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Similar 24-h Pattern and Rate of Carbon Dioxide Production, by Indirect Calorimetry vs. Stable Isotope Dilution, in Healthy Adults under Standardized Metabolic Conditions^1,2,

Laboratory of Human Nutrition, School of Science and Clinical Research Center, Massachusetts Institute of Technology, Cambridge, MA 02139 ^* Rockefeller University, New York, NY 10021-6399

We investigated the applicability of the bicarbonate stable isotope dilution technique for accurate determination of 24-h energy expenditure in humans and in reference to the conduct of short-term (8 h) metabolic studies. Five healthy adult subjects consumed for 4 d a standard diet providing -188 kJ·kg^-1·d^-1 and 1.0 g·kg^-1·d^-1 of egg protein. From d 4 at 1800 h to d 5 at 1800 h, a 24-h metabolic study, combining indirect calorimetry with an intravenous infusion of ¹³C-labeled sodium bicarbonate, was performed under standardized conditions of 12 h fasting-12 h feeding. "Measured" CO₂ production (VCO₂) (indirect calorimetry) over 24 h was not significantly different from "predicted" VCO₂ (bicarbonate dilution) (218.31 ± 20.91 vs. 221.51 ± 19.44 mmol CO₂·kg^-1·d^-1, respectively) (P = 0.34). Further, 24-h VCO₂ and energy expenditure were determined by extrapolating from "predicted" VCO₂ during the last hour of fasting (15th hour following last meal) and the fifth hour of the small hourly meal-feeding phase. The maximum difference for an individual subject between the above-calculated 24-h energy expenditure and that from 24-h indirect calorimetry measurements was <4%. These results support use of the present experimental protocol, and the bicarbonate dilution technique, lasting 8 h, to obtain reliable quantitative estimates of 24-h CO₂ production and energy expenditure in healthy adult humans.

KEY WORDS: • carbon dioxide production • humans • stable isotope dilution • sodium bicarbonate

¹ Supported by NIH Grants RR88 and DK 42101.

² The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 USC section 1734 solely to indicate this fact.

³ To whom correspondence and reprint requests should be addressed.

Manuscript received 28 October 1993. Revision accepted 7 March 1994.

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