What are the biologic and medical causes of obesity?
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Obesity results when the body consumes more energy than it uses. Research points to several different factors that may influence weight gain. About 90% of people who diet gain every pound back that they lose regardless of their weight-loss method. Some evidence suggests that every person has an inherited weight range that varies by only about 10% either up or down from some set point. (For instance, a man whose “genetically-determined” weight is 200 pounds would tend to swing from 180 to 220 pounds, but would be unlikely to lose or gain more than this.) Genetic factors that influence fat metabolism and regulate certain hormones and proteins that affect appetite may play some part in 70% to 80% of obesity cases.
The Biologic Pathway to Appetite
Appetite, and, thereby weight, is determined by processes that occur in both the brain and gastrointestinal tract. Eating patterns are regulated by feeding and satiety centers located in the hypothalamus and pituitary glands of the brain that respond to signals indicating high fat stores and hunger. A number of molecules are produced that further control this process by stimulating or suppressing appetite. In some cases genetic factors may produce imbalances in these chemicals:
Specific Genetic Factors
There are at least seven known genetic mutations that have been associated with specific and uncommon cases of severe obesity. A few are as follows:
Genetics also determine the number of fat cells a person has, and some people are simply born with more.
The Thrifty Gene
Although genetic abnormalities may make it harder or easier to lose weight, the prevalence of obesity has dramatically increased over the past two decades, and genes cannot have changed within that short amount of time. The human metabolism evolved over centuries so that it could conserve energy and store fat during times of famine. Most cases of obesity occur now in people with normal physiology who live in industrialized nations where food is overly plentiful, and it is easy to avoid expending enough energy to burn the excess calories. One theory that combines genetic and environmental factors suggests that type 2 diabetes and the obesity that usually accompanies this disorder are derived from genetic actions that were once important for survival.
- Some experts postulate the existence of a so-called “thrifty” gene, which regulates hormonal fluctuations to accommodate seasonal changes. Theoretically, it works in the following manner:
- In certain nomadic populations, hormones are released during seasons when food supplies have traditionally been low, which results in resistance to insulin and efficiently increased fat storage.
- The process is reversed in seasons when food is readily available.
- Because modern industrialization has made high-carbohydrate and fatty foods available all year long, the gene no longer serves a useful function and is now harmful because fat, originally stored for famine situations, is not used up.
Such a theory could explain the high incidence of type 2 diabetes and obesity found in Pima tribes and other Native American tribes with nomadic histories and Western dietary habits. The traditional low-fat high-fiber foods (corn, lima beans, white and yellow teparies, mesquite, and acorns) of the Pima people may have protected this genetically susceptible population in the past from the high incidence of obesity and Type 2 diabetes they are experiencing now.
Medical or Physical Causes of Obesity
A number of medical conditions may contribute to being overweight, although rarely are they a primary cause of obesity.
- Some overweight people may believe their weight problem is due to hypothyroidism; patients with an underactive thyroid, however, generally show only a moderate weight increase of five to 10 pounds, mainly due to accumulation of fluid.
- Very rare genetic disorders, including Froehlich’s syndrome in boys, Laurence-Moon-Biedl, and the Prader-Willi syndromes, cause obesity.
- Abnormalities or injury to the hypothalamus region in the brain can cause a condition called hypothalamic obesity.
- Cushing’s disease is a rare condition caused by high levels of steroid hormones, which results in obesity, a moon-shaped face, and muscle wasting.
- Obesity is also linked with polycystic ovarian syndrome, a common hormonal disorder in women.
Effects of Certain Medications
Some prescription medications contribute to weight gain, usually by increasing appetite. Such drugs include the following:
- Corticosteroids.
- Some female hormone treatments, including some oral contraceptives (usually temporary) and certain progestins (such as Megestrol) used to treat cancer.
- Antidepressants, and other psychoactive drugs, including certain antipsychotics, lithium, and antiseizure agents (such as valproate).
- In a particularly unfortunate conflict of interest for obese individuals with type-2 diabetes, the use of insulin and insulin-stimulating drugs used to treat the condition often leads to weight gain.
- Certain anti-seizure agents used in epilepsy and bipolar disorder can cause significant weight gain.
- Certain antipsychotics.
- Although drugs are not usually the primary cause of obesity or of being overweight, some people may be mistakenly tempted to stop taking their medications without their doctors’ knowledge.
Revision date: June 11, 2011
Last revised: by David A. Scott, M.D.
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