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Causes of obesity

Obesity results when there is an imbalance between energy intake and energy expenditure. In other words, you consume more calories than you expend in your daily activities. Weight gained during certain critical periods of your life more commonly lead to an increased number (as opposed to increased size) of fat cells and make obesity more difficult to treat. These time periods are:

  • Between 12 and 18 months of age.
  • Between 12 and 16 years of age.
  • Adulthood when a person gains in excess of 60% of their ideal body weight.
  • Pregnancy.

During these periods, an excessive amount of weight gain causes an increased number of fat cells. Once a fat cell is formed, you generally cannot get rid of it.

What causes one person to gain weight while another person maintains their weight? There are differences in people, and several factors that contribute to these differences have been identified:


As a general rule, as you grow older, your metabolic rate slows down and you do not require as many calories to maintain your weight. People frequently state that they eat the same and do the same activities as they did when they were 20 years old, but at 40, are gaining weight. This will happen. Metabolism slows down with advancing age.


Gender is also an important factor. Males have a higher resting metabolic rate than females, so males require more calories to maintain their body weight. This higher resting metabolic rate is primarily due to the increased lean body mass (mainly muscle tissue) males have compared to women.

Activity level

Active individuals require more calories than less active ones. Physical activity tends to diminish appetite in obese individuals while increasing the body's ability to preferentially metabolize fat as an energy source. It is believed that much of the increase in obesity in the last 30 years has resulted from the decreased level of physical activity in everyday life (such as emailing coworkers instead of walking over to their desks). However, the relationship between physical activity and weight is more complex and is discussed in more detail in another section.

Body weight

Heavier people require more calories to maintain their body weights than lighter ones. For example, a middle-aged male weighting 250 lbs. doing minimal amounts of physical activity may require 2700 calories to maintain his body weight. If this person goes on a 2000 calorie-per-day diet, he will lose weight. Eventually, however, even if he stays with a 2000-calorie daily diet, his weight will stabilize because his metabolic rate will gradually decrease. When this man reaches approximately 200 lbs., he will require perhaps only about 2000 calories per day to maintain his new weight. This reduction in metabolic rate is a normal process and takes place in all individuals.

Food preferences/High saturated fat diet

High saturated fat foods are obesity-promoting in animals and humans. In the last 40 years, the ready availability of these high saturated fat foods (such as, "fast foods"), combined with the decreased calorie requirements from decreased physical activity, is felt to be the major factor in the sharp rise in the prevalence of obesity. Some fats, especially the very long-chain omega-3 fats EPA (eicosapentaenoic acid) and DHA (docosahexaenoic acid), commonly found in fatty fish, are not obesity promoting and can, when incorporated into the diet, actually help reduce body weight and improve health (please see information under the SCIENCE tab for further information).


Certain medications prescribed for inflammatory conditions, seizures, and mental illness tend to increase appetite and may also decrease metabolic rate.

Hereditary factors affecting appetite and metabolism

Heredity is associated not only with obesity, but also with thinness. It most closely correlates with the biological mother's weight. If the biological mother is heavy as an adult, there is approximately a 75% chance that her children will be heavy. If the biological mother is thin, there is also a 75% chance that her children will be thin. It is related to metabolic processes inherited primarily from the biological mother. These differences are independent of thyroid activity which, incidentally, is a relatively rare cause of obesity.

A tendency to be obese is actually of survival value. If a person tends to be obese, he or she may have a more efficient metabolism, which means that they can survive on less food. In times of famine, people with this more efficient metabolism can survive on fewer calories. In most countries, adequate food, much of it high fat, is relatively plentiful for the majority of the population, making a more efficient metabolism a disadvantage contributing to the obese state. Hereditary factors are a major factor contributing to obesity.

Incidentally, inheriting a tendency toward obesity does not mean that you cannot lose weight - it just means that you have to work harder at it.

Updated: 27 December 2011

Copyright © 1996 -2011 Michael D. Myers, M.D., Inc.
All rights reserved.

Disclaimer Statement

The above information is for general purposes only and should not be construed as definitive or binding medical advice, diagnosis or treatment. Because each person is medically different, individuals should consult their own personal physicians for specific information and/or treatment recommendations.