Thermogenesis associated with fermentable carbohydrate in humans, validity of indirect calorimetry, and implications of dietary thermogenesis for energy requirements, food energy and body weight

BACKGROUND: Animal studies and theory show fermentable carbohydrate (FC) intake causes appreciably thermogenesis, but a similar occurrence in humans is controversial. HYPOTHESES: (a) That indirect calorimetry (IDC) is a valid method to assess thermogenesis during fermentation. (b) That a consistent and rigorous approach to the analysis of published IDC data from human studies will establish a representative thermogenic response to FC. (c) That conventional estimates of food energy and energy requirements can mismatch appreciably, more especially when thermogenesis is ignored. PURPOSE: To derive information and understanding of IDC, thermogenesis and energy balance in relation to food energy and energy requirement estimates. METHODS: (a) The validities of IDC equations that estimate the heat of reaction and carbohydrate utilization were assessed for various types of FCs under various circumstances. (b) Pooled analysis of eight published randomized cross-over studies in humans with elevation of FC intake. Studies were analysed for the first time or reanalysed according to a consistent approach with appropriate corrections for confounders. (c) Some 1500 regular and 'special' diet compositions were examined to assess the extent to which Atwater general food energy factors and updated estimates of energy requirements mismatch due to variation in substrate-associated thermogenesis and substrate-associated faecal+urinary energy losses. Impact of such mismatches on BMI was assessed under conditions of all else being equal. RESULTS: (a) Indirect calorimetry was valid, providing robust estimates of heat production during various types of fermentation; only small correction factors were necessary. By contrast, IDC equations for carbohydrate utilization sometimes applied poorly to FC. (b) A best estimate of thermogenesis in humans due to fermentation, above that due to oral glucose as a reference standard, was 0.39 (s.e.m. 0.14) kJ per kJ net metabolizable energy (NME; P<0.05, n=8 studies, total 72 humans) compared with 0.34 kJ/kJ from theory. Six sources of bias were identified; all had potential to underestimate FC thermogenesis. (c) Mismatches in energy availability and requirement estimates were often marked and translated into long-term differences in body mass index from ~20 to 33 kg/m(2) in average-height middle-aged initially obese women, and from ~22 to a non-survivable 13 kg/m(2) in initially slim women. CONCLUSIONS: (a) Indirect calorimetry is valid for the present purpose. (b) Thermogenesis in response to FC is real in humans and is comparable to that in animals and in theory. (c) Mismatches between estimates of energy requirements and dietary energy as metabolizable energy means the two expressions are not directly comparable, which has implications for the expression of food energy, energy requirements and the conduct and interpretation of research related to body weight.