|MadSci Network: Biochemistry|
It may be prudent to clarify the basic metabolic processes by setting some boundaries: Sustenance is the maintenance of a homeostatic, a steady state, condition of intake and caloric output in terms of Nitrogen, Hydrogen, Carbon and Oxygen( N and CHO). In starvation, a depletion of N and CHO from less essential processes toward the more essential processes is indicated.
We usually think of these as metabolic balances maintained by intake, output and the endocrine functions that regulate those. They are not separate cycles or “modes”, but run simultaneously on the ends of a homeostatic balance.
If a body is in a homeostatic condition, with proper exercise, N supply (protein) and a minimal caloric intake; the body will tend to utilize CHO as depot fat or carbohydrate. N is a requirement to make protein for muscle repair. The body will remodel, grow muscle and lose fat mass (1,2). This is the premise of many diet plans.
If the body is held in starvation while exercising, N and CHO all are derived solely from the body. The body will deplete muscle and depot fat while attempting to maintain a homeostatic condition (2,3).
To complete the clinical picture: If the body is not exercised and caloric intake is N and CHO, muscle will deplete from disuse, N will be excreted via the urine and the remaining CHO and protein will make fat. The usual picture is that protein, carbohydrate and fat are ingested while not exercising and thus the equilibrium shifts toward depot fat while muscle loses mass (2,3).
Thank you for your interesting question!
Peter E. Hughes, Ph.D. Mad Scientist
1) Romijn, E. F. Coyle, L. S. Sidossis, A. Gastaldelli, J. F. Horowitz, E.
Endert, and R. R. Wolfe
“Regulation of endogenous fat and carbohydrate metabolism in relation to exercise intensity and duration”
American Journal of Physiology – Endocrinology and Metabolism, September 1, 1993 vol. 265 no. 3 E380-E391.
2) Robert R Wolfe
“The underappreciated role of muscle in health and disease”
Am J Clin Nutr September 2006 vol. 84 no. 3 475-482.
3) Angelo Tremblay, Jean-Aimé Simoneau and Claude Bouchard
“Impact of exercise intensity on body fatness and skeletal muscle metabolism”
Metabolism July 1994, Volume 43, Issue 7, Pages 814–818.
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