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What causes obesity?

Obesity is a complex disorder involving an excessive amount of body fat, and usually results from a combination of causes and contributing factors. A study found that more than two-thirds of Americans are either overweight or obese.

The American Medical Association recognizes obesity as a disease. It is an endocrine-related disease with multiple interactions among hormonal and neural pathways that regulate food intake and body fat mass.

The causes and contributing factors of obesity include the following:

  • genetics
  • family lifestyle
  • inactivity
  • unhealthy diet
  • medical problems, such as Prader-Willi syndrome, Cushing syndrome and other conditions
  • certain medications
  • social and economic issues
  • age
  • pregnancy
  • lack of sleep

One hypothesis explains why gaining weight is easy: you start eating high fat/high carb food that causes hypothalamic injury in the brain, so the brain can’t tell how much fat is stored and how much food is eaten. This leads to a reduced sense of satiety and to craving that causes increased food intake and weight gain.

Obesity has a variety of causes that vary depending on the individual. However, the main cause of obesity is consuming more calories than you can burn through daily activity. This occurs in some individuals because of a sedentary lifestyle, an unhealthy diet, pregnancy and tiredness. Specific medications like antidepressants, diabetes medication, antipsychotics, steroids and anti-seizure medication can also cause an increase in your weight. Certain conditions like Prader-Willi syndrome and Cushing's syndrome sometimes contribute to an unhealthy weight. Factors like family history and ethnicity can also increase your chances of becoming obese.

Scripps Health Admin
Administration Specialist

The culprits are varied. Larger meal portions and higher consumption of cheap, tasty and readily available processed carbohydrates, fast foods and “junk” foods add up to more calories. Meanwhile, electronic devices, computer games and ever-growing television screens contribute to more sedentary lifestyles. Biological and environmental factors can also play a role.

An excess of body fat—the defining characteristic of obesity—results from an imbalance between the amount of calories you take in and those you expend. The reasons for this imbalance are unclear, and the relationship between energy intake/expenditure and body fat storage and distribution varies from person to person. Factors that promote obesity include:

  • genetic predisposition
  • family history of obesity
  • age
  • behavioral factors (including a high-fat or high-calorie diet and sedentary lifestyle)
Enas Shakkour
Nutrition & Dietetics Specialist

Obesity is caused by the consumption of more calories than one burns. Food contains varying amounts of calories, and each person has a variable amount of calories that he or she burns each day. The amount of calories burnt depend on gender, height, weight, muscle mass, metabolism and activity level. A registered dietitian can give you an estimate of how many calories you burn each day. Certain foods contain more calories than other foods. When we consume more calories we burn, our body converts the unused calories to fat. Excessive fat storage leads to obesity.

Some scientists argue that our national obesity stems largely from our food choices. We choose to eat a calorically dense diet, one that is high in saturated fats and trans fats, rich in sugar and low in fiber and nutrients. Others argue that we are genetically predisposed to obesity. Yet others believe that in our sedentary society, obesity is due to the abundance, variety, availability and palatability of the food we can eat. That is to say, we overeat. I say it's double trouble. Even taking genetic factors into account, we eat too much, and we eat the wrong things. In addition, we don't get enough exercise, especially strength-building exercise.

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Theories of the underlying causes of obesity are tied to genetics, low brain serotonin levels, impaired diet-induced thermogenesis (heat production) and the inner workings of fat cells. All of these models support the notion that obesity is not just a matter of overeating. They explain why some people can eat large quantities of food and not increase their weight substantially, while for others, just the reverse is true. For example, a certain amount of the food we consume is immediately converted to heat, which is known as diet-induced thermogenesis. Diet-induced thermogenesis is the method by which the body "wastes" calories. There is evidence that the level of diet-induced thermogenesis is what determines whether an individual is likely to be overweight. In lean individuals, a meal may stimulate up to a 40 percent increase in heat production. In contrast, overweight individuals often display only a 10 percent or less increase in heat production. The food energy is stored as fat instead of being converted to heat.

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The main cause of obesity is external factors, which means:

  • your beliefs and practices about food (food culture)
  • how much food you eat
  • your exercise habits

There is a lot of interest in how and if genes cause obesity. Certain genes can make you more likely to develop obesity, but this only affects a very small number of people who are obese in the United States.

As people evolved, they didn't know where their next meal was coming from, so they tried to hold on to all of their calories to help their species survive. Nowadays, things have changed. Many of us live a sedentary lifestyle, sitting in cubicles all day or sitting in cars going to and from work. We have access to a lot of calorie-dense foods. It's a much different lifestyle from a few thousand years ago when people actively looked for food. This is a fundamental issue in obesity.

Americans are eating more as well. Compared to 1960, we're eating 500 calories more per day, according to the Centers for Disease Control and Prevention (CDC).

Experts have concluded that the chief causes of obesity are a sedentary lifestyle and overconsumption of high-calorie food:

  • Sedentary lifestyle—researchers have found a strong correlation between lack of physical activity and obesity.
  • Diet—a diet high in calories and/or fat appears to be an important factor in obesity.

This answer is based on source information from the National Cancer Institute.

Dr. Melissa B. Bagloo, MD
Bariatric Medicine (Obesity Medicine) Specialist

The exact causes of morbid obesity are not understood, but there are, most likely, many factors involved. In obese people, the set point of stored energy is too high. This altered set point may result from a low metabolism with low energy expenditure, excessive caloric intake or a combination of both. Some scientific data indicate that 80 percent of obesity may be inherited, strongly indicating a genetic cause. The most probable contributing factors to obesity are genetic, psychological, environmental, social and cultural influences. Severe obesity is absolutely not caused by a lack of self control. Because of the multi-faceted nature of this disorder, there is no single quick fix of the problem, nor is there a single way to achieve permanent weight loss for every individual. That is why a comprehensive approach to the problem is absolutely necessary.

Dr. William L. Wilson, MD
Family Practitioner

The traditional view is that obesity is a simple math problem—to many calories consumed and not calories burned up through basic metabolism and exercise. A recent series of articles in the journal Lancet suggests that there isn't a linear relationship between reduction of calories and the amount of weight lost. After measuring over 18,000 direct body composition readings in primary care, I also learned that the issue of the cause of obesity is much more complex than the simple calories in/calories out theory. I also learned that what my patients were eating sometimes had a lot more to do with how much fat they stored than the amount of food they were eating.

There is emerging evidence that excessive fructose primarily from sugar and high fructose corn syrup combined with high glycemic (rapidly absorbed) carbohydrates play a direct role in obesity. There is also some evidence that a healthy brain helps to auto-regulate total body fat stores. Because many patients with obesity have a subtle degree of reversible brain dysfunction, it appears that the excessive fructose/high glycemic carbohydrate combination may adversely affect brain function, ultimately contributing to obesity.

Thus paying attention to what you eat may be as important as how much you eat. So what's the bottom line when it comes to preventing or treating obesity? Eat a reasonable amount of real food and reduce your intake of sugar, high fructose corn syrup and high glycemic carbohydrates. Throw in a reasonable amount of exercise and you're well on your way to improved health.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.