At the simplest level, obesity is caused by consuming more calories than you burn.
Obesity, however, is a complex condition caused by more than simply eating too much and moving too little.
The environment you live in and your community's social norms surrounding food, eating, and lifestyle strongly influence what, when, and how much you eat.
Similarly, your environment affects whether, where, and how you are able to be physically active.
Diet and Lifestyle
Changes in American dietary habits and lifestyle have contributed to today's high prevalence of obesity.
Those changes include:
- More adults in the workforce, combined with long work hours and commutes, have led to fewer meals prepared at home.
- More Americans eat more meals in restaurants, which often serve oversized portions of calorie-dense foods.
- Portion sizes of packaged foods, such as snacks and soft drinks, have gotten larger over the years.
- Children spend more hours watching television, using computers, or playing electronic games and less time engaging in active play and recreation.
- Adults have gotten more sedentary as fewer perform physical labor on the job.
Environment
The way communities, workplaces, and schools are structured in much of the United States has contributed to the country's high rate of obesity.
Some of the changes seen in the past few decades include:
- Food (especially junk food) is now sold in places such as gas stations and office supply stores that historically did not sell food. The end result is that food is available almost constantly.
- Food products and restaurants are marketed intensively on television, radio, online, and elsewhere.
- Many communities have no safe routes for walking or bicycling, or safe places to play outdoors.
- Most jobs present few opportunities for physical activity.
- Many schools provide little or no recess periods or gym classes.
- Poor neighborhoods are often "food deserts," with no purveyors of fresh, healthy foods.
- There are many television shows dedicated to food, restaurants, and cooking that show no regard for the health consequences of the food being featured.
Stress
Stress contributes to obesity in a few ways:
- People who are stressed tend to make bad food choices and to eat too much.
- Stress causes the release of stress hormones including cortisol, which triggers the release of triglycerides (fatty acids) from storage and relocates them to fat cells deep in the abdomen. Cortisol also increases appetite.
Genes
Some people have a genetic predisposition to being overweight or obese.
However, in most cases, those people do not become obese unless they also have an energy imbalance — meaning they consume more calories than they burn.
A genetic tendency toward obesity often becomes apparent only when a person's or group's lifestyle or environment changes significantly.
Genetic syndromes such as Prader-Willi, Alstrom, Bardet-Biedl, Cohen, Börjeson-Forssman-Lehman, Frohlich, and others can also lead to obesity.
Such syndromes are rare, however, and they typically include other abnormalities besides obesity.
Medical Conditions
A variety of medical conditions are associated with being overweight and obese, including:
- Cushing's syndrome (a rare syndrome that results from excess production of cortisol by the adrenal glands)
- Eating disorders, especially binge eating disorder, bulimia nervosa, and night eating disorder
- Growth hormone deficiency
- Hypogonadism (low testosterone)
- Hypothyroidism (underactive thyroid)
- Insulinoma (a tumor of the pancreas that secretes insulin)
- Polycystic ovarian syndrome
In some cases it's not clear whether obesity causes the medical condition, or whether the condition causes obesity.
Drugs That Contribute to Obesity
Certain drugs have been shown to encourage weight gain — often by increasing appetite — and contribute to obesity.
These drugs include:
- Diabetes drugs, including insulin, thiazolidinediones (Actos and Avandia), and sulphonylureas(glimepiride, glipizide, and glyburide)
- Drugs for high blood pressure, including thiazide diuretics, loop diuretics, calcium channel blockers, beta blockers, and alpha-adrenergic blockers
- Antihistamines (used for allergies), particularly cyproheptadine
- Steroids, including corticosteroids and birth control pills
- Psychotherapeutic medications, including lithium, antipsychotics, and antidepressants
- Anticonvulsant drugs (used for epilepsy and some other conditions), such as sodium valproate and carbamazepine
In some cases, other drugs can be substituted for those that encourage weight gain, or a lower dose can be used.
However, don't stop taking prescribed medications on your own.
Discuss your options with your doctor, and make a decision together about what's best for you.
If you must take a medicine that increases your appetite, behavioral measures such as learning to count calories and eat slowly can help to limit weight gain