Prolonged bed rest decreases skeletal muscle and whole body protein synthesis

Abstract

We sought to determine the extent to which the loss of lean body mass and nitrogen during inactivity was due to alterations in skeletal muscle protein metabolism. Six male subjects were studied during 7 days of diet stabilization and after 14 days of stimulated microgravity (-6 degrees bed rest). Nitrogen balance became more negative (P < 0.03) during the 2nd wk of bed rest. Leg and whole body lean mass decreased after bed rest (P < 0.05). Serum cortisol, insulin, insulin-like growth factor I, and testosterone values did not change. Arteriovenous model calculations based on the infusion of L-[ring-13C6]-phenylalanine in five subjects revealed a 50% decrease in muscle protein synthesis (PS; P < 0.03). Fractional PS by tracer incorporation into muscle protein also decreased by 46% (P < 0.05). The decrease in PS was related to a corresponding decrease in the sum of intracellular amino acid appearance from protein breakdown and inward transport. Whole body protein synthesis determined by [15N]alanine ingestion on six subjects also revealed a 14% decrease (P < 0.01). Neither model-derived nor whole body values for protein breakdown change significantly. These results indicate that the loss of body protein with inactivity is predominantly due to a decrease in muscle PS and that this decrease is reflected in both whole body and skeletal muscle measures.

Nutrition and sarcopenia: evidence for an interaction

“…While the atrophy of disuse, such as occurs with protracted bed rest is well described (8,10,16) it is less clear cut as to the extent to which variation in levels of physical activity within the normal population influences sarcopenia, mainly because of the difficulty of measurement. Low physical activity at work has been identified as a risk factor for sarcopenia (17) and a recent study indicated that muscle mass and strength in the elderly does respond to aerobic exercise (18) .…”

High sodium chloride intake exacerbates immobilization-induced bone resorption and protein losses

“…Specific disuse-induced changes include a negative nitrogen balance, a decrease in whole body protein synthesis, and an increase in lower-limb proteolysis leading to decreased amounts of contractile protein in skeletal muscle (11,20,44,57,67).…”

Efeitos da estimulação elétrica neuromuscular sobre o membro posterior imobilizado de ratos durante 15 dias: análises metabólicas e morfométricas

“…O desuso muscular provocado por condições de períodos longos no leito, colocação de órteses ou fixações em membros e microgravidades induzem resistência à insulina e a um estado catabólico nos musculoesqueléticos afetados de humanos 7 . Esse quadro de resistência à insulina pode explicar a redução das reservas de glicogênio inerente à imobilização em todos os músculos analisados nesse estudo.…”

“…A homeostasia metabólica das fibras musculares também pode ser comprometida, induzindo o quadro de resistência à insulina e a um estado catabólico nos musculoesqueléticos afetados de humanos 7,8 . Porém, ainda não está claro como o desuso muscular crônico ou a imobilização alteram a sinalização de insulina 9 .…”