Findings from some past studies have suggested that the human body becomes less sensitive to leptin as people gain weight, causing an increase in leptin secretion, explains senior author Virend K. Somers, M.D., D. Phil. However, among the African populations in the current study, higher body fat was not clearly associated with increased leptin levels.

"Regardless of body fat or body mass index, 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."

Body mass index is a measure of body fat based on height and weight. The investigators also used skin-fold thickness to assess body fat. The average index among the people in the study, regardless of diet, was 20. A value from 18.5 to 24.9 is considered healthy; values from 25.0 to 29.9 indicate overweight, and values of 30 or greater signal obesity.

Somers says leptin levels among 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 tribal populations in Tanzania. The two groups are essentially the same tribe but they are separated geographically. One group lives close to a lake; the other lives inland. The inland-dwelling tribe primarily eats a diet high in fruits and vegetables, whereas 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 on the vegetarian diet, comparing average daily calorie intake and food consumption, body mass index, 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 of11.2 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."

"These results add to the increasing body of evidence pointing to the benefits of fish consumption," he says.