Wednesday, October 29, 2008

Saturated Fat and Health: a Brief Literature Review, Part II

I'm aware of twelve major controlled trials designed to evaluate the relationship between saturated fat and risk of death, without changing other variables at the same time (e.g., increased vegetable intake, omega-3 fats, exercise, etc.). Here is a summary of the results:
  • Two trials found that replacing saturated animal fat with polyunsaturated vegetable fat decreased total mortality.
  • Two trials found that replacing saturated animal fat with polyunsaturated vegetable fat increased total mortality.
  • Eight trials found that reducing saturated fat had no effect on total mortality.
Of the two trials that found a benefit of saturated fat reduction, neither was properly controlled. The first was conducted in Sweden and published in 1965. The intervention group reduced saturated animal fat and increased polyunsaturated vegetable fat. The control group was significantly older than the intervention group, confounding the results. In addition, physicians regularly monitored the intervention group while the control group went off their radar, thus the intervention group was getting better care. This is the definition of an improperly controlled trial.

The second study to "support" the idea that saturated fat increases total mortality was the
Finnish mental hospitals trial. In this trial, two mental hospitals in different towns fed their patients different diets and monitored their health. One diet was low in animal fat and high in polyunsaturated vegetable fat, while the other was higher in saturated fat. Patients eating the polyunsaturated diet had a greatly reduced death rate, mostly due to a reduction in heart attacks. The study design was pitiful. They included all patients in their analysis, even those who stayed at the hospital for only one month or who checked in and out repeatedly. Furthermore, they used a "crossover" design where the hospitals switched diets halfway through the study. This was designed to control for location, but it means we don't know whether the increase in deaths after switching to the control diet was due to the saturated fat or the vegetable oil diet that preceded it for 6 years! The only reason I included this poor study in my list is that it's commonly cited as evidence against saturated fat.

The first study to show an increase in deaths from replacing saturated animal fat with polyunsaturated vegetable fat was the tragically named
Anti-Coronary Club study. After four years, despite lowering their cholesterol substantially, the intervention group saw more than twice the number of deaths as the control group. Amazingly, rather than emphasizing the increased mortality, the study authors instead focused on the cholesterol reduction. This study was not properly controlled, but if anything, that should have biased it in favor of the intervention group.

The second study to show an increase in deaths from replacing saturated animal fats with polyunsaturated vegetable fats was the
Sydney Diet-Heart study. This was one of the larger, longer, better-conducted trials. After five years, the intervention group saw about 50% more deaths than the control group.

I should also mention that one of the studies in the "no effect" category actually saw more than a four-fold increase in deaths after replacing saturated fat with corn oil, but somehow the result didn't achieve statistical significance (the paper states that p= 0.05-0.1, whatever that means). It may have simply been due to the small size of the study.

Overall, the data from controlled trials are clear: replacing animal fat with vegetable oil does not reduce your risk of dying! The same is true of reducing total fat. The main counterpoint to this conclusion is that the trials may have been too short to pick up the effect of saturated fat. However, two years was enough time to detect the effect of fish oil on death in the DART trial, and the trials I'm writing about lasted up to 8 years (not including the Finnish mental hospital trial or the Swedish one). There's also the fact that the greatest consumers of saturated fat in the world eat it for their entire lives and don't seem to suffer from it. Proponents of the theory that saturated fat is unhealthy have the burden of proof on their shoulders, and the data have failed to deliver.

Most trials of this nature are designed with cardiovascular outcomes in mind. Out of the twelve studies mentioned above, nine measured coronary heart disease mortality.
  • Two found it was reduced when saturated fat was replaced with polyunsaturated vegetable fat.
  • One found that is was increased when saturated fat was replaced with polyunsaturated vegetable fat.
  • Six found no effect.
Of the two that found an effect, the first was the Finnish mental hospital study. See above. The second was the L.A. Veterans Administration study, which was actually a good, eight-year study. However, it's worth noting three things about it: first, there were significantly more heavy smokers in the control group; second, overall mortality was the same in both groups, partly because of an increased cancer risk in the diet group; and third, it's the only well-conducted study of its kind to find such a result.

The study to find an increase in cardiovascular deaths was again the unfortunately-named Anti-Coronary Club trial. The Sydney Diet-Heart trial did not report cardiovascular mortality, which was almost certainly increased. Also, the study mentioned above that saw a "non-significant" four-fold increase in deaths on corn oil also saw a similar increase in cardiovascular deaths. I included it in the "no effect" category.

So not only do the best data not support the idea that saturated fat increases the overall risk of death, they don't even support the idea that it causes heart disease! In fact, the body seems to prefer saturated fat to unsaturated fats in the bloodstream. Guess what your liver does with carbohydrate when you eat a low-fat diet? It turns it into saturated fat (palmitic acid) and then pumps it into your bloodstream. We have the enzymes necessary to desaturate palmitic acid, so why does the liver choose to secrete it into the blood in its saturated form? Kitavan lipoproteins contain a lot of palmitic acid, which is not found in their diet. Are their livers trying to kill them? Apparently they aren't succeeding.

Eat the fat on your steaks folks. Just like your great-grandparents did, and everyone who came before.

No comments:

Post a Comment