A new study underscores the vital importance of including certain kinds of fat in your diet if one of your goals is to
reduce bodyfat. The study, reported in the American Journal of Physiology (270:E353- E362, 1996), looked at the effects
of substituting six grams of fish oil a day for saturated fat.
A diet rich in saturated fat causes insulin resistance, which in turn promotes increased bodyfat deposition. Omega-3 fatty acids (fish oils) and omega-3 precursors such as alpha-linoleic acid, which is found in flax oil, prevent insulin resistance, even during higher saturated fat intake. This is thought to result from the beneficial effect of omega-3 fats on cell-membrane fluidity, which fosters a better connection between insulin and cell receptors. Another possible mechanism involves a decreased triglyceride (fat) content in muscle promoted by omega-3 fats.
In the new study, five human subjects not only substituted six grams of fish oil for saturated fat, but also took either fructose or glucose loads to determine how the fish oil affected the handling of carbohydrates. The two sugars were chosen because of their differing absorption qualities. Fructose is a low glycemic-index carb that doesn't initially cause a pronounced insulin flow; glucose, a rapidly absorbed sugar, does.
The results showed that fish oil increased blood glucose, as well as producing a whopping 40% decrease in insulin output. The insulin resistance that may have been responsible for this occurred in the liver. On the other hand, fish oil may have stimulated an increased efficiency of glucose transport. Most interesting, however, was the 35% increase in whole-body lipid oxidation (fat-burning) induced by fish oil!
The increased fat-burning effect of fish oil was probably related to the decreased insulin level. High insulin levels depress all known fat-mobilizing systems in the body. In short, when insulin is secreted, fat is either not released or stored.
This study underscores the importance of including beneficial fat sources, such as fish oil, under dieting conditions to obtain maximum fat-burning capacity while maintaining optimal blood-glucose levels. Precisely how much fish oil is required to accomplish this remains a matter of conjecture. I'd hazard a guess that the answer is somewhere between three to five grams a day, with the lower figure pertaining to people who also consume natural sources of omega-3 fats, such as those found in fatty fish (e.g., salmon) or flaxseed oil.