Low capacity to oxidize fat and body weight

Abstract

For a given positive energy balance, a low capacity to oxidize fat could contribute to weight gain (low fat oxidation hypothesis). This hypothesis is based on the arguments that for a given stable diet and food quotient (FQ), the respiratory quotient (RQ) is higher in obesity prone (OP) than in obesity resistant individuals (OR) and that a high RQ predicts higher future weight gain. A review of 42 studies shows that there is no convincing experimental support to these arguments and thus for the low fat oxidation hypothesis. A power analysis also shows that this hypothesis might be impossible to experimentally confirm because very large numbers of subjects would be needed to reject the null hypotheses that the 24-h RQ is not different in OP and OR or that future weight gain is not different in individuals with a low and high 24-h RQ at baseline. A re-examination of the significance of the 24-hour and fasting RQ also shows that the assumption underlying the low fat oxidation hypothesis that a high RQ reflects a low capacity to oxidize fat is not valid: For a stable diet, the 24-h RQ entirely depends on FQ and energy balance, and the fasting RQ mainly depends on the FQ and energy balance and on the size of glycogen stores. KEYWORDS food quotient, fuel partitioning, obesity, respiratory quotient 1 | INTRODUCTION It has been hypothesized that fat accumulation and weight gain could be favoured by two defects in fat oxidation: (a) a low intrinsic capacity to oxidize fat, 1-3 which characterizes "low fat oxidizers" 3,4 ("low fat oxidation hypothesis" of weight gain), or (b) a low ability to quickly adjust fat oxidation to an increase in fat intake, 5-8 which is one of the characteristics of individuals with "metabolic inflexibility" 9 ("metabolic inflexibility hypothesis" of weight gain). According to the low fat oxidation hypothesis of weight gain, for a given positive energy balance, fat oxidation is lower and fat accumulation is higher in individuals with a low than a high intrinsic capacity to oxidize fat, and over time, this could result in larger weight gains. 3 According to the "metabolic inflexibility hypothesis" of weight gain, for a given energy intake (EI), the slow adjustment of fat oxidation to an increase in fat intake in individuals with metabolic inflexibility favours fat storage with a compensatory increase in glucose oxidation from carbohydrate (CHO) stores. The result is a reduction in glycogen stores that could be a signal for increasing food intake and EI through the glycogenostatic model of the control of appetite. [5][6][7][8] Because of constant day-to-day variations in the percentage of fat in the diet, individuals with metabolic inflexibility could repeatedly accumulate small amounts of fat, which over time could lead to weight gain.List of abbreviations: ACC2, acetyl-CoA carboxylase 2; AICAR, 5-aminoimidazole-4-carboxamide ribonucleotide; AMPK, AMP-activated kinase; BMI, body mass index; CHO, carbohydrate; d, difference in 24-h RQ estimated between OP and OR; d/SD, standardized difference; EE, ...

Pathophysiological role of metabolic flexibility on metabolic health

“…In other words, after a long enough period, not only 24‐h energy expenditure matches 24‐h energy intake, but 24‐h RQ has to be equal to food quotient (FQ). The 24‐h RQ for an individual is calculated from the total volume of CO 2 and O 2 exchanged over a day, while 24‐h FQ corresponds to the expected volume of CO 2 and O 2 exchanged due complete oxidation of the dietary macronutrients ingested over a day 54,55 . Thus, a high‐carbohydrate diet (i.e., high FQ) results in an elevated carbohydrate oxidation with concomitant suppression of lipid oxidation (i.e., high 24‐h RQ).…”

Direct Relationship Between Metabolic Flexibility Measured During Glucose Clamp and Prolonged Fast in Men

“…The pre‐clamp RQ was not measured in 1 participant because of technical difficulties; thus, the sample size was 17 for the clamp‐associated ΔRQ and its derivatives measures. Because the diet of the participants the day before the challenges may have influenced prechallenge RQ (9,10), the ΔRQ was adjusted for prechallenge RQ.…”

“…Low fasting RQ would reflect enhanced lipid oxidation due to elevated lipid availability induced by fasting. Fasting RQ is, however, influenced by the preceding energy balance and dietary macronutrient content (9,10). Even controlling for these factors, fasting RQ remains just as an index of relative substrate oxidation (11).…”

Similar rates of fat oxidation during graded submaximal exercise in women of different body composition

“…Still, randomized intensity stages may be a useful addition to future studies using graded tests for similar purposes. Lastly, we did not quantitatively control for habitual diet and physical activity before the submaximal test, which are factors known to affect measures of macronutrient contribution to total energy energy expenditure to some extent [12].…”

“…Overweight and obesity are associated with insulin resistance [7][8][9] and increased intramuscular triglycerides [8]. A reduced ability to utilise free fatty acids (FFA) as a substrate at rest and during exercise [7,9] has been cited as a contributing factor to further the progression of obesity [10,11], although this fat oxidation hypothesis (the inability to utilise fat driving weight gain upward) is criticised [12].…”