BMI Is a Terrible Measure of Health. Here's What to Track Instead.

BMI — Body Mass Index — is calculated by dividing your weight in kilograms by your height in meters squared. It was developed in the 1830s by Adolphe Quetelet, a Belgian mathematician and statistician who was studying population averages, not individual health.

Quetelet was explicit that his index wasn't meant to assess individual health. He was describing the average adult male, mathematically. Somewhere along the way, medicine adopted it as a health screening tool, embedded it in electronic health records, and used it to determine insurance premiums and treatment eligibility.

The problems with BMI as a health metric are well-documented, frequently criticized, and almost completely ignored in practice. Here's why it matters — and what to use instead.

The Core Problems With BMI

It doesn't distinguish muscle from fat. A 200-pound person with 15% body fat and a 200-pound person with 35% body fat have identical BMIs. Their metabolic risk profiles are completely different. Athletes and highly muscular individuals routinely fall into "overweight" or "obese" categories despite exceptional metabolic health.

It doesn't measure fat distribution. Where you carry fat matters enormously for metabolic risk. Visceral fat — stored deep in the abdomen, surrounding organs — is metabolically active and associated with insulin resistance, cardiovascular disease, and inflammation. Subcutaneous fat — stored under the skin at the hips, thighs, and arms — is far less metabolically dangerous. Two people with the same BMI can have radically different visceral fat levels and correspondingly different health risks.

It was derived from white European males. This isn't a minor limitation. Different ethnic groups have different body composition at the same BMI. South and East Asian populations have higher visceral fat percentages at lower BMIs, meaning the traditional "overweight" threshold (25) understates risk. African American individuals often have higher bone and muscle density than average, meaning BMI overstates risk. The cut-points are biologically meaningless for large portions of the population.

It creates false categories. BMI categories (underweight, normal, overweight, obese) have sharp cut-offs that have no equivalent in biology. Someone at BMI 24.9 ("normal") isn't meaningfully different from someone at 25.1 ("overweight"). But one gets a clean bill of health and one gets a referral and a lecture.

The metabolically obese normal weight problem. Roughly 20-30% of people in the "normal" BMI range are metabolically unhealthy — with insulin resistance, elevated visceral fat, dyslipidemia, or early-stage cardiovascular changes. They're invisible to BMI screening. Conversely, 20-35% of people in the "overweight" and "obese" BMI categories are metabolically healthy. They get the diagnosis and the stigma, often without the associated risk.

What Actually Predicts Metabolic Risk

Waist circumference and waist-to-height ratio.

Waist circumference is a simple, cheap, and clinically validated proxy for visceral fat. The evidence is clear: waist circumference predicts cardiovascular risk better than BMI.

Thresholds with clinical relevance: - Men: >40 inches (102 cm) — elevated risk - Women: >35 inches (88 cm) — elevated risk - European men: >37 inches (94 cm) — elevated risk

Waist-to-height ratio (waist circumference ÷ height, both in the same unit) is even better. A ratio below 0.5 is generally associated with good metabolic health across populations. Above 0.5, risk increases progressively.

The practical advantage: a tape measure costs nothing, takes 30 seconds, and gives more actionable information than a BMI calculation.

DEXA body composition scan.

Dual-energy X-ray absorptiometry (DEXA) — the same technology used to measure bone density — can precisely measure lean mass, fat mass, and visceral fat separately. It's the gold standard for body composition assessment.

What DEXA gives you that BMI doesn't: - Exact body fat percentage - Lean mass by body segment (left arm vs. right arm, identifying muscle asymmetries) - Visceral adipose tissue (VAT) volume — the most clinically significant fat depot - Bone density

DEXA is increasingly accessible at specialized fitness facilities, performance clinics, and some telehealth providers. Cost ranges from $50-200. For people interested in body composition tracking, it's the most information-dense measurement available.

Fasting insulin and HOMA-IR.

Insulin resistance — the precursor to type 2 diabetes and a driver of obesity, cardiovascular disease, and cognitive decline — doesn't show up in fasting glucose until the condition is advanced. By the time blood sugar is elevated, insulin resistance has typically been developing for years.

Fasting insulin and HOMA-IR (Homeostatic Model Assessment for Insulin Resistance, calculated from fasting glucose and fasting insulin) identify insulin resistance far earlier. Standard reference ranges for fasting insulin are often too permissive — fasting insulin above 7-8 μIU/mL is associated with elevated risk even when blood glucose is normal.

This is the test that catches the "metabolically obese normal weight" person that BMI misses. And it catches early insulin resistance in people who think they're fine because their BMI is 23 and their blood sugar is normal.

Triglyceride-to-HDL ratio.

One of the most underused risk markers in standard labs. The triglyceride-to-HDL ratio is a powerful predictor of insulin resistance and cardiovascular risk.

• Ratio <2: Favorable (insulin sensitive, low cardiovascular risk)
• Ratio 2-4: Borderline
• Ratio >4: Significant insulin resistance and elevated cardiovascular risk

This can be calculated from a standard lipid panel (no extra cost) and is more predictive of heart attack risk than LDL alone. Yet it's rarely discussed in routine primary care visits.

VO2 max / cardiorespiratory fitness.

Cardiorespiratory fitness (CRF) — measured as VO2 max — is arguably the single strongest predictor of all-cause mortality. The research is unambiguous: low fitness is a more powerful predictor of death than obesity, smoking, hypertension, or diabetes in most studies.

VO2 max can be estimated (not precisely, but usefully) from a 1.5-mile run test, a 12-minute Cooper test, or wearable device algorithms. Precision measurement requires a metabolic cart and clinical testing, but directional assessment is accessible.

Fitness category targets (approximate): - Men <45: VO2 max >45 ml/kg/min - Men 45-55: VO2 max >40 ml/kg/min - Women <45: VO2 max >40 ml/kg/min

People with high BMIs but high fitness have substantially better outcomes than lean but unfit individuals. This is the "fat but fit" finding — not an endorsement of obesity, but a demonstration that fitness is independently and powerfully protective.

Grip strength and muscle mass.

Sarcopenia — low muscle mass and function — is a stronger predictor of mortality than obesity in people over 50. Grip strength, measured with a simple dynamometer, is a validated proxy for whole-body muscle function and is associated with cardiovascular disease, cognitive decline, and all-cause mortality risk independent of BMI.

Peak strength targets (varies by age and sex, roughly): - Men aged 20-40: Dominant hand >48 kg - Women aged 20-40: Dominant hand >29 kg

Muscle mass (measured by DEXA or bioelectrical impedance) and maintaining that mass as you age is a health intervention that BMI completely ignores.

A Better Screening Protocol

If you want to actually understand your metabolic health, this panel costs under $200 and takes 30-60 minutes:

1. Waist circumference — tape measure, 30 seconds
2. Standard metabolic panel + fasting insulin — blood draw
3. Lipid panel (calculate triglyceride/HDL ratio yourself)
4. HbA1c — 3-month average blood glucose
5. Blood pressure

Optional but high-value additions: - DEXA scan (body composition + visceral fat) - VO2 max test (fitness level) - Grip strength test

This panel gives you a far more accurate picture of metabolic health, cardiovascular risk, and where you actually need to make changes than any BMI calculation.

What This Means for Treatment Decisions

The BMI-centered approach to obesity medicine has consequences. People in the "normal" range with significant metabolic dysfunction go untreated because their BMI looks fine. People in the "obese" range with excellent metabolic health face stigma, interventions they may not need, and arbitrary insurance coverage barriers.

At Marrow, our intake assessment doesn't end at BMI. We ask about waist circumference, available labs, family history, symptoms of insulin resistance (energy crashes, sugar cravings, brain fog, difficulty losing weight despite caloric restriction). The goal is to understand the actual metabolic picture — because that's what determines whether treatment is appropriate and what treatment will be most effective.

BMI is a starting point, not a destination. Use it to roughly understand where you are. Don't let it be the last word.