Updated: 12 November 2026, 01:34 UTC.
A large UK Biobank analysis published in 2026 found that a classification combining body-fat percentage and waist circumference identified risk patterns that BMI alone did not. In the study, 32.6% of people in the highest-risk adiposity group had a BMI in the normal-to-overweight range. The result reinforces a practical point for UK readers: BMI can help screen, but it cannot diagnose an individual’s health or reliably describe where fat is carried.
Current NHS advice already pairs BMI with waist-to-height ratio for many adults, while NICE advises lower BMI thresholds for several ethnic groups and caution when interpreting BMI in older adults and people with high muscle mass. These approaches add context; they do not make any single measurement a complete health assessment.
Body mass index is calculated by dividing weight in kilograms by height in metres squared. It is quick, inexpensive and useful for describing patterns across large populations. Its simplicity is also its limit: two people with the same BMI can have different proportions of muscle and fat, and fat in different places.
A muscular person may fall into a higher BMI category without having excess body fat. Conversely, someone whose BMI is in the usual healthy range may carry more fat around the abdomen or have less muscle than the number suggests. BMI also cannot show blood pressure, blood sugar, cholesterol, fitness, mobility or symptoms. It is therefore a screening signal, not a verdict on whether a person is healthy.
The 2026 longitudinal analysis used data from 489,311 UK Biobank participants and followed health outcomes for a median of 13.1 years. Researchers combined body-fat percentage with waist circumference to create five adiposity risk groups, then compared these with BMI categories. The highest-risk group had higher observed rates of type 2 diabetes, chronic kidney disease and major cardiovascular events than the reference group. More than three in ten people in that highest-risk group had BMI in the normal-to-overweight range.
This is evidence of discordance between measures, not proof that BMI causes missed diagnoses in every clinic or that the alternative classification should replace current practice. UK Biobank is an observational cohort, and its participants do not represent every age or population equally. The study supports taking a broader view of adiposity and risk, while further work is needed to establish how combined measures should be used in routine care.
The NHS says that for most adults a BMI of 25 to 29.9 is classed as overweight and 30 or above as obese. It explicitly notes that BMI calculators cannot distinguish muscle from fat. For adults with BMI below 35, it suggests waist-to-height ratio as an additional way to assess central fat. The NHS rule of thumb is to keep waist measurement below half of height.
NICE says cardiometabolic risk can occur at a lower BMI among people with South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean backgrounds. For these groups, its practical thresholds are BMI 23 to 27.4 for overweight and 27.5 or above for obesity. NICE also recommends caution for adults with high muscle mass and for people aged 65 and over, considering other conditions and functional capacity.
Use BMI as one piece of information. If appropriate, measure waist-to-height ratio using a consistent method, and discuss the result with a GP or other clinician alongside your medical history, symptoms and measurements such as blood pressure, glucose and cholesterol. Body-composition tests may be useful in some clinical settings, but they are not necessary for everyone and can have their own limitations.
Do not make a health decision from one number alone, and do not assume that a BMI in the “healthy” range rules out risk. If you have concerns about weight, diabetes risk, cardiovascular health, mobility or an eating disorder, speak with a healthcare professional. NHS guidance recommends seeking advice when a higher BMI is accompanied by concerns such as a waist greater than half your height or conditions including high blood pressure, pre-diabetes or sleep apnoea.
Information checked: 12 November 2026, 01:34 UTC. The study is observational and does not establish an individual diagnosis.
No. BMI estimates weight relative to height, but it cannot distinguish fat from muscle or show fat distribution.
NHS guidance recommends considering waist-to-height ratio for many adults with BMI below 35. A waist measurement below half your height is the simple guide.
No. Treat it cautiously and discuss the full picture with a clinician, including symptoms, medical history and other relevant measurements.
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