sarcopenia in the USA are estimated as $18·5 billion ($10·8 billion in men and $7·8 billion in women), about 1·5 % of total health care expenditure⁽ Reference Sayer ^{66 ,} Reference Janssen, Shepard and Katzmarzyk ^{68 )}.

In addition to the direct estimated costs of sarcopenia, sarcopenia contributes to fractures (through the impact on falls) which are a major public health problem, costing £2·3 billion per year in health and social care in the UK alone and $17 billion per year in the USA⁽ Reference Harvey, Dennison and Cooper ^{69 , 70 )}.

Sarcopenic obesity can also exist (high fat mass and low muscle mass) because the reduction in muscle mass associated with ageing can be accompanied by accumulation of fat mass⁽ Reference Silva, Karnikowski and Funghetto ^{71 –} Reference Baumgartner ^{73 )}. Estimates of the prevalence of sarcopenic obesity are between 34 and 48 % in the elderly although these estimates depend on the method of definition⁽ Reference Silva, Karnikowski and Funghetto ^{71 ,} Reference Batsis, Barre and Mackenzie ^{74 )}. A recent study in about 5000 men and women from the National Health and Nutrition Examination Survey study found estimates of sarcopenic obesity varied between 4·4–84 % in men, and 3·6–94 % in women and recommended that methods of estimating prevalence of sarcopenia needed to be more consistent⁽ Reference Batsis, Barre and Mackenzie ^{74 )}. Obesity is a risk factor for poor health, reduced functional capacity and quality of life, as is loss of muscle mass and strength. Sarcopenic obesity combines the effects and mechanisms of both sarcopenia and obesity, potentially exacerbating both in older people⁽ Reference Rolland, Lauwers-Cances and Cristini ^{28 ,} Reference Stenholm, Harris and Rantanen ^{36 ,} Reference Batsis, Barre and Mackenzie ^{74 –} Reference Li and Heber ^{78 )}. Both sarcopenia and obesity are associated with inflammation and increased insulin resistance and these mechanisms may act synergistically to worsen both obesity and muscle loss in overweight elderly people⁽ Reference Stenholm, Harris and Rantanen ^{36 ,} Reference Batsis, Barre and Mackenzie ^{74 –} Reference Roubenoff ^{77 )}. Thus being overweight and having lower muscle mass and strength is not only a risk factor for reduced functional capacity but may also act in a vicious circle to potentiate and worsen the situation, with ever increasing fat mass and lower muscle mass in those with sarcopenic obesity⁽ Reference Stenholm, Harris and Rantanen ^{36 ,} Reference Batsis, Barre and Mackenzie ^{74 –} Reference Roubenoff ^{77 )}.

Maintenance of muscle mass is therefore highly important for maintaining metabolic health, for potentially preventing the onset of insulin resistance and obesity and osteoporosis and for prevention of the onset of frailty and sarcopenia and their consequences on fractures and on morbidity and mortality.

Protein

Protein has been the most intensively studied nutrient in relation to muscle mass in terms of absolute or percentage of energy intake or as amino acids.

Protein turnover is integral to muscle and as discussed previously, in the section on mechanisms, the balance between anabolism and catabolism is important for maintenance of skeletal muscle mass. The major metabolic pathway through which essential amino acids act to produce an anabolic response is the mTOR pathway and current opinion is that this is regulated by the branched chain amino acid leucine⁽ Reference Dillon ^{142 )}. Recent evidence suggests that the threshold concentration of circulating amino acids required to produce an anabolic response in skeletal muscle synthesis may be increased in older muscle, and that increasing concentrations of leucine are required to maintain robust anabolic responses⁽ Reference Dillon ^{142 )}. A number of recent publications have extensively reviewed the literature on interventions and mechanisms of protein and amino acid intake in relation to muscle mass, with evidence on total protein intake, type and quantity of amino acid and type of protein (casein/whey v. soya) to which the reader can refer for more detail⁽ Reference Dillon ^{142 –} Reference Cermak, de Groot and van Loon ^{145 )}.

Cross-sectional studies and protein intake

Studies associating protein intake and muscle mass in population cross-sectional studies have not been previously reviewed extensively but those included here have found contradictory associations⁽ Reference Baumgartner, Waters and Gallagher ^{1 ,} Reference Scott, Blizzard and Fell ^{15 ,} Reference Meng, Zhu and Devine ^{22 ,} Reference Mitchell, Haan and Steinberg ^{146 –} Reference Stookey, Adair and Popkin ^{148 )}. Protein intake was not associated with muscle mass in older men and women in two studies⁽ Reference Baumgartner, Waters and Gallagher ^{1 ,} Reference Mitchell, Haan and Steinberg ^{146 )} but was positively related in another⁽ Reference Scott, Blizzard and Fell ^{15 )}, with a difference of 1·3 kg between quartiles 1 and 4 (P = 0·002)⁽ Reference Scott, Blizzard and Fell ^{15 )}; also at follow-up it was associated with change in appendicular lean mass over a two and a half-year period⁽ Reference Scott, Blizzard and Fell ^{15 )}. A lower percentage protein intake was associated with greater loss of mid-arm muscle area (<10·4 % protein as percentage energy compared with 10·4–12·1 % energy) over 4 years, in Chinese men and women aged 50–69 years⁽ Reference Stookey, Adair and Popkin ^{148 )}. In addition, in men and women aged 70–79 years, energy adjusted protein intake was associated with 3-year changes in lean mass⁽ Reference Houston, Nicklas and Ding ^{147 )}. Those with a higher percentage protein intake had a lower rate of loss of lean mass and appendicular lean mass, by ∼40 %, compared with the lowest⁽ Reference Houston, Nicklas and Ding ^{147 )}. Finally, in Australian women aged 72–78 years lean mass, appendicular lean mass and upper arm muscle area were all positively and significantly associated with baseline protein intake⁽ Reference Meng, Zhu and Devine ^{22 )}. Compared with the lowest tertile of protein intake (<66 g/d v. >87 g/d) there were 5·4–6·0 % differences in whole body and appendicular lean mass⁽ Reference Meng, Zhu and Devine ^{22 )}.

Interventions with protein and amino acids

A large number of intervention trials with protein, whey, casein and mixed essential or individual amino acids have been performed, although very few have been dietary or supplementation studies alone⁽ Reference Malafarina, Uriz-Otano and Iniesta ^{144 )}. Of the few studies of amino acid or protein supplementation only, in interventions with essential amino acids, two studies found increases in lean body mass over the intervention period⁽ Reference Dillon ^{142 –} Reference Malafarina, Uriz-Otano and Iniesta ^{144 ,} Reference Dillon, Sheffield-Moore and Paddon-Jones ^{149 ,} Reference Borsheim, Bui and Tissier ^{150 )}. Generally, the response to protein and amino acid supplementation (with or without exercise interventions) has not been consistent and this may be due to differences in nutritional status at baseline, differing methods of measuring outcomes and geographical and racial differences⁽ Reference Malafarina, Uriz-Otano and Iniesta ^{144 ,} Reference Casperson, Sheffield-Moore and Hewlings ^{151 )}. Supplementation studies with protein have not always enhanced the effects of resistance training⁽ Reference Candow, Forbes and Little ^{152 ,} Reference Kukuljan, Nowson and Sanders ^{153 )}.

In a recent meta-analysis of twenty-two randomised controlled trials of protein supplementation studies (in the form of protein, whey, casein and mixed essential or individual amino acids) accompanied by resistance exercise interventions, protein supplementation had an overall positive effect on FFM and leg strength⁽ Reference Cermak, Res and de Groot ^{154 )}. Protein supplementation had a similar effect in both younger and older participants (>50 years) and also increased the gain in type II muscle fibre cross-sectional area overall, but in further analysis this was not apparent in older subjects⁽ Reference Cermak, Res and de Groot ^{154 )}.

The research to date from both cross-sectional and intervention studies indicates that sufficient protein of high quality is undoubtedly crucial for muscle mass, with leucine being the most effective amino acid. However, other nutritional factors may also influence muscle mass as well as protein but have been much less studied. The following sections cover these less researched areas relating nutrition to age-related muscle loss.

Vitamin D

The mechanisms that explain the relationship of vitamin D with muscle mass and strength may be direct or indirect either through calcium handling and signalling and accumulation in the sarcoplasmic reticulum (influencing the involvement of calcium in muscle contraction) or through the activation of vitamin D receptors found in muscle⁽ Reference Ceglia and Harris ^{155 )}. Vitamin D deficiency is associated with atrophy of type II muscle fibres⁽ Reference Ceglia and Harris ^{155 )}. However, in contrast to prevailing opinion and previous studies, a recent study was unable to detect vitamin D receptors in human skeletal muscle, suggesting the activity of vitamin D on muscle may be indirect⁽ Reference Wang and DeLuca ^{156 )}. Nevertheless, an earlier study found that vitamin D receptor polymorphisms in muscle were associated with lower FFM in older men and women⁽ Reference Scott, Blizzard and Fell ^{157 )}. Vitamin D status has also been associated with muscle strength in a number of studies, i.e. better vitamin D status associated with greater muscle strength⁽ Reference Ceglia and Harris ^{155 )}.

Comparatively few studies have assessed vitamin D status and associated it with muscle mass, although two early intervention studies found a positive effect of vitamin D supplementation on type IIa or II muscle fibres⁽ Reference Ceglia and Harris ^{155 )}. Four cross-sectional studies found a positive association between vitamin D status and total or appendicular muscle mass in men and women⁽ Reference Szulc, Duboeuf and Marchand ^{122 ,} Reference Marantes, Achenbach and Atkinson ^{158 –} Reference Visser, Deeg and Lips ^{160 )}, although in one of the studies the association was found only in women under the age of 65 years; not in men and older women⁽ Reference Marantes, Achenbach and Atkinson ^{158 )}. Two of these studies, after 2·6 or 3 years follow-up, found that protection from muscle loss was associated with baseline vitamin D status⁽ Reference Scott, Blizzard and Fell ^{159 ,} Reference Visser, Deeg and Lips ^{160 )}. A further cross-sectional study did not find an association between vitamin D status and muscle area (using computed tomography) but did find that infiltration of fat into muscle was greater in those with lower vitamin D status (plasma≤29 ng/ml)⁽ Reference Gilsanz, Kremer and Mo ^{161 )}.

Although vitamin D is highly likely to influence muscle mass further research is needed to establish definitively the extent to which vitamin D can positively influence muscle mass.

Alcohol

Extreme habitual consumption of alcohol (>80 g alcohol/d) can lead to chronic alcoholic myopathy, development of muscle weakness and wasting⁽ Reference Fernandez-Sola, Preedy and Lang ^{162 –} Reference Preedy, Paice and Mantle ^{164 )}. Even in healthy volunteers drinking about 22 units daily over a month induces substantial muscle damage⁽ Reference Song and Rubin ^{165 )}. The frequency of alcoholic myopathy is estimated as being between 40 and 60 % in those who are alcoholic⁽ Reference Wijnia, Wielders and Lips ^{166 )}. An acute alcoholic myopathy can also occur after substantial alcohol consumption, even in healthy people, and is accompanied by muscle aching and tenderness, and often muscle cramps⁽ Reference Wijnia, Wielders and Lips ^{166 ,} Reference Hewitt and Winter ^{167 )}. The extreme loss of muscle mass that occurs in alcoholic patients may be reversed by becoming abstinent, as one study found significantly increased total lean mass in alcoholics who abstained from drinking (∼600 g over 6 months)⁽ Reference Martin-Gonzalez, Gonzalez-Reimers and Santolaria-Fernandez ^{168 )}.

The structural damage to skeletal muscle that occurs with alcohol includes reduced diameter of type II fibres, particularly the type IIb fibres (which do not include mitochondria) whereas the type I fibres appear unaffected, and in the early stages show compensatory hypertrophy⁽ Reference Preedy and Emery ^{163 ,} Reference Wijnia, Wielders and Lips ^{166 )}. The extent of alcoholic myopathy is sufficiently severe to be measurable by changes to mid-arm circumference and DXA measurements⁽ Reference Preedy and Emery ^{163 ,} Reference Martin-Gonzalez, Gonzalez-Reimers and Santolaria-Fernandez ^{168 )}.

The mechanisms involved in alcoholic myopathy include reduced rates of protein synthesis and breakdown, loss and redistribution of ribosomal RNA, increased RNase activities, membrane damage, altered Ca²⁺ regulation and the generation of free radicals⁽ Reference Preedy and Emery ^{163 ,} Reference Preedy, Paice and Mantle ^{164 ,} Reference Pacy, Preedy and Peters ^{169 )}. Alcohol can also inhibit hepatic stimulation of IGF-1 and disrupt the growth hormone/IGF-1 axis⁽ Reference Ronis, Wands and Badger ^{170 )}. Secretion of glucocorticoids and inflammatory cytokines are other mechanisms underlying alcoholic myopathy⁽ Reference Ronis, Wands and Badger ^{170 )}.

Although extreme loss of muscle mass has been identified in alcoholics, very little population research on impact of high alcohol consumption has been done. In one study of Korean men, older than 60 years, and one in France in older men, found no association between alcohol intake and sarcopenia⁽ Reference Szulc, Duboeuf and Marchand ^{122 ,} Reference Kim, Kim and Hwang ^{171 )}. However, the distribution of high alcohol consumption was relatively low in the Korean population with the highest category of alcohol consumption being >7 drinks at a time >2 d/week which may explain the lack of findings in that study⁽ Reference Kim, Kim and Hwang ^{171 )}.

Alcohol consumption could contribute to the loss of skeletal muscle at the population level as in some population groups such as young men in the UK aged 18–24 years 40 % drink more than the recommended 21 units per week and 25 % of women more than 14 units per week⁽ Reference Preedy and Emery ^{163 )}. This could be a potential future problem given that alcohol use is increasing, and further research is required.

Dietary acid–base load

Metabolic acidosis is one of the major causes of skeletal muscle loss in chronic kidney disease, and the mild metabolic acidosis that occurs within populations may also be relevant for loss of skeletal muscle⁽ Reference Mitch ^{172 –} Reference Welch, Macgregor and Skinner ^{174 )}.

The acid–base balance in blood is partly maintained by the balance between H⁺ and bicarbonate ions. Metabolic acidosis is associated with a more acidic blood acid–base composition and, while the acid–base equilibrium is maintained within narrow limits, the pH in blood becomes more acidic with age⁽ Reference Frassetto, Morris and Sebastian ^{175 )}. This is because maintenance of the blood acid–base system is dependent on excretion of H⁺ ions by the urine, and as renal function gradually declines with age there is potential for mild metabolic acidosis to increase⁽ Reference Frassetto, Morris and Sebastian ^{175 )}.

Diet has the potential to contribute to mild metabolic acidosis in the general population through consumption of potentially acidogenic and alkalinogenic forming foods. Potentially acidogenic foods are protein containing foods that contain the sulphur-containing amino acids (cysteine and methionine) which when metabolised lead to the production of hydrogen ions that can lower blood pH⁽ Reference Welch, Mulligan and Bingham ^{176 –} Reference Wynn, Krieg and Aeschlimann ^{180 )}. The major contributing acidogenic foods are meats, fish, eggs, cereals and dairy foods and these foods have been positively associated with a more acidic urine pH in population studies⁽ Reference Welch, Mulligan and Bingham ^{176 –} Reference Wynn, Krieg and Aeschlimann ^{180 )}. The alkalinogenic foods, fruits and vegetables that balance the H⁺ ions generated via metabolism of acidogenic foods, through the carbonate compounds present in them, can lower increase blood pH⁽ Reference Welch, Mulligan and Bingham ^{176 –} Reference Wynn, Krieg and Aeschlimann ^{180 )}. The overall dietary balance between acidogenic foods (protein-containing foods) and alkalinogenic foods (fruits and vegetables), that supply base precursors, is the dietary acid–base load, and a number of population and intervention studies have demonstrated associations between diet and measures of metabolic acidosis. There are a number of methods of estimating diet-dependent net acid load and the balance between acidogenic and alkalinogenic foods and these include estimated net endogenous acid production or potential renal acid load⁽ Reference Frassetto, Lanham-New and Macdonald ^{181 )}. Potential renal acid load includes contributions from nutrient categories that relate to the major determinants of their acid or base-forming potential, i.e. protein and phosphorus (acidogenic) and calcium, potassium and magnesium (alkalinogenic)⁽ Reference Welch, Mulligan and Bingham ^{176 –} Reference Frassetto, Todd and Morris ^{178 )}.

A number of mechanisms have been proposed for the loss of muscle mass with metabolic acidosis, largely derived from animal studies and studies on those with chronic kidney disease⁽ Reference Mitch ^{172 –} Reference Welch, Macgregor and Skinner ^{174 )}. Metabolic acidosis potentially accelerates proteolysis and amino acid catabolism, probably through activation of caspase-3 and the ubiquitin proteasome system or via effects on the growth hormone/IGF-1 axis⁽ Reference Perrini, Laviola and Carreira ^{97 ,} Reference Dawson-Hughes, Harris and Ceglia ^{182 –} Reference Thomas and Mitch ^{186 )}.

The relationship between supplementation with bicarbonate compounds, designed to attenuate metabolic acidosis, has been investigated in a few studies of middle- and older-aged populations and two of these studies found attenuation of nitrogen excretion, with the more recent study suggesting this might potentially be mediated by IGF-1⁽ Reference Frassetto, Morris and Sebastian ^{86 ,} Reference Dawson-Hughes, Harris and Ceglia ^{182 ,} Reference Dawson-Hughes, Castaneda-Sceppa and Harris ^{187 –} Reference Dawson-Hughes, Harris and Palermo ^{189 )}. A reduction in nitrogen excretion indicates more nitrogen has been retained for the anabolic processes in muscle. Two intervention studies designed to understand the effect of reducing the metabolic acidosis of chronic renal disease, using sodium bicarbonate as the source of bicarbonate, found improvements in nitrogen balance and in serum albumin and mid-arm muscle circumference after supplementation for 2 years⁽ Reference Papadoyannakis, Stefanidis and McGeown ^{190 ,} Reference de Brito-Ashurst, Varagunam and Raftery ^{191 )}.

However, to date the effects of dietary metabolic acid–base load on muscle mass in healthy participants have only been investigated in two studies⁽ Reference Welch, Macgregor and Skinner ^{174 ,} Reference Dawson-Hughes, Harris and Ceglia ^{182 )}. In one an alkaline diet, as measured by potassium excretion was positively related to muscle mass and change in lean body mass over a 3-year period⁽ Reference Dawson-Hughes, Harris and Ceglia ^{182 )}. In the other, a more alkalinogenic diet was positively related to indices of muscle mass and the proportion of fruits and vegetables to alkaline-forming foods (meat, fish, eggs and dairy)⁽ Reference Welch, Macgregor and Skinner ^{174 ,} Reference Dawson-Hughes, Harris and Ceglia ^{182 )}. The associations remained significant after accounting for total protein intake and other confounders that influence muscle mass. The scale of the association ranged between a fifth and one-half of the observed relationship with 10 years of age, i.e. the alkalinogenic effect of diet was about half as important as the effect of age on muscle loss⁽ Reference Frassetto, Morris and Sebastian ^{86 ,} Reference Dawson-Hughes, Harris and Ceglia ^{182 ,} Reference Dawson-Hughes, Castaneda-Sceppa and Harris ^{187 –} Reference Dawson-Hughes, Harris and Palermo ^{189 )}. In that study the optimal ratio of alkalinogenic to acidogenic foods was about 1·4.

These results suggest that fruits and vegetables as well as protein may be important for conservation of muscle; however, intervention studies to investigate the effects of a more alkaline diet on muscle loss in populations have yet to be done.

Interestingly, a recent rodent study designed to understand the interaction between vitamin D and alkaline status on the loss of muscle mass, found an interaction between vitamin D and alkaline status⁽ Reference Ceglia, Rivas and Pojednic ^{192 )}. Vitamin D status modified the effect of potassium bicarbonate supplementation (alkalinising compound) on muscle mass (after 12 weeks)⁽ Reference Ceglia, Rivas and Pojednic ^{192 )}. It also influenced Akt activation (Akt regulates several signalling pathways in skeletal muscle)⁽ Reference Ceglia, Rivas and Pojednic ^{192 )}. The data suggested that alkali supplementation was effective in increasing muscle but that this effect was more enhanced when vitamin D status was adequate⁽ Reference Ceglia, Rivas and Pojednic ^{192 )}. Therefore, future work in this field also needs to take into account vitamin D status.

Dietary fat composition

The association between dietary fat and muscle mass has been relatively under investigated which, given the central role of fat in muscle metabolism, is surprising. The fatty acids derived from dietary fat are the major source of energy for resting and working muscle and fats are an integral component of myocellular membranes⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 ,} Reference Welch, MacGregor and Minnihane ^{81 ,} Reference Turner, Lee and Bruce ^{125 ,} Reference Kiens ^{193 –} Reference Kien ^{204 )}. Dietary fat composition also influences inflammation and insulin resistance, mechanisms recently identified as relating to potential loss of FFM⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 ,} Reference Kiens ^{193 –} Reference Frayn ^{195 ,} Reference Kien ^{204 –} Reference Holloway, Luiken and Glatz ^{208 )}.

During exercise fatty acid entry into cells is principally regulated by the proteins fatty acid translocase and fatty acid binding protein, although at high intensities of exercise the contribution of fat to oxidative metabolism declines⁽ Reference Kiens ^{193 ,} Reference Frayn ^{194 ,} Reference Holloway, Bonen and Spriet ^{205 ,} Reference Holloway, Luiken and Glatz ^{208 ,} Reference Dyck ^{209 )}. It is also known that fatty acids are differentially oxidised (from whole-body studies), with unsaturated fatty acids and oleic acid oxidised preferentially over SFA⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 ,} Reference Kien ^{204 ,} Reference Kien, Bunn and Ugrasbul ^{206 ,} Reference DeLany, Windhauser and Champagne ^{210 ,} Reference Jones and Schoeller ^{211 )}. Moreover, in vitro studies have found EPA increases fatty acid oxidation in myotubules⁽ Reference Kalupahana, Claycombe and Moustaid-Moussa ^{212 ,} Reference Wensaas, Rustan and Just ^{213 )}.

As the proportion of dietary fatty acids varies within populations and is associated with differences in the fatty acid composition of myocellular membranes, the different types of dietary fatty acids could affect lipid messengers and cellular signalling within the membrane. They could also affect membrane fluidity and the positioning of proteins, as well as levels of muscle ceramide, diacylglycerol, TAG and acylcarnitines⁽ Reference Corcoran, Lamon-Fava and Fielding ^{39 ,} Reference Blachnio-Zabielska, Baranowski and Zabielski ^{214 ,} Reference Kien, Everingham and Stevens ^{215 )}.

As inflammation is associated with loss of muscle mass in the elderly and since dietary fat composition can influence inflammation, through stimulation of eicosanoid production, dietary fat may influence muscle loss (saturated and trans-fatty acids are considered pro-inflammatory and the n-3 and n-6 fatty acids, anti-inflammatory)⁽ Reference Schaap, Pluijm and Deeg ^{105 ,} Reference Kalupahana, Claycombe and Moustaid-Moussa ^{212 ,} Reference Kalogeropoulos, Panagiotakos and Pitsavos ^{216 –} Reference Rallidis, Paschos and Liakos ^{228 )}. Fatty acids are associated with insulin resistance in the same direction as inflammation⁽ Reference Honors and Kinzig ^{6 ,} Reference Waterlow ^{229 )}.

Total dietary fat (as opposed to individual fatty acids) may also have an effect on muscle mass through a number of mechanisms, including decreased hepatic and skeletal muscle oxidative capacity and by increasing the availability of fatty acids through skeletal muscle for oxidation⁽ Reference Hancock, Han and Chen ^{230 –} Reference Newsom, Schenk and Li ^{234 )}. In animal models, a high-fat diet leads to impaired protein turnover and muscle hypertrophy, through attenuated activation of Akt and S6K1 (kinases in the mTOR pathway)⁽ Reference Sitnick, Bodine and Rutledge ^{235 )}.

Recent human studies have found that supplementation with long chain n-3 PUFA increases rates of protein synthesis and augments the muscle protein anabolic response, and may also attenuate acute muscle loss (cachexia)⁽ Reference Kumar, Kazi and Smith ^{236 –} Reference Fiaccavento, Carotenuto and Vecchini ^{241 )}. One small observational study found a negative association with appendicular lean mass and saturated fat, and a comprehensive study of dietary fat composition and muscle mass found positive associations with the PUFA:SFA ratio and negative associations with saturated and trans fatty acids⁽ Reference Scott, Blizzard and Fell ^{15 ,} Reference Welch, MacGregor and Minnihane ^{81 )}.

Antioxidant micronutrients

Oxidative stress and the accumulation of ROS potentially contribute to age-related muscle loss so, consumption of antioxidant nutrients may reduce oxidation in muscle⁽ Reference Liochev ^{115 )}. However, the use of dietary antioxidants for the purpose of attenuating oxidative damage in muscle may not be entirely beneficial as although antioxidants may reduce oxidative damage in mitochondria they also reduce the redox signalling, initiated by ROS, required for muscle contraction⁽ Reference Jackson ^{119 )}.

Of the nutrients considered to be antioxidant (vitamins C, E and carotenoids and the trace elements: Cu, Mn, Se and Zn) the few intervention and observational studies available have mainly investigated the relationship with vitamins C and E. Skeletal muscle is the major body store of vitamin C, estimated to contain up to 67 % of total body vitamin C. Therefore, vitamin C is likely to be essential for muscle structure and function due to its role as an antioxidant and enzyme cofactor for collagen and carnitine synthesis⁽ Reference Carr, Bozonet and Pullar ^{242 )}. A recent intervention study in men found skeletal muscle was highly responsive to increased vitamin C intake with relative uptake being greater in muscle than for leucocytes⁽ Reference Carr, Bozonet and Pullar ^{242 )}.

A number of studies have related decline in physical function or frailty to serum micronutrient concentrations of vitamin E or carotenoids or Se, and also low intake of fruit and vegetables has been associated with functional limitations in mid-life and older men and women⁽ Reference Bartali, Frongillo and Guralnik ^{243 –} Reference Lauretani, Semba and Bandinelli ^{248 )}. One study in women aged 18–79 years found a positive relationship between intake of vitamin C, and total carotene and FFM index and no association with vitamin E and Se⁽ Reference Kelaiditi, jennings and Macgregor ^{249 )}. A further cross-sectional study found that in older women a higher antioxidant nutrient intake (vitamin C, β-carotene, Se) was associated with certain aspects of physical performance (walking and chair rise times)⁽ Reference Martin, Aihie Sayer and Jameson ^{250 )}. One longitudinal observational study found a protective relationship between vitamin C intake and loss of muscle mass in older men and women over 2·6 years⁽ Reference Scott, Blizzard and Fell ^{15 )}. Supplementation studies in human subjects have varied in methodology and in mixtures of nutrients, with one study finding that daily supplementation with 600 mg vitamin E and 1000 mg vitamin C improved FFM when combined with resistance training, more than resistance training alone⁽ Reference Bobeuf, Labonte and Dionne ^{251 ,} Reference Labonte, Dionne and Bouchard ^{252 )}. Although there was a small increase in FFM in the group receiving nutritional supplements, only, this was NS⁽ Reference Bobeuf, Labonte and Dionne ^{251 ,} Reference Labonte, Dionne and Bouchard ^{252 )}. In rats, supplementation with rutin, vitamins A and E, Zn and Se improved the anabolic response to the amino acid leucine, and reduced inflammation, but the beneficial effects of antioxidants on the leucine response may result from a systemic rather than a localised reduction in oxidative stress and inflammation⁽ Reference Mosoni, Balage and Vazeille ^{100 ,} Reference Marzani, Balage and Venien ^{253 )}.

The limited research to date suggests that antioxidant nutrients would be worthwhile investigating further in relation to prevention of muscle loss.

Minerals

Magnesium status has effects on muscle performance, through its roles in energy metabolism and trans-membrane transport and 27 % of magnesium in the body is stored in skeletal muscle⁽ Reference Lukaski ^{254 ,} Reference Swaminathan ^{255 )}. Low serum magnesium has been associated with lower muscle strength, but the role of magnesium has so far only been found to be positively related to muscle mass in two studies⁽ Reference Scott, Blizzard and Fell ^{15 ,} Reference Kelaiditi, jennings and Macgregor ^{249 ,} Reference Dominguez, Barbagallo and Lauretani ^{256 )}. In a 2·6-year follow-up study, magnesium was a positive predictor of change in appendicular lean mass, as was iron, phosphorus and zinc, indicating a role for minerals in conservation of lean mass, i.e. better mineral intake was associated with less muscle loss over time⁽ Reference Scott, Blizzard and Fell ^{15 )}. However, further work is needed in this area.

Bioactive compounds

There is recent research indicating that certain bioactive compounds may influence age-related skeletal muscle loss; however, these have so far only investigated relationships with muscle strength or exercise performance. An olive oil derived anti-oxidant mixture was effective, in aged rodents, in restoring a number of sarcolemma ion channels in muscle⁽ Reference Pierno, Tricarico and Liantonio ^{257 )}. Circumin (a phenol compound found in the spice turmeric) has been used as a supplement in, in vivo studies, in relation to contractile function, although the results have so far been equivocal⁽ Reference Chopard, Hillock and Jasmin ^{10 )}. And finally, a recent review of nitrate supplementation from beetroot juice or inorganic nitrate found results suggestive of a moderate improvement in constant load time to exhaustion tests⁽ Reference Hoon, Johnson and Chapman ^{258 )}. However, the relationship with muscle mass does not yet appear to have been investigated. Bioactive compounds that influence muscle performance and strength may also be relevant for prevention of age-related muscle loss and so deserve further investigation.

Population advice

Current dietary recommendations for prevention of sarcopenia suggest protein intake should be 0·8 g/kg/d (US dietary guidelines Dietary Recommended Allowance) and the maximum intake in the UK is currently set at twice the current reference nutrient intake, i.e. 1·5 g/kg/d which equates to not more than 90 g/d in a woman weighing 60 kg. Recent suggestions are that older adults should consume 1·0–1·2 g/kg/d, although this is debated⁽ Reference Wolfe, Miller and Miller ^{259 –} Reference Waters, Baumgartner and Garry ^{263 )}. It has also been suggested that older people potentially divide protein intake during the day to consume 25–30 g high quality protein per meal in order to maximise protein synthesis⁽ Reference Symons, Sheffield-Moore and Wolfe ^{264 ,} Reference Bernstein and Munoz ^{265 )}. At present, there is no recommendation for younger adults although the recommendation to consume 0·8 g/kg/d protein would appear to be wise. Although there is limited research on the relationship between antioxidant nutrients and dietary acid–base load and conservation of muscle mass, consumption of adequate amounts of fruit and vegetables, i.e. following the dietary guidelines for five portions of fruits and vegetables per day would likely contribute to conservation of muscle. Given the potential adverse effects of high alcohol consumption, it would also be advisable to follow UK government guidelines for alcohol consumption (2 units/d for women and 3/d for men). Consumption of foods containing vitamin D and regular, appropriate, exposure to sunlight would also be advisable⁽ Reference Myint and Welch ^{266 )}.

For management of sarcopenia, advice is for total protein intake to be 1·0–1·5 g/kg body weight/d with measurement of 25(OH)D levels and supplementation, where required, to achieve circulating levels of 100 nmol/l⁽ Reference Morley, Argiles and Evans ^{14 )}.

However, the role of diet in the maintenance and prevention of muscle loss in ageing populations needs to be further characterised.