Tuesday May 19 2009
The 'obesity paradox' needs further research
“Overweight heart attack victims should stay fat as they are more likely to live longer”, the Daily Mail reported. It said that the controversial claim that being fat can be useful for heart attack patients has come from a review published in a journal.
This news report is based on a review that looked at selected studies describing the ‘obesity paradox’. The paradox is that, in some studies, overweight people have a better survival rate after heart disease than those who are a normal weight. The review itself does not claim that staying fat is good for health, but describes a few studies that have shown trends in people who already have heart disease.
The ‘obesity paradox’ will need more systematic research. People who have had a heart attack are advised to enrol in cardiac rehabilitation programmes where they will receive a physical activity programme and advice on the ideal diet for preventing future heart problems.
Where did the story come from?
The research was carried out by Dr Carl J Lavie, Richard V Milani and Hector O Ventura from the Ochsner Medical Center in New Orleans, Louisiana. Funding for the study is not reported. The study was published in the peer-reviewed Journal of the American College of Cardiology.
What kind of scientific study was this?
In this review, doctors from a cardiac rehabilitation and exercise laboratory discuss the impact of obesity on the risk of heart disease, including heart failure (HF), coronary heart disease (CHD), sudden cardiac death and atrial fibrillation, and how it is associated with reduced overall survival. They also investigate a phenomenon they refer to as the “obesity paradox”, in which overweight and obese people have a better survival rate after heart disease than those who are normal weight or underweight.
The authors do not report the methods they used in this review, but do refer to 87 other papers on obesity, its harmful effects and the various biological mechanisms and hormones involved.
What were the results of the study?
The authors begin by discussing how obesity might have an effect on cardiovascular health. They suggest that the hormone leptin, which is produced by fat cells and controls food intake and energy metabolism, may be particularly related to heart disease. They also refer to studies that suggest that C-reactive protein, a marker of inflammation that is associated with an increased risk of major cardiovascular events, such as heart attacks, may also play a role.
The authors list the adverse effects that obesity has on blood pressure, the lipids (fats) in the blood, the metabolism of glucose, the heart muscle and the lining of the blood vessels, and how it has an association with sleep disorders, arthritis and cancer. They say that obesity has reached global epidemic proportions in both adults and children. They say that, due to the harmful effect it has on various heart disease risk factors, obesity is also strongly associated with diseases such as heart failure, coronary heart disease, sudden cardiac death and atrial fibrillation. In addition, obesity is associated with increased overall mortality.
The authors say that numerous studies have documented an “obesity paradox” in which overweight and obese people who already have heart diseases, such as high blood pressure, heart failure or heart attacks, have a better prognosis compared with non-overweight or non-obese patients. They say that high blood pressure usually leads to a thickening of the muscular walls of the heart without increases in the volume of the pumping chamber (ventricle). However, obesity is conversely associated with an increase in chamber dilation without marked increases in wall thickness.
They refer to several studies that illustrate the “obesity paradox”. One example they mention is a recent study of 22,576 patients with known heart disease and high blood pressure. The study found that, after two years, overweight and obese patients had 30% lower deaths from all causes. This was despite their blood pressure being less well controlled than the normal weight group. They also refer to similar findings from selected studies in people with heart failure and peripheral arterial disease (narrowing of the arteries in the legs). However, the authors say that the pattern was not seen in studies of people with stroke, irregular heart beats or sleep disorders, which are all conditions that increase the risk of death for the obese.
What interpretations did the researchers draw from these results?
The researchers say that “overwhelming evidence supports the importance of obesity” as a cause of and contributor to the progression of heart disease. They say that, although a paradox exists, the data still support “purposeful weight reduction in the prevention and treatment of CV diseases”.
What does the NHS Knowledge Service make of this study?
This narrative review gives the authors’ views and also gives a presentation of both sides of the debate from selected papers on the topic. The suggestion that obesity may be protective in some cases is not new, and has been highlighted previously. There are several points to note about this research:
- As a narrative review it is not clear how the referenced papers were selected and if all the research, positive and negative, has been systematically identified and evaluated. This would require a systematic review.
- Although some suggestions are made about how obesity might protect the heart, these biological mechanisms are not yet certain and more research will be needed.
- The review is mostly based on observational studies. These types of study are subject to confounding, which means that there may be other explanations for the improved survival of obese people with heart disease. For example, obese people may be preferentially referred for cardiac rehabilitation or treated more intensively because of associated risk factors such as diabetes.
As the authors confirm, more research is needed in all of these areas. They warn that, if the current obesity epidemic continues, “we may soon witness an unfortunate end to the steady increase in life expectancy”.