🧭 When to suspect

Obesity is a chronic, relapsing condition of excess adiposity that drives type 2 diabetes mellitus (T2DM), cardiovascular disease (CVD), several cancers, obstructive sleep apnoea (OSA), fatty liver and mechanical joint disease. The National Institute for Health and Care Excellence (NICE) guideline NG246 (2025) reframes it as a long-term condition needing ongoing, non-stigmatising support – not a one-off failure of willpower – and replaces the older CG189.

Identify and stratify using two measures together: body mass index (BMI), and – in any adult with a BMI under 35 – waist-to-height ratio (WtHR), which captures the central adiposity that BMI misses. Interpret BMI with caution: thresholds are lower in several ethnic groups, high muscle mass inflates it, and it should be read cautiously in people aged 65 and over. The two clinical tasks are to stratify risk and to exclude a secondary or sinister cause before attributing weight to lifestyle alone.

Classification	BMI (kg/m²)	Higher-risk ethnic groups*
Healthy weight	18.5–24.9	18.5–22.9
Overweight	25.0–29.9	23.0–27.4
Obesity – class 1	30.0–34.9	≥ 27.5
Obesity – class 2	35.0–39.9	32.5–37.4
Obesity – class 3 (severe)	≥ 40.0	≥ 37.5

*Apply lower thresholds (reduce by ~2.5 kg/m²) for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean family background, in whom cardiometabolic risk rises at a lower BMI.

Central adiposity (BMI < 35)	Waist-to-height ratio	Health risk
Healthy	0.4–0.49	No increased risk
Increased	0.5–0.59	Increased risk
High	≥ 0.6	Further increased risk

WtHR uses the same cut-offs for both sexes and all ethnicities and is summarised for patients as: “keep your waist to less than half your height” (ratio under 0.5). Be especially alert in those with a family history of obesity or T2DM, weight-promoting medication (antipsychotics, some antidepressants, corticosteroids, insulin/sulfonylureas), polycystic ovary syndrome (PCOS), hypothyroidism, poor sleep, low mood, and social deprivation.

Source: NICE NG246

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