Tuesday May 3 2011
The study looked at people with coronary artery disease
“People with coronary artery disease have an increased risk of death if they have fat around the waist,” BBC News has reported.
This news story is based on a systematic review that combined five observational studies which looked at different measures of obesity (BMI, waist circumference and waist-hip ratio) and the risk of mortality in almost 16,000 people with coronary artery disease. The research found that total weight measured by BMI was not associated with an increased risk of dying over the average 2.3-year study follow-up, but did find that storing fat around the waist increased the risk of death, even in people within a normal weight range.
This research is in keeping with advice that people should try to maintain a healthy weight, but it raises the question of whether weight around the middle – the apple shape – poses a particular risk. It also adds to the debate over whether waist-hip ratio and waist circumference are of equal, or possibly greater, importance than BMI – an unresolved issue that has been examined by numerous pieces of past research.
Where did the story come from?
The study was carried out by researchers from the Mayo Clinic in the US. No sources of external funding were reported. The study was published in the peer-reviewed Journal of the American College of Cardiology.
This study was reported accurately by the BBC News.
What kind of research was this?
This systematic review assessed which measures of obesity most accurately predicted survival rates in people with coronary artery disease.
Obesity is associated with increased risk of cardiovascular death, as well as of death due to other causes in the general population. There are various ways in which obesity is measured, including the body mass index (BMI), waist circumference (WC) and waist-hip ratio (WHR), which may better describe body fat distribution.
The researchers say that although obesity has been found to be associated with the risk of developing coronary artery disease (CAD), some studies have reported that lower BMI is associated with a higher risk of dying from CAD. This is known as the ‘obesity paradox’, and has largely been attributed to residual confounders (where other factors have contributed to the increased risk of death from CAD in thinner individuals).
The researchers were interested in how the risk of death from CAD is associated with WC and WHR, as these may be better indicators of ‘central obesity’ (fat around the middle of the body) than BMI.
What did the research involve?
The researchers searched various scientific and medical databases for entries between 1980 and 2008 that reported the association of either WC or WHR with mortality in patients who had established CAD.
The researchers looked at data from prospective cohort studies that had measured the WC or WHR of people with CAD and followed participants for at least six months. They also looked at studies where the risk of mortality during follow-up had been calculated using these obesity measurements.
The researchers pooled the data from five studies in order to calculate the risks of CAD associated with increased WC or WHR.
What were the basic results?
Of the five included studies, three had information on both WC and WHR, one measured WC only, and one measured WHR only. In the pooled analysis, there were 14,282 participants for which WC data was available and 12,835 subjects in which WHR data was available. In total, data was available for 15,923 participants.
The average age of all participants was 66 years, and 59% of them were men. Out of the 15,923 participants 6,648 were normal weight (BMI 18.5 to 24.9), 6,879 were overweight (BMI 25 to 29.9) and 2,396 were obese with a BMI of over 30.
On average, the participants were followed for 2.3 years, during which time there were 5,696 deaths.
The researchers looked at the range of values in the three measurements and divided the participants into three groups based on their WC, WHR and BMI, grouping them into the highest third, middle third and lowest third categories.
In men the cut-offs were:
- WC: lowest third below 89cm, second third over 89cm, highest third over 99cm
- WHR: lowest third below 0.94, second third over 0.94, highest third over 0.98
- BMI (kg/m2): lowest third below 24.1, second third over 24.1, highest third over 27.1
In women the cut-offs were:
- WC: lowest third below 84cm, second third over 84cm, highest third over 96cm
- WHR: lowest third below 0.86, second third over 0.86, highest third over 0.93
- BMI: lowest third below 23.7, second third over 23.7, highest third over 27.9
The researchers adjusted the participants’ data for age, gender smoking, diabetes, high blood pressure, heart failure and BMI. They found there was an association between having a WHR or WC in the highest or middle third and an increased risk of death compared to those people whose measurements were in the lowest third:
- highest WHR had a 69% increased risk (hazard ratio [HR] 1.69, 95% confidence interval [CI] 1.55 to 1.84)
- highest WC had a 29% increased risk (HR 1.29, 95% CI 1.20 to 1.39)
However, similar to the findings of some previous research studies, they found that the risk of death decreased with increasing BMI.
The researchers combined the WHR and WC data into a measure of ‘central obesity’ and found that people who were in the top two-thirds of carrying fat around their middles, had a 30.8% increased risk of mortality (43.2% in women, 19.4% in men). They also looked at participants who were of normal weight but carried more weight around their middle. They found that the risk of mortality associated with central obesity was 33.1% (61.5% for women and 19.9% in men).
People who had a high WC and a high WHR were 75% more at risk of dying during follow-up than people with low WC and WHR measurements (HR 1.75, 95% CI 1.57 to 1.95).
How did the researchers interpret the results?
The researchers found that carrying weight around the centre of the body was associated with increased risk of mortality in people with CAD, and this pattern was found in both people who were obese and those who were of normal weight but carried their weight predominantly around their middles.
The researchers found that BMI, which measures your weight relative to your height, was inversely associated with mortality in people with CAD, meaning that people with lower BMIs were at higher risk of mortality. They say that a relationship between increasing BMI and mortality has been demonstrated in the general population, but in people with CAD the association is more complex. The researchers say that the ‘association between fatness and mortality is complex and might rely more on measures of fat distribution than on the amount of body fat’, highlighting that, in their study ‘central obesity is associated with higher mortality even in individuals with a normal BMI’.
This systematic review pooled data from five studies and demonstrated that central obesity, measured by waist circumference or a higher waist-to-hip ratio, was associated with higher mortality in people with coronary artery disease. The research also showed that this increased risk was not seen with increasing BMI, and suggests that fat distribution rather than total fat is important in determining mortality risk in this group of patients with CAD.
The systematic review benefited from being able to pool data from a large number of individuals. However, as the data came from different studies, the participants’ characteristics and how data was collected may have varied greatly.
Overall, this study has shown that central obesity may be associated with an increased risk of mortality in patients with CAD. It is recommended that people maintain their weight within a healthy range to lower the risks of a multitude of diseases. This study again questions whether it is weight around the middle (the ‘apple’ shape) that is a particular risk factor, and whether waist-to-hip ratio and waist circumference are of equal, or possibly greater, importance than BMI – an issue that has been debated in numerous past research studies.