Dietary Fats and Heart Disease – Exposing the Villain
“I would only argue that Senators don’t have the luxury that the research scientist does, of waiting until every last shred of evidence is in.” These words were spoken by Senator George McGovern 35 years ago when confronted by Dr. Robert Olson from St. Louis University. Senator McGovern´s Select Committee on Nutrition and Human Needs went ahead and published their famous report, “Dietary goals for the United States” in 1977. Doctor Olson, however had pleaded for “more research on the problem before making announcements to the American public”. The main message from the McGovern report was to reduce overall fat consumption, reduce the consumption of saturated fat and cholesterol, and to substitute polyunsaturated fatty acids (PUFA) for saturated fat. Decreased consumption of meat, butterfat, eggs and other high cholesterol sources was recommended. McGovern’s words reflect the fact that the report was not based on strong scientific data.
The committee’s members acknowledged that their recommendations relied on epidemiological and observational data at best. There were no prospective randomized studies available to support their recommendations. Dr. D.M. Hegsted, professor of nutrition at Harvard School of Public Health, who assisted in the preparation of the report wrote: “The question to be asked, therefore, is not why should we change our diet but why not. What are the risks associated with eating less meat, less fat, less saturated fat, less cholesterol, less sugar, less salt and more fruits, vegetables, unsaturated fat and cereal products – especially whole grain cereals. There are none that can be identified and important benefits can be expected”. What the committee’s members probably did not realize however, was that their report laid the foundation for worldwide dietary guidelines for the next 35 years. Ever since , “eat less saturated fat and cholesterol…” , has been the cornerstone recommendation from medical professionals, for patients with heart disease or those who want to reduce their risk of heart attack.
Although the McGovern committee’s report turned out to be influential, several other important expert panels had already provided similar recommendations. The advise to limit saturated fats was already alive in the 1950s. In 1957 the American Heart Association suggested that a decreased consumption of saturated fats might reduce the risk of heart disease. The Inter-Society Commission for Heart Disease Resources and The American Heart Association had already in the early 1970s suggested a reduction in dietary cholesterol to less than 300 mg a day. However, the experts did not agree. In October 1977, the Canadian Department of National Health and Welfare concluded that: “Evidence is mounting that dietary cholesterol may not be important to the great majority of people… Thus a diet restricted in cholesterol would not be necessary for the general population”. However, the wheels were turning fast and food manufacturers were already adopting new strategies. Low fat food varieties of all possible kinds spread throughout the world. Butter consumption decreased and vegetable oils became popular. Cholesterol and saturated fat were to be eliminated from our dishes. The villains had been exposed and had to be taken care of.
Trans fat joins the villains
The French scientist Paul Sabatier developed the hydrogenation process in the 1890s. In 1902, Wilhelm Normann found that liquid oils could be hydrogenated to form trans fatty acids. This turned out to be the first man made fat. During the second world war the use of trans fats rose because of the increased use of margarine. In the early 1980s there was a strong campaign by consumer advocacy groups against using saturated fat for frying in fast-food restaurants. In response, most fast-food companies began using partially hydrogenated oils containing trans fat instead of beef tallow and tropical oils high in saturated fats.
A lot of evidence indicates that the fear of saturated fats during the 1950s through the 1980s, lead to an increased use of trans fats. In the 1990s it became clear however that trans fats might be harmful. Numerous research studies during the 1990′s revealed correlations between trans fatty acids and increased LDL-cholesterol and a higher incidence of heart disease. In 1993 health advocacy groups started calling for fast-food restaurants to stop using partially hydrogenated oils in their deep fryers. Trans fat was the new villain. In 2003, Denmark was the first country in the world to limit the use of trans fats by law. Trans fat labeling became mandatory in the United States in 2006. The same year, The American Heart Association became the first major health organization to specify a daily limit: less than 1 percent of calories from trans fat. Later in the year, New York became the first U.S. city to pass a regulation limiting trans fat in restaurants. Multiple cities and states have since proposed similar regulations.
Is polyunsaturated fat (PUFA) a healthier option than saturated fat?
The recommendation to substitute PUFA for saturated fat was one of the main goals of the McGovern report in 1977. This probably originated from the research of the influential Minnesota epidemiologist, Ancel Keys. Keys is often acknowledged as the father of the diet-heart hypothesis. He published many studies in the 1960′s and 70′s demonstrating higher cholesterol levels among patients with heart disease. His studies also showed higher cholesterol levels among people who consumed food rich in saturated fat and cholesterol. From his famous “Seven Countries Study” he concluded that dietary fat was the single most important cause of heart disease because it elevates blood cholesterol. However, the study has been criticized for the method in which populations were selected for the study, and the way that the population (ecologic) correlations were carried out. Keys believed, like so many others, that a Mediterranean-style diet low in animal fat and rich in PUFA protected against heart disease.
Advice to replace animal fats rich in saturated fats by vegetable oils rich in PUFA has been a cornerstone of worldwide dietary guidelines for half a century. However, when the advice originated fifty years ago, PUFAs were regarded as single molecular category with one highly relevant biological effect – to reduce blood levels of cholesterol. At that time, omega-6 (linoleic acid) was the best known PUFA. Since then, it has been recognized that PUFAs comprise multiple species of omega-3 and omega 6, each with unique biochemical properties, and perhaps different cardiovascular effects.
What was the evidence behind recommending decreased consumption of saturated fat and increased consumption of PUFA? Interestingly, there were three prospective studies performed during the 1960′s and 70′s comparing the effects of these two types of fats. The studies involving a total of 1300 men with heart disease in England, Norway and Australia used diets with high ratios of PUFAs to saturated fats, limited dietary cholesterol, and low levels of monounsaturated fat. The trials lasted five years, and despite achieving lower cholesterol levels, people on the experimental diet did not do any better than their counterparts did on the control diet. Therefore, these studies did not support the hypothesis that substituting PUFA for saturated fat was beneficial. Somehow, the medical community did not seem to take notice. Interestingly, one of these studies, The Sydney Diet Heart Study was recently recovered, reevaluated and published in the British Medical Journal. The results have received huge attention.
Linoleic acid (omega – 6) – The new villain
The Sidney Diet Heart Study was a randomized trial conducted from 1966 – 1973. It tested the effect of replacing saturated fat with linoleic acid (omega-6) from safflower oil. Safflower oil is a concentrated source of linoleic acid, containing no other PUFAs. The study population consisted of 458 men aged 30-59 years with a recent history of heart attack (acute myocardial infarction or acute coronary insufficiency).The men were randomized to an intervention group (n=221) and a control group (n=237).
The intervention group received instruction to increase their PUFA intake to about 15% of food energy, and to reduce their intake of saturated fat and dietary cholesterol to less than 10% of food energy and 300 mg per day, respectively. To achieve these targets, intervention participants were provided with liquid safflower oil and safflower oil polyunsaturated margarine (“Miracle” brand, Marrickville Margarine). Liquid safflower oil was substituted for animal fats, common margarines, and shortenings in cooking oils, salad dressings, baked goods, and other products, and was also taken as a supplement. Safflower oil polyunsaturated margarine was used in place of butter and common margarine.The control group received no specific dietary instruction. However, some participants began substituting polyunsaturated margarine for butter after their coronary event.
The results of the study were quite striking. Compared with the control group, the intervention group had an increased risk of all cause mortality (17.6% vs. 11.8%), cardiovascular mortality (17.2% vs. 11.0%) and mortality from coronary heart disease (15.3% vs. 10.1%). This difference existed despite the fact that serum total cholesterol decreased more in the intervention group than in the control group (13.3% vs 5.5%). These unfavorable effects of omega-6 are consistent with two other randomized controlled trials, in which saturated fats were replaced with omega-6 and common margarines were replaced with corn oil. By reanalyzing the original Sidney Diet Heart Study data, the authors were able to include the study into their previous meta-analysis. The meta-analysis covers secondary prevention studies where saturated fats were replaced, either by omega-6 selective PUFA interventions or a combination of omega-6 and omega-3. In short; replacing saturated fats with PUFAs containing mainly omega-6 fatty acids appears to increase coronary heart disease mortality, while replacing saturated fats with a combination of omega-3 and omega-6 decreases coronary heart disease mortality. This may explain why a diet rich in omega-3, such as the Mediterranean diet appears to positively affect the risk of heart disease.
Omega-6 is the most abundant fatty acid in low density lipoprotein (LDL) particles. Oxidized linoleic acid metabolites (OXLAMs) are the most abundant oxidized fatty acids in oxidized LDL. The authors of the British Medical Journal paper suggest that a diet induced increase in the production of bioactive OXLAMs may contribute to atherosclerosis and cardiovascular disease pathogenesis. This hypothesis supports the possibility that atherosclerosis may occur through mechanisms that are very distantly related to plasma cholesterol.
Is it all due to the trans fats?
Australia’s National Heart Foundation has claimed the recent Sidney Diet Heart Study is misguided. Dr Robert Grenfell who is National Health Director at the Heart Foundation points out that “in the 60s and 70s margarine still contained trans fats which we now know are extremely harmful to heart health. Replacing saturated fat with a product that was high in trans fat would never be recommended now”. Bill Shrapnel, the Deputy Chairman of the Sidney University Nutrition research Foundation agrees that “the study was not objective because margarine no longer contains the trans fatty acids it did at the time of the trials”. He adds: When the study began, Miracle margarine contained approximately 15 per cent trans fatty acids which have the worst effect on heart disease risk of any fat. The adverse effects of the intervention in this study was almost certainly due to the increase in trans fatty acids in the diet.”
The authors of the recent British Medical Journal article have a completely different view on the trans fat issue. In their discussion of the results they point out that the trans fatty acid content of the participant’s diets was not recorded. On the other hand they argue that the restriction of common margarines and shortenings (major sources of trans fatty acids) in the intervention group would be expected to substantially reduce consumption of trans fatty acids compared with the control group. Thus, the consumption of trans fatty acids may indeed have been higher among the control patients than the intervention patients. Trans fatty acids are known to raise cholesterol levels. The fact that cholesterol levels decreased more in the intervention group than in the control group does not indicate that the intervention group had more trans fatty acids in their diet.
Dietary fat and heart disease – Exposing the villain
I have always been a fan of Agatha Christie’s famous crime novels. Agatha was the master of twists and turns. You never knew until the last page who was the villain. In one chapter you were led to believe it was the gardener or the butler. In the next chapter you definitively suspected the beautiful widow. The two things you could be certain of was that there were always going to be more twists and turns, and the end would take you by surprise. I somehow have a similar feeling about fat and heart disease. There will be more twists and turns before we know the truth. For the time being though, it appears that the doomed outlaw, saturated fat may not be the worst villain. Indeed, the evidence against it is fairly weak. At least, saturated fat appears more innocent than trans fat and omega-6. I wonder, could Ancel Keys have been studying an effect of trans fat instead of saturated fat in his epidemiological research. Since he never separated these two we will probably never know.