BMI, Waist Size, and Stroke Risk

Summarized by Robert W. Griffith, MD
August 28, 2007

Summary

A large Finnish study shows that adiposity increases the risk for total and ischemic stroke in both sexes, and higher waist circumference and waist-hip ratio predicts higher stroke risk in men, but not in women.

Introduction

Overweight has been recognized as a risk factor for cardiovascular disease for some time. But is it associated with a higher risk of stroke? And if so, what kind of overweight? And what kind of stroke? The answers to these three questions are to be found in a new study reported in the Archives of Internal Medicine.

A stroke is the loss of brain function due to an interruption in the flow of blood to all or part of the brain. There are two kinds of stroke: Ischemic stroke, caused by a clot that has traveled to the brain or within the brain and blocked the flow of blood, and hemorrhagic or bleeding stroke, caused by the rupture of a blood vessel within the brain. Ischemic stroke is the most common type, accounting for almost 80% of all strokes.

Body fat distribution is also an important risk factor for obesity-related diseases. The accumulation of fat in the middle part of the body gives rise to an android, or "apple shape" body, whereas accumulation in the hips and thighs produces a gynecoid, or "pear shape" body. Abdominal (or central) fat is associated with cardiovascular disease.

In the study, Finnish researchers analyzed 3228 strokes among almost 50,000 Finns aged 25 to 74. They wanted to examine the association of 3 different indicators of overweight - body mass index (BMI), waist circumference, and waist-hip ratio - with the risk of all strokes and different stroke types. Here's a summary of their findings.

What was done

Six separate surveys - in 1972, 1977, 1982, 1987, 1992, and 1997 - were done in Finland in 5 different areas of the country. The volunteers had to be free of a history of coronary heart disease or stroke at baseline. The final sample consisted of 23,967 men and 26,029 women.

Baseline data collected included information on socioeconomic factors, family history of stroke, and the presence of diabetes. Examinations done at baseline included height, weight, and blood pressure; waist and hip circumference were measured in the final 3 surveys only, along with administration of a modified food frequency questionnaire.

Information on deaths and non-fatal events were obtained from national Finnish sources. Strokes were classified into subarachnoid hemorrhage, intracerebral hemorrhage, and intracerebral infarction (death of brain tissue following blockage of an artery). The endpoints of interest in the analyses were 'incident stroke events', defined as either the first nonfatal stroke event or stroke-related death without a preceding nonfatal event.

For the purpose of analysis, the participants were classified at baseline according to their BMI: below 18.5 ('lean'), 18.5 to 25 ('normal'), 25.0 to 29.9 ('overweight'), and 30 or above ('obese'). They were also classified into 4 groups (quartiles) based on their waist circumference and into 4 quartiles using their waist-hip ratio.

What was found

The average follow-up period was 19.5 years. The average age at baseline was 46. As expected, the overweight and obese were slightly older, on average, and had higher blood pressures, cholesterol levels, a history of diabetes, a family history of stroke, and increased waist size, compare to those with a normal BMI. The latter, compared with the overweight and obese, had more education, smoked more, drank more alcohol, ate more vegetables, and were more physically active.

After adjusting the results to compensate for possibly confounding factors such as those listed above, it was found that increasing BMI was a risk factor for total and ischemic stroke in both men and women, compared with participants of normal weight. The hazard ratios¹ are given in the table:

	BMI = Normal	BMI = Overweight	BMI = Obese
Men
Total strokes	1.0 (reference)	1.23	1.59
Ischemic stroke	1.0 (reference)	1.27	1.70
H'agic stroke	1.0 (reference)	1.09	1.20
Women
Total strokes	1.0 (reference)	1.08	1.30
Ischemic stroke	1.0 (reference)	1.11	1.41
H'agic stroke	1.0 (reference)	1.00	0.97

It can be seen that the differences in total stroke risks were due chiefly to ischemic strokes; hemorrhagic strokes showed no increased risk at all in overweight and obese women, as defined by the BMI.

There were significantly increased risks of total and ischemic stroke with increased waist circumference and waist-hip ratio - seen in men, but not in women.

What the findings mean

We can now answer the three questions we posed in the Introduction to this article, based on the results of this study. Overweight is clearly associated with an increased risk of stroke. The type of overweight (or adiposity) appears to be abdominal, at least in men, given the positive association between waist size and waist-hip ratio with increased risk of stroke. And the type of stroke most closely linked to overweight is clearly ischemic, rather than hemorrhagic.

The findings support the view that overweight people have an increased likelihood of atherosclerosis, which can affect the vessels of the brain, among other areas. Abdominal adiposity is particularly risky (at least for men), and should trigger attempts at management: weight control through exercise and appropriate diet (using a low glycemic approach).

Source

• Body mass index, waist circumference, and waist-hip ratio on the risk of total and type-specific stroke. G. Hu, J. Tuomilehto, K. Silventoinen,  et al., Arch Intern Med , 2007, pp. 1420--1427

Footnotes
1. The hazard ratio describes the relative risk of the incident stroke event in comparison to the rate of the event in the 'normal' BMI group.

Related Links
Your Waist Guidelines
Can Eating Fruit and Vegetables Lower Your Risk of a Stroke?
Preventing a first heart attack or stroke with a statin

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