Obesity is increasing in prevalence in many developed and affluent countries. By convention, the body mass index or BMI is used as an indicator of body fat and a measure of obesity. The BMI is derived from the formula of weight in kilograms divided by the square of the height in metres. The BMI is categorised as underweight (<18.5 kg/m2), normal (18.5 to 24.9 kg/m2), overweight (25 to 29.9 kg/m2), obese (30 to 34.9 kg/m2), severely obese (35 to 39.9 kg/m2) and very severely obese (> 40 kg/m2).
Obesity is associated with many medical conditions including heart disease, high blood pressure, diabetes mellitus, sleep related breathing disorders and joint disorders. Despite the adverse association with medical conditions, many studies have shown that obese patients with heart disease have better outcomes than those who are non obese.
Better survival in overweight heart patients
Overweight heart patients appear to have a lower risk of death compared with normal weight patients. A study involving more than a quarter million patients with heart disease by Romero-Corral in 2006 showed that those with a low BMI had the highest risk for death while overweight patients had the lowest risk.
Obese and severely obese patients had no increased risk compared with the normal BMI group. Other than using BMI as a measure of obesity, studies measuring body fat have demonstrated that in heart patients, low body fat is an independent predictor of an approximately three-fold higher risk of death.
Overweight and obese patients who undergo procedures or surgery for blocked heart arteries have better outcomes. A study by Oreopoulos published in 2008 in the American Heart Journal that included about 200,000 patients with heart disease showed that overweight and obese patients who had undergone a procedure to open the heart arteries with catheter based techniques had a lower short-term (one month) and long-term (up to five years) death from all causes compared with patients with a normal weight. Overweight and obese patients who had undergone heart artery bypass surgery had significantly lower short-term death and similar long-term death compared with patients with a normal BMI.
Lower incidence of complications for obese hypertensives
For heart patients who were medically managed, those who were overweight or obese had a significantly lower incidence of death compared with patients with a normal BMI. A 2009 study by Oreopoulos on more than 30,000 patients with proven heart artery disease on medical management, underweight patients had the highest risk of death, while patients who were overweight or obese (30.0 to 40 kg/m2) had the most favourable outcomes.
A study by Uretsky published in The American Journal of Medicine in 2007 on more than 22,000 hypertensive patients with heart artery disease showed that compared to those with a normal BMI, the occurrence of death, heart attack, or stroke was 30 per cent lower in the overweight, obese and severely obese patients. The obese patients had the lowest incidence of complications despite having smaller blood pressure reduction compared with patients of normal BMI at 24 months. Hypertensives with the lowest BMI had the highest risk of death.
Better outcomes in obese heart failure patients
Many studies have shown that obese heart failure patients had a better outcome than those with normal weight. In a study by Oreopoulos in 2008 involving more than 28,000 patients with heart failure, compared to normal BMI patients ,overweight and obese heart failure patients had reductions in cardiovascular death (decrease of 19 per cent and 40 per cent, respectively) and total death (decrease of 16 per cent and 33 per cent, respectively).
Skin fat versus abdominal fat
The mechanism for this obesity paradox effect, where being overweight or mildly obese appears to confer some benefit for heart patients as compared to underweight or normal weight patients, is not completely understood. In a 2012 American Heart Association update by Jean-Pierre Després, it was proposed that fat distributed in the subcutaneous layer of the skin around the hips had a different impact on the heart as compared to fat distributed around the waist and in the abdomen below the subcutaneous skin layer, and these differences in body fat distribution may explain the obesity paradox.
The presence of this excess intra-abdominal fat has been reported to be associated with diabetes mellitus, an unfavourable cholesterol profile, inflammation, increased risk of clot formation and abnormal blood vessel wall function.
Obesity and AF
Although an increased BMI appears to be beneficial in those with heart disease, for the general population, obesity does not confer a protective benefit and is associated with an increased likelihood of developing diseases and sudden death.
Obese patients have a higher risk of sudden cardiac death (SCD). In the Framingham Heart Study, the annual rate of SCD in the obese was nearly 40 times higher than the non obese group. The study also showed that for every 1kg/m2 increase in BMI, the risk of heart failure increased 5 per cent in men and 7 per cent in women. A review of 78,602 patients by Wanahita to assess the impact of obesity on an irregular rhythm, atrial fibrillation, which increases the risk of stroke significantly showed that obese patients had a nearly 50 per cent increased risk of developing atrial fibrillation.
Obesity also increases the risk of stroke with every 1kg/m2 increase in BMI being associated a 4 per cent increase in the risk of stroke (without bleeding) and 6 per cent increase in stroke with bleeding in the brain. Obese patients tend to have upper airway obstruction and decreased oxygen during sleep, a condition called obstructive sleep apnoea (OSA). OSA is associated with an increased risk of high blood pressure, abnormal heart rhythms, heart attacks, stroke and death.
Hence on balance, the apparent benefit obesity confers on heart patients may not be present in those with predominant intra-abdominal fat and will never surpass the benefit of not developing the medical conditions associated with obesity and not having an increased risk of SCD.
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