Thermic effect of food (TEF) is the amount of energy expenditure above the resting metabolic rate due to the cost of processing food for use and storage. The effect varies substantially for different food components. The mechanism is unknown.
A commonly used estimate of the thermic effect of food is about 10% of one's caloric intake.
The primary determinants of daily thermic effect are:
the total caloric content of the meals,
the macronutrient composition of the meals ingested.
The thermic effect of food is the energy required for digestion, absorption, and disposal of ingested nutrients, and depends on the composition of the food consumed:
Protein: 20-35 % of the energy consumed,
Carbohydrates and fats: 5-15 %.
Meal frequency has little to no thermic effect.
Thermic effect also 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. Both insulin resistance and obesity are independently associated with impaired thermic effect of food at rest, but "the responsiveness of thermogenesis to exercise before a meal is related to the obese state and not independently to insulin resistance per se."
The thermic effect of food is marginally increased by 7-8 calories per hour with exercise:
aerobic training of sufficient duration and intensity
and by anaerobic weight training.
"Negative" Caloric Balance
Celery, grapefruit, lemon, lime, apple, lettuce, broccoli, and cabbage are often claimed to have negative caloric balance, requiring more energy to digest than recovered from the food. There is no scientific evidence to show that any of these foods have a negative caloric impact.
Antioxidants - substances that may prevent or delay some types of cell damage. Examples of antioxidants include beta-carotene, lutein, lycopene, selenium, vitamins A, C and E.
Vegetables and fruits are rich sources of antioxidants. There is good evidence that eating a diet with lots of vegetables and fruits is healthy and lowers risks of certain diseases. But it isn't clear whether this is because of the antioxidants, something else in the foods, or other factors. Most clinical studies of antioxidant supplements have not found them to provide substantial health benefits.