Obese and very obese patients have a lower risk of dying after they have been treated for heart attacks than do normal weight patients, according to new research.
Researchers in Germany and Switzerland found that amongst patients who had received initial treatment for a specific type of heart attack, those that were obese or very obese were less than half as likely to die during the following three years as patients who had a normal body mass index (BMI).
Dr Heinz Buettner, head of interventional cardiology at Herz-Zentrum, Krozingen, Germany, who led the study, said: "Although there is no doubt that people who are overweight, obese and very obese have a higher risk of developing diabetes, hypertension and coronary artery disease, the evidence from our study shows once a coronary event has occurred and been optimally treated, obese patients switch to a more favourable prognosis compared to normal weight patients."
Dr Buettner and his colleagues followed 1,676 consecutive patients in a prospective study who had been admitted to hospital suffering from unstable angina/non-ST-segment elevation myocardial infarction between 1996 and 1999. Coronary angiography was performed to diagnose the extent of the problem and then the patients were usually treated with early coronary revascularisation, primarily by inserting stents to widen the relevant artery, or by coronary artery bypass grafting.
A third of the patients (551) had a normal BMI, half (824) were overweight, and 18% (292) were obese or very obese . The obese and very obese patients tended to be younger, more likely to have hypertension and diabetes already, but less likely to have suffered an earlier heart attack. They were also more likely to be discharged from hospital after treatment with prescriptions for statins, ACE-inhibitors and beta-blockers.
"After three years of follow-up, we found that obese and very obese patients had less than half the long-term mortality when compared with normal BMI patients; 9.9% of normal BMI patients and 7.7% of overweight patients had died at the end of the three years, but only 3.6% of obese patients had died and no very obese patients had died. The reduction in mortality rates was consistent among all sub-groups and persisted after adjustment for a number of variables. These findings contrast with primary prevention studies that implicate BMI as a strong risk factor for mortality.
"The findings of this study complement and extend our knowledge regarding the impact of obesity on cardiovascular disease by suggesting that the prognostic impact of obesity is confounded by a cardiovascular event such as a heart attack. Approximately 2-2.5 million patients worldwide are hospitalised for unstable angina/non-ST-segment elevation myocardial infarction each year. Until now, the impact of obesity on outcomes after revascularisation treatment was unknown."
Dr Buettner said his study was limited to describing the association observed between obesity and improved survival, and it was not possible to say what might be causing it. "Further research is necessary to elucidate the underlying pathophysiological mechanisms responsible for the more favourable outcome in obese patients. Possible mechanisms might include treatment differences, lower age, endogenous cannabinoids, lower platelet counts and excess triglyceride content in heart tissue."
However, he believes that the adjustments they made when analysing the data rule out the possibilities that the survival effect could be a result of the obese patients being younger and more likely to be prescribed statins, ACE-inhibitors and beta-blockers on discharge from hospital. While the researchers did not have information on whether the obese patients embarked on a more vigorous programme of improved life style and weight loss after their discharge, Dr Buettner thought this probably did not affect their results either.
"Even without this additional information, our findings have considerable clinical impact as risk is typically assessed according to actual BMI rather than BMI changes. Survival in the different BMI groups started to differ significantly early on during follow-up; if body weight changes were likely to influence prognosis, a more delayed effect would be expected. Other studies have shown that even five years after coronary patients have been given advice on life-style changes, their overall BMI remains unchanged. However, our study is ongoing, and we hope that a larger cohort of patients will enable us to discover whether shifts in body weight have an additional impact on prognosis."
He said the clinical role of other potential mediators, such as the endogenous cannabinoid system (levels of endogenous cannabinoids were higher in obese patients), platelet count (which were lower in obese patients and which can affect clotting), or excess triglyceride (fat) content in heart tissue (higher levels of triglyceride might protect the damaged heart), were yet to be determined.
"Obesity was an independent predictor for reduced mortality. So any differences between obese and non-obese patients, which might have prognostic implications, are of interest. Current evidence suggests a central role of the endogenous cannabinoid system in obesity. The stimulation of the CB1 cannabinoid receptor subtype in the brain seems to be a key component in the development of diet-induced obesity and the brain level of endogenous cannabinoids increases with greater intake of food. A growing line of evidence indicates that endogenous cannabinoids can have protective roles in pathophysiological conditions such as shock, ischaemia and myocardial infarction."
He said that while the findings of his study provided important prognostic information for obese patients, people who were obese should not wait to have a heart attack before starting to make an effort to reduce their weight.
"Not all patients can be treated with early revascularisation because an acute coronary syndrome always has the risk of sudden cardiac death. It is well known that even a modest intentional weight loss can improve or prevent obesity-related cardiovascular risk factors like diabetes mellitus and arterial hypertension," he said.
Reference: The impact of obesity on mortality in UA/non-ST-segment elevation myocardial infarction. European Heart Journal, doi:10.1093/eurheartj/ehm220
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