Losing Weight on Prozac

The Prozac story began in 1987, when the Eli Lilly pharmaceutical corporation released a drug aimed at fighting depression. The drug, whose trademark name is fluoxetine, had an unexpected, but serendipitous side-effect. Unlike other antidepressants, which often cause weight gain, some patients reported minor and significant weight loss from taking Prozac.

How Prozac Works

Prozac belongs to the selective serotonin reuptake inhibitor family of antidepressants, explains Mayoclinic.com. These drugs influence the brain's chemical messengers, called neurotransmitters. Serotonin is a neurotransmitter associated with mood, sleep and appetite. After a nerve cell releases serotonin, a uptake pump re-absorbs some of it into the brain. A serotonin reuptake inhibitor prevents the re-absorption of serotonin, keeping it more available on an ongoing basis. Since serotonin helps control appetite, normalizing its levels might eventually lead to weight loss.

Prozac and Satiety

The word satiety refers to after-meal feelings of fullness. Researchers at the Department of Psychology, School of Medicine at University of Leeds, UK speculated that Prozac causes weight loss by increasing post-meal satiety. They recruited 12 obese, non-depressed females to test their theories. During the two-week experiment, one group took 60 mg of Prozac, while the control group took a placebo. The subjects taking Prozac consumed an average of 532 during a meal. In contrast, the control group consumed 730 calories. The Prozac group lost about 4.4 lbs. during the two-week period, whereas the control group only lost 0.8 lb. Lead author C.L. Lawton published the study in the 1995 edition of "Obesity Research."

Prozac vs. Paxil

The serotonin inhibitor theory of why Prozac controls appetite is certainly logical, but there is one significant caveat. Researchers at Massachusetts General Hospital in Boston found that Paxil and Zoloft, which are brands of selective serotonin reuptake inhibitors, may actually cause weight gain. Lead author M. Fava randomly assigned patients to Prozac, Paxil or Zoloft treatment. After 32 weeks of treatment, the Paxil and Zoloft patients showed minor to significant weight gain, whereas the Prozac patients showed weight loss. The "Journal of Clinical Psychiatry" published the study in November 2000.

Prozac and Baseline Weight

Researchers at Mclean Hospital in Belmont Massachusetts speculated that Prozac weight loss is a function of the patient's baseline weight. They tested depressed patients over a six-month period. Each patient took between 20 and 80 mg of Prozac. Lead author M.H. Orzack reported that while all subjects experienced decreased depression, overweight patients were the only ones who lost weight. Patients at their ideal weight actually gained an average of 4.4 lbs., while underweight patients did not show any significant trends. "Psychopharmocology Bulletin" published the study in 1990.

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