A diet rich in fish may lower levels of the fat-regulating hormone leptin, scientists say. Previous findings have linked elevated levels of leptin, which is produced by fat cells in the body, to obesity and cardiovascular disease. The substance seems to tell the body when it has consumed enough food, and researchers posit that obese people somehow lose the ability to recognize these chemical cues. But exactly how the system works and what other factors influence the hormones levels are unknown. The new work, published today in the journal Circulation, suggests that diet plays a key role.

Scientists have known for some time that fish or fish oil seems to provide some protection against cardiovascular disease in humans. And earlier studies in rats indicated that unsaturated fatty acids in fish may affect leptin levels. Mikolaj Winnicki of the Mayo Clinic and his colleagues thus wanted to see if a fish-rich diet has a similar effect on the hormone in humans. To do this, the team examined the body mass index (a relationship between height and weight), fat content, age, gender, diet, and leptin levels of about 600 individuals from the same tribe in Tanzania. Half of the subjects lived on a lake and ate a lot of fish; the others were vegetarians. The scientists found that for every study characteristic except diet and leptin levels the two groups were identical. The fish-eaters, however, possessed significantly lower levels of the hormone than did their inland counterparts, even though body mass index--typically an important indicator of leptin levels--was the same for both groups. Additionally, although women generally possess higher levels of the hormone than men do, the investigators found the leptin levels of women who ate fish to be less than half that of both the female and male vegetarians. "We speculate that a fish diet may change the relationship between leptin and body fat and somehow help make the body more sensitive to the leptin message," remarks team member Virend Somers, also at the Mayo Clinic.

The authors caution against extrapolating diet recommendations from these results, however. "These are African individuals living in a fairly rural environment," Somers notes. "We don't know if the findings will apply to a semi-overweight, urban-dwelling North American population." The researchers plan to further probe this relationship by looking at whether leptin levels change in people who increase their fish consumption.