Whether or not your weight changes depends on a simple rule:
Weight change = calories in - calories out
If you burn as many calories as you take in each day, there's nothing
left over for storage in fat cells and weight remains the same. Eat
more than you burn, though, and you end up adding fat and pounds. Many things influence what and when you eat and how many calories you
burn. These turn what seems to be a straightforward pathway to excess
weight into a complex journey that may start very early in life.
Genes: Some people are genetically predisposed to
gain weight more easily than others or to store fat around the abdomen
and chest. It's also possible that humans have a genetic drive to eat
more than they need for the present in order to store energy for future.
This is called the thrifty gene hypothesis.
It suggests that eating extra food whenever possible helped early
humans survive feast-or-famine conditions. If such thrifty genes still
exist, they aren't doing us much good in an environment in which food is
constantly available.
Diet: At the risk of stating the obvious, the
quantity of food in your diet has a strong impact on weight. The
composition of your diet, though, seems to play little role in weight—a
calorie is a calorie, regardless of its source.
Physical activity: The "calories burned" part of
the weight-change equation often gets short shrift. The more active you
are, the more calories you burn, which means that less energy will be
available for storage as fat. Exercising more also reduces the chances
of developing heart disease, some types of cancer, and other chronic
diseases. In other words, physical activity is a key element of weight control and health
Read more at http://www.hsph.harvard.edu/nutritionsource/healthy-weight
