The thermic effect of food is the increase in heat production up to eight hours following a meal, that results from the metabolism of macronutrients to form a storage substrates or a usable form of fuel. Therefore the thermic effect increases with increasing meal size but evidence suggests that this response is not linear. A high protein meal creates a higher thermic response than either carbohydrate or fat meals because nitrogen containing compounds require extra work during metabolism. Altering meal patterns may also have a pronounced effect on subsequent metabolic response and the thermic effects of the ingested food. For example, changes to the meal frequency and size have been researched with respect the ability to alter the postabsorptive thermogenic response. Evidence suggests that a single large meal produces a larger thermic response than a number of small meals of equal size.
For example, researchers1 have investigated the effects of meal size and frequency on the thermic effects of food in seven healthy normal weight women. Subjects were randomly assigned to consume a single meal of 750kcal over 10 min or in 6 equal portions of 125kcal at 30 min intervals over a 3 hour period. The meals were identical and comprised of 54.5% carbohydrate, 14.0% protein and 31.5% fat. The study was designed as a cross over so all subjects consumed both meal types after a washout period. The results showed that the thermic effect of food was significantly higher in the large meal, when compared to the smaller divided meal. Expressed as a percentage of the total energy in the meal, the large meal schedule produced a thermic effect of 7.68% of total energy whereas the smaller divided meal created a thermic effect of 5.56% of total energy.
These results are interesting as they suggest that the metabolic work required to process a meal is dependent on the rate that the meal is ingested. The greater influx of nutrients may overwhelm the capacity of the body to oxidise the nutrients and so result in a greater initial storage via a larger release of insulin This would be metabolically more costly than the oxidisation of nutrients as they are absorbed. Grazing has been shown to reduce serum triglyceride levels presumably because the energy is oxidised as it is absorbed rather than processed by the liver to form triglycerides for storage in adipose tissue. However, because the energy from a large meal may need to be subsequently mobilised, additional metabolic processes may negate any benefit initially derived from grazing on account of creating a greater thermic effect, that in this study accounted for around 48kcal per day.
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