The sale of BCAAs as nutritional supplements has become a multi-million dollar business. At the center of the marketing for these products is the widely-believed claim that consumption of BCAAs stimulates muscle protein synthesis, and as a result elicits an anabolic response. BCAAs may also be consumed for the purpose of improving “mental focus”, but we will not consider that application. The primary purpose in this paper to evaluate the assertion that BCAAs alone are anabolic is adequately supported either theoretically or empirically by studies in human subjects. Implicit in our assessment will be the examination of whether or not the phosphorylation state of the eukaryotic initiation factors plays a rate-controlling role in the regulation of muscle protein synthesis in humans.
Muscle protein turnover and dietary protein intake
Muscle protein is in a constant state of turnover, meaning that new protein is continuously being produced while older proteins are being degraded. The anabolic state has no specific definition, but generally refers to the circumstance in which the rate of muscle protein synthesis exceeds the rate of muscle protein breakdown. The results in a gain of muscle mass. Conventionally the anabolic state is considered to be driven by a stimulation of muscle protein synthesis, but theoretically could also result from an inhibition of muscle protein breakdown.
The overriding metabolic goal of consuming BCAA supplements is to maximize the anabolic state. It is widely asserted that BCAAs induce an anabolic state by stimulating muscle protein synthesis. An abundant availability of all EAAs is a requisite for a significant stimulation of muscle protein synthesis [7]. Muscle protein synthesis will be limited by the lack of availability of any of the EAAs, whereas a shortage of NEAAs can be compensated for by increased de novo production of the deficient NEAAs [7]. In the post-prandial state following a meal containing protein, all of the EAA precursors required for new muscle protein synthesis can be derived from either the elevated plasma concentrations resulting from digestion of the consumed protein or from recycling from protein breakdown. In this circumstance of abundant availability of EAAs the rate of muscle protein synthesis exceeds the rate of breakdown, thereby producing an anabolic state. In the post-absorptive state the plasma EAA levels fall below the post-prandial values because amino acids are no longer being absorbed. As a result, EAAs are no longer taken up by muscle, but rather released by muscle into plasma [8]. This catabolic state of muscle protein in the post-absorptive state enables continued availability of EAAs for other tissues to maintain the rate of protein synthesis at the expense of muscle protein, which can be considered to play a role as the reservoir of EAAs for the rest of the body to draw upon.
Since EAAs cannot be produced in the body and there is a net release of EAAs from muscle, in the post-absorptive state the only source of EAA precursors for muscle protein synthesis is intracellular EAAs derived from muscle protein breakdown [8]. In addition to being reincorporated into muscle protein via synthesis, some EAAs released from muscle protein breakdown may be partially oxidized within muscle, thereby making them unavailable for reincorporation into muscle protein. EAAs released from muscle protein breakdown that are not reincorporated into muscle protein or oxidized within muscle tissue are released into plasma, whereupon they can either be taken up by other tissues as precursors for protein synthesis or irreversibly oxidized [9]. Thus, the rate of muscle protein synthesis will always be lower than the rate of muscle protein breakdown in the post-absorptive state, owing to the net flux of EAAs from protein breakdown into plasma and to oxidative pathways. Expressed differently, it is impossible for muscle protein synthesis to exceed the rate of muscle protein breakdown when the precursors are derived entirely from protein breakdown, and thus an anabolic state cannot occur in the absence of exogenous amino acid intake.
Are BCAAs anabolic in the post-absorptive state?
Theoretical considerations
吸収後状態での筋タンパク質合成のためのすべてのEAA前駆体は、筋タンパク質の分解に由来します。正常な吸収後のヒトでは、筋タンパク質の分解速度が筋タンパク質合成の速度を約 30% 上回ることが一貫して報告されています [10]。BCAAのみの消費(つまり、他のEAAなし)は、血漿に放出されたり酸化されたりするのではなく、タンパク質の分解からタンパク質合成へのEAAのリサイクル効率を高めることによって、吸収後の状態での筋タンパク質合成を増加させるだけです。これは、筋肉タンパク質を生成するには9つのEAA(および11のNEAA)すべてが必要であり、EAAは体内で生成できないためです。BCAAの消費の場合のように、3つのEAAしか消費されない場合、タンパク質の分解は、筋タンパク質合成の前駆体として必要な残りのEAAの唯一の供給源です。したがって、BCAAのみを摂取して筋タンパク質合成が筋タンパク質の分解を超える同化状態を作り出すことは理論的には不可能です。BCAAの摂取により、筋タンパク質の分解から筋タンパク質合成までのEAAのリサイクル効率が50%向上するという寛大な仮定がなされた場合、これは筋タンパク質合成率の15%の増加に変換されます(基礎状態でのリサイクルの30%×リサイクルの50%の改善=合成の15%の増加)。さらに、筋肉から血漿への EAA の放出が 50% 減少すると、遊離 EAA の血漿および細胞内プールも減少します。図1は、これらの原理を概略的に示しています。リサイクル効率の50%向上は妥当な最大限界程度であるため、筋タンパク質合成の最大刺激は15%を超えることができないことを意味します。これは、筋肉の合成率が基礎状態での約0.050%/hの基礎値から0.057%/hに増加することに対応し、タンパク質の合成率(FSR)のこの差を正確に測定することは困難です[11]。
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