Thursday April 8 2010
A stroke can have a devastating effect
“Strolls can cut stroke in women,” according to the Daily Mirror, which said that brisk walking for longer than two hours a week cuts the risk of a stroke by more than a third for women. According to the newspaper, walking is better at combating strokes than more vigorous forms of exercise.
The study behind this report followed nearly 40,000 women for 12 years, looking at the association between their exercise habits and their risk of having a stroke. The findings of the study are difficult to interpret as they were only of borderline significance and the research has some shortcomings. The researchers say that their results are surprising, as vigorous exercise did not seem to be linked to reduced stroke risk.
A number of factors raise the risk of having a stroke, including age, being male, family history of strokes, diabetes, high blood pressure, high cholesterol and smoking. Modifying lifestyle factors, such as reducing alcohol consumption, staying active and eating a balanced diet, may help reduce the risk of weight gain and, in turn, the risk factors associated with vascular diseases, such as strokes. While the association between exercise and strokes may not be firmly established, other research has provided a body of evidence supporting the wider health benefits of exercise.
Where did the story come from?
The study was carried out by Dr Sattelmair and colleagues from Harvard School of Public Health and other academic institutions in Boston, USA. The study was funded by the US National Institutes of Health and published in the peer-reviewed medical journal Stroke.
Newspapers generally reflected the findings of this research accurately.
What kind of research was this?
This was a cohort study that examined the association between physical activity levels and the risk of having a stroke. It followed 39,315 healthy American women aged over 44 who had participated in a previous randomised controlled trial (RCT), called the Women’s Health Study. During the study, the participants were followed for about 12 years and the links between various factors, including exercise and stroke outcomes, were assessed. The researchers suggest that exercise is a “promising modifiable risk factor” for strokes, but studies assessing the link have so far had inconsistent results.
This study was designed to investigate the association between physical activity and strokes in a large group of women and to explore whether different types of physical activity are linked with different types of stroke.
What did the research involve?
The Women’s Health Study was a randomised controlled trial conducted between September 1992 and May 1995, which investigated the effect of low-dose aspirin and vitamin E on risk of cardiovascular disease and cancer. Eighty-eight per cent of the women in the original study agreed to continue participating in the longer-term observational study, which provided the data used in this research paper.
The data available to the researchers was the baseline physical activity data, collected using a survey given to all participants at the beginning of the study. The survey asked for details of average time spent on eight recreational activities – such as walking or hiking, dancing, cycling, aerobic exercise and swimming – during the previous year. Similar questions about activity were asked at 36, 72 and 96 months and again at the end of the randomised controlled trial, then during the observational follow-up period. The researchers then estimated the energy expended on each of the activities.
Other information available from the baseline survey included age, weight, height, smoking, diet, menopause, number of children and medical history. Women were categorised as being of normal weight, overweight or obese. Other variables were used in the analyses to adjust for confounding factors that may affect the link between exercise and strokes. Stroke outcomes were ascertained by reviewing the women’s medical records, including death records to measure fatal strokes.
The researchers used an analytical method called survival analysis. This is used to determine how strongly an exposure (in this case exercise) is associated with an outcome (in this case having a stroke). This is an appropriate method because it allows researchers to make adjustments to account for the influence of confounding factors, which may affect the relationship being studied. Generally, the association between physical activity and strokes was measured by examining the link between women’s energy expenditure in leisure-time activity and their risk of having a stroke. In these analyses, energy expenditure was grouped into four ranges (expressed in kcal/week): less than 200, 200-599, 600-1,499 and 1,500 or more per kcal/week.
The researchers also examined the link between strokes and vigorous physical activities, comparing categories of women expending various amounts of energy on vigorous activity with those who did no vigorous activity and spent little energy doing other activities. They also did a separate analysis of the link between walking (a moderate-intensity activity) and stroke risk using data only on those women who did not report any vigorous activity (22,862 women). In this analysis, women were placed into four groups depending on the total time spent walking each week and their usual walking pace.
Other analyses looked at the role of body mass index (BMI) in the associations and how changes in walking activity affected stroke risk.
What were the basic results?
During the follow-up, a total of 579 strokes occurred in the 39,315 women. When the researchers fully adjusted for all measured confounders (including age, treatment received in the RCT, smoking, alcohol consumption, diet and medical history), they found no statistically significant link between the risk of a stroke and any of the four leisure-time energy expenditure levels. The findings were similar when the authors analysed the results by the type of stroke: haemorrhagic (caused by a bleed) or ischaemic (caused by a blood clot). Neither overall stroke risk nor risk of individual stroke types was associated with weekly energy expenditure during vigorous physical activity.
When assessing the link with walking, the researchers reported a significant trend between increased time spent walking, increased pace of walking and the overall risk of a stroke, although this association was weakened when the analysis was fully adjusted for confounders. When analysing types of stroke separately, the trend seemed only apparent for haemorrhagic stroke. Compared to the women who did not walk regularly, those who walked for two or more hours a week were 0.43 times less likely to have a stroke (multivariable-adjusted relative risk 0.43, 95% confidence interval 0.20 to 0.89).
How did the researchers interpret the results?
The researchers say that they have found an association of borderline significance between total leisure-time physical activity and stroke risk. They also note that both time spent walking and usual walking pace had a significant relationship with the overall risk of a stroke and risk of a haemorrhagic stroke. Energy expenditure also had a borderline significant link with an ischaemic stroke.
The researchers conclude that the study shows a “tendency for leisure-time physical activity to be associated with lower stroke risk in women. In particular, walking was generally associated with lower risks of total, ischaemic, and haemorrhagic stroke”.
This large cohort study assessed the association between the risk of having a stroke and levels of energy expenditure. There are several strengths to the study, including the large number of participants and the fact that the levels of physical activity were updated during the course of follow-up (it was not assumed that women’s energy expenditure at the start of the study would remain constant throughout the course of the study).
However, there are a number of points to consider when interpreting these findings:
- The researchers found few statistically significant results overall. In the fully adjusted models, which took into account a full range of important confounders, there was no statistically significant link between any intensity of leisure-time activity and the risk of a stroke (total stroke or haemorrhagic/ischaemic).
- The study did find two significant associations: between greater than two hours of walking (compared with not walking regularly) and the risk of a haemorrhagic stroke, and between a usual walking pace of 4.8km/hour or more (compared with not walking regularly) and the risk of a haemorrhagic stroke. However, haemorrhagic strokes are the less common type of stroke, so these analyses were in small groups (10-31 cases) and should be interpreted with caution.
- The researchers have reported results inconsistently, sometimes focusing on the partially adjusted results and sometimes on the fully adjusted results. Generally, in the fully adjusted models, the associations between energy expenditure and stroke risk were weakened.
- The researchers state that it is “not entirely clear” why an association between walking and stroke risk was observed but not an association between vigorous-intensity activity and stroke risk.
- Some important confounders were not assessed in this study, including participants’ blood pressure (although the women would have reported whether or not they had a history of hypertension). The researchers raise the possibility of residual (unmeasured) confounding as an issue. They also say that another potential weakness is the reliance on self-reported measures of physical activity and other confounders.
In general, this study suggests at best that there is a limited association between physical activity and stroke risk and the findings are difficult to interpret given the borderline significance in most of the analyses.