Most of the food we eat contains energy measured in the form of calories.An individual's weight is steadily maintained when calories taken in are balanced by those calories being used. A persistent shortage of calories to balance expenditure may lead to weight loss, while an excess of calories is stored in the body, mostly as fat. For every 9.3 excess calories, 1 gram of fat is formed.

Types and Sources of energy-yielding nutrients:

• Carbohydrates yield four calories of energy per gram
• Lipids (fats) yield nine calories of energy per gram
• Proteins yield four calories of energy per gram
• Alcohol yield seven calories of energy per gram

Human's Body Weight is composed mainly of:

• About 60% is water. Water is the most essential nutrient and provides approximately 2/3 of our body weight, it is also the medium for nutrient and waste and transportation of oxygen and nutrients, and plays a critical role in temperature regulation.
• 6% to 22% is protein (lean tissue)
• 3% is minerals (lean tissue)
• Remaining weight is mostly fat

Weight Gain. Studies suggest that the relationship between calorie intake and energy expenditure is not always straightforward. Studies of genetic obesity in animals indicate that in some humans there may also be a genetic component at work in excessive weight gain. It has been suggested that inheritance, along with other factors such as levels of exercise and calorie intake, may influence just how many calories are consumed before the body increases its metabolic rate and tries to burn off the excess. In some individuals the body may have a relatively low tolerance for extra calories, allowing these people to remain thin, while in obese individuals the body may have a very high tolerance for excess calories, allowing fat to build before the metabolism speeds up.

Weight Loss. Weight loss remedies are a multi-billion dollar business that preys on the individual looking for a quick fix. In order to lose weight, more calories must be lost through expenditure than gained from a diet. Therefore, exercise and reduction of calorie intake, are important when considering weight loss. However, it is essential for individuals to maintain a proper nutrient balance even while reducing calories. If an individual's nutrient intake is continually below the level required, malnutrition will occur. Chronic malnutrition can result in starvation, characterized by a rapid loss of weight and muscle mass, accompanied by weakness and mental depression.

Even if a person can eat whatever they want and whenever they want without gaining weight, this should not be abuse because it is not good for the individual's health. The goal of eating should be to replenish and refuel the body to be able to build new structural materials, repair damaged cells, and fuel the activities of our body.

Function of Body Fat: The average male should have between 12-20% body fat, while the average female should have between 20-27% body fat. For males they should not get below 3% body fat, and for females they should not drop below 12% body fat. Sufficient stores of body fat are critical for hormone production, cushioning of the vital organs, insulation from extreme cold, and it serves as a reserve of fuel.