Can this food help you lose fat... and keep it off for good?
Losing weight isn't as difficult as it sounds -- it's trying to keep it off for good that's the real challenge.
Eating is a powerful drive. If you've lost a lot of weight, you're probably familiar with that intense sense of hunger, which just won't go away. The usual advice to just "eat less" is a lot easier for me to write than it is for you to do.
One of the hormones responsible for the increase in hunger is known as leptin. Leptin (the Greek term for thin) is a hormone released by your fat cells. When you lose fat, leptin levels drop. When you gain fat, leptin levels rise.
Once leptin has been secreted by your fat cells, it travels to the hypothalamus. This is the part of your brain that controls eating behavior. Once it's there, leptin activates anorectic nerve cells, which suppress your appetite. At the same time, leptin prevents orexigenic cells from stimulating your appetite.
The good news is that some exciting new research suggests that you might be able to change the way your body responds to leptin... simply by eating more fish!
Fish
Scientists have known for some time that fish oil affects leptin levels in rats. But Dr. Mikolaj Winnicki and his colleagues from the Mayo Clinic wanted to see if a fish-rich diet has a similar effect in humans.
The findings, published in the journal Circulation, come from a comparison of two neighboring tribes near Lake Nyasa in Tanzania.
In the population of the village Lupingu, located near to a river, fish provides almost 25% of the total daily calorie intake. The villagers in Madilu, a nearby village located inland, eat very little fish.
Both tribes consume a similar number of calories per day, and lead very similar lifestyles.
Average daily calorie intake was 2196 for the fish diet and 2109 for the no-fish diet. The fish diet consisted of 300-600 grams of fish per day, with 60-120 grams of maize (corn), 40-60 grams of beans, 20-40 grams of spinach, 40-60 grams of potatoes and 30-50 grams of rice.
The no-fish diet included negligible amounts of fish with 150-350 grams of maize, 70-140 grams of beans, 60-100 grams of spinach, 100-200 grams of potatoes and 80-120 grams of rice.
Leptin
Regardless of their body fat levels, the fish-eating tribe was found to have much lower levels of leptin.
That's despite the fact that body fat -- usually an important indicator of leptin levels -- was similar in both groups. Leptin is made in fat cells; the more fat you have, as a rule, the more leptin you make.
In the tribe eating fish, men had an average leptin level of 2.5 nanograms per milliliter, compared with 11.2 in the mainly vegetarian tribe.
Although leptin levels are usually higher in women, the researchers found the leptin levels of women who ate fish to be less than half that of both the female and male vegetarians.
Although it might seem counterintuitive, many obese individuals actually have high levels of leptin. On the face of it, this appears to blow all of the theories regarding leptin and weight loss out of the water.
After all, if obese people have high levels of leptin, surely they shouldn't be obese in the first place.
While obese people do have high levels of leptin circulating in their body, the leptin isn't active where it needs to be -- the hypothalamus. Instead, they seem to have developed a form of leptin resistance, where leptin is unable to make the "jump" from the blood to the brain.
This latest study suggests that a fish-rich diet somehow makes your body more sensitive to leptin, and might play a small (but important) role in helping you lose weight and keep it off for good.
Questions
As with all research, this study does have limitations.
First, only lean black Africans living in a rural environment were studied. We don't know if the findings will apply to a semi-overweight, urban-dwelling population. Fish-rich diets might affect leptin differently in obese subjects and in different races.
What's more, this study only describes a link or association between a fish-rich diet and lower leptin levels. It doesn't show that one is causing the other. Remember that association does not mean causation. And leptin is just one of many factors influencing hunger and calorie intake.
According to the diet questionnaires, the Lupingu villagers were eating 300-600 grams of fish per day. That's a lot of fish.
If you were to eat this amount of fish every day, you'd probably end up getting a lot of mercury. Some forms of mercury -- such as methylmercury -- are toxic to both humans and animals. Even wild fish contain mercury, although the levels vary depending on the type of fish.
Although fish oil supplements contain no mercury, tests show that some of them contain organic pollutants such as dioxins and dioxin-like polychlorinated biphenyls. A review of the fish oil supplements that have passed quality-control tests (as well as the names of three that failed) is available here.
The bottom line
There are still many more questions than answers about fish oil and leptin. It would be hard to make a solid case for eating more fish based on this study alone.
However, the long-chain omega-3 fatty acids in fish have been linked with everything from a reduction in the risk of heart disease to improved mental function. In the right doses, they can also raise your metabolic rate and help you burn fat faster.
If you're not eating oily fish with a relatively low mercury content (such as salmon) several times a week, or using a high-quality fish oil supplement, now would be a good time to start.
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