Proteinaceous tissue, including skeletal muscle, is in a constant state of flux. In this regard skeletal muscle is constantly restructured and the size of the muscle pool within the body is therefore determined by the balance between the catabolic and anabolic processes at work in the muscle tissue. Generally catabolism occurs continually, and this process is accelerated or slowed under particular conditions. Likewise, anabolic mechanisms can be fairly continual, but they alternate between high and low activity, depending on the prevailing nutritional, hormonal and exercise conditions. Anabolism tends to be greatest during the day for one obvious reason. namely, nitrogen is supplied in the form of dietary protein and glucose in the form of dietary starch, and this stimulates the release of insulin which in turn stimulates protein synthesis. During the night the withdrawal of the building material and main stimulatory mechanisms for insulin release reduce anabolism in muscle tissue to some considerable extent.
This diurnal cycle of anabolism and catabolism has been investigated by researchers using the intravenous (1-14C)leucine method. This involves an intravenous infusion of a leucine isotope, which allows the metabolic pathways followed by the leucine to be traced. From this the whole body protein utilisation can be estimated. This has been said to be superior to the (15N)glycine isotope infusion method. For example, in one study1 obese subjects received a (1-14C)leucine infusion for 24 hours. During the night, when no food was ingested, the rate of whole body protein synthesis was 67 % of that during the day when meals were taken hourly. During the day, around 30 % of the ingested protein was immediately incorporated into body protein, with the remaining 70 % being immediately oxidised. In contrast during the night, The rate of protein oxidation fell to 38 %, as this reflected the undersupply of protein that in turn decreased the protein surplus, that was free to be oxidised.
During the night the total contribution of protein oxidation to total energy expenditure therefore fell. The authors estimated that this reduction was from about 27 % in the day to about 13 % at night. Concomitant to this was a reduction in the oxygen consumption. During the study, the rate of body protein breakdown remained constant. This study highlights the mainly anabolic processes that occur during the day when energy and nitrogen is available in the form of food, and the lower activity of these anabolic processes that occur during the night, and which coincide with a reduction in the total metabolic rate. As protein turnover and skeletal muscle mass are the primary drivers of the metabolic rate, it is no surprise that metabolic rate drops with the reduction in protein synthesis and protein oxidation. Protein synthesis therefore follows a discontinuous pattern. Whether muscle growth could be stimulated by consumption of protein during the night is therefore an interesting question.
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