Thermic effect of food (also commonly known simply as thermic effect when the context is known), or TEF in shorthand, is the increment in energy expenditure above resting metabolic rate due to the cost of processing food for storage and use.[1] It is one of the components of metabolism along with the resting metabolic rate, and the exercise component. Another term commonly used to describe this component of total metabolism is the specific dynamic action (SDA). A common number used to estimate the magnitude of the thermic effect of food is about 10% of the caloric intake of a given time period, though the effect varies substantially for different food components. Dietary fat is very easy to process and has very little thermic effect, while protein is hard to process and has a much larger thermic effect.[2]
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The thermic effect of food is increased by both aerobic training of sufficient duration and intensity and by anaerobic weight training. However, the increase is marginal, amounting to 7-8 cal per hour.[1] The primary determinants of daily TEF are the quantity and composition of the food ingested.
Although some believe that TEF is reduced in obesity, discrepant results and inconsistent research methods have failed to validate such claims.[3]
Raw celery and grapefruit are often claimed to have negative caloric balance (requiring more energy to digest than recovered from the food), presumably because the thermic effect is greater than the caloric content due to the high fiber matrix that must be unraveled to access their carbohydrates. However, there has been no research carried out to test this theory and a significant amount of the thermic effect depends on the insulin sensitivity of the individual, with more insulin-sensitive individuals having a significant effect while individuals with increasing resistance have negligible to zero effects.[4][5]
Australia's Human Nutrition conducted a study on the effect of meal content in lean women's diets on the thermic effect of food and found that the inclusion of an ingredient containing increased soluble fibre and amylose did not reduce spontaneous food intake but rather was associated with higher subsequent energy intakes despite its reduced glycaemic and insulinemic effects.[6]
The thermic effect of food should be measured for less than or equal to five hours.[7]
The American Journal of Clinical Nutrition published that TEF lasts beyond 6 hours for the majority of people.[7]
Research has found that the thermic effect of food contributes to the fact that calories may not all be equal in terms of weight gain. Sadie Barr showed that the consumption of processed foods led to a 50% decrease in postprandial energy expenditure.[8]