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Health & BMI FAQs

Q: Who invented BMI and what was it originally used for?

BMI was developed in the 1830s by Belgian mathematician Adolphe Quetelet, who called it the Quetelet Index. He was a statistician studying population averages, not a doctor diagnosing patients. The metric was popularised for clinical screening in the 1970s by physiologist Ancel Keys, who chose it because it only requires height and weight, making it cheap and quick to use across very large populations.

Q: How often should I check my BMI?

For most adults, recalculating BMI every 3 to 6 months is plenty unless your weight is changing rapidly. Daily fluctuations mostly reflect water, food and bowel content rather than fat. Weighing weekly under the same conditions (in the morning, after the loo, before food) and recalculating monthly gives a far clearer trend than obsessing over single readings.

Q: Is BMI accurate for muscular people or athletes?

No, BMI consistently overestimates body fat in muscular people because muscle is denser than fat. A rugby player at 1.83 m and 105 kg has a BMI of 31.4, classifying them as obese, even when their body fat sits below 12 percent. If you lift weights regularly or play a strength sport, pair BMI with a body fat percentage estimate or a waist measurement for a more honest picture.

Q: Can my BMI be normal but I still be unhealthy?

Yes, this is sometimes called normal-weight obesity. Someone with a BMI of 22 but very little muscle and 30 percent body fat can have insulin resistance, raised cholesterol and elevated cardiovascular risk despite a healthy-looking number. A normal BMI also does not protect against high blood pressure, smoking, poor sleep or sedentary behaviour, all of which independently affect long-term health.

Q: Why are BMI cut-offs lower for Asian populations?

People of South Asian, Chinese and Japanese descent tend to carry more visceral fat at any given BMI, so health risks like type 2 diabetes and heart disease appear at lower numbers. Many Asian health authorities use an overweight threshold of 23 instead of 25 and an obesity threshold of 27.5 instead of 30. The 18.5 to 24.9 normal range still applies broadly, but the upper end deserves more caution.

Q: How is BMI different for children and teenagers?

Children and teens use BMI-for-age percentile charts rather than the fixed adult categories, because their bodies are still growing. A child between the 5th and 84th percentile is considered a healthy weight, 85th to 94th is overweight, and 95th and above is obese. The same BMI number can be perfectly healthy at 8 years old and overweight at 18, so charts must be specific to age and sex.

Q: When should I see a doctor about my BMI?

Book an appointment if your BMI is below 18.5 or above 30, if it has changed by more than 2 to 3 points in a few months without you trying, or if you have a family history of heart disease or diabetes. Symptoms like persistent fatigue, breathlessness or joint pain alongside an unusual BMI also warrant a check. A healthcare provider can combine BMI with blood tests and waist measurement for a fuller assessment.

Q: What is the difference between "clinical" and "preclinical" obesity?

In 2025, a Lancet Commission proposed splitting obesity into two categories instead of relying on BMI alone. "Clinical obesity" means excess body fat that is already causing organ or tissue dysfunction — for example reduced mobility, breathlessness, or metabolic complications. "Preclinical obesity" means excess fat is present but has not yet impaired how the body works, representing a raised risk rather than an active illness. The idea is to diagnose and treat based on actual health impact rather than a number on the BMI scale.

Q: What is BMI and how is it calculated?

BMI (Body Mass Index) is a numerical value calculated by dividing your weight in kilograms by your height in metres squared: BMI = kg/m². For imperial units: BMI = (lbs ÷ in²) × 703. A BMI of 18.5–24.9 is normal weight, 25–29.9 is overweight, and 30+ is obese. BMI was created by Adolphe Quetelet in the 1830s and adopted by the WHO as the standard weight classification system. It cannot distinguish between muscle and fat.

Q: Can you have a normal BMI but high body fat?

Absolutely, and the condition is often called normal-weight obesity or being skinny fat. A sedentary 1.78 m office worker weighing 71 kg has a BMI of 22.4, but their body fat could be 30 percent or higher if they have very little muscle. This pattern carries the same metabolic risks as visible obesity, including insulin resistance and unfavourable cholesterol, even though the scale and BMI chart say everything is fine.

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