‘Heart disease’ is a term usually used to describe the gradual ‘furring up’ of the arteries around the heart due to a process known as atherosclerosis (or arteriosclerosis). Eventually, one or more artery may become completely blocked, starving some of the heart muscle of blood. If this persists, heart muscle will die, and this is what a heart attack is.
Atherosclerosis can occur in other blood vessels too, including those supplying blood to the brain. This condition, termed ‘cerebrovascular’ disease, can ultimately result in a stroke through essentially the same process that underlies heart attacks. These forms of strokes are termed ‘ischaemic’ strokes (ischaemia is a term that refers to a lack of blood supply).
A minority of strokes are not caused by blockage of a blood vessel, but by bleeding from one. In this case, strokes are referred to as ‘haemorrhagic’ strokes.
‘Raised’ cholesterol levels are said to be a risk factor for heart disease. Given that most strokes are caused by the essentially the same process as heart attacks, one might imagine that raised cholesterol is a risk factor here too. However, in reality cholesterol levels appear to have little or no relationship to stroke risk [1,2]. In fact, previous evidence has found that lower levels of cholesterol, including ‘unhealthy’ LDL-cholesterol, are associated with an increased risk of haemorrhagic stroke .
I recently came across a new study that assessed the relationship between cholesterol levels and stroke. In it, researchers assessed the relationship between cholesterol levels in those who had suffered an ischaemic stroke and their survival over time. Individuals were classified as either having ‘high’ cholesterol (> 4.6 mmol/L/178 mg/dl) or ‘low’ cholesterol (levels lower than this), and survival was assessed 3 months, 1 year and 5 years later .
For those with low cholesterol, overall survival at these respective time points were 92, 87, 57 per cent. Survival for those with high cholesterol was significantly better, though: 100, 98, 84 per cent respectively. Overall, lower levels of cholesterol were associated with an increase in risk of death of about 90 per cent.
In summary, we have evidence that:
- cholesterol levels have little or no relationship with overall stroke risk
- low cholesterol levels are associated with increased risk of haemorrhagic stroke
- higher cholesterol levels are associated with improved survival in those who have suffered an ischaemic stroke
All of this evidence is ‘epidemiological’ in nature, which means that while it tells us about the association between cholesterol levels and health outcomes, it cannot be used to conclude that higher cholesterol levels have a protective effect. However, these facts clearly call into question the conventional wisdom regarding the role of cholesterol in cardiovascular diseases such as heart disease and stroke. They also should cause us to question the idea that lower cholesterol levels are something all of us should aspire to.
1. Cholesterol, diastolic blood pressure, and stroke: 13,000 strokes in 450,000 people in 45 prospective cohorts. Prospective studies collaboration. Lancet 1995;346(8991-8992):1647-53.
2. Imamura T, et al. LDL cholesterol and the development of stroke subtypes and coronary heart disease in a general Japanese population: the Hisayama study. Stroke 2009;40(2):382-8.
3. Wang X, et al. Cholesterol levels and risk of hemorrhagic stroke: a systematic review and meta-analysis. Stroke. 2013;44(7):1833-9.
4. Markaki I, et al. High Cholesterol Levels Are Associated with Improved Long-term Survival after Acute Ischemic Stroke. J Stroke Cerebrovasc Dis. 2013 Oct 5 [Epub ahead of print].
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