Body mass index

BMI calculator

The evidence clearly demonstrates a non-linear relationship between BMI and mortality, with both insufficient and excessive body weight associated with increased mortality risk and reduced life expectancy.

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Understanding BMI and Life Expectancy

How body weight affects health, longevity, and disease risk

TL;DR

Both low and high BMI are linked with higher mortality; the lowest risk is around 22–25 kg/m² for most adults.
In older adults, slightly higher BMI (25–30 kg/m²) may be protective.
BMI is a simple screening tool but doesn’t distinguish between muscle and fat.
Waist size, fat distribution, and metabolic health are also important.
Long-term stability and overall lifestyle matter more than a single measurement.

What Is BMI?

BMI (Body Mass Index) is a simple number that helps estimate body fat based on height and weight.
It’s calculated by dividing weight in kilograms by height in meters squared:

BMI = weight (kg) / height (m)²

Although BMI doesn’t measure body fat directly, it’s widely used because it’s quick, inexpensive, and correlates with general health risks.

BMI Range (kg/m²)	Category	Common Interpretation
<18.5	Underweight	Higher risk of frailty, malnutrition
18.5–24.9	Normal weight	Lowest all-cause mortality
25–29.9	Overweight	Mildly increased risk; sometimes protective in older adults
30–34.9	Obesity class I	Increased risk of CVD, diabetes
35–39.9	Obesity class II	High risk of metabolic disease
≥40	Obesity class III	Severe health risk, reduced life expectancy

BMI and Longevity

Across hundreds of studies, BMI follows a U-shaped or J-shaped relationship with life expectancy.
Both very low and very high BMI values are associated with shorter lifespan. The lowest overall mortality risk is typically seen at BMI 22–25 kg/m² for most adults.

In people over 65, slightly higher BMI — 25–30 kg/m² — is often associated with better survival, likely due to greater muscle and energy reserves.

BMI Range	Mortality Risk Pattern	Notable Findings
<18.5	Increased risk	Linked to malnutrition, frailty, underlying illness
18.5–24.9	Lowest risk	Optimal longevity range for adults
25–29.9	Neutral to slightly increased	May be protective in older adults
≥30	Increased risk	Higher risk of heart disease, diabetes, cancer

How BMI Is Measured and Used

BMI is a screening tool, not a diagnostic one.
It should be interpreted alongside:

Waist circumference
Muscle mass and body composition
Blood pressure, glucose, and lipid levels

Alternative or complementary measures include:

Waist-to-hip ratio – reflects fat distribution
Body fat percentage – distinguishes fat from lean tissue
A Body Shape Index (ABSI) – adjusts for body shape and composition

Why BMI Matters for Health

A very high BMI increases the likelihood of:

Cardiovascular disease
Type 2 diabetes
Certain cancers (e.g., colon, breast, endometrial)
Sleep apnea and fatty liver disease

A very low BMI is linked with:

Nutrient deficiencies
Weak immune response
Increased infection risk
Muscle loss and frailty, especially in older adults

The “Obesity Paradox”

In people with chronic illnesses like heart failure, COPD, or kidney disease — and in older adults — being overweight or mildly obese sometimes predicts better survival.
This “obesity paradox” is partly due to:

Loss of muscle mass in low-BMI individuals
Reverse causation (illness causing weight loss)
Protective metabolic effects of extra fat reserves during illness

Factors That Influence BMI

Factor	How It Affects BMI
Genetics	Determines fat storage patterns and metabolism
Diet	Influences calorie balance and body composition
Physical activity	Builds muscle, improves insulin sensitivity
Age	Muscle mass declines, fat redistributes
Hormones	Thyroid and sex hormones affect metabolism
Stress and sleep	Alter appetite-regulating hormones (leptin, ghrelin)
Medications	Some antidepressants, steroids, and antidiabetics raise weight
Socioeconomic and cultural factors	Affect diet, stress, and lifestyle patterns

Testing and Tracking

BMI should be checked:

Every 1–2 years in adults
More often if you have weight-related conditions (e.g., diabetes, hypertension)
Alongside waist and metabolic markers for a full picture

Use consistent tools — same scale, similar time of day, and note trends, not one-off results.

Managing BMI Healthily

Approach	Typical Impact	Why It Works
Regular physical activity	↓ 2–5 BMI points over time	Burns fat, preserves muscle mass
Balanced diet (more fibre, fewer processed foods)	↓ BMI gradually	Improves energy balance and satiety
Sleep 7–8 hours	Supports stable weight	Regulates appetite hormones
Stress reduction	Prevents overeating	Lowers cortisol and emotional eating
Medical evaluation	Rule out thyroid or hormonal causes	Addresses underlying imbalances

Weight management should focus on health and function, not only numbers.
In older adults, preventing unintentional weight loss is just as important as reducing obesity.

Key Insights

BMI reflects general fatness, but not fat quality or distribution.
The healthiest BMI for longevity is about 22–25 kg/m².
Older adults may benefit from slightly higher BMI (25–30 kg/m²).
Extreme leanness and severe obesity both shorten life expectancy.
Track waist size and muscle mass, not just weight.

Limitations of BMI

Doesn’t separate fat from muscle (athletes may appear “overweight”)
Doesn’t reflect visceral fat distribution
Can misclassify individuals with high muscle or low muscle mass
Must be interpreted in context with other markers (e.g., glucose, lipids)

Resources for Further Reading

Full References

Sun Y et al. 2019. The BMJ. https://doi.org/10.1136/bmj.l1042
Bhaskaran K et al. 2018. Lancet Diabetes & Endocrinology. https://doi.org/10.1016/S2213-8587(18)30288-2
Aune D et al. 2016. The BMJ. https://doi.org/10.1136/bmj.i2156
Whitlock G et al. 2009. Lancet. https://doi.org/10.1016/S0140-6736(09)60318-4
Angelantonio E et al. 2016. Lancet. https://doi.org/10.1016/S0140-6736(16)30175-1
De González A et al. 2010. NEJM. https://doi.org/10.1056/NEJMoa1000367
Winter J et al. 2014. AJCN. https://doi.org/10.3945/ajcn.113.068122
Javed A et al. 2020. Obesity Reviews. https://doi.org/10.1111/obr.13035
Ng T et al. 2017. PLoS ONE. https://doi.org/10.1371/journal.pone.0180818
Carr P et al. 2023. Scientific Reports. https://doi.org/10.1038/s41598-023-29586-w
Ryg J et al. 2021. JAMDA. https://doi.org/10.1016/j.jamda.2021.07.015
Lin Y et al. 2020. Nutrition, Metabolism & Cardiovascular Diseases. https://doi.org/10.1016/j.numecd.2020.08.014
Lee D et al. 2018. The BMJ. https://doi.org/10.1136/bmj.k2575
Li J et al. 2021. PLoS ONE. https://doi.org/10.1371/journal.pone.0253696
Bioletto F et al. 2024. Nutrients. https://doi.org/10.3390/nu16203433
Nowak M et al. 2024. Journal of Clinical Medicine. https://doi.org/10.3390/jcm13082305
Veronese N et al. 2016. The BMJ. https://doi.org/10.1136/bmj.i5855
Ler P et al. 2022. BMC Public Health. https://doi.org/10.1186/s12889-022-13082-3
Kim N et al. 2015. PLoS ONE. https://doi.org/10.1371/journal.pone.0139924
Khan S et al. 2018. JAMA Cardiology. https://doi.org/10.1001/jamacardio.2018.0022
Lee E et al. 2017. Diabetes Care. https://doi.org/10.2337/dc16-1458
Qiu Z et al. 2025. International Journal of Obesity. https://doi.org/10.1038/s41366-024-01710-4
Tenda E et al. 2024. European Respiratory Review. https://doi.org/10.1183/16000617.0261-2023
Jones N et al. 2023. Heart. https://doi.org/10.1136/heartjnl-2023-322459
Zhang J et al. 2018. Clinical Research in Cardiology. https://doi.org/10.1007/s00392-018-1302-7
Xu F et al. 2025. NPJ Primary Care Respiratory Medicine. https://doi.org/10.1038/s41533-025-00431-z
Lv Y et al. 2022. Nature Aging. https://doi.org/10.1038/s43587-022-00201-3
Padwal R et al. 2016. Annals of Internal Medicine. https://doi.org/10.7326/M15-1181
Zaccardi F et al. 2016. Diabetologia. https://doi.org/10.1007/s00125-016-4162-6
Visaria A & Setoguchi S. 2023. PLoS ONE. https://doi.org/10.1371/journal.pone.0287218
Wiebe N et al. 2023. Obesity Reviews. https://doi.org/10.1111/obr.13588
Burgess S et al. 2024. BMJ Open. https://doi.org/10.1136/bmjopen-2023-081399
Lai Y et al. 2025. BMC Public Health. https://doi.org/10.1186/s12889-025-22932-9
Wu Y et al. 2025. Diabetes. https://doi.org/10.1111/dom.16325
Sun Y & Hu Y. 2025. Frontiers in Endocrinology. https://doi.org/10.3389/fendo.2025.1529004
Wu C et al. 2025. Endocrinology. https://doi.org/10.1210/endocr/bqaf040
Chiu P et al. 2023. PLoS ONE. https://doi.org/10.1371/journal.pone.0295919
Wei S et al. 2025. Diabetology & Metabolic Syndrome. https://doi.org/10.1186/s13098-025-01614-x

Disclaimer: This summary is informational only and not a substitute for medical advice.
Scientific data were collected and synthesised using the AI models and verified peer-reviewed studies.