The thermic effect of food (TEF) is defined as the energy used in the digestion, absorption, transport, metabolism and storage of macronutrients following a meal. This energy can be measured as a rise in body temperature. The thermic effect of food contributes around 10 % to the total energy expenditure in man (around 250 kcals from a total energy expenditure of 2500 kcals in a 70 kg healthy male). The thermic effect of food varies depending on the macronutrient composition of the diet, as certain foods are more costly to digest, absorb, transport, metabolise and store compared to other foods. For example, protein has a particularly high cost associated with its postprandial utilisation and thus high protein diets tend to cause a large thermogenic effect postprandially. The ability of different carbohydrate sources to produce differing thermogenic effects postprandially is also well known, and a number of studies have compared the thermogenic effect of various carbohydrate sources.
For example, in one study1 researchers investigated the postprandial thermogenic effect from glucose and sucrose in healthy lean subjects. Subjects consumed either 5 grams per kilogram of ideal body weight glucose or sucrose and the heat produced postprandially was measured. The results showed that a significantly larger thermogenic effect was evident from sucrose compared to glucose. The reason for this is not clear as the authors had hypothesised that as sucrose is an obesogenic agent, that it would produce a lesser thermogenic response postprandially. One explanation is that health subjects are able to metabolise the fructose component of sucrose more rapidly than glucose due to the different metabolic pathways they follow. Fructose for example is more likely to enter the liver and be converted to triglycerides or synthesised to glycogen following large intakes compared to glucose, the latter being more preferentially used as an energy source for skeletal muscle and the central nervous system.
Interestingly when the authors repeated this experiment in obese subject they found that the postprandial metabolic response was higher following glucose compared to the lean healthy subjects. The result of this was that there was no significant difference in the thermogenic response to glucose or sucrose in the obese subjects. These results are interesting because some research suggests that the obese have blunted thermogenic responses postprandially. The difference between the results for the study cited here and other research is not clear, but obesity is a highly complex disease and so a number of reasons may be responsible for this discrepancy. For example, it might be that the thermic effect of food is blunted in obese subjects than have been through periods of dieting compared to obese subjects who have never dieted. Evidence for this viewpoint comes from the higher resting metabolic rates in obese subjects compared to lean subjects, but only in obese subject who have not been through cycles of dieting.
Dr Robert Barrington’s Nutritional Recommendation: This study is interesting in that it suggests that sucrose may have a higher metabolic cost associated with its digestion, absorption, transport, metabolism and storage compared to glucose. However, while this interesting from a theoretical point of view it should be remembered that sucrose is a driver of obesity. High intakes of fructose (sucrose is one molecule of fructose bonded to one molecule of glucose) are associated with the development of insulin resistance, whereas glucose does not have these same detrimental effects. The results from this study are also interesting as they suggest that the thermic effect of food is not blunted in all obese individuals. This may relate to the presence of periods of previous dieting which are known to damage the metabolism semi-permanently. Fructose is however a useful dietary agent for athletes if used in moderation due to it propensity to increase hepatic glycogen resynthesis rates when compared to glucose.
RdB
