The way the news media tell it, you would think that the last nail has been hammered into the coffin of low-fat diets. Such is not the case. If it were true, it is unlikely that the Japanese, who traditionally have followed a very low-fat diet, would have the longest life expectancy of any people in the world.
A balanced review of the medical literature does, in fact, provide a good deal of additional evidence that a low-fat diet is conducive to health. Conversely, high-fat diets are associated with a number of negatives.
Oils are a refined food, lacking in most nutrients, and could therefore be considered a source of empty calories. Of course, they are not truly empty calories because they can be a source of vitamin E or, as exemplified by olive oil, potentially beneficial antioxidant phytochemicals. It is true that a small amount of fat in the form of essential fatty acids (polyunsaturated fatty acids) is required in the diet. But the fat that is naturally present in whole foods provides more than enough of these nutrients.
The main problem with higher fat diets is that people consuming such diets usually end up with a higher caloric intake than people who consume low-fat diets. If you perform studies on people who are confined to a metabolic ward and you control for the number of calories they consume, it is true that those consuming more fat will not gain weight. But the real world is not like that because most people cannot exercise that kind of control on their own. If they could, we would not have all the overweight people that we do.
Many people have the mistaken belief that fat promotes satiety (sense of fullness). They believe that people consuming low-fat meals will not feel full and will compensate by gorging themselves with additional servings and snacks. However, studies have shown that fat does not promote satiety any more than carbohydrate does. In fact, eating high-fat foods may have the opposite effect, as implied by Bert Lahr's line from the old commercial, "Bet you can't eat just one [Lay's potato chip]! If anything, higher protein intakes may have a satiating effect, although more research needs to be done. The Chinese restaurant meal syndrome of becoming hungry not long after eating may have something to do with consuming a meal high in refined grains and fat but relatively low in protein. Low-fat diets promote satiety in another way, however. Fat is more energy-dense than either carbohydrate or protein. It has been shown that when people consume foods with a low energy density, fewer calories are required to achieve satiety than when foods of high energy density are consumed. In other words, the volume of foods consumed helps determine satiety along with the number of calories. Cooking without oil produces low energy-dense foods and can therefore help reduce caloric intake.
The importance of reducing caloric intake goes well beyond that of reducing body fat. Cutting calories may greatly benefit people of normal weight as well, lowering cholesterol, blood pressure, and blood sugar and reducing the risk of many chronic diseases. This subject is discussed on the Dietary Restriction page.
Although the importance of the ratio of omega-6 to omega-3 fatty acids in the diet remains controversial, many authorities feel it should be lower than it is in the average U.S. diet. Low-fat diets have been shown to reduce the omega-6 to omega-3 fatty acid ratio, not only by decreasing omega-6 fatty acid intake but also by increasing the absolute amount of omega-3 fatty acids in the blood via an uncertain mechanism.1
High-fat diets have other deleterious consequences as well. For example, a diet deriving 40% or more of its calories from fat, regardless of the kind of fat, increases the coagulability of the blood, that is, the tendency to form blood clots.2,3 This tendency is also felt to play a role in the development of atherosclerosis (hardening of the arteries).
A single high-fat meal can impair endothelial function, that is, the ability of blood vessels to dilate when increased circulation is needed, for up to four hours.4 This phenomenon, which can be blocked by antioxidant vitamins, could increase the risk for heart disease.
Cooking foods in oil causes the production of large amounts of chemical compounds called advanced glycoxidation end products (AGEs), which are felt to play a role in aging and in diabetic complications. Actually, all cooking produces these compounds, and the longer food is cooked and the higher the heat being used, the more AGEs form. But steaming or microwaving food results in far less AGE production than frying or sauteing.
The question of the relationship of high-fats diets to the risk of cancer is a controversial one. In the case of prostate cancer, high-fat diets may not increase the incidence of the disease but may promote the aggressiveness of the tumor. But that is really what is important with respect to prostate cancer. Essentially 100% of men everywhere who are 75 or 80 years old have evidence of prostate cancer on a microscopic level. But the fact that Japanese men on their traditional low-fat diet have a death rate from prostate cancer that is a small fraction of that in American men provides some support for the notion that fat affects survival.
With regard to breast cancer, the dietary fat hypothesis has been around for some time, and the low death rate from breast cancer among Japanese women as compared with American women seemed to support the hypothesis. However, studies on American women with different levels of fat consumption have not shown a relationship with the incidence of breast cancer. Caloric intake may be more important. On the other hand, a recent clinical trial in which women with breast cancer were asked to reduce the fat content of their diet showed improved survival among those making this dietary change as compared with a control group. The greatest effect was seen in women with estrogen receptor-negative tumors, indicating that hormones were not playing a role. However, it is possible that increased fruit and vegetable consumption or lower caloric intake was responsible for the beneficial effect rather than the reduced amount of fat in the diet.
Proponents of higher fat diets often argue that such diets are associated with lower triglyceride levels than seen with high-carbohydrate, low-fat diets. High triglyceride (fat) levels represent a risk factor for heart disease. First, it should be noted that not all carbohydrate is the same. The complex carbohydrates found in starchy foods do not have the same effect as refined carbohydrates, which increase triglyceride levels to a greater extent. Second, the kind of triglyceride generated by consumption of carbohydrate is not the same as that produced when fats are consumed, including the monounsaturated fatty acid oleic acid that predominates in olive oil. A high carbohydrate diet produces fewer "big" VLDL (triglyceride-containing) particles in the blood (which could increase the risk of atherosclerosis) than a high monounsaturated fat diet does in diabetics (diabetics are especially prone to high triglyceride levels).5 Finally, it should be noted that exercise prevents the large rise in triglyceride-rich fats in the blood of people consuming a high carbohydrate, low-fat diet, so such a rise may be primarily a "couch potato" effect.
A documented negative effect of a low-fat diet is a decrease in mineral absorption, which can be seen even when a diet deriving 30% of calories from fat is compared with a 40% of calories from fat diet. Ultra-low fat diets, those deriving fewer than 10% of calories from fat, suffer from lower than optimal intakes of fat-soluble nutrients, such as vitamin E, and phytochemicals.
In general, although a certain amount of flexibility is desirable, I favor a diet deriving about 70% of calories from carbohydrate (primarily complex), 15% from protein, and 15% from fat. Such a level of fat is easily achieved simply by minimizing the use of added oils in food preparation.
1Raatz SK, Bibus D, Thomas W, Kris-Etherton P. Total fat intake modifies plasma fatty acid composition in humans. J Nutr 2001;131:231-234.
2Larsen LF, Bladbjerg E-M, Jespersen J, Marckmann P. Effects of dietary fat quality and quantity on postprandial activation of blood coagulation factor VII. Arterioscler Thromb Vasc Biol 1997;17:2904-2909.
3Hunter KA, Crosbie LC, Horgan GW, Miller GJ, Dutta-Roy AK. Effect of diets rich in oleic acid, stearic acid and linoleic acid on postprandial haemostatic factors in young healthy men. Br J Nutr 2001;86:207-215.
4Plotnick GD, Corretti MC, Vogel RA. Effect of antioxidant vitamins on the transient impairment of endothelium-dependent brachial artery vasoactivity following a single high-fat meal. JAMA 1997;278:1682-1686.
5Georgopoulos A, Bantle JP, Noutsou M, Hoover HA. A high carbohydrate versus a high monounsaturated fatty acid diet lowers the atherogenic potential of big VLDL particles in patients with type I diabetes. J Nutr 2000;130:2503-2507.