More than just a number: Why BMI doesn’t tell the whole story about your health

It is no longer about fitting into your old jeans when your weight can determine your risk of obesity-related diseases such as diabetes, heart disease and certain cancers. And for that matter, accuracy is important.

So, why then is the Body Mass Index or BMI still being used when it is known to have limitations? (A recap: The BMI is calculated by dividing your weight in kilogramme by the square of your height in metre – and hope it falls within the healthy zone of 18.5kg/m2 to 22.9kg/m2.)

The answer is this: “The advantages of the BMI measurement are that it is cheap, quick, reproducible and easy to perform,” said Dr Lee Phong Ching, a senior consultant with Singapore General Hospital’s Department of Endocrinology and the director of its Obesity Centre.

Dr Benjamin Lam, the head of Section, Family and Community Medicine at Khoo Teck Puat Hospital, also opined that the “BMI is a useful starting point”. “It’s easy to calculate and it has been widely used for population-level screening.”

However, detractors of the BMI system claim that it “can underestimate health concerns in older adults and anyone who has lost muscle”. Here’s a look at how that happens and what it means for you at your next health check.

WHAT IS BIOELECTRICAL IMPEDANCE ANALYSIS (BIA) IN THE FIRST PLACE?

You might recall standing barefoot on a platform with metal plates while holding a pair of handles. A few minutes later, your fat mass, muscle mass and water weight are ready, and emailed or texted to you.

Bioelectrical impedance analysis or BIA is a “quick and non-invasive method of estimating body composition such as body fat, muscle mass and water content”, said Dr Lee.

As its name implies, the BIA works by measuring how much impedance or resistance the different tissues such as muscle, fat and water create when a very mild current is passed through your body. “By measuring the resistance to this current, the machine estimates your body composition,” said Dr Lam.

By providing estimates of body fat and muscle mass, said Dr Lee, the BIA can potentially be more accurate than the BMI in diagnosing obesity, especially in athletes and older adults. However, he emphasised that the “BIA only provides an estimation of body composition”.

WHAT ARE THE LIMITATIONS OF BMI AND BIA?

Both have their inherent limitations, said the experts.

“The BMI has often been found to misclassify muscular individuals as ‘overweight’ or ‘obese’,” said Dr Lam. “For example, gym-goers and military personnel have often been assessed with BMI values in the overweight range (BMI 27 to 30) when these individuals are metabolically healthy and weight loss is not required.”

Furthermore, the BMI “does not account for gender or ethnicity differences”, said Louise Yap, a senior dietitian with Mount Elizabeth Novena Hospital. For instance, men tend to naturally have a higher muscle mass and lower body fat than women.

There are also significant variations in obesity prevalence among the ethnic groups in Singapore, according to the Health Promotion Board - Ministry of Health Clinical Practice Guidelines 2016, which the BMI doesn’t take into account. For example, Malays have the highest prevalence at 21 per cent, followed by Indians at 14 per cent and Chinese at about 6 per cent.

Another downside of the BMI is that it can’t measure muscle mass and bone mass, which the BIA can, Yap said. “As such, it may underestimate the health risks in older adults with low muscle mass.” 

BIA is not without its drawbacks either. “BIA readings are sensitive to hydration status (dehydration overestimates fat), recent meals and exercise, so the results will differ depending on the time of day that the readings are taken, even on the same day,” said Dr Lee.

And while newer BIA models can estimate visceral fat, they have limitations, said Yap. “They also do not measure subcutaneous fat accurately; the skinfold calliper testing remains a gold standard for that.”

SO WHICH IS MORE RELIABLE?

“I would say the BMI gives a less accurate reading in my practice,” said Yap. But the BMI isn't left in the dust just yet. “The BMI remains a useful, quick screening tool for obesity. It is validated, easily accessible and part of standard health checks,” said Dr Lee.

“The BIA adds nuance as it estimates body fat, muscle and water composition,” he continued. “However, the accuracy of BIA devices varies widely; hence, care is needed when evaluating the BIA results. Each method has their own limitations and we should always interpret these readings in conjunction with other health parameters such as blood pressure, glucose, cholesterol levels and quality of life.”

Dr Lam also advocated for both tools. “Rather than replacing the BMI with BIA, the better approach is to use them together, along with waist circumference, metabolic markers and clinical judgement to assess and predict health,” he said.

“Information from both tools is typically considered and results are interpreted in the context of your overall health profile and goals,” said Dr Lam. “Their respective limitations are acknowledged and factored into the assessment.”

ARE HOME BIA MACHINES LESS ACCURATE?

Not all BIA machines are the same, said Dr Lee. “Consumer-grade, foot‑to‑foot scales are generally less accurate than professional-grade BIA machines.” He explained that the “accuracy varies widely depending on the device specifications and modelling algorithms”.

The number of electrodes or sensors used plays a part as well. “Professional-grade BIA machines use tetrapolar or eight-point devices (involving hand and feet electrodes), and multiple currents at varying frequencies to provide segmental body composition and better accuracy,” said Dr Lee.

“Meanwhile, home BIA machines are typically single-frequency, foot‑to‑foot only. They generally rely on broad algorithms and empirical estimations, and hence, are less reliable.”

SO, WHAT IS THE TAKEAWAY?

If there is an opportunity to perform a BIA, take it, said Yap. “It does give a more in-depth breakdown of what your body comprises of. For individuals who lead a less active lifestyle, a quick calculation of their BMI would help classify them in the respective category to determine their at-risk disease status.”

Both Dr Lee and Dr Lam recommended using the BMI and BIA for a more complete picture of your health. “The BMI is useful for screening. The BIA provides a more detailed look at body composition, helping guide individualised care and long-term monitoring," said Dr Lam.

And "we should always interpret these readings in conjunction with other health parameters such as blood pressure, glucose, cholesterol levels and quality of life," added Dr Lee.

At the end of the day, remember that “no single number defines your health”, said Dr Lam. “What matters most is your lifestyle, metabolic health, functional ability and overall well-being."