Understanding BMI and weight classification
| BMI (kg/m²) | Classification | Health risk |
|---|---|---|
| <18.5 | Underweight | Increased (malnutrition) |
| 18.5–24.9 | Normal weight | Low |
| 25.0–29.9 | Overweight | Mildly increased |
| 30.0–34.9 | Obese (Class I) | Moderately increased |
| 35.0–39.9 | Obese (Class II) | Severely increased |
| ≥40 | Morbidly Obese (Class III) | Very severely increased |
Note: BMI has limitations. It does not distinguish muscle from fat. A muscular person may have a high BMI without excess fat. For Asian populations, risk thresholds are lower (overweight: BMI ≥23; obese: BMI ≥27.5).
Waist circumference: why it matters more than weight
| Measurement | Men | Women |
|---|---|---|
| Low risk | <94 cm (<37 in) | <80 cm (<31.5 in) |
| Increased risk | 94–102 cm | 80–88 cm |
| High risk | >102 cm (>40 in) | >88 cm (>35 in) |
Central obesity (excess belly fat around organs) is more metabolically dangerous than subcutaneous fat (under the skin). A high waist circumference indicates visceral fat and predicts metabolic risk better than BMI alone.
Blood tests essential in obesity
| Test | Why ordered |
|---|---|
| Fasting glucose + HbA1c | Diabetes / pre-diabetes: extremely common in obesity |
| Fasting insulin + HOMA-IR | Insulin resistance (often precedes diabetes by years) |
| Lipid profile | Obesity raises triglycerides, lowers HDL, raises LDL |
| TSH | Hypothyroidism can contribute to weight gain |
| LFT (ALT, AST) | NAFLD (fatty liver) is present in ~75% of obese patients |
| Uric acid | Obesity raises uric acid and gout risk |
| Vitamin D | Deficiency is extremely common in obesity (stored in fat) |
| Sleep study (if snoring/apnoea) | Sleep apnoea affects 40% of obese adults |
Health conditions caused or worsened by obesity
- Type 2 diabetes: 80% of people with T2D are overweight or obese
- Cardiovascular disease: heart attack and stroke risk doubled
- Hypertension: 60% of obese people have high blood pressure
- Non-alcoholic fatty liver disease (NAFLD): 75% prevalence in obesity
- Obstructive sleep apnoea: most cases are obesity-related
- Osteoarthritis: each kg of weight lost reduces knee load by 4 kg
- Cancers: breast, colon, uterine, kidney and oesophageal cancer all increased
- PCOS: obesity worsens hormonal imbalance and insulin resistance
- Depression and anxiety: bidirectional relationship with obesity
Questions to ask your doctor
- What is my BMI and waist circumference?
- Do I have pre-diabetes or insulin resistance?
- Should I have a sleep study for sleep apnoea?
- Am I a candidate for weight loss medication (GLP-1 agonists)?
- Would I benefit from bariatric surgery?
References
The clinical information on this page is drawn from peer-reviewed sources indexed by the US National Library of Medicine. Links go to the source so you can read it yourself.
- Obesity and Comorbid Conditions. In: StatPearls. Treasure Island (FL): StatPearls Publishing. NCBI Bookshelf NBK574535
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Medical Disclaimer: This page is for general educational purposes only and does not constitute medical advice. Always consult a qualified doctor for diagnosis and treatment.