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 total metabolism along with the resting metabolic rate, and the exercise component. 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

Raw celery and grapefruit are often claimed to have negative caloric balance (they take more energy to digest than they contain in food energy), presumably because the thermic effect is greater than the caloric content.

The thermic effect of food is increased by both aerobic training of sufficient duration and intensity and by anaerobic weight training. ¹

References

1. Denzer CM - The effect of resistance exercise on the thermic effect of food - International Journal of Sport Nutrition and Exercise Metabolism - 01-SEP-2003; 13(3): 396-402
2. Christensen, Peter. "What is the thermic effect of food?". Retrieved March 28, 2005.