You get energy or calories from food and drinks. This energy is called energy IN. Your body needs a certain amount of energy for activities like breathing, digesting, and being physically active. This energy is called energy OUT. When energy IN is more than energy OUT, you gain weight. In the similar way when the energy OUT is more that energy IN, you lose weight. If you want to maintain the same weight, your energy IN and OUT should be equal.

Obesity definitely runs in families. Furthermore, identical twins usually maintain weight levels within two pounds of each other throughout life if they live under similar conditions or within five pounds of each other if their conditions of life differ markedly.

This might result partly from eating habits engendered during childhood, but it is generally believed that this close similarly between twins is genetically controlled.

The genes can direct the degree of feeding in several different ways including a genetic abnormality of the feeding center to set the level of nutrient storage high or low, abnormal hereditary psychic factors that either whet the appetite or cause the person to eat as a ?release? mechanism.

Childhood over-nutrition is also important factor responsible for obesity. The rate of formation of new fat cells is especially rapid in the first few years of life, and the greater the rate of fat storage the greater also becomes the number of fat cells. In obese children the number of fat cells is often as much as three times that that in normal children. However, after adolescence the number of fat cells remains almost identically the same throughout the remainder of life. Therefore, it has been suggested that overfeeding the children, especially in infancy and to a lesser extend during the older years of childhood, can lead to a lifetime of obesity. The person who has excess fat cells is though to have a higher setting of fat storage by the hypothalamic feedback auto regulatory mechanism for adipose tissues. In less obese persons, especially those who become obese in middle or old age, most of the obesity results from hyperbiophy (enlargement) of already existing fat cells. This type of obesity is far more susceptible to treatment than is the life long type. An interesting peculiarity of this type of obesity is that these persons have excess secretion of insulin, similar to the excess insulin secretion that occurs in animals with lesions in the ventromedial nuclei of the hypothalamus.