We’ll probably all be familiar with the idea that raised cholesterol levels cause heart disease. The idea here is that high levels of cholesterol cause fatty deposits on the inside of the arteries around the heart. If one or more of vessels should block off completely, the heart muscle is starved of blood and, if this persists, the part of the heart supplied after the point of blockage will die. This is what a heart attack is, and the technical term for it is a ‘myocardial infarction’.
Narrowing of arteries doesn’t just occur around the heart. It can happen in other vessels too including those that supply blood to the brain. Blocking off of these vessels here can cause what is known as a stroke. However, a minority of strokes are caused not by blockage of vessels, but by bleeding from them. These two types of stroke are called ‘ischaemic stroke’ and ‘haemorrhagic stroke’ respectively.
In a way, we can think of ischaemic strokes as a ‘heart attack of the brain’. So, we might expect there to be about as strong a relationship between cholesterol levels and risk of stroke as there is said to be between cholesterol levels and risk of heart attack. Actually, previous research has found very weak or non-existent relationships between cholesterol levels and stroke [1,2]. The suggestion here, therefore, is that cholesterol does not cause strokes. In which case, it’s unlikely to cause heart attacks either.
Some researchers have suggested that the lack of a relationship between cholesterol levels and strokes is because studies have included haemorrhagic strokes as well as ischaemic strokes in their analyses. The idea here is that cholesterol does cause ischaemic stroke, but the effect is masked by including haemorrhagic strokes that are unlikely to have anything to do with cholesterol.
Well, now a study has been published which casts considerable doubt on this theory. It also asks serious questions about the conventional advice that our diet should be low in fat and high in carbohydrate.
The study in question was published earlier this month in the journal Annals of Neurology [3]. In this study, about 14,000 men and women were followed for more than 30 years. The relationship between cholesterol levels and risk of stroke was analysed. The analysis was confined to ischaemic stroke only (so no chance of haemorrhagic strokes diluting the results here).
The results showed:
- no relationship at all in women
- no increased risk of stroke in men unless cholesterol levels were extremely raised (9.0 mmol/l or more – equivalent to 348 mg/dl or more)
In other words, overall, there was little or no relationship between cholesterol levels and risk of ischaemic stroke. These findings suggest that cholesterol has little or no causative role to play in the development of ischaemic stroke.
This study also analysed the relationship between levels of blood fats known as triglycerides and stroke risk. The result? The higher the triglyceride levels, the higher the risk of stroke. In other words, risk of stroke was much more closely linked with triglycerides levels than cholesterol levels.
From a so-called ‘epidemiological’ study of this nature we cannot know if triglycerides cause ischaemic stroke, or are merely associated with an increase risk of this condition. However, it is worth bearing in mind that previous evidence has linked triglycerides with enhanced risk of heart disease and stroke [4].
So, what are triglycerides and where do they come from? Triglyceride is a form of fat used to transport fats around the bloodstream. Triglycerides are also the form of fat used to store fat in our fat tissue as well as elsewhere. Triglycerides in the bloodstream are made in the liver. The major stimulus for their manufacturing the liver is not fat, but carbohydrate (sugar and starch).
It’s well known that if individuals adjust their diet to contain less fat and more carbohydrate (as they’re often encouraged to do), triglyceride levels go up. In short, it seems conventional nutritional advice induces changes in the blood stream associated with enhanced risk of stroke.
The idea that we should be taking out saturated fat from our diets and replacing it with carbohydrate is based on the belief that saturated fat is bad for the heart and circulation and carbohydrate is better. I wouldn’t be too sure about that.
References:
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. Varbo A, et al. Nonfasting triglycerides, cholesterol, and ischemic stroke in the general population. Annals of Neurology. Article first published online: 18 FEB 2011
4. Kannel WB, et al. Triglycerides as vascular risk factors: new epidemiologic insights. Curr Opin Cardiol. 2009 Jul;24(4):345-50.
