BMI For People Over 40 May Not Be Accurate

You’ve probably heard of BMI or body mass index. Your BMI is the ratio between your weight in kilograms and your height in centimetres squared.

Check your BMI here

In many ways, BMI is a helpful screening tool for obesity-related health risks. The higher your BMI, the greater the risk. A BMI above 30 is as risky as smoking 10 cigarettes a day.

But while your height and weight help us calculate your BMI, new research suggests we need to consider another important factor when interpreting the results. That’s your age. It seems traditional BMI calculations may not be accurate for over-40s.

Body Composition Changes With Age

As you get older, your body composition changes. You’re more likely to be carrying fat than muscle. So, while you may have a relatively normal BMI, you may have a higher percentage of fat than a younger person with the same BMI.

It’s already well-established that a certain body fat percentage means obesity and that these measurements change with age and gender.


Body fat percentage that indicates obesity





28% or higher

40% or higher


30% or higher

42% or higher

Is it possible to be classed as obese based on your body fat but not your BMI? Italian and Lebanese researchers decided to find out how the two measuring systems compared.

They calculated the body fat percentage of 4,800 people aged 40-80 using data from dual x-ray absorptiometry (DEXA) scans. That enabled them to work out how many were obese according to their body fat and how many were obese according to their BMI.

There was a significant difference in who met the criteria for obesity depending on whether they assessed BMI or body fat percentage. Far more people could be considered obese based on their body fat percentage.


Obese by BMI measurement

Obese by body fat percentage







As a result, the researchers recommended reassessing BMI categories for people over 40. Instead of defining obesity as a BMI above 30, we should consider using a BMI above 27 for over-40s.

The Pros and Cons of BMI

BMI is relatively easy to calculate and likely to remain a common measurement used to inform medical practice.

But it is definitely not the only measurement we need to consider. Your waist-hip ratio, for example, may be a better predictor of your risk of developing obesity-related comorbidities like cardiovascular disease.

Clearly, your body fat percentage is another, especially if you’re over 40.

Why Does This Matter?

Because we may be underdiagnosing obesity – a condition that poses serious risks to health and longevity.

Historically, the most common form of malnutrition was having too little to eat. Over the last 40-50 years, though, we’ve reached a tipping point. Obesity is now one of the largest contributors to poor health in most countries.

The latest Global Burden of Disease Study published in the Lancet has demonstrated that obesity is becoming a much more prevalent cause of death around the world. From 2000-2021, we’ve successfully reduced risks relating to unsafe water and sanitation but metabolic risks rose nearly 50% over the same period. That includes high BMI, high blood pressure and high blood sugar.

How Can Dr Lockie Help?

These studies may seem a little deflating if you’re struggling to manage your weight despite your best efforts. Your weight is influenced by many different factors including your genetics, environment, medications, overall health and lifestyle behaviours.

That’s why Dr Lockie leads a multidisciplinary approach to weight loss, where we address behavioural, dietary, psychological, physical and medical considerations to help you achieve and sustain a healthy weight.

If you’d like to explore how bariatric surgery could help you, we’d love to share our expertise. Please book an appointment with SCOPE Certified Shirley Lockie today if you would like to explore your options.


All information is general and is not intended to be a substitute for professional medical advice. Dr Phil Lockie can consult with you to confirm if a particular procedure or treatment is right for you. All surgery carries risks.


AHPRA disclaimer

*All information is general in nature, patients should consider their own personal circumstances and seek a second opinion. Any surgical or invasive procedure carries risks

Note From Dr Lockie

Medications will be assessed pre-operatively and post-operatively. With weight-loss and particularly after surgery, comorbidities can change for the better, particularly e.g., hypertension or diabetes. It is essential for your health that medications are discussed with you, your GP and/or any other specialists such as Cardiologist or Endocrinologist etc.

In addition, use of multivitamins, and alternative supplements should be discussed with the practice to promote your better health.

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