Commentary: Protection from stroke by eating animal foods? Surely not!

Gary Fraser

Loma Linda University, Loma Linda, CA 92350, USA.

Many vegetarians, like myself have almost come to expect the data to indicate that they have an advantage, whatever the disease that is being considered. Thus, it is disquieting to find evidence in a quite different direction for at least one subtype of stroke. This evidence is not yet compelling and has some inconsistencies, but cannot easily be dismissed. Could it really be true, as the data presented by Sauvaget et al. in this edition of the International Journal of Epidemiology suggest, that eating more animal products reduces risk of some types of stroke?1

Over a number of years, reports about Japanese in Japan & Hawaii, and from others, have suggested a negative association between consumption of fish,2–5 dairy foods,6,7 multiple animal foods,8,9 animal protein,8,10–13 animal fat,11,12,14 and either total stroke, or haemorrhagic or ischaemic stroke. The study of Sauvaget et al.1 adds to this evidence. However, there are a few contrary reports. One study from Japan showed a positive association with saturated fats,15 and at least one prospective16 and one case-control17 study have not confirmed the protective association with fish consumption. There is also a small body of work suggesting that vegetable products and associated nutrients and vitamins are protective for stroke.15,18–23

Although most of the studies referenced above are prospective and hence methodologically stronger, they still suffer the difficulties of measurement error and confounding, hence residual confounding, that are common to all observational nutritional population studies.24 However, confounding does not seem very likely as a complete explanation of these associations given the number of studies reporting relatively strong effects. Such confounding would require one or more strong risk factors that are very closely correlated with the use of animal foods and no such confounders are easily identified at present. There are also diagnostic problems (see below). Despite these difficulties, is it possible to put this evidence together in a rational way?

More than 30 years ago Kuller & Reisler25 proposed a model that still seems a good scheme on which to base discussion and further research. Briefly this points out that strokes may result from disease in three types of arteries: (1) large extracranial and intracranial vessels subject to atherosclerotic disease similar to that found in coronary vessels; (2) arteries of the Circle of Willis and other larger vessels over the base of the brain where, except in diabetics, disease is usually characterized by proliferative intimal fibrosis rather than lipid infiltration; (3) small intracerebral vessels where disease resulting in stroke is characterized by proliferative fibrosis involving the adventitia and/or the media. Amyloid deposits may also cause intracerebral haemorrhage.

Hypertension appears to be a risk factor for all kinds of stroke and is more likely to result in cerebral haemorrhage. Lipid abnormalities mainly predict disease in the large vessels proximal to the Circle of Willis.

Thus it is very tempting to relate this situation to the somewhat unusual dietary data discussed above. Given other well-known evidence, it is not plausible that animal product consumption would protect against atherosclerotic disease of the large extra- and intra-cranial vessels. But it is possible that these foods (or other exposures associated with them) may protect against small vessel disease or vascular disease of the Circle of Willis. In some studies, the association with animal foods, animal fats, and protein is non-linear, suggesting that it is particularly those with very low intake of these foods or nutrients that have the elevated risk,11,12,14 although this was not always the case.8,13,26

Animal foods probably do not act through effects on traditional risk factors as these foods generally raise low density lipoprotein cholesterol and also blood pressure levels. The possible protective effect of dairy products seems to be independent of their calcium content, and of any effects they have on blood pressure.6 Thus, the mechanism underlying the suggested protection from animal foods is not at all clear, although several possibilities have been mentioned by others. Arachadonic acid which is mainly obtained from animal sources has been proposed as an explanation, based on its structural and physiological roles in the blood vessel wall.27 As summarized by Iso et al.12,28 animal work can provide other plausible hypotheses to support an elevated risk of stroke in those with very low serum cholesterol levels or protein deficiency.

The associations of diet or blood lipid levels with specific subtypes of stroke are particularly confusing and inconsistent. Although an effect of animal foods to reduce risk of intracerebral hemorrhage has often been observed, a similar association has often also been seen with cerebral infarction4–6,10,11,26 and sometimes only for this endpoint.4–6,26 It seems probable that the common categorization of stroke to ischaemic-thrombotic or haemorrhagic variants, while diagnostically convenient and therapeutically relevant, may not be helpful in aetiological research. Diseases of both large and small vessels may produce ischaemic and haemorrhagic strokes, with the main, but inconsistent difference being the size of the cerebral lesion produced. Cerebral infarcts, either from thrombosis of large vessels or the intraparenchymal small lacunar infarcts, may secondarily become haemorrhagic. Further progress may require different definitions of the stroke subtypes, possibly excluding the confusing larger haemorrhagic lesions that may have initially resulted from disease of either large or small vessels.

In Western society these results are probably most relevant to vegans who eat no animal fat or protein. It is not known whether vegans have an increased risk of certain types of strokes; however, this certainly deserves investigation. If further research identifies a protective factor in the animal foods, it is to be hoped that a diet can be found that still protects against atherosclerotic cardiovascular disease without increasing risk of stroke from disease in small and midsize cerebral vessels.


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 References
 
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