Genes play a part in affecting our susceptibility to gaining weight. Offspring of obese parents have a higher predisposition to become obese as compared to offspring of lean parents.
Several hormones and peptides are involved in body weight regulation. For example, the hormone leptin, which is produced by fat cells, is supposed to send signals to the part of our brain that controls food intake (by telling us whether we feel full or need to stop eating after a meal). Its main role is to regulate how many calories we eat and burn, as well as how much fat we carry in our body.
obese people face a condition called leptin resistance, which scientists increasingly believe to be the main driver of obesity in humans. though obese people have a high level of leptin, the hormone does not function as it is supposed to, causing the brain to erroneously think that the body is starving even though it has more than enough energy stored.
Due to factors like affluence, a sedentary lifestyle, the easy availability and convenience of processed food and junk food, and aggressive marketing tactics, people in developed, industrialised societies tend to have a higher propensity towards addictive eating behaviour that leads to obesity.
Certain pharmaceutical drugs can cause weight gain as a side effect by altering the function of the body and brain; for instance, to selectively store fat instead of burning it.