Common knowledge is something we all regard as plain truth and does not need to be debated or discussed. Go for a walk on a rainy day and you´ll get wet. Staying for to long in the sun will make your skin burn. Drinking coffee before you go to sleep may keep you awake. Smoking may damage your lungs and so on.
But, is there a common knowledge regarding diet and heart disease? I asked a friend of mine recently what food he would associate with the risk of getting heart disease. He said: “Butter, cream, bacon, fat cheese, french fries, red meat and steaks”. I said: “How do you know that?” He replied: “Well doctor, it´s common knowledge. Everybody knows this. Animal fats and cholesterol! This is the stuff that´s blocking our arteries. That´s what you doctor´s have been telling us.”
For years I have been recommending my patients to stay away from saturated fats. This is what I learned in medical school and during my training as a cardiologist. The concept sounds quite simple indeed. I´ll explain it to you:Cholesterol is a risk factor for heart disease. The higher your blood cholesterol, the higher your risk for heart attack. Lowering cholesterol therefore should be beneficial. Furthermore, we have been told that saturated fats raise blood cholesterol. Therefore, obviously; Eat less saturated fats if you want to avoid heart attacks.
However, considering the complexity of the human body, this sequence of events is probably too simple to be true. Moreover, total cholesterol and LDL – cholesterol (the “bad” cholesterol) are absolutely not the only factors that matter when it comes to diet and heart disease. Why should they be? Nature seldom provides us with such simplicity. What about refined carbohydrates? What about obesity? What about insulin resistance and the metabolic syndrome? What about blood levels of triglycerides and HDL – cholesterol (the “good” cholesterol)? What about LDL particle size which may be negatively affected by sugars. What about inflammation which many specialists nowadays believe plays a role in heart disease? So, what is the case against saturated fat?
Let´s look at the theories on cholesterol a little closer. Basically they are twofold. One is the so-called lipid hypothesis which simply implies that an elevated level of cholesterol causes heart disease and therefore lowering cholesterol will be helpful. The other is the diet – heart hypothesis which implies that eating saturated fats and cholesterol will increase the risk of heart disease, supposedly by raising blood levels of cholesterol. I am not going to discuss the lipid hypothesis this time, but rather focus on the latter, the diet – heart hypothesis.
The main reason saturated fats have been linked to an increased risk of heart disease is their tendency to elevate LDL – cholesterol. We´ve learned that LDL is the bad cholesterol. The lower the blood levels of this substance, the less risk of heart disease. This has been the main focus of dietary recommendations in cardiovascular prevention and treatment for decades. Why such a huge effort has been put in promoting the risk of saturated fat,s and their possible effects on blood cholesterol is hard to understand, not least because the scientific basis behind it is indeed fairly weak.
It is important not to confuse saturated fat with trans fat. Saturated fat is a natural fat found in animal products such as meat, milk and cheese. Saturated fat is also typically found in tropical oils such as coconut, palm and palm kernel oils. On the other hand, the vast majority of trans fats in our food are manufactured by adding hydrogen bonds to unsaturated fats. This makes the fat more stable, so it doesn’t spoil as quickly. These fats are usually referred to as “hydrogenated fats” or “partially hydrogenated fats”. There is evidence linking consumption of trans fats with increased risk of heart disease.
Although saturated fats are often thought of as a group, it is important to keep in mind that they are not all the same. There are many different types of saturated fats and they may have different health effects. Grouping them all together is based on their chemical nature which implies that all the hydrogen bonds in the fat molecule are used. Saturated fats with carbon chain lengths of 14 (myristic) and 16 (palmitic), which are chiefly found in full fat dairy products and red meat, appear most likely to increase blood levels of cholesterol. Stearic acid (18 carbon) which is also found in beef, and is the main fatty acid of cocoa butter, appears to increase cholesterol less.
Now, let´s say we are going to cut down the consumption of saturated fats. What do we replace them with? Are we going to increase carbohydrate consumption instead? Or, are we going to eat more monounsaturateded or polyunsaturated fatty acids instead? Data from some clinical trials have shown that substitution of polyunsaturated fat for saturated fat may result in a reduced incidence of coronary artery disease. However, this is not true for all studies.
Many studies have linked the so-called Mediterranean diet with a reduced incidence of coronary artery disease. This diet generally involves an increased intake of the omega-3 fatty acid alpha-linolenic acid. Alpha-linolenic acid is a kind of omega-3 fatty acid found in plants. It is similar to the omega-3 fatty acids that are in fish oil, called eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA).
The Lyon Diet Heart Study compared the Mediterranean diet to a one involving higher consumption of saturated fats in patients with a prior history of heart attack (myocardial infarction). In the Mediterranean diet group there was a slightly more consumption of carbohydrates and fiber. In this study there was a 72% reduction in recurrent coronary events in the group receiving the Mediterranean diet. Secondary analysis has indicated that this positive effect correlates with the increased consumption of alfa-linolenic acid. However, the fact that omega – 3 fatty acids may be beneficial does not in itself imply that saturated fats are dangerous. Why should it?
Let´s look at the largest controlled intervention trial on diet and heart disease to date, the Women´s Health Initiative. This trial randomly assigned more than 48 thousand women, 50 – 79 years old, to a low-fat intervention or a comparison group. Saturated fat intake was lower in the intervention group as was dietary polyunsaturated fat. However, dietary carbohydrates were higher in the intervention group. After six years of follow-up there were no differences between the groups in the incidence of coronary heart disease and stroke. So, replacing fat with carbohydrates does not appear to be beneficial.
A metaanalysis, published 2010 in The American Journal of Clinical Nutrition, including 16 prospective observational cohort studies has indicated that there is no association between the intake of saturated fats and coronary artery disease. There is another study addressing the intake of saturated fats in childhood that found no association with adult coronary heart disease mortality.
There is evidence that for a large proportion of the population, high carbohydrate diets may create a metabolic state which is characterized by elevated triglycerides, reduced HDL – cholesterol and increased concentrations of small, dense LDL particles. This is especially true for those who are overweight, have insulin resistance or diabetes. Recent studies indicate that reducing carbohydrate intake in this population, but not saturated fat, ma be beneficial.
Therefore, the common knowledge that saturated fats are associated with cardiovascular disease lacks scientific evidence. Furthermore, the common wisdom that reducing the intake of saturated fats will reduce the risk for heart disease has a very weak scientific support. In light of the available scientific evidence it is hard to understand how we have managed to create those misconceptions. The case against saturated fat could not be won in any court.
Furthermore, the message to reduce the intake of dietary fat may have stimulated consumers and manufacturers to choose foods that may be potentially harmful and could have contributed to the so-called obesity epidemic and increased incidence of type 2 diabetes. Over consumption of sugar and refined carbohydrates probably plays the biggest role, the case against those indeed appears very strong.
Graham
1 comment:
Thank you. Another informative article.
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