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Dietary Carbohydrate Deprivation Increases 24-Hour Nitrogen Excretion without Affecting Postabsorptive Hepatic or Whole Body Protein Metabolism in Healthy Men

Departments of Endocrinology and Metabolism (P.H.B., H.P.S.), Biochemistry (A.J.M.), Academic Medical Center, University of Amsterdam, 1100 DE Amsterdam, The Netherlands; Department of Metabolic Diseases (M.G.M.d.S.-v.d.V.), University Medical Center Utrecht, 3500 AB Utrecht, The Netherlands; Center for Liver (F.K., F.S.), Digestive and Metabolic Diseases, Academic Hospital Groningen, 9713 GZ Groningen, The Netherlands; and Department of Endocrinology (P.H.B., J.A.R.), Leiden University Medical Center, 2333 AZ Leiden, The Netherlands

Address all correspondence and requests for reprints to: J. A. Romijn, M.D., Department of Endocrinology and Metabolism (C4Q), Leiden University Medical Center, Albinusdreef 2, 2333 AZ Leiden, The Netherlands. E-mail: J.A.Romijn{at}lumc.nl.

Because insulin is an important regulator of protein metabolism, we hypothesized that physiological modulation of insulin secretion, by means of extreme variations in dietary carbohydrate content, affects postabsorptive protein metabolism. Therefore, we studied the effects of 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. Whole body protein metabolism was assessed by 24-h urinary nitrogen excretion, postabsorptive leucine kinetics, and fibrinogen and albumin synthesis by infusion of [1-¹³C]leucine and [1-¹³C]valine.

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: 15.3 ± 0.9 vs. 12.1 ± 1.1 (P = 0.03) and 10.8 ± 0.5 g/24 h (P = 0.005), 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, eucaloric 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.

This study was supported by the Dutch Diabetes Foundation, Grant 96.604.

Abbreviations: c.v., Coefficient of variation; FSR, fractional synthesis rates; HCHF, high-carbohydrate, high-fat; ICIF, intermediate-carbohydrate/intermediate-fat; KIC, ketoisocaproic acid; KIV, ketoisovaleric acid; LCHF, low-carbohydrate, high-fat; L_ox, leucine oxidation; L_Ra, rate of appearance of leucine; VCO₂, carbon dioxide production.

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