What Causes Weight Gain?
Weight gain or weight loss is determined by the “calorie equation.” If you burn as many calories as you take in each day, there’s nothing left over for storage in fat cells and your weight remains constant. But if your calorie-intake exceeds your calorie-expenditure, the extra calories are stored somewhere on your body and you gain weight. For a more detailed explanation of how surplus calories (from fats, protein and carbs) are stored as body fat, please see: Body Fat/Adipose Tissue – Why We Gain Fat
Your calorie-intake is simply the total calorie-content of the food (or drink) you consume. The composition of your diet seems to play little role in weight – a calorie is a calorie, regardless of whether it is from fat, carbs or protein.
Although you might think that physical exercise accounts for most of the calories you burn, it actually accounts for only about one third. In general, your calorie expenditure depends on three things:
(1) Your Basal Metabolic Rate. This accounts for about 60 percent of all calorie expenditure. Basal (or resting) metabolic rate is the amount of energy or calories your body consumes while at rest, in order to power the thousands of chemical reactions required to maintain health – such as, temperature, cell-repair, cardiovascular system and so on. Put simply, basal metabolic rate is an estimate of how many calories you would burn if you were to lie asleep for 24 hours, with measurement beginning 12 hours after your last meal to ensure that your digestive system was inactive and not burning calories digesting food.
(2) Your Physical Activity. Typically this accounts for about 30 percent of calorie expenditure.
(3) Dietary Thermogenesis. This is the calories you burn while digesting food. It accounts for about 10 percent of the total calories you burn.
See also What Influences Weight Gain?