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The Questionable Benefits of Exchanging Saturated Fat with Polyunsaturated Fat

The Contradictions

For many years we have been told that to prevent cardiovascular disease (CVD), we must lower our intake of saturated fatty acids (SFAs) and instead eat more carbohydrates and polyunsaturated fatty acids (PUFAs).

The main argument for using this diet has been that it lowers the concentration of cholesterol in the blood and thus prevents CVD. This idea was proposed for the first time by Keys. He presented data from 16 cohorts in 7 countries in an article that has been used as an argument till today. However, although there was a weak association between the intake of SFAs and heart mortality when all cohorts were analyzed together, there were substantial differences within each country in spite of similar intakes of SFAs. Most contradictory was the observation that coronary heart disease (CHD) mortality on the Greek island Corfu was 16 to 17 times higher than that on Crete, although the intake of SFAs was the same on both the islands. Furthermore, in a recent analysis of the data from this study, the authors found that processed foods, primarily carbohydrates, were classified as saturated fats. 

The diet-heart recommendations of the American Heart Association (AHA), published in 1982 and based mainly on Keys' hypothesis, were already questioned a year later, and since then, many more objections have been presented. The main arguments are as follows:

  • In clinical experiments, the effect of a high intake of SFAs on serum cholesterol is weak and transient, and 10 randomized controlled or crossover trials have found that a high intake of SFAs, even up to 50% of the total caloric intake, has little effect or none at all on total or low-density lipoprotein (LDL) cholesterol.
  • Two meta-analyses of prospective epidemiologic studies found lack of an association between CVD mortality and SFA intake. Even more contradictory were the results from 10 cohort studies of patients with stroke. In 3 of them, no difference in SFA intake was seen between patients with stroke and healthy people; in 7 studies, patients with stroke had eaten significantly less SFAs.
  • A meta-analysis of 16 observational studies found that a high-fat dairy intake was inversely associated with adiposity and was unassociated with diabetes and CVD.
  • Meta-analyses of the dietary trials have found only trivial or no benefit at all from decreasing the intake of SFAs and/or increasing the intake of PUFAs.

 

The Irrational Advice Continues

These facts have had no effect on the official guidelines. Although the authors of the recent WHO/Food and Agriculture Organization of the United Nations guidelines stated that 'the available evidence from cohort and randomised controlled trials is unsatisfactory and unreliable to make judgment about and substantiate the effects of dietary fat on risk of CHD,' they found no reason to change the advice regarding the intake of SFAs.

 

At a recent international symposium, a panel of dietary experts concluded that 'the evidence from epidemiologic, clinical, and mechanistic studies is consistent in finding that the risk of CHD is reduced when SFAs are replaced with polyunsaturated fatty acids' without specifying which types of PUFAs. Their main arguments were as follows:

  • A pooled analysis of 11 cohort studies established that for a 5% lower energy intake from SFAs and a concomitant higher energy intake from PUFAs, there was a significant inverse association between the intake of PUFAs and the risk of coronary events. However, the authors had excluded more than a dozen cohort studies included in the above-mentioned meta-analyses, which reported no difference in SFA intake between people with and without CHD, and they had ignored the above-mentioned cohort studies of patients with stroke.
  • A meta-analysis of 7 dietary trials established a significantly lower number of coronary events in the treatment groups. However, the authors of this analysis had excluded the trial of Rose et al and the Sydney trial, both of which resulted in a higher mortality in the treatment group. The meta-analysis is also in conflict with the results from a recent report of 4 unsuccessful trials in which SFAs were exchanged with omega-6 PUFAs only.

 

The Consequences

The dietary recommendations, according to which SFAs should be exchanged with carbohydrates, were introduced more than 30 years ago for the US population and have been followed in many countries. In retrospect, the current epidemics of obesity, metabolic syndrome, and type 2 diabetes that started shortly afterward may be an effect of this diet.

To exchange SFAs with PUFAs is not a wise decision either. Today, food rich in PUFAs is dominated by vegetable oils from soybeans, corn, and sunflower, all of which are rich in linoleic acid.

Already in 1991 Scott Grundy warned against eating too much omega-6 PUFAs. According to Grundy, there was no epidemiological support for this advice; it suppressed the immune system; it lowered HDL, it promoted LDL oxidation, it increased the risk of cholesterol gallstones, and it promoted cancer in laboratory animals. Since then, many studies have confirmed his warnings. Associations have been found between omega-6 PUFAs and prostate, pancreas, colon and breast cancer. Several cohort studies have found that women with a high intake of omega-6 PUFAs run a higher risk of breast cancer, and several studies have also found that women with a low omega-3/omega-6 ratio in their adipose tissue have the highest risk of breast cancer. (892 szó)

Mayo Clinic Proceedings

Volume 89, Issue 4, Pages 451–453, April 2014