Causes
In regards to adipose tissue increases, a person generally gains fat-related weight by increasing food consumption, becoming physically inactive, or both. When energy intake exceeds energy expenditure (when the body is in positive energy balance), the body can store the excess energy in a dense, high-energy form as fat. One pound of fat represents 3500 calories, so over time, excessive energy intake and/or lack of exercise can contribute to fat gain and obesity. A study, involving more than 12,000 people tracked over 32 years, found that social networks play a surprisingly powerful role in determining an individual's chances of gaining weight, transmitting an increased risk of becoming obese from wives to husbands, from brothers to brothers and from friends to friends.
Weight gain is a common side-effect of certain psychiatric medications.
Read more about this topic: Weight Gain