Polyunsaturated fats: the dietary equivalent of low-tar cigarettes?

Recently I read an article on the website of the Pritikin Longevity Center in Florida, entitled What’s Wrong with Olive Oil? This makes the case for the hypothesis that there is no such thing as a “healthy fat” – only bad fats and very bad fats – likening the situation to comparing low-tar cigarettes with normal cigarettes. Just because the low-tar cigarettes produce less disease than the regular variety does not mean they are good for you. It simply means they are less bad for you. The same, it is argued by some, is true for monounsaturated and even polyunsaturated fats.

I’m interested to look a little more closely at the science behind these claims, which are very much at odds with current standard advice and the popular and growing movement promoting “healthy fats” whilst demonising sugar. Here I’m going to discuss one of the references cited in the Pritikin article: a paper published in 1995 in the journal Arteriosclerosis, Thrombosis and Vascular Biology entitled “Compared with dietary monounsaturated and saturated fat, polyunsaturated fat protects African Green Monkeys from coronary artery atherosclerosis” (1).

I should note that the field of lipid metabolism and atherosclerosis research is vast, and I am in no way familiar with the full body of literature in this field. I am posting this for interest only, and I am going to try and make it as straightforward and understandable as possible.


So, what did they do?

They took 44 adult male African Green (Grivet/Vervet) Monkeys, which are evolutionarily close to humans and have been shown to respond to food challenges similarly in terms of their blood lipids, and divided them into three groups. They fed them “monkey chow” for a ten-week washout period, followed by an atherogenic (meaning resulting in the formation of atherosclerosis, or clogging of the arteries) diet containing 0.8mg/kcal of cholesterol and 40% of calories as saturated fat (mainly lard), for the next 11 weeks. Several tests of blood lipids were done during the challenge period to ensure that all monkeys responded to this diet in a similar way. The monkeys were also similar in terms of their body weight, blood pressure and blood glucose levels. After a further six weeks on monkey chow, the monkeys were divided into three groups of 15, 15 and 14 animals, and groups were assigned to diets containing 0.8mg/kcal cholesterol and 35% of total calories from either saturated, monounsaturated or polyunsaturated fat.

The amount of cholesterol included in all diets was noted to be “sufficient to induce an average level of hypercholesterolaemia of between 200 and 400mg/dl, the range of high risk in the North American population, and a range that would permit evaluation of atherosclerosis in these animals at the end of the study” (2).

For the first year the monkeys were fed a “mixed fat diet”, while for the remainder of the study they were fed a “pure fat diet”*. The mixed fat diets consisted of oleic-acid (omega 9) rich safflower oil (monounsaturated) and linoleic-acid (omega 6) rich safflower oil (polyunsaturated), as well as lard (saturated) for all three groups. The pure fat diet did not include lard, and the saturated fat was derived from palm oil. Olive oil itself was not studied.

[*N.B. This is highly dubious scientific practice – they started out with one diet, and when it did not seem to be producing the desired results they modified it halfway through the study]


And what did they find?

At the end of the five-year period the surviving animals (not all of them survived) were killed and their coronary arteries dissected. Overall, 10 of 13 (77%) animals were affected by atherosclerosis in the saturated group, 11 of 13 (85%) animals in the monounsaturated group, and 7 of 12 (58%) animals in the polyunsaturated group.

Coronary artery atherosclerosis severity was presented as the maximum percentage lumen stenosis (narrowing). The most affected group was the saturated group, with seven animals with more than 10% narrowing, four with more than 20% narrowing, and one with about 80% narrowing. In the monounsaturated group four animals had more than 10% narrowing and three had more than 25%. In the polyunsaturated group three animals had more than 10% narrowing and one had more than 20%.

They also measured the average area of the lining of the coronary arteries that was affected by atherosclerosis. There was a wide range within each group. The original paper does not give the actual figures for the affected areas in each animal, but rather presents composite bar charts that indicate that all three groups were affected to a similar degree, although there is a slight gradient, with the saturated group being the worst affected and the polyunsaturated group being the least affected. The authors state that the area affected was similar in the monounsaturated fat and saturated fat groups (no statistically significant difference), and was less (p=0.07 for polyunsaturated vs. saturated, and p=0.09 for polyunsaturated vs. monounsaturated) in the polyunsaturated fat group. This is the origin of the erroneous conclusion that also forms the title of this paper. It is also why the paper was quoted by the Pritikin article in support of their contention that olive oil (mainly monounsaturated, but also 14% saturated by weight) is just as bad for you as saturated fat.

Why am I saying that the authors’ conclusion (that polyunsaturated fat is protective against atherosclerosis) is erroneous? Well, the monkeys fed polyunsaturated fat developed less furring than those fed monounsaturated fat or saturated fat. But there was no control group of monkeys who were fed no additional fat at all. Given that the natural diet of African green monkeys* contains very little fat of any sort (even monkey chow, which is a highly processed dry feed, contains only 5% fat as a minimum), this is a significant oversight. You can only conclude that polyunsaturated fats protect against atherosclerosis if that group was demonstrated to have less clogging of the arteries than animals fed their natural diet (but then, animals consuming their natural diet don’t get atherosclerosis** – a slight technical hitch there – which also begs the question, what is the natural diet for human beings?). Well then can we conclude that polyunsaturates protect against atherosclerosis in monkeys already consuming an artificially high amount of cholesterol (which may be analogous to a Western meat/dairy/egg-based diet)? No, because there was no control group consuming cholesterol only, with no other added fats, so no direct comparison could be made. We simply do not know how much atherosclerosis the “cholesterol-only control” group or the “natural diet control” group would have developed, under these specific laboratory conditions. For the conclusion to be true, we would have to have observed a level of atherosclerosis development in a cholesterol-only control group that fell in between the levels observed for the polyunsaturated group and the monounsaturated and saturated groups. If a cholesterol-only control group had produced levels significantly less than all three fatty acid supplementation groups then we could have concluded that all three types resulted in increased atherogenesis (plaque formation), with polyunsaturates being the least bad. Or vice versa… perhaps all three types are protective (highly unlikely, from what we know from other studies). We simply cannot say from this, which is a crying shame: a waste of an opportunity to learn something useful, and a waste of 44 lives.

[*African green monkeys are primarly frugivorous (they derive the majority of their calories from fruit), but they also consume plenty of leaves, flowers, seeds and seed pods. They eat insects such as termites, and will raid nests and eat eggs and chicks. They will sometimes take small mammals and birds. 

[**The authors state that “several studies have been completed in which low-cholesterol diet groups were maintained for as long as 5 years without coronary artery atherosclerosis development”. This suggests that the majority of the observed disease was actually due to the cholesterol in the diet and not the supplemental fatty acids anyway.]


A final word about animal rights…

This study confined 44 adult monkeys in individual cages for five years. It sounds as if they were able to see each other, but not touch each other (I fail to see why this was necessary). They were fed an unnatural diet, with raw carrots as a snack only three times a week. At the end of the period they were killed. For some people this is never justifiable. For some people it might be justified if the information gained was of vital significance for humanity. However to do this for a study that it was clear from the outset (at the design stage) was going to produce fairly meaningless results is particularly tragic in my view. There are much better ways of gaining the information that we need on which to base our dietary choices. Arguably, we already have all the information we need (it just hasn’t yet been brought together yet in any structured way), and I sincerely hope that no more monkeys will be made to suffer like this.

I started out with high hopes that this study would help me to understand more about the relationship between fat consumption and atherogenesis. I am sad to say that it has not really done so, and I am left feeling rather disappointed after all my efforts. The one interesting fact which was stated at the start of the study was that it is already known that a certain amount of cholesterol in the diet of African Green Monkeys reliably produces atherosclerosis within a relatively short time-frame, and it seems reasonable to think that the same may be true for human beings. It’s not conclusive of course, so (as with all studies carried out on animals) you can discount it if you like, but I for one will be continuing with my cholesterol-free (i.e. animal-fat-free) diet.

[N.B. This study does not really shed any light on the debate, amongst those already consuming a cholesterol-free plant-based diet, about whether it is safe (or even beneficial) to consume plant fats such as extracted oils, nuts, avocadoes and coconut, and if so in what quantities.]



1. Compared with dietary monounsaturated and saturated fat, polyunsaturated fat protects African Green Monkeys from coronary artery atherosclerosis. Lawrence L et al. Arteriosclerosis, Thrombosis and Vascular Biology 1995;15:2101-2110

2. Effects on plasma lipoproteins of monounsaturated, saturated and polyunsaturated fatty acids in the diet of African green monkeys. Lawrence L et al. The Journal of Lipid Research, 31, 1873-1882. October 1990.

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