Tuesday December 23 2008
“Being just a little overweight dramatically raises the risk of heart attacks,” reports the Daily Express. The paper claims that it is not only the obese who have a higher risk of heart attacks, but that “podgy” people also face an 11% increase in the risk of coronary disease.
The large study behind this story assessed data on 21,000 male doctors, which was collected over 20 years on average. Researchers wanted to see whether the doctors’ body mass index (BMI) and physical activity levels at the beginning of the study were linked to their risk of developing heart failure. They found that risk of heart failure increased in relation to excess weight.
The study has some shortcomings, but in general the findings are not unexpected: that there is an optimum weight for health (not underweight or overweight), and that physical activity is good for the circulatory system. The researchers sensibly conclude that public health initiatives which promote these facts may go some way to limiting the “scourge of heart failure”.
Where did the story come from?
This research was conducted by Dr Satish Kenchaiah, Dr Howard Sesso and Dr J. Michael Gaziano from Brigham and Women’s Hospital, Harvard Medical School, Massachusetts Veterans Epidemiology Research and the Veteran Affairs Boston Healthcare System.
The study was funded by the National Heart, Lung, and Blood Institute and the National Cancer Institute in the US, and it was published in the peer-reviewed medical journal Circulation.
What kind of scientific study was this?
This was a prospective cohort study investigating how BMI and physical activity levels might contribute to the risk of developing heart failure by following up 21,094 male doctors between 1982 and 2007.
Heart failure occurs when the heart becomes less efficient at pumping blood around the body. It can have severe consequences and may lead to death. Several problems can lead to heart failure, including valve diseases, high blood pressure or disease of the heart muscle itself.
Previous research has established that obesity (BMI of more than 30) increases the risk of heart failure. However, less is known about how physical activity and being overweight (or preobese) affect the risk of heart failure.
In this study, researchers followed up doctors already participating in the larger Physicians' Health Study (PHS), which was evaluating the use of low-dose aspirin and beta carotene for primary prevention of cardiovascular disease and cancer.
As part of the PHS study, baseline information on the doctors’ weight and height was taken. Their average age at entry into the study was 53 years. The doctors’ level of physical activity was also determined at baseline through a single question, which asked how often the doctors did sweat-inducing exercise each week. Possible answers were: rarely/never; one to three times a month; once a week; two to four times a week, five to six times a week or daily.
Through the PHS, the doctors reported health outcomes (including signs and symptoms of heart failure) every six months in the first year, and annually thereafter.
For this subsequent publication, researchers included those doctors who had participated in the PHS study and had information available on BMI and physical activity at baseline.
Researchers excluded men who reported heart failure before baseline, or were missing other information, including age, family history of heart disease, smoking status, alcohol consumption, and history of various conditions such as high blood pressure, diabetes mellitus and high cholesterol. This group comprised the 21,094 men included in this analysis.
The researchers determined whether the men’s baseline BMIs and their levels of reported physical activity was linked to their risk of heart failure during follow-up. They made several different calculations but took into account other factors that may be linked to heart failure risk, including age, smoking, alcohol, parental history of heart disease, treatment received during the original study, levels of exercise and health history.
What were the results of the study?
During the 20-year follow-up, 1109 men developed heart failure. The risk of heart failure increased in accordance with increasing BMI, with each 1kg/m2 associated with a 13% increase in the risk of heart failure.
When compared with lean men, preobese men were 1.49 times more likely to experience heart failure, while obese men were 2.8 times more likely. This pattern did not change when taking into account the amount of physical activity each man did.
The study also found that vigorous physical activity at least one to three times a month reduced the risk of heart failure by 18% after accounting for other factors that might explain this reduction. These factors included BMI, high blood pressure, diabetes and high cholesterol.
What interpretations did the researchers draw from these results?
The researchers conclude that a higher BMI was associated with a greater risk of heart failure in men. Vigorous physical activity was conversely associated with reduced risk of heart failure. Lean, active individuals had the lowest risk of heart failure, while obese, inactive people had the highest risk.
The authors say that while the majority of their findings are consistent with those of previous research, the link between preobesity and heart failure is significant, and has not been seen before in previous large studies.
What does the NHS Knowledge Service make of this study?
This large prospective cohort study followed male doctors for 20 years on average, and linked their baseline levels of physical activity and BMI with their risk of developing heart failure during that time.
The researchers have taken into account the fact that other variables such as cardiac symptoms, age and family history might be responsible for the increased risk of outcome, and they have adjusted accordingly for these. However, this study has its shortcomings, some of which the researchers acknowledge:
- Firstly, the study population were all male doctors, meaning the results may not be applicable to women and other social or economic groups (doctors may be generally more healthy, of higher socioeconomic status and have better access to healthcare, etc.).
- BMI and physical activity were only measured at one point in time, at the start of the study. These measures are unlikely to have remained constant during 20 years of follow-up. Individuals may have become more or less active, or have put on or lost weight during that time.
- There were too few underweight doctors in this study to conduct meaningful analysis of this group. Therefore, the effects of being underweight on heart failure risk remain unknown within this population.
- Also, although the researchers could show from their study that vigorous physical activity as little as one to three times a month reduces risk of heart failure, they cannot specify exact details on this exercise, such as the type of activity, the duration of exercise, or whether this activity was for work or leisure.
In general, the findings from this study are not unexpected: there is an optimum healthy weight (between underweight and preobese), and physical activity benefits the circulatory system.
The researchers sensibly conclude that public health initiatives which promote these facts may go some way to limiting the “scourge of heart failure”.