by William R. Ware, Ph.D. :The belief among the general public that fat is bad is almost universal. All fats, the saturated, monounsaturated, polyunsaturated and trans-fats are lumped together and regarded with a jaundiced view if not outright condemnation. According to the conventional wisdom, fat is implicated in heart disease, stroke, cancer, obesity, and diabetes. However, not all nutritional scientists and experts in the etiology of heart disease and cancer agree with this view of fat. In fact, public opinion appears to be about five to ten years behind the results of modern science in this regard.
The problem of the relationship between dietary fat and health is exceedingly complex. Since the principal health question regarding fat relates to its association with the risk of coronary heart disease (CHD), this becomes the central issue that must be examined. Not only are there several distinct types of fat, but there are a number of different consumers, i.e., old, young and in between, men and women, pre- and postmenopausal women, individuals with the so-called metabolic syndrome or Syndrome X, the obese and the overweight, those with normal and those with abnormal blood lipids (the several cholesterol fractions and triglycerides), diabetics, individuals who have had a heart attack and individuals who are thought to be free of all coronary heart disease, smokers and non-smokers, etc. Most studies look at only a few sub-sets of this general population. Since fat consumption alters cholesterol levels, the fat-heart disease question must also involve a consideration of the relationship between fat consumption and cholesterol on the one hand, and cholesterol and heart disease on the other. Thus the subject is far from simple.
The history of the anti-fat movement will be reviewed first, followed by an examination of what modern nutritional epidemiology and nutritional science has revealed about the merits and risks of dietary fat, especially in the context of coronary heart disease. Then we will discuss low-fat and high-fat diets with these results in mind.
THE ANTI-FAT MOVEMENT
In the last twenty-five to thirty years, fat has gone from being taken for granted as a normal constituent of Western diets to being branded public enemy number one. This change came about through the efforts of the US Government, mainly the USDA, and various groups such as the American Heart Association, the National Academy of Science, and the American Dietetics Association. The anti-fat movement was also greatly assisted by the medical profession and the media, both of whom were active in spreading the word. The battle cry–fat causes heart disease, strokes and cancer–was, as might be expected, very effective. The food industry picked up on this movement very early and an ever-increasing stream of low-fat, lite, low- cholesterol, and zero-fat products have appeared on the shelves of the supermarkets. Fat substitutes have become popular–they taste like fat but are not digestible. Books on low-fat diets have appeared by the dozens. An almost zero-fat diet was claimed to reverse heart disease, which the general public has been lead to believe is caused by dietary fat and cholesterol particles plugging up the coronary arteries (unfortunately, it is not that simple). The USDA came out with its famous food pyramid, which basically favors a high-carbohydrate and low-fat diet. Today the notion that fat is bad is securely established in the minds of the general public. In fact, the movement has many of the attributes of an organized religion. It is therefore quite interesting that the lumping of all fat into one category and labeling it as bad has virtually no basis in modern scientific studies, and the elimination of some fats from the diet can have very serious health consequences. Gary Taubs, in his recent feature article in the journal Science (1) called it "The Soft Science of Dietary Fat." His article contains an excellent review of the anti-fat movement.
A problem with the early studies that led to the condemnation of fat was that the various fats were lumped together as simply fat, without regard for the fact that there were thought to be good fats and bad fats. The need for and actions of the good fats were, to some extent, recognized and understood even at the start of the anti-fat campaign, and this was the basis of the opposition from a number of scientists. It turned out that their concerns were ignored. Today it is absolutely clear that any discussion or study concerning fats and health must deal with the individual fats, that is, the saturated fats, the monounsaturated fats, the polyunsaturated fats (with special attention to the omega-3 and omega-6 fats), and the trans-fats, which are mostly man-made (2). This obviously complicates matters quite considerably. In fact, when the decision was made to condemn all fats, part of the motivation was the view that the general public was not able to deal with these complexities, which was probably correct. A decrease in consumption of the good fats was considered the price that had to be paid to decrease the consumption of what were viewed as the bad fats. The molders of public opinion considered that on balance there would be benefit, and they would not listen to those who disagreed and pointed out the risks. Also, how bad the trans-fats were was not fully appreciated in the early days of the low-fat movement, when part of the gospel was to replace butter with margarine, which was high in trans-fat. More on this later.
Another problem with the early studies that led to the condemnation of all fat was that, by modern standards, they were rather crude. The science of nutritional epidemiology has changed significantly during the last twenty years, with the result that study design has vastly improved, statistical power increased, and the ability to correct for confounding factors has reached a very high level (3). An example of the confounding factor problem is as follows. Let’s say a study finds that heavy drinking of alcohol is positively associated with lung cancer. But since heavy drinkers also tend to be smokers, frequently heavy smokers, the data must be corrected for the known risk of lung cancer associated with smoking. Otherwise the observed association of drinking with lung cancer is meaningless. Studies of dietary fat and health are confounded by numerous factors associated with both lifestyle and other nutritional factors. Early studies by and large failed to deal effectively with this problem.
Finally, in the period when the early studies were being performed, little was known about the relationship between the various blood lipid fractions such as HDL, LDL, and the triglycerides, and the risk of cardiovascular disease. Only total cholesterol was measured. At present, there is considerable information on the relationship of the various lipid fractions to CHD risk, and this continues to be an active area of research.
Today, the early studies of the relationship between fat and health are regarded by many nutritional epidemiologists as mainly of historical interest, although these studies are still quoted by proponents of the "all fat is bad" religion as they struggle to maintain their viewpoint. In what follows, the subject of dietary fat will be explored in some detail, with special emphasis on potential health problems associated with low-fat diets. The questions that need to be addressed are as follows:
* Which fats, if any, are bad, which fats are good? What is the actual evidence?
* Do fats really cause heart disease and cancer?
* Are there dangers associated with low and very low fat diets?
* On the basis of modern nutritional epidemiology, what are the recommendations as regards optimum balance of the various types of fat, carbohydrate and protein in the diet if the goal is to remain healthy?
FAT CONSUMPTION AND HEART DISEASE
There are three types of study that are of importance in this context. One involves following large groups of individuals, the so-called cohorts, using food frequency questionnaires and in some cases interviews. Subjects are followed for a number of years, and the participants in any given study may number in the thousands to tens of thousands. Associations are sought between diet and the incidence of one or more health problems that occur at a later date (the prospective, cohort study). The second type of study involves an intervention. In the present context, the two common ones are (a) reducing the total intake of fat in general, and (b) replacing saturated fat with unsaturated fat, keeping the total fat calories approximately constant. In connection with the diet-heart hypothesis, the investigators then look for changes in the incidence of events related to heart disease, such as a survived or fatal heart attack. From such studies, one can determine if there is an association between fat intake and the incidence of adverse coronary events. A third type of study, called the case-control study, involves recruiting both a group of individuals with a specific problem or problems and a control group presumed free of the problem or problems. Factors are then sought that are present in the group with problems and absent in the controls.
In connection with the questions of whether or not scientific studies support the hypothesis that there is an association between adverse heart events and the consumption of either saturated or polyunsaturated fat, Willett (3) has reviewed 21 prospective cohort studies involving over a quarter-million participants. Four of the nineteen studies supported the hypotheses that saturated fat was bad, 10/19 showed no significant association, and the balance provided insufficient information. In connection with the hypotheses that polyunsaturated fat was beneficial, 4/19 supported this hypothesis, 9/19 showed no association, and the balance provided insufficient information. There are two other similar reviews in the recent literature (4,5). One essentially agrees with the Willett analysis, the other finds a slight suggestion that saturated fat may have an adverse association, but the association is weak. There is a small but significant variation in the studies included in these two last mentioned reviews, but the general picture is clear. If saturated fat were tried before a judge and jury on the basis of this evidence, it would no doubt be acquitted! The evidence is simply inconclusive.
Nine dietary intervention studies have also been reviewed by Hu and coworkers at Harvard (6). In the studies reviewed, groups of individuals were randomly assigned to an intervention or control group, and the incidence of events in the intervention group was compared with the number found in the control group. Two of the studies involved presumably healthy individuals, and seven involved patients who had experienced a heart attack. Two of the studies involved reducing total fat. One found a benefit (reduced adverse coronary events), the other found none. In both cases the changes were very small. Of the seven other studies included in the review, in six the intervention involved decreasing saturated fat and increasing unsaturated fat. In one study, the intervention involved adopting a Mediterranean type diet augmented with alpha- linolenic acid, one of the omega-3 essential fatty acids. Six of the seven studies found a significant benefit whereas one found none.
The study that found the largest decrease in adverse coronary events was the one that involved switching to a Mediterranean diet. In this study the changes in total cholesterol were also tabulated. The change in the incidence of adverse coronary events failed completely to correlate with the change in the serum cholesterol. If the data are plotted, one obtains a scatter diagram with no visible trend or correlation. In the study where there was the largest decrease in adverse coronary events (73%), there was no on-average change in the serum cholesterol. Note that seven of the nine studies involved heart attack patients–so- called secondary prevention. Also note that in the studies where saturated fat was replaced with unsaturated fat, two variables are being simultaneously changed. Also, only two of the studies where saturated fat was replaced by unsaturated fat involved healthy individuals (primary prevention), and one of these found no effect. These studies nevertheless are universally viewed as providing convincing evidence that the action of replacing saturated fat with unsaturated fat is a beneficial move in the context of heart disease, especially preexisting heart disease. In fact, it is hard to argue with the overall benefit observed, which was quite large. However, such studies fail to prove that saturated fat is bad, since that question was not studied in isolation. The prospective cohort studies mentioned earlier attempted to pull out saturated fat as an independent variable but failed to provide any conclusive or consistent evidence that it was bad. Conclusive evidence that polyunsaturated fat was good was also not obtained in these cohort studies. The advice to decrease saturated fat and increase polyunsaturated fat is now incorporated in most modern recommendations as a presumably beneficial dietary modification.
It seems clear that on the basis of the above studies, one cannot conclude that the consumption of saturated fat has been proven to be bad in the context of heart disease. In fact, it would appear close to neutral. If it is indeed dangerous to the heart, the risk is too small to stand out even in a large number of studies. After all, the prospective cohort studies discussed above involved over a quarter-million subjects! The fact that this point of view is at variance with the older studies has been explained by the failure to take into account confounding factors. Also, selection of only data favorable to the fat-heart disease hypothesis appears to have taken place. In the most notorious case, a comparison of the incidence of heart disease, as a function of fat consumption for a group of different nations, led to a remarkable, straight-line correlation where, in fact, when all the data available at the time are used, a scatter plot with no correlation is obtained (7). Yet this study was for years the basis of the belief that fat consumption caused heart disease and is still quoted by those who adhere faithfully to this dogma.
Polyunsaturated fats, as a class, include the so-called trans-fats. Most of the trans-fats encountered in the modern diet are man-made by the partial hydrogenation of polyunsaturated fats using high temperature and a metal catalyst. While the question of whether or not saturated fat is bad will no doubt continue to be debated, there appears no question whatsoever that trans-fats are bad, in fact really bad. Yet they permeate the modern diet. Part of the motivation for their creation was vastly extended shelf life, since natural unsaturated fats oxidize easily and thus become rancid. Also, this was a way to make liquid fat into solid fat, such as Crisco and stick margarine. The body treats trans-fats like real fats, and, among other things, incorporates them into the cellular lipid bilayer. They are found in fried foods such as French fries, baked goods, and in many prepared foods. There has been a lot of research on the problems caused by the consumption and metabolism of trans-fats. For example (2):
* Trans-fatty acids lower HDL in a dose response fashion, i.e. the more you eat, the lower it gets (the wrong direction for the so-called good cholesterol).
* Trans-fatty acids increase LDL (the so-called bad blood lipid).
* Trans-fatty acids raise lipoprotein(a), which is regarded as undesirable
* Trans-fatty acids raise total cholesterol in the blood by as much as 20 to 30 mg/dL.
* Trans-fatty acids decrease the levels of testosterone in males and increase the number of abnormal sperm.
* Trans-fatty acids inhibit the function of a number of essential enzymes in the body associated with the chemistry of omega-3 and omega-6 fatty acids.
* Trans-fatty acids interfere with the action of enzymes that metabolize toxic chemicals.
* Trans-fatty acids decrease the response of cells to insulin. They hamper the proper function of the insulin receptors by changing the fluidity of the lipid bilayer and other cellular membranes. Thus they may contribute to insulin resistance.
This is a horrible report card for something that today is a major constituent of the typical North American diet. It is estimated that for a typical teenage diet with 3000 calories and 35% from fat, that 40% of the fat calories are from trans-fats. It may be even higher. Trans-fats totally permeate the processed food production. Until the trans-fat content is included as a part of the label on all processed foods, all one can do is look for the words partially hydrogenated and avoid all products that contain such ingredients. Individuals wishing to avoid trans-fats should avoid all commercial baked goods, reject margarine totally (unless it is the new trans-fat free product) and avoid all deep fried foods as well unless they are deep fried at home, using fresh unsaturated oils at a minimum temperature to accomplish the cooking. Some scientists who have studied the trans-fat phenomenon in detail consider it to be potentially one of the major health problems of the 21st century.
Emeritus Professor of Chemistry, University of Western Ontario, London, Ontario, Canada