Dr. Willett does recommend less saturated fat and more mono- and poly-unsaturated fat, although he quotes studies that say that saturated fat had no effect on heart disease risk, but his overall opinion is music to the lowcarbers ears--that people should stop making sweeping comments about eating fat being bad. He shows that a low-fat diet increases risk of heart disease and that replacing saturated fat with carbs also increases risk of heart disease.

Proc Soc Exp Biol Med 2000 Dec; 225(3):187-90.

Will High-Carbohydrate/Low-Fat Diets Reduce the Risk of Coronary Heart Disease?

Walter C. Willett
Chairman of the Department of Nutrition, Harvard

…During the 1980s, a subtle shift developed in dietary recommendations that led to an emphasis on reducing fat intake in general, with less distinction about the type of fat. Thus, the first recommendation of the American Heart Association became to reduce total fat intake, and similar emphasis on total fat reduction has been part of the general US dietary guidelines as well. This emphasis on total fat reduction is epitomized by the U.S. dietary pyramid, which indicates that all types of fat should be used sparingly, and complex carbohydrates should be consumed in large amounts. . .

Although replacement of total and saturated fat with carbohydrate has been at the centerpiece of recommendations to reduce heart disease, the degree of risk reduction to be expected has received little attention. Recently, the Nutrition Committee of the American Heart Association provided an estimate of the reduction in risk that would be expected were the total U.S. population to decrease their intake of saturated fat to the Step 1 goal of 10% of energy. This calculation involved a two-step process using data from controlled feeding studies to estimate the effect of saturated fat reduction on serum cholesterol to risk of coronary heart disease. The committee concluded that “equations developed from carefully conducted clinical studies indicate that reducing saturated fat from the current average intake of 12% to 14% of calories can lead to an average reduction of 3% to 5% in CHD risk in the population as a whole.” One might reasonably ask whether such a modest reduction should be a sufficient reason for individuals to make major changes in their life, but there is good reason to suspect that even this estimate may be a serious overstatement because the same dietary change will have an adverse effect on lipid fractions other than total serum cholesterol.

The effect on blood lipids of replacing saturated fat with carbohydrate has been evaluated in numerous controlled feeding studies with consistent results. With this dietary change, serum total cholesterol and LDL decline, but HDL cholesterol also decreases, and triglyceride levels are elevated. Thus, the ratio of total cholesterol divided by HDL is minimally affected by replacing saturated fat with carbohydrate, suggesting that there is little if any benefit to be expected. On the other hand, replacing saturated fat with olive oil or another fat high in mono-unsaturated fatty acids produces a similar decline in total and LDL cholesterol, but HDL cholesterol is not reduced, and triglycerides do not increase. Thus, these lipid changes would predict that the replacement of saturated fat with carbohydrate would have little effect on CHD risk, but that replacement with monounsaturated fat would be beneficial. The adverse effects of high-carbohydrate intake appear to be worse among individuals with underlying insulin resistance, as would be expected in a population that was over-weight an sedentary.

The effects of dietary fats on blood lipid fractions are further complicated because the isomeric position of double bonds can have a major impact on function of fatty acids including their effects on blood lipids. In a controlled feeding study, Mensink and Katan found that, compared with monounsaturated fat, trans fatty acids and saturated fat had similar effects on LDL cholesterol, but trans fat uniquely lowered HDL cholesterol. Thus the adverse effect of trans fat on the total cholesterol to HDL ratio was approximately twice as detrimental as that of saturated fat. This finding has been reproduced consistently in other metabolic studies, and further evidence indicates that trans fatty acids modestly elevate serum triglycerides and increase Lp(a) compared with saturated fat.

The effects of dietary fat on blood lipids described above would suggest that reduction in fat intake per se is not likely to be beneficial. Further, were monounsaturated or polyunsaturated fats replaced with carbohydrate, risk of coronary heart disease might actually be increased. …

…We recently also documented that total fat intake as a percentage of calories was strongly inversely associated with fasting blood triglyceride levels as predicted by metabolic studies (unpublished data)….

[Note: This last sentence means in English that the less your percentage of fat in your diet is the higher your triglycerides i.e. inversely associated (the more of one thing, the less of the other)]

…total fat intake was not associated with risk of coronary heart disease….

Similarly, substitution of saturated fat with carbohydrates was associated with only a small and not statistically significant lower risk….

[Note: This last sentence means that there is absolutely no difference in lowering risk of heart disease if you take saturated fat out of the diet and substitute it with carbs. Not statistically significant means that it is within the margin for error (the standard deviation) which means it has no meaning statistically.]

…The notion that total fat intake is not an important risk factor for coronary heart disease may appear heretical as this is inconsistent with the general advice conveyed to the U.S. public. However, it is hardly a revolutionary conclusion. The Executive Summary of the major 1989 review on diet and health by the National Academy of Sciences concludes that “intake of total fat per se, independent of the relative content of different types of fatty acids, is not associated with high blood cholesterol levels and coronary heart disease”. In particular, this message is inconsistent with the emphasis on total fat reduction in the U.S. dietary pyramid. The likelihood that coronary heart disease risk will actually be increased by replacing mono- and poly-unsaturated fats with carbohydrate is important because unsaturated fats actually comprise the majority of dietary fat in the United States….

…On the basis of metabolic and epidemiologic evidence, replacing unsaturated fats with carbohydrates would be expected to increase risk of coronary heart disease. However, some have argued that decreasing the percentage of energy from fat in the diet will reduce weight importantly, so that the overall effect would be beneficial. Despite this argument, an important benefit of dietary fat reduction on body weight has not been supported in longer-term, randomized trials. …However, in studies lasting one or more years, the weight loss described above appears to be transient, and the effect on weight over the longer time is minimal, typically about 1-3kg. …Although more research is needed on the effects of long-term fat intake on body weight control, the available evidence is quite clear that there will be little if any effect on body weight with reductions in dietary fat, at least over the range of about 18%-40% of energy from fat.
Although further studies are warranted relating specific types of fat to risk of coronary heart disease, the available evidence suggests several conclusions. First, coronary heart disease rates can be dramatically reduced by nutritional means, but this will not be achieved by replacing saturated fat with carbohydrates. Second, we should abandon recommendations regarding the percentage of energy from fat and pejorative references to fat or fatty foods. Finally, advice about dietary fat should focus on replacement of saturated and trans fatty acids with unsaturated fats, including sources of alpha-linolenic acid.