Opioid regulation of food intake and body weight in humans.

Relatively few studies of humans have evaluated the effects of opioids on food intake and body weight. Most have focused on the potential role of opioids in the etiology of obesity. Measurements of endogenous opioids in plasma or spinal fluid of humans reveal higher levels, particularly of beta-endorphin, in obese subjects. Opioid agonists such as methadone and butorphanol tartrate stimulate food intake, and all studies with naloxone, an opioid antagonist, demonstrate a reduction of short-term food intake in obese or lean humans. Long-term studies with naltrexone, an antagonist similar to naloxone, show no effect on food intake or body weight. Opioid agonists or antagonists have little effect on nutrient selection in humans. The effects on feeding-related hormones is equivocal. Further studies with more specific opioid receptor activities are needed.