Unlike most complications, obesity doesn't have a single cause; there are several. Recent research suggests that the rise in obesity is primarily due to altered, sedentary lifestyles, energy-dense diets and low-levels of physical activity.

Listed below are the most common causes of obesity:

Heredity
Statistics suggests that heredity influences the distribution of fat tissue. Generally, heavy newborns grow into heavy adolescents, more so when either parent is overweight. Moreover, weight regulation in the human body depends upon various genetically determined factors such as hormones. Any abnormality in these factors could result in substantial weight gain. Almost 60% of obese people are said to have inherited this condition. There are several genetic conditions that also contribute or lead to weight gain.

Endocrinological causes
In rare cases, some people are genetically predisposed to obesity due to hormonal imbalance or glandular problems. Cushing syndrome, hypothyroidism, hypogonadism in men and polycystic ovarian syndrome in women, hypothalamic lesions like tumours, infections or severe trauma are some of the genetic causes that are known to lead to obesity.

Medication
Certain drugs are now known to accelerate weight gain such as corticosteroids, sulfonylureas for diabetes, steroidal contraceptives and anticonvulsants such as valproate used in epileptic therapy. Antipsychotics, antidepressants, mood stabilizers like lithium are also known to possess the same properties.

Psychological causes
The relationship between obesity and one's emotional set-up is now widely recognized. Several patients reduce depressive symptoms by eating hence they gain weight. Over time this becomes a repetitive process and eventually leads to obesity.

Dietary factors
The world today is more affluent than it ever was; this means that more people have access to a multitude of dietary options. People nowadays are also less active than their predecessors, however the calorific content of their diet hasn't decreased; instead it has increased. Diets around the world have drastically changed; we have transitioned from a high-protein, high-fat diet to a high-carbohydrate, high-fat diet.

Moreover, the consumption of empty-calorie foods like alcohol, aerated drinks, candies etc. has also risen sharply. All of this, coupled with a sedentary lifestyle, makes the ideal combination for susceptibility to obesity and diabetes.