‘I’m A Nutritionist – Here’s Why We Should Be Looking Beyond BMI’

For over a century Body Mass Index (BMI) has been used to assess whether someone is a healthy weight, but a new study has brought the effectiveness of the tool into question.

Led by scientists at University College London, the research found that the BMI framework both overestimates and underestimates how excess body fat may impact health. In the UK alone, 2.4 million people could have dangerous levels of body fat that BMI fails to pick up, while a further 1.3 million people may have been wrongly classed as obese. So should we be scrapping the health test altogether? We asked our resident nutritionist Kim Pearson whether BMI still has use in modern society – and which markers might be more effective in painting an accurate picture of health.

What is BMI and how is it calculated?

BMI stands for Body Mass Index. It’s a calculation used to assess body weight and establish whether people are a ‘healthy weight’. It’s based on a person’s weight in relation to their height and is calculated by dividing weight in kilograms by height in metres squared.

For adults, the conventional classifications are:

• Below 18.5: Underweight
• 18.5 to 24.9: Healthy weight
• 25 to 29.9: Overweight
• 30 and above: Obesity

BMI is widely used because it is quick, inexpensive and easy to calculate, but it has some significant limitations when applied to individuals.

What does the latest study suggest?

A recent study published in Nature, involving more than 844,000 adults across the UK, China, the US and Spain, provides further evidence that BMI alone is not sufficient to accurately identify obesity or assess associated health risks. An analysis of data found that using BMI to diagnose obesity failed to identify a considerable proportion of people with elevated health risks caused by excess body fat. BMI isn’t a good way to assess obesity in individuals.

One of the main issues with BMI is that it doesn’t distinguish between fat mass and lean mass. Someone with a high proportion of muscle, a body builder for example, may have an elevated BMI despite having a relatively low body fat percentage. Conversely, someone can have a BMI within the conventionally healthy range but carry excess body fat, particularly around their abdomen.

This is important because where we store fat impacts our health. Excess visceral fat, which accumulates around the internal organs, is associated with increased cardiometabolic risk, including type 2 diabetes and cardiovascular disease. Someone with a relatively normal BMI but significant abdominal fat may therefore have suboptimal metabolic health not identified by their BMI measurement.

Ultimately, BMI tells us very little about an individual’s actual body composition, fat distribution or metabolic health. We need to move towards a more comprehensive assessment rather than relying on a single measure.

When should you worry about a high BMI?

A high BMI shouldn’t automatically be a cause for concern, but it shouldn’t be dismissed either. If someone has a BMI above 30, it is worth investigating whether this reflects excess body fat, particularly if there is also increased abdominal fat or evidence of metabolic dysfunction. I’d want to look at the bigger picture, including waist circumference, blood pressure, blood glucose regulation and lipid profile.

It’s also important to consider ethnicity, age and muscle mass. For example, people of South Asian, Chinese and other Asian backgrounds can experience increased cardiometabolic risk at lower BMI levels. NICE recommends lower BMI thresholds for identifying overweight and obesity in these populations.

The important distinction is between simply having a higher body weight and having excess body fat that is negatively affecting health. 

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What other markers of measuring body composition might be worth looking into?

There are several simple measurements that can provide a more comprehensive picture than BMI alone. Alongside my clinical practice, my team and I also deliver workplace health screenings for corporate clients. 

As well as measuring HbA1c to assess blood glucose regulation and diabetes risk, and a full lipid panel to provide insights into cardiovascular health, we also assess the following important markers:

• Body composition, which includes body fat and muscle mass. Assessing body composition gives us a much more informative picture than weight or BMI alone. It allows us to look at the proportion of body weight made up of fat and lean mass, including muscle. Body fat percentage can help identify individuals who have relatively high levels of body fat despite having a BMI within the conventionally healthy range. It can also identify elevated visceral fat. 
• Muscle mass is also important, particularly as we get older. Maintaining muscle is vital for metabolic health, physical function and healthy ageing. Someone can lose weight and see their BMI decrease, but if a significant proportion of that weight loss comes from muscle, this isn’t necessarily a positive outcome.
• Waist to height ratio – this is one of the simplest measurements I’d recommend incorporating into routine health assessments. It is calculated by dividing waist circumference by height, using the same units for both. A useful general recommendation is to aim for a waist circumference of less than half your height. In other words, keep your waist to height ratio below 0.5. NICE recommends this alongside BMI for adults with a BMI below 35.
• Waist circumference – this is a simple and easy way to assess abdominal fat accumulation. Increased waist circumference is associated with greater cardiometabolic risk, independently of BMI. The thresholds differ according to sex and ethnicity. For white European men, below 94 cm is considered lower risk, 94–102 cm indicates increased risk, and above 102 cm indicates substantially increased risk. For women, below 80cm is considered lower risk,cou 80–88 cm indicates increased risk, and above 88cm indicates substantially increased risk. For South Asian men, a waist circumference of 90cm or above indicates increased risk. For South Asian women, a waist circumference of 80cm or above indicates increased risk.

I would always advise looking beyond BMI. A combination of anthropometric measurements, body composition assessment and relevant blood biomarkers gives us a much clearer understanding of an individual’s health and metabolic risk.

The goal shouldn’t just be to achieve a particular number on the scales. It should be to improve body composition, preserving or increasing muscle mass and reducing excess fat, especially visceral fat, while improving the less visible markers of metabolic health too.

Kim Pearson is Country & Town House’s contributing health editor. A qualified nutritionist with almost two decades of clinical experience, and author of The Nozempic Diet, Kim specialises in weight loss, metabolic health and healthy ageing. To find out more about Kim’s work, visit her website kim-pearson.com or follow her on Instagram.