Why BMI is misleading for adults over 40, and what to measure instead.

If you've ever been told you're "overweight" or "obese" based on a BMI reading, despite feeling fit, exercising regularly, and eating reasonably well, there's a good chance the problem wasn't you. It was the measurement.

BMI has been the dominant health screening tool for decades. It's used by GPs, insurance companies, public health campaigns, and practically every health calculator on the internet. And for adults over 40 specifically, it is a deeply flawed proxy for health, one that misclassifies millions of people and may be causing more harm than good.

Where BMI came from, and what it was actually designed for

BMI was developed in the 1830s by Adolphe Quetelet, a Belgian mathematician. He was not a physician or physiologist. He was a statistician interested in describing the average physical characteristics of populations, not assessing the health of individuals.

Quetelet's formula (weight in kilograms divided by height in metres squared) was a mathematical convenience for population-level statistics. It was never intended, and was explicitly not designed, to assess individual health risk. The idea of using it as a clinical screening tool was largely a 20th-century invention, adopted primarily because it was easy to calculate without any equipment.

The BMI thresholds most people know (18.5, 25, 30) were somewhat arbitrarily set, and the research supporting them was conducted predominantly on white European men. The tool has known accuracy limitations across different ethnicities, body types, and ages.[1]

Why BMI fails specifically after 40

Even accepting BMI's limitations as a broad screening tool, it becomes particularly unreliable after 40 for several compounding reasons.

Body composition shifts without weight changing

As we covered in our article on metabolic changes after 40, body fat increases by approximately 1% per year after 40, while lean muscle tissue gradually declines. This happens even without any change in total body weight. The result is what researchers call "normal weight obesity", a person whose BMI is "healthy" but who carries a significantly higher proportion of body fat than they did at 30, concentrated particularly in the visceral (internal organ) region.

BMI sees none of this. It only sees your weight relative to your height, which hasn't changed.

Muscle mass inflates BMI misleadingly

For adults over 40 who exercise regularly and particularly those who resistance-train, BMI is almost certainly an overestimate of health risk. A 55-year-old woman who has maintained significant muscle mass through consistent training may register as "overweight" on BMI while having excellent insulin sensitivity, cardiovascular markers and bone density.

In this case, her BMI is penalising her for the very behaviour (doing resistance training, muscle preservation etc.) that most strongly protects against the health risks BMI claims to flag.[2]

Visceral fat, the real risk factor, is invisible to BMI

The health risks most associated with overweight and obesity, including type 2 diabetes, cardiovascular disease, metabolic syndrome, are driven primarily not by total body fat but by visceral fat: the fat that accumulates around the abdominal organs. Visceral fat is metabolically active in a way subcutaneous fat (the fat under your skin) is not. It produces inflammatory cytokines, disrupts insulin signalling, and is directly associated with elevated cardiometabolic risk.[3]

BMI cannot detect visceral fat. Someone can have a "healthy" BMI while carrying dangerous levels of visceral fat, sometimes called "skinny fat", and BMI will give them a clean bill of health it hasn't earned.

The better measurements for adults over 40

Two relatively simple measurements capture what BMI misses, and both are meaningfully more predictive of cardiometabolic risk in mid-life and beyond.

1. Waist-to-height ratio

Waist-to-height ratio (WHtR) divides your waist circumference by your height. The target is straightforward: keep your waist to less than half your height, giving a ratio below 0.5.

A 2012 systematic review published in PLOS ONE analysed data from over 300,000 participants and found that waist-to-height ratio was a significantly better predictor of cardiovascular disease, type 2 diabetes and hypertension than BMI across all age groups and ethnicities.[4] A 2020 meta-analysis reached similar conclusions, noting that WHtR's advantage over BMI was most pronounced in older age groups, precisely the adults over 40 who are most commonly being misled by BMI measurements.[5]

Unlike BMI, WHtR directly captures central adiposity, specifically the visceral fat that actually drives metabolic risk. It's also self-correcting in the way BMI is not. A strength-trained person with low visceral fat will have a low waist circumference and therefore a healthy WHtR, regardless of how much muscle they carry.

2. Biological age estimation

Beyond single-measurement tools, biological age estimation gives a broader picture of how your lifestyle is affecting the ageing process overall. Rather than measuring one aspect of body composition, it integrates multiple markers including exercise habits, sleep quality, diet, stress, smoking history and social connection, to estimate how old your body actually functions relative to your calendar age.

Research using epigenetic clock analysis (blood-based biological age testing) has consistently found that these lifestyle factors predict biological age with meaningful accuracy and more importantly, that interventions in these areas can measurably reduce biological age markers over months.[6]

While a lifestyle-based estimator like ours is less precise than an epigenetic clock test, it reflects the same underlying science and provides actionable direction for the specific factors most strongly driving biological ageing.

What to actually do with this information

If you've been told your BMI is elevated, the first thing worth doing is calculating your waist-to-height ratio. If your WHtR is below 0.5, there's a reasonable argument that your "overweight" BMI is reflecting muscle mass rather than metabolic risk. Doubling down on resistance training is more useful than trying to lower the number on the scale.

If your WHtR is elevated (above 0.5, and particularly above 0.6), then you do have real central adiposity to address. The solution is the same regardless of what your BMI says. A protein-prioritised diet in a moderate calorie deficit, combined with consistent resistance training, is the most evidence-based approach to reducing visceral fat specifically.

This is exactly what the Over40Macros calculator is designed to support. It sets your protein at 1.6g per kg — the upper end of the evidence-based range — and your calorie target based on a genuine understanding of how metabolism and body composition work after 40.

The bottom line

BMI is not useless. As a rough population-level screening tool, it has its place. But for adults over 40 making decisions about their own health and body, it is an unreliable guide that misclassifies people in both directions, and one that ignores the very shift in body composition (muscle loss, visceral fat gain) that defines health risk in this age group.

Waist-to-height ratio and a holistic biological age assessment are both more informative, more actionable, and far better suited to the physiological reality of being over 40. Neither requires a GP visit, laboratory test, or expensive equipment. Just a tape measure and five minutes.