HOME
Low cholesterol increases stroke risk
Introduction
Stroke is defined as the sudden rupture or blockage by clotting of a blood vessel to or within the brain. Symptoms of stroke can range from minor dizziness or momentary temporary loss of function that is barely perceptible to paralysis or death. Strokes were generally confined to the very elderly, but recently strokes are being increasingly seen in younger people.
Asians traditionally eat a diet which is poor in animal products; they have low levels of cholesterol in their blood - and have a very high level of strokes. Japanese scientists believed that their diet could be the cause of their high stroke risk. United States scientists, on the other hand, were sceptical of the possibility, believing that there were few if any risks associated with low blood cholesterol. This difference in viewpoint was thought worthy of further investigation to resolve the question.
Stroke trials
Because medical trials are expensive, they tend to involve only a relatively small number of people. But there are exceptions. One such arose after World War II, when Japan's eating habits, influenced by American dietary habits, changed dramatically: from 1958 to 1995 fat consumption increased four-fold and animal protein intake in the form of meat, eggs and dairy products increased by 22%. At the same time, they ate less rice.
Not unexpectedly, cholesterol levels rose — by an average 0.57 mmol/L (22 mg/dL) in men and 0.75 mmol/L (29 mg/dL) in women.
This change of circumstances after the war was a perfect opportunity for scientists to witness any changes in disease patterns that this change of diet would bring.
They found that it brought dramatic benefits. During the period, cholesterol levels rose from averages of 3.9 mmol/l to 5.0 mmol/l,[1] and the numbers of strokes of both types fell by 85 % from 1344 to 205 per 100,000 per year.[2]
Deaths from cerebral haemorrhage also declined by 65% in men and 94% in women aged 40-69.[3]
And, even though blood cholesterol went up, there was no increase in heart disease deaths in either sex.
The authors conclude that the benefits to health in Japan were a result of blood pressure fall and a rise in total cholesterol.
Follow-up studies
A follow-up study by the Institute of Community Medicine, University of Tsukuba, Japan, over a 30-year period confirmed the earlier findings. By that time, heart disease deaths had also fallen by 20%.[4] A later study of both men and women aged 35 to 89 years over the period 1984 to 2001 showed clearly that the really beneficial part of the Japanese diet was their increased intakes of animal fat and cholesterol.[5] In this study, the risk of a stroke was reduced by almost two-thirds (63%) in those who had the highest intake of animal fat and cholesterol, compared to those with the lowest. Animal protein was not significant — just the fat and cholesterol.
These results were not confined to Japan or even to Asia. In 1997, the results of a follow-up of 350,000 men screened for the Multiple Risk Factor Intervention Trial in the United States showed that the excess risk of death from cerebral haemorrhage was 6 times higher in middle-aged men with low blood cholesterol levels.[6]
Scientists at the Cardiovascular Health Research Unit, Washington University, conducted an analysis of 13 randomised controlled trials to look at the effects of lowering blood cholesterol on diseases other than CHD.[7] They found that while lowering cholesterol resulted in a slight lowering of non-fatal strokes, fatal strokes were increased by 32%, and where cholesterol was lowered by a drug, the death rate from stroke rose by a huge 264%.
Because strokes and cerebral haemorrhages are rare in younger people, there has been little research done at these lower ages. However, cerebral haemorrhages are now affecting people younger than 40. Doctors at the Stroke Clinic, Instituto Nacional de Neurologia y Neurocirugia, Mexico City, looked into this new phenomenon.[8] They found a number of 'risk factors' including smoking, drinking alcohol and raised blood pressure. You might expect that high blood pressure posed the most risk, but that only accounted for 13%. The highest risk, at 35%, was low blood cholesterol levels.
The connection between low cholesterol levels and haemorrhagic stroke cannot be dismissed.
Low cholesterol also increases ischaemic stroke risk
There is no doubt the risk of haemorrhagic stroke is increased by low cholesterol levels, but that still leaves ischaemic strokes. Although the effect is similar to a haemorrhagic stroke, the cause is different. Instead of a blood vessel leaking in the skull, in this case, an artery is blocked by a clot.
For the same reason that heart attacks are blamed on cholesterol blocking coronary arteries, so are ischaemic strokes blamed on a fatty diet. But the theory that eating too much saturated fat increases the risk of a stroke, also seems to have no foundation. In 1997, we got a Christmas present from the long-running Framingham study when it published findings that for every 3% more calories from fat, ischaemic stroke risk went down by 15%.[9]
Confirmation came 6 years later in yet another study. This time, scientists at the Department of Nutrition, Harvard School of Public Health, studied 43,732 men aged 40-75 years who were free from cardiovascular diseases and diabetes in 1986 and followed them for 14 years.[10]
During that period they studied intakes of every conceivable type of fat: animal fat, fish oil, vegetable oil, saturated, mono-unsaturated, polyunsaturated, trans fat, as well as the amounts of each that were consumed. They found that none of them made any difference whatever and conclude:
'These findings do not support associations between intake of total fat, cholesterol, or specific types of fat and risk of stroke in men.'
To sum up, the published evidence refutes the accepted wisdom that high levels of cholesterol and a fatty diet lie at the root of strokes. It turns out that the opposite is true. Low cholesterol increases the risk of both types of stroke. Which is probably why the number of cases has been increasing — and being seen in ever younger people — since we were introduced to 'healthy eating'.
References
1. Adachi H, Hino A. Trends in nutritional intake and serum cholesterol levels over 40 years in Tanushimaru, Japanese men. J Epidemiol 2005; 15: 85-9.
2. Liu L, Ikeda K, Yukio Yamori Y. Changes in Stroke Mortality Rates for 1950 to 1997. A Great Slowdown of Decline Trend in Japan. Stroke 2001; 32: 1745
3. Shimamoto T, et al. Trends for Coronary Heart Disease and Stroke and Their Risk Factors in Japan. Circulation. 1989; 3: 503-15.
4. Iso H, Shimamoto T, Kitamura A, Iida M, Komachi Y. Trends of cardiovascular risk factors and diseases in Japan: implications for primordial prevention. Prev Med 1999; 29: S102-5.
5. Sauvaget C, Nagano J, Hayashi M, Yamada M. Animal Protein, Animal Fat, and Cholesterol Intakes and Risk of Cerebral Infarction Mortality in the Adult Health Study. Stroke 2004; 35: 1351.
6. Gillman MW, et al. Inverse association of dietary fat with development of ischemic stroke in men. JAMA 1997; 278: 2145-50.
7. Atkins D, Psaty BM, Koepsell TD, et al. Cholesterol reduction and the risk of stroke in men. A meta-analysis of randomized, controlled trials. Ann Int Med 1993; 119: 136-45
8. Ruiz-Sandoval JL, Cantu C, Barinagarrementeria F. Intracerebral hemorrhage in young people: analysis of risk factors, location, causes, and prognosis. Stroke 1999; 30: 537-41.
9. Gillman MW, et al. Inverse association of dietary fat with development of ischemic stroke in men. JAMA 1997; 278: 2145-50.
10. He K, et al. Dietary fat intake and risk of stroke in male US healthcare professionals: 14 year prospective cohort study. BMJ 2003; 327: 777-782
Last updated 1 December 2011
Related Articles
