July 2, 2002 DALLAS, July 2 – In a study of neighboring African tribes, a tribe eating a fish-rich diet had lower levels of the hormone leptin than a tribe eating a primarily vegetarian diet, researchers report in today's rapid access issue of Circulation: Journal of the American Heart Association.
Leptin is secreted by fat tissue. In humans, leptin is a satiety factor, which in normal-weight people tells their bodies when they have consumed enough food. High leptin levels have been associated with obesity and increased risk for cardiovascular disease.
As people gain weight, the body may stop listening to the leptin message, so more leptin is produced, explains senior author Virend K. Somers, M.D., D. Phil., professor of medicine in the division of cardiovascular disease and hypertension at the Mayo Clinic, Rochester, Minn. However, among the African populations in this study, higher body fat was not clearly associated with increased leptin levels.
"Regardless of body fat or body mass index (BMI), leptin levels were substantially lower among the fish-eaters than among vegetarians," says Somers. "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."
BMI is a measure of body fat based on height and weight. Skin-fold thickness was also used to assess body fat. The average BMI among the people in the study, regardless of diet, was 20. A BMI value from 18.5 to 24.9 is considered healthy; BMI from 25.0 to 29.9 is overweight; and a BMI value of 30 or greater is obese.
Somers says leptin levels among the women were particularly noteworthy. Women usually have higher leptin levels than men, but in this study, women who ate the fish-rich diet had lower leptin levels than both men and women on the vegetarian diet.
The researchers compared leptin levels in two closely related African tribal populations living in Tanzania. The two groups are essentially the same tribe but they're separated geographically. One group lives close to a lake and the other lives inland. The inland-dwelling tribe primarily eats a diet high in fruits and vegetables, while the tribe living by the lake eats freshwater fish as a main component of their diet.
The researchers studied 279 people on the fish diet and 329 who ate the vegetarian diet. They compared average daily calorie intake and food consumption, BMI, body fat content, age and gender. Leptin, insulin and glucose levels were measured after an overnight fast.
Average daily calorie intake was similar for both groups – 2196 for the fish diet and 2109 for the vegetarian diet. The fish diet consisted of 300-600 grams (g) of fish per day, with 60-120g of maize (corn), 40-60g of beans, 20-40g of spinach, 40-60g of potatoes and 30-50g of rice. The vegetarian diet included negligible amounts of fish with 150-350g of maize, 70-140g of beans, 60-100g of spinach, 100-200g of potatoes and 80-120g of rice.
For those on the fish diet, men had average leptin levels of 2.5 nanograms per milliliter (ng/mL), and women had an average of 5.0 ng/mL. Among the tribe eating primarily vegetables, men had average leptin levels of 11.2 ng/mL, and women had average levels of 11.8 ng/mL.
Somers says the finding fits with earlier studies that showed diets high in fish were associated with an improved cardiovascular risk profile. "However, this study only describes an association," he says. "We don't know if lowering leptin by itself can improve outcomes. These findings suggest the need for more research to answer that question."
He adds that the results may not apply to a non-African population. "These are African individuals living in a fairly rural environment. We don't know if the findings will apply to a semi-overweight, urban-dwelling North American population."
Fish consumption is very low in most American's diets, and it is unlikely that most Americans would make it a mainstay of their daily diet, Somers says. He would not recommend such a change based exclusively on these results.
"These results add to the increasing body of evidence pointing to the benefits of fish consumption," he says.
The American Heart Association recommends at least two servings of fish a week.
Co-authors are Mikolaj Winnicki; Bradley Phillips; Valentina Accurso; Massimo Puato; Paolo Palatini; and Paolo Pauletto.
This research was funded in part by the National Institutes of Health.
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