The thermic effect of food (TEF) is the increment in energy expenditure above resting metabolic rate associated with the cost of absorption and processing food for storage i.e. the energy your body requires during digestion.
A general calculation for the thermic effect of foods is to multiply the total calorie consumption by 10%. This is a general estimation; the thermic effect for different foods can range from 3-30%. Dietary fat is easily processed and turned into body fat, thereby having a low thermic effect of 2 to 3%. Fibre-rich foods like fibrous vegetables, whole grains, beans, oatmeal, brown rice have a thermic effect of about 20%. Processing proteins requires the greatest expenditure of energy, around 30%. Thus, if you eat an equal number of calories from carbohydrates, proteins and fats, the calories from the fat are more likely to be stored in the body as fewer of them are burned off by the thermic effect.
Raw celery, lettuce, asparagus, grapefruit are claimed to have a negative calorie balance because their thermic effect is greater than their caloric content i.e. they require more energy to be digested than what energy they provide.
A significant amount of the thermic effect depends on the insulin sensitivity of an individual. More insulin sensitive people have a significant effect whereas individuals with insulin resistance have negligible effects.
TEF is enhanced by aerobic endurance exercise of sufficient duration and intensity and also resistance exercise. However, the increase is marginal; about 7-8 calories per hour. The primary determinants of daily TEF are the quantity and composition of the food ingested.