Obesity occurs over time when you eat more calories than you use. The balance between calories-in and calories-out differs for each person. It is the result of a positive energy balance, whereby energy intake exceeds expenditure, resulting in the storage of energy, primarily as lipids in white fat cells. Energy balance is modulated by food consumption and physical activity, as well as by the dissipation of energy as heat generated by mitochondria in brown fat cells. This heat production or thermogenesis, is triggered by mechanisms within the cells themselves or by the sympathetic nervous system, in response to exercise, diet, or exposure to cold. 2
Factors that might affect your weight include your genetic makeup, overeating, eating high-fat foods, and not being physically active. Obesity affects more than 500 million people worldwide and contributes to type 2 diabetes, cardiovascular disorders, and cancer. If you are obese, losing even 5 to 10 percent of your weight can delay or prevent some of these diseases.
Body-mass index (BMI) has a strong genetic component (40 to 80% heritability) involving several genes with roles in appetite regulation. The strongest genome-wide association signal lies in the FTO gene.