Obesity is the interaction of genes and environment.

Genes are one of several factors that influence your weight. Our ancestors stored energy as fat, enabling them to survive during lean times. But your ancestors and genes are not always to blame for extra weight. Many people who have the obesity gene are not obese. Your environment and lifestyle choices have the greatest influence on how much you weigh.

Everyone is Different

Obesity and overweight occur when there is an energy imbalance caused when you take in more calories or energy than you expend. In almost all cases, the cause may simply be eating too much and exercising too little. Two people can follow the same diet and exercise routine, and one may be overweight while the other maintains a healthy weight. There are many reason why this is true, including:

• Environment has a major influence on obesity. We live in a fast-paced culture where high-fat, high-sugar diets, and limited exercise are the norm and technology contributes to physical inactivity. In our fast-food culture, we have many conveniences that keep us from being physically active.
• Psychological and behavioral factors influence weight gain. When stressed, bored, or angry, some people cope by eating. This creates an association between an emotion and food. Depression and stress, the leading causes of eating disorders, can lead to obesity.
• Fat storage is regulated by complex biological systems that involve input and feedback from fatty tissues, the brain, and endocrine glands (pancreas and thyroid). Even a small imbalance over a long period of time can affect metabolism and predispose to overweight and obesity.
• Men have more lean body mass (muscle) and less fat than women. Since lean body mass uses more energy than fat, men burn up to 20% more calories than women.  Women are more susceptible to gaining weight than men if they eat the same quantity of food as men.
• Some people are genetically predisposed to gain weight. They have a chronic imbalance that favors energy input over energy output. If you tend to retain weight, preventing weight gain may be the best approach to maintaining a normal weight.
• Some illnesses can lead to weight gain or obesity. These include hypothyroidism, Cushing's syndrome , depression, and some neurological problems. Medications such as steroids and some antidepressants may also are related to weight gain. Your doctor can tell you whether you have underlying medical conditions that are making it difficult for you to lose weight.
• For women, the balance of female sex hormones during adolescence, pregnancy, and menopause influences body fat levels.
• Women, on average, exercise less than men, and drop out of exercise programs sooner than men, contributing to weight problems.

Understanding the specific factors that contribute to obesity is an important step toward maintaining a normal body weight.