The most basic dietary guideline is to balance energy intake with energy output. This means that as long as you burn what you eat, your weight will stay the same. If you want to lose weight, however, you need to burn more energy than you take in through the food you eat.
Most of the body's energy is set free in the form of heat, but before this happens, the energy is utilized in the intricate processes that take place in the zillions of body cells. These processes include the complex chemical reactions that accomplish the maintenance, repair and growth of body tissues, the smooth operation of the nerves, muscle activity and heat production to keep the body at a healthy 37 degrees Celsius.
Energy usage in the body can be divided into three categories:
Resting energy expenditure (basal metabolic rate) – the amount of energy your body uses while you're resting. Unless you're an extremely active sport enthusiast, this constitutes the largest portion (60% to 75%) of your body's total energy expenditure.
Physical activity – this includes everything from stroking your cat to sprinting 100 metres. It is not surprising then that physical activity is the most variable component of the body's energy usage. In a sedentary person, the expenditure might be as little as 100 calories per day, while a very active person might utilise more than 3 000 calories in a single day.
Thermic effect of food – the energy your body uses to digest the food you eat. Approximately 10% of total energy expenditure is used to digest, absorb and metabolise nutrients. This value also accounts for the manufacturing and storage of protein, fat and carbohydrate.
When more calories are consumed than are used, fat is gained. An accumulation of 7 000 extra calories in the adipose tissue is stored as 1 kg of fat. Unfortunately, the only way to get rid of this excess fat is to exercise more and eat less.