Body mass index (BMI) is a numerical measure used to classify body weight relative to height. It is calculated by dividing weight in kilograms by height in metres squared (kg/m²). While it is not a direct measure of body fat or health, it is widely used as a population-level screening tool to identify underweight, healthy weight, overweight and obesity.
The standard BMI classifications used in the UK are: underweight (below 18.5), healthy weight (18.5 to 24.9), overweight (25 to 29.9), obese class I (30 to 34.9), obese class II (35 to 39.9) and obese class III or severe obesity (40 and above). For people of Black African, African-Caribbean, South Asian, Chinese and other Asian backgrounds, the health risks associated with excess weight occur at lower BMI values, and NICE recommends using lower thresholds (overweight from 23, obese from 27.5) for these groups.
Obesity is a significant cardiovascular risk factor. Excess body weight is associated with raised blood pressure, dyslipidaemia (abnormal cholesterol), type 2 diabetes, sleep apnoea and inflammation, all of which contribute to atherosclerosis and increase the risk of heart attack and sudden cardiac arrest. Weight management is therefore part of secondary prevention advice for many cardiac arrest survivors. Even modest weight reduction (5 to 10 percent of body weight) can produce meaningful improvements in cardiovascular risk factors.
BMI has recognised limitations: it does not distinguish between fat mass and lean muscle mass (so a muscular person may have a high BMI without excess fat), and it does not account for the distribution of body fat. Central adiposity (excess fat around the abdomen), measured by waist circumference or waist-to-hip ratio, may be a better predictor of cardiovascular risk than BMI alone. A waist circumference above 88 cm in women or 102 cm in men indicates substantially increased cardiovascular risk.
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