The Daily Mail today reports that a new study has blamed a third of heart disease deaths on being fat. It also said that “one in seven non-fatal heart disease cases could be attributed to people being overweight or obese”.
This large 10-year study of about 20,000 people in the Netherlands found a link between obesity and higher waist circumference and risk of both fatal and non-fatal cardiovascular disease (CVD), such as heart attacks and strokes. The study’s strengths include its relatively large size and the fact that its BMI and waist data were collected using standard methods by trained staff. Its limitations include its reliance on national registries to identify cases of CVD. In addition, the results may not be representative of other populations, particularly the figures estimating how much CVD in a population is due to obesity and being overweight.
Although the exact figures may differ for different populations, this study highlights the importance of maintaining a healthy weight in reducing CVD.
Where did the story come from?
The research was carried out by Dr Ineke van Dis and colleagues from the National Institute for Public Health and the Environment, Wageningen University and the Netherlands Heart Foundation in the Netherlands. The study was funded by the Ministry of Health, Welfare and Sports of the Netherlands and the National Institute of Public Health and the Environment. It was published in the peer-reviewed European Journal of Cardiovascular Prevention and Rehabilitation.
This story has been reported by the Daily Mail, The Daily Telegraph and the Daily Mirror. The Daily Mail provides a reasonable report of the research, and the Mirror’s story is very brief but accurate. The Daily Telegraph initially focuses on the effects of waist circumference, saying that abdominal obesity is as bad for the heart as just being heavy, and that it increases four-fold the risk of dying from heart disease. This figure actually pertains to obesity measured by BMI and is from an analysis that took into account the participants gender and age.
What kind of research was this?
This was a prospective cohort study called the Monitoring Project on Chronic Disease Risk Factors (MORGEN). The researchers were particularly interested in how body mass index (BMI) and waist circumference are linked to non-fatal CVD. They say that previous studies looking at this question have largely relied on self-reported BMI and waist circumference, and that people may underestimate these measurements, which could affect results.
This type of study is the best design for investigating links between people’s characteristics and their risk of disease. Setting up a study to collect data from a group of people at a future date means that the data collected is more likely to be complete and accurate than if researchers had to rely on data previously collected by other people for other purposes. Because the different groups compared may differ in factors other than the one of interest (in this case BMI), such studies should always measure factors that could also be having an effect on the outcome and take these into account in their analyses.
What did the research involve?
The researchers enrolled about 20,500 adults (aged 20–65 years) between 1993 and 1997 in the Netherlands. These people had their height, weight and waist circumference measured using standard methods by trained staff. They also gave blood samples, had their blood pressure measured and filled in questionnaires about their health, lifestyle and other factors such as education. Over the follow-up period to the start of 2007 (an average of 10 years), the researchers identified people who had died and their cause of death, as well as any cases of non-fatal CVD from national registers.
Obesity was defined as having a body mass index of 30kg/m2 or over, and being overweight as having a body mass index of between 25 and 30kg/m2. A high waist circumference (indicating abdominal obesity) for women was defined as 88cm or higher, and a normal waist circumference was less than 80cm. A high waist circumference for men was defined as 102cm or higher, and a normal waist circumference was less than 94cm. Having measurements taken by trained staff in a standardised way increases the reliability of these measurements.
The researchers then looked at what effect being overweight or obese or having a high waist circumference had on the risk of fatal or non-fatal CVD. They took into account the participants’ gender and age when looking at the proportion of people who had CVD. When comparing the risk of CVD between the different groups, they also took into account smoking, educational level, blood pressure, cholesterol level and whether the participant reported having diabetes. They also calculated what proportion of obese or overweight people’s risk of CVD was caused by their weight, and also how much of the general population’s risk of CVD was related to weight.
The researchers lost 556 people to follow up, mainly because of emigration. They also excluded people who already had a history of heart attack at the start of the study, pregnant women, people with missing data and people with a BMI of less than 18.5kg/m2. This left 9,062 men and 10,572 women for analysis.
What were the basic results?
Overall, 42% of men and 30% of women in the study were overweight, and 10% of men and
11% of women were obese. Just over a quarter of women (26%) had a high waist circumference (abdominal obesity), and just under a fifth of men (19%). On average, the participants were followed for 10 years, during which time 728 men and 403 women were hospitalised for non-fatal CVD and 81 men and 30 women died from CVD.
Over 10 years:
- Non-fatal CVD occurred in: 16% of obese men and 6.8% of obese women; 8.7% of overweight men and 5.1% of overweight women; 5.9% of normal weight men and 2.6% of normal weight women.
- Fatal CVD occurred in: 2.6% of obese men and 0.9% of obese women; 1.1% of overweight men and 0.3% of overweight women; 0.4% of normal weight men and 0.2% of normal weight women.
For obese people, the risk of non-fatal CVD was about 1.3 times higher than for normal weight people, after taking into account factors that could affect the results (hazard ratio [HR] 1.3, 95% confidence interval [CI] 1.1 to 1.5). Obese people were also three times more likely to die from CVD (HR 2.9, 95% CI 1.7 to 5.1).
People with abdominal obesity had about 1.2 times the risk of non-fatal CVD and 2.1 times the risk of fatal CVD, compared to those with normal waist measurements.
The researchers calculated that about a half of the risk of fatal CVD and a quarter of the risk of non-fatal CVD in people who were obese or overweight was caused by their weight. In the general population, about a third of deaths from CVD and one in seven cases of non-fatal CVD were caused by being overweight and obese.
How did the researchers interpret the results?
The researchers conclude that the link between BMI and waist circumference and risk of CVD “were equally strong”. They also say that the link between those who are overweight and obese and fatal CVD is stronger than the link with non-fatal CVD.
This study provides further support for the link between being overweight or obese and an increased risk of heart disease. The strengths of this study include its relatively large size, standardised measurement of BMI and waist circumference by trained staff, and that it adjusted for factors that could have affected the results. Some of its limitations include the following:
- Although a number of factors that can affect the risk of CVD were taken into account, it is possible there is some residual effect of these or other unconsidered factors on the results.
- Outcomes were assessed based on national records. It is possible that these records may be missing for some people, or may contain some inaccuracies.
- About 1,000 people from the original sample were not included in the final analyses due to missing data, previous heart attack or loss to follow up. Inclusion of these people may have affected the results.
- Non-fatal CVD was identified by looking at hospital records, this would miss people who had CVD but did not go to hospital.
- The results may not be representative of different populations. The authors point out that their population was younger (average age 43) than those used in other studies. Also, in the general population, the proportion of fatal and non-fatal CVD that is caused by people being overweight and obese will depend on how common these conditions are. So, for example, if overweight and obesity are more common in a population they may account for more of the risk, but if they are rare they may account for less of the risk.
The study suggests that about a third of deaths from CVD and one in seven cases of non-fatal CVD could be prevented in the general population if people avoid becoming overweight or obese. These figures highlight the importance of maintaining a healthy weight.