Why BMI Isn’t the Whole Story

June's BMJ had a useful review of BMI. Here are my notes.

Introduction

When I first stepped on the scales and calculated my body-mass index (BMI), I thought I had a clear verdict on my health. Yet in 2025 researchers and health professionals alike are raising serious questions about this simple formula. Weight-loss jabs such as semaglutide and tirzepatide have thrust BMI into the spotlight—revealing both its appeal and its flaws. In this post I’ll explain what BMI can and can’t tell you, share expert insights, and suggest better ways to measure your health.

What Is BMI, and Why Did It Catch On?

BMI dates back to the 19th century as a quick way to estimate whether someone’s weight is “healthy” for their height. You divide your weight (in kilograms) by the square of your height (in metres), then check a chart:

• Underweight: BMI < 18.5

• Healthy weight: 18.5–24.9

• Overweight: 25–29.9

• Obesity: BMI ≥ 30

Its simplicity made BMI a favourite in clinics, research papers and public guidelines. No fancy machines or blood tests required—just a scale and a tape-measure.

The Flaws in a Simple Number

But simplicity comes at a cost. BMI doesn’t distinguish between muscle, bone and fat, so a bodybuilder with high muscle mass could be “obese” by BMI and a sedentary person with low muscle mass might be “healthy.” That’s not just academic: it affects real-life diagnoses, treatment decisions and how we view ourselves.

1. Misleading for Individuals

   • Athletes, pregnant women and older adults can all be mis-categorised.

   • Ethnicity and age affect how body fat distributes—yet BMI uses one standard chart for everyone.

2. Limited View of Health

   • BMI says nothing about where fat sits in the body (visceral fat around organs is more harmful than subcutaneous fat under the skin).

   • It ignores blood-pressure, cholesterol, blood-sugar levels and fitness.

3. Reinforces Weight Stigma

   • “You can’t judge a book by its cover,” says Dr Fatima Stanford (Massachusetts General Hospital). BMI was never meant to be a summary of health, yet it’s often used that way, fuelling stigma when people don’t fit the “normal” range.

Expert Calls for Change

In January 2025 the Lancet Diabetes & Endocrinology commission urged us to move beyond BMI as a disease metric. They recommend combining BMI with other tools—among them:

• Waist-to-Height Ratio: Waist circumference divided by height; a ratio above 0.5 can indicate excess abdominal fat.

• Waist-Hip Ratio: Highlights distribution of fat around midsection versus hips.

• Body-Composition Scans: Devices such as DEXA or bioelectrical impedance give a split of muscle, bone and fat mass.

• Clinical Assessments: Blood tests for lipids, glucose and inflammation markers.

Dr Adrian Brown (NIHR advanced fellow, University College London) points out that combining simple measurements makes for a more accurate health picture. “We shouldn’t just use BMI,” he says. “We should measure adiposity directly and combine metrics.”

Understanding ‘Metabolically Healthy Obesity’

Some people with a BMI over 30 have normal blood-pressure and insulin sensitivity. Health professionals call this “metabolically healthy obesity.” But it’s still debated—long-term risks may creep up even if today’s tests look fine.

Putting It Into Practice: What You Can Do

Here’s how to move beyond BMI in your own life:

1. Measure Your Waist

   • Use a tape measure at the narrowest part of your waist—or just above your hip-bones.

   • Aim for a waist-to-height ratio under 0.5.

2. Track Body Composition

   • Many gyms and clinics offer bioelectrical impedance scans; some smart scales do too.

   • Look at trends, not single readings.

3. Know Your Numbers

   • Ask your GP for blood-pressure, cholesterol and HbA1c (blood-sugar) tests.

   • Keep a simple log—seeing improvements can be more motivating than watching your BMI.

4. Focus on Fitness and Strength

   • Regular strength training builds muscle, which burns calories even at rest.

   • Cardiovascular exercise supports heart health regardless of weight.

5. Adopt a Balanced Diet

   • Plenty of vegetables, whole grains and lean proteins help regulate blood-sugar levels.

   • Remember that healthy eating isn’t just about weight—it influences energy and mood.

Conclusion

BMI served us well as a rough guide, but health is far more complex than a single number. By combining waist measurements, body-composition scans and clinical tests, we get a richer picture of how our bodies function. I’ve personally found tracking my waist-to-height ratio and blood-pressure far more motivating than fixating on BMI. If you’re curious about your own health, I encourage you to measure more than just weight - because you’re worth understanding in full.