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Research Department of Human Nutrition Royal Veterinary and Agricultural University Rolighedsvej 30 DK-1958 Frederiksberg Denmark E-mail: pma{at}kvl.dk
Dear Sir:
In their recent study, Kris-Etherton et al (1) compared an average American diet (AAD) rich in saturates (containing 34% total fat) with the American Heart Association (AHA) Step II diet (25% total fat) and 3 moderately fatty diets high in monounsaturates and poor in saturates (34–36% total fat) that were based on olive oil, peanut oil, or peanut butter (1). The AHA Step II diet and the monoene diets were associated with identical reductions in LDL cholesterol compared with the AAD. Similar to earlier studies, the low-fat Step II diet was found to raise triacylglycerol compared with the higher-fat diets. HDL cholesterol differed only marginally and insignificantly between diets. The authors concluded that "a high-MUFA, cholesterol-lowering diet may be preferable to a low-fat diet" in prevention of coronary heart disease. The same opinion was expressed by other authors (2).
We would like to warn people against considering high-fat monoene diets to be the most healthy diets. There are several reasons that high-fat diets—even those based on monoene fats—should be avoided in modern societies in which physical activity and energy requirements are low. First, any high-fat diet increases the likelihood of developing obesity, as do most other highly energy-dense diets (3). Even the olive oil–consuming Greeks have become more obese during the past decades because of their increasingly sedentary lifestyle (4, 5).
Second, in the recent study by Kris-Etherton et al and in most earlier trials comparing diets with various fat contents, energy intake was fixed and constant (ie, isoenergetic conditions were maintained) to keep body weight constant. This design is inappropriate because spontaneous energy intake would normally differ if diets with different fat contents were eaten ad libitum (6). It was shown very elegantly in a meta-analysis by Kris-Etherton's own group that fat-reduced diets cause a dose-dependent decrease in energy intake and body weight (7). The spontaneous weight loss that would be expected with a low-fat Step II diet was thus inhibited by the design used in the recent study (1). Accordingly, the blood lipid response was importantly biased. It is well known that weight loss causes triacylglycerol to decline and that concomitant increases in HDL cholesterol are often seen (8). In addition, earlier long-term trials of healthy and hyperlipemic persons showed that diets comparable with the Step II diet had no adverse effects on triacylglycerol if eaten ad libitum (9, 10).
Third, not only blood lipids but also several other cardiovascular risk factors are influenced by diet and therefore need to be considered in the overall evaluation of the health effect of a diet. We and other researchers showed that low-fat, high-fiber diets may affect blood coagulation and fibrinolysis strongly in an antithrombotic manner (11, 12). The effects on the hemostatic system seem to rely heavily on the carbohydrate quality of the diet, ie, the fiber content and the glycemic index (13). Therefore, it is unfortunate that Kris-Etherton et al did not report anything about these aspects of their experimental diets.
Fourth, there is strong epidemiologic evidence that high intakes of fruit and vegetables are associated with less coronary heart disease and cancer morbidity. High-fat diets prevent high intakes of fruit and vegetables because of the low energy ceiling of modern sedentary societies. Remember that the Greeks of the 1950s and 1960s were very physically active fishermen and farmers and that their high-fat, olive oil–based diets still allowed consumption of a large amount of bread, vegetables, and fruit (5, 14).
For these 4 reasons (more could be added), we believe it is incorrect to consider high-fat monoene diets the most healthy choice for sedentary people. We can take good care of our body weight, our blood lipids, our hemostatic system, and our need for trace elements and unknown nonnutrients present in foods only if we allow plenty of our energy to be supplied from foods with low fat contents. Where to set the fat limit is a matter of discussion, but the epidemic of obesity tells us that we still eat more fat than is appropriate. We consider a population average fat intake of 30% of total energy intake to be a wise recommendation.
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