Guides
Every section we cover, with the explainers that go with it.
BMI and health risk
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BMI and cardiovascular risk
BMI is associated with cardiovascular disease, but most of the risk is mediated through blood pressure, lipids, and glucose. Central adiposity markers add information BMI misses.
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BMI and mortality: the curve
Observational studies show a J-shaped relationship between BMI and mortality. Reverse causation and smoking confound the low end; Mendelian randomisation suggests a more linear causal effect.
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BMI and type 2 diabetes risk
Type 2 diabetes risk rises with BMI in all populations, but the gradient begins at a lower BMI in Asian groups due to greater visceral fat and insulin resistance at the same weight.
Body composition
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Body fat percentage and healthy ranges
Body fat percentage ranges differ by sex, age, and measurement method. Learn the published ranges, why a BIA number is not a DXA number, and the error margins that come with every method.
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Waist circumference and what it means
Waist circumference thresholds differ by issuing body and population. Learn the NIH, WHO, and IDF cut-offs, where the tape goes according to each protocol, and the measurement error you should expect.
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Waist-to-height ratio
The NICE CG189 (2022) guideline advises keeping waist-to-height ratio below 0.5. Learn what this ratio predicts, how it differs from BMI, and the correct waist measurement protocol with its error margins.
Children and teenagers
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BMI through puberty
Why a teenager's BMI changes during puberty, the normal trajectory of adiposity rebound and growth spurt, and why adult cut-offs do not apply.
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How child BMI percentiles work
Why a child's BMI is compared with other children of the same age and sex, what a percentile actually means, and why it is not a body fat measurement.
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Reading a growth chart
How to read CDC and WHO growth charts, what centile lines represent, and what crossing percentiles means for a child's growth trajectory.
Measuring it properly
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The calculators
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The BMI calculator: what it does
The formula a BMI calculator runs, where the WHO 1995 classification categories came from, and how rounding during calculation can shift a person across a category boundary.
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The TDEE calculator: which equation and why
The TDEE calculator uses the Mifflin-St Jeor equation with FAO/WHO/UNU 2001 activity multipliers. Individual error against doubly labelled water is typically ±15–25%.
Understanding BMI
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Quetelet, Keys and how BMI became the standard
BMI began as Adolphe Quetelet's 19th-century statistical description of the average man. Ancel Keys named it in 1972, and the WHO adopted it as a screening standard in 1995.
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The BMI categories and where the cut-offs come from
The WHO adult BMI categories were fixed by an expert committee in the 1990s using epidemiological cut-points, not physiological thresholds. Every band names its source and year.
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What BMI is and how it is calculated
What What BMI is and how it is calculated claims, what owners report, and the one trade-off that decides whether it is the right machine.
Weight and energy balance
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Basal metabolic rate, explained
Basal metabolic rate is measured under strict conditions; prediction equations estimate it with individual error typically ±10–20%, and some groups face systematic bias.
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Total daily energy expenditure
Total daily energy expenditure has four components; the physical activity multiplier is the weakest link, introducing individual error typically ±15–25% against doubly labelled water.
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What a calorie deficit actually does
The 3500-calorie-per-pound rule is a static 1958 estimate; dynamic models show energy expenditure falls as mass is lost, and the first week's loss is mostly water.
When BMI reads wrong
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Athletes and muscular builds
How far BMI overstates health risk in athletes and muscular builds, with measured examples from published studies and alternative body composition methods.
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BMI in older adults
Why the healthiest BMI range shifts upward in older adults, how sarcopenia complicates BMI interpretation, and what measurements add useful information.
