Carbohydrate is required for optimal utilization of protein...
Because insulin is an important regulator of protein metabolism,
physiological modulation of insulin secretion, by means of extreme
variations in dietary carbohydrate content, affects postabsorptive
protein metabolism.
A study compared three isocaloric diets with identical protein
content and low-carbohydrate/high-fat (2% and 83% of total energy,
respectively), intermediate-carbohydrate/intermediate-fat (44% and
41% of total energy, respectively), and high-carbohydrate/low-fat
(85% and 0% of total energy, respectively) content in six healthy
men [1].
The low-carbohydrate/high-fat diet resulted in lower absorptive
and postabsorptive plasma insulin concentrations, and higher rates
of nitrogen excretion compared with the other two diets over a 24-hour
period (15.3 vs. 12.1 ± 1.1 and 10.8 grams, respectively).
Postabsorptive rates of appearance of leucine and of leucine oxidation
were not different among the three diets. In addition, dietary carbohydrate
content did not affect the synthesis rates of fibrinogen and albumin.
In conclusion, carbohydrate deprivation increases 24-h nitrogen
loss but does not affect postabsorptive protein metabolism at the
hepatic and whole body level.
By deduction, dietary carbohydrate is required for an optimal regulation
of absorptive, rather than postabsorptive, protein metabolism.
To define the mechanism of insulin's anticatabolic action, the
effects of three different dosages of insulin (0.25, 0.5, and 1.0
mU x kg(-1) x min(-1)) versus saline on protein dynamics across
splanchnic and skeletal muscle (leg) beds were determined in 24
healthy subjects [2].
After an overnight fast, protein breakdown in muscle exceeded protein
synthesis, causing a net release of amino acids from muscle bed,
while in the splanchnic bed protein synthesis exceeded protein breakdown,
resulting in a net uptake of these amino acids.
Insulin decreased muscle protein breakdown in a dose-dependent
manner with no effect on muscle protein synthesis, thus decreasing
the net amino acid release from the muscle bed.
In contrast, insulin decreased protein synthesis in the splanchnic
region with no effect on protein breakdown, thereby decreasing the
net uptake of the amino acids.
In addition, insulin also decreased leucine nitrogen flux substantially
more than leucine carbon flux, indicating increased leucine transamination
(an important biochemical process for nitrogen transfer between
amino acids and across the organs), in a dose-dependent manner,
with the magnitude of effect being greater on skeletal muscle than
on the splanchnic bed.
Muscle is in a catabolic state in human subjects after an overnight
fast and provides amino acids for synthesis of essential proteins
in the splanchnic bed. Insulin achieves amino acid balance across
splanchnic and skeletal muscle beds through its differential effects
on protein dynamics in these tissue beds.
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Reference
1. Bisschop PH, De Sain-Van Der Velden MG, Stellaard F, Kuipers
F, Meijer AJ, Sauerwein HP, Romijn JA. Dietary carbohydrate deprivation
increases 24-hour nitrogen excretion without affecting postabsorptive
hepatic or whole body protein metabolism in healthy men. J
Clin Endocrinol Metab. 2003 Aug;88(8):3801-5
2. Meek SE, Persson M, Ford GC, Nair KS. Differential regulation
of amino acid exchange and protein dynamics across splanchnic and
skeletal muscle beds by insulin in healthy human subjects. Diabetes.
1998 Dec;47(12):1824-35
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