As presented here, PUFAs have the potential to affect a large number of metabolic processes and, therefore, these fatty acids are beneficial in obesity and its related diseases. The most important effect of omega-3 PUFAs, and in particular EPA and DHA, is the triglyceride-lowering effect observed in humans (Connor et al., 1993). Lowering circulating triglycerides has been proven to protect against coronary heart disease and the use of fish oil or increased consumption of fish after myocardial infarction reduced reinfarc-tion and mortality (Calder, 2004). The American Heart Association have presented guidelines for dietary fish intake, proposing that patients without documented coronary heart disease should eat a variety of fish, preferably oily fish, twice a week (Kris-Etherton et al., 2002). Patients with documented coronary heart disease should consume dietary supplementation of at least 1 g EPA and DHA per day. Long-chain omega-3 PUFAs derived from fish and fish oils have beneficial effects in people with pre-existing cardiovascular heart disease. One serving of fish per week may decrease the risk of mortality in heart failure by up to 40%. Patients with type 2 diabetes are often overweight or obese. To better control this disease it is important to eat healthily and to control fat intake. The American Diabetes Association recommend two to three servings of fish per week and the use of vegetable oils instead of butter in cooking (www.diabetes.org). Furthermore, the Department of Health and Human Services (HHS) and the Department of Agriculture (USDA) publish annually the Dietary Guidelines for Americans (www.healthierus.gov/dietaryguidelines/) providing advice on good dietary habits, which can promote health and reduce risk for major chronic diseases.
The role of PUFAs for body weight control is, however, at the present time not fully understood. Most studies indicate no change in body weight after increased intake of PUFAs. However, incorporation of a fish meal in a weight-loss programme improved the response to the diet with a tendency to increased weight loss, although this was not statistically significant (Mori et al., 1999). Fasting insulin levels were significantly reduced and the lipid profile improved with decreased triglyceride levels and increased high-density lipoprotein levels. A dietary recommendation that is valid for all individuals is to consume a diet with: (1) decreased total fat content; (2) reduced saturated fat content; (3) reduced omega-6 : omega-3 fatty acid ratio by increasing the intake of omega-3 PUFAs. This would be recommended to maintain body weight and whole-body glucose and lipid metabolism.
The positive effects of fish oil and omega-3 PUFAs in cardiovascular disease and mortality are apparent. However, most studies with increased fish intake or addition of dietary fish oil supplementation do not take into account that there may be other components present in fish that may be important in cardiovascular health. The recommendation by the American Heart Association is that the degree of PUFAs in the diet should be increased and that the ratio of omega-6 : omega-3 PUFAs needs to be reduced. This will be achieved by increased consumption of plant-derived a-linolenic acid and marine-derived EPA and DHA (Kris-Etherton et al., 2002). Another source of essential omega-3 PUFAs is walnuts, which despite being energy dense have no implications in inducing altered energy balance. Hence, incorporation of nuts into diets with low levels of saturated fats may have beneficial effects on weight reduction and maintenance (Gillen et al., 2005).
In summary, different fat types have different effects on satiety. To be able to make clear recommendations regarding dietary PUFA supplementation, more studies are needed to evaluate the correct composition of fatty acids that induces satiety and maintains energy balance. Methods for prevention of metabolic abnormalities such as overweight, obesity and type 2 diabetes are urgently required and a primary long-term strategy to cure these diseases is to improve dietary habits. The use of PUFAs in the diet may, by stimulating the use of fat as an energy substrate and by reducing the accumulation of fat in the adipose tissue, significantly contribute to the maintenance or reduction of body weight.
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