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Post-MI mortality risk lower among overweight/obese but increased for underweight individuals

A meta-analysis suggests post-MI mortality is reduced in those with BMI above normal but increased where an individual’s BMI is below normal

The post-myocardial infarction (post-MI) mortality risk is significantly lower among individuals deemed to be overweight or obese yet much higher where an individual’s body mass index (BMI) is below the normal range. This was the conclusion of a systematic review and meta-analysis by an international team led by researchers from Aberdeen University, Scotland, UK.

Body mass index (BMI) is used as a measure of nutritional status in adults and defined as a person’s weight in kilograms divided by the square of the person’s height in metres (kg/m2). Using the BMI, an individual is considered to be of normal weight when their BMI is being 18.5 and 24.9. According to the World Health Organisation, individuals with a BMI > 25 are classed as overweight and obese where their BMI is equal to or > 30). The American Heart Association has suggested that the presence of obesity leads to the development of cardiovascular disease and mortality, independently of other cardiovascular risk factors. Nevertheless, there is increasing evidence to suggest that patients with an elevated BMI may have better outcomes if they develop cardiovascular or renal disease and this represents the so-called “obesity paradox”. Despite this, meta-analyses show a J-shaped relationship between mortality and BMI in patients with coronary artery disease, i.e., the risk is highest for underweight and overweight individuals and lowest for those with a normal BMI.

Whether or not this same J-shaped relationship also applies to post-MI mortality is unknown and was subject of the present meta-analysis. The researchers looked for studies in adults with a previous myocardial infarction where the BMI had been measured and which reported on outcomes such as all-cause mortality, recurrence of an adverse cardiovascular event or hospital re-admission. Hazard ratios (HRs) were used to assess the risk of mortality and the risks to calculated with reference to the normal BMI (18.5 – 24.9)

Post-MI mortality and body mass index

A total of 27 articles with 308,430 participants and with follow-up periods ranging from 6 months to 17 years were included in the analysis.

Among individuals who were classed as overweight (BMI 25 – 29.9) compared to those of normal BMI and based on 8 studies, there as a 15% lower risk of post-MI mortality (Hazard ratio, HR = 0.85, 95% CI 0.76 – 0.94, p = 0.002). In 14 studies that reported odds ratios (ORs) rather than hazard ratios, there was also a reduced mortality risk (OR = 0.72, 95% CI 0.64 – 0.81, p < 0.0001).

Among obese patients (BMI > 30) the hazard ratio was 0.86 (95% CI 0.81 – 0.91, p < 0.0001) and for morbidly obese patients, although the HR was reduced, it was not statistically significant (HR = 0.89, 95% CI 0.78 – 1.01, p = 0.08).

In contrast, among those with a BMI < 18.5, there was a 42% higher risk of post-MI mortality (HR = 1.42, 95% CI 1.25 – 1.62, p < 0.0001) and again, where studies reported odds ratios, this was also significantly increased (OR = 2.48, 95% CI 1.77 – 3.47, p < 0.0001).

There were no significant effects when comparing the risk of hospital re-admission for those who were overweight, obese and underweight.

The authors concluded that individuals who have a BMI less than the normal range are at a significantly higher risk of post-MI mortality. They called for future studies to examine the prognostic utility of other markers of nutritional status in MI to better identify those at a higher risk of poor clinical outcomes.

De Paola L et al. Body Mass Index and Mortality, Recurrence and Readmission after Myocardial Infarction: Systematic Review and Meta-Analysis J Clin Med 2022

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