Is BMI really important?

What is BMI and why do we use it?

BMI is a measure used to assess the weight status of a person in relation to their height. This is done by calculating an individual's weight in kilograms by the square of their height in meters (BMI = weight (kg) / height (m)^2). Whilst the calculations may sound complex, BMI simply assesses whether you are the correct weight for your height. The measure’s primary purpose is to provide a quick and easy way to place individuals into weight categories, such as underweight, normal weight, overweight, and obese.

BMI is used across the world for the following purposes:

• To determine and shape public health policies for populations.
• To help to identify people’s potential health risks for cardiovascular disease, type 2 diabetes and certain cancers by GP surgeries, hospitals and other healthcare practices.
• To provide calculations for energy burn and intake requirements (calories) for an individual to help them gain, maintain and lose weight.

Is it right for everyone?

While BMI is a convenient and adequate tool for many, it may not accurately assess individuals with higher muscle mass, such as bodybuilders and athletes. This is because muscle is denser than fat, meaning it weighs more per volume but takes up less space. So, if you’re really into the gym and your BMI figure categorises you as 'overweight,' it could be due to your muscular physique rather than excess fat. And having more muscle mass can be incredibly beneficial for overall health and fitness, even if it bumps up your BMI!

Additionally, BMI may not accurately reflect health risks in older adults or individuals from different ethnic backgrounds who can have varied body compositions even at the same BMI level. This diversity highlights the challenge of using a universal BMI scale across all populations. But that said, research still supports BMI as a reliable starting point for the majority of individuals looking to lose, gain or maintain weight.

What else can be used?

In addition to BMI, there are also a variety of measurement methods to not only determine how much fat we have but also where we store fat in our body (something BMI can’t do!) It is important to consider these alongside BMI, to ensure that we are using the most appropriate and helpful method to assess people.

Other ways to measure weight-related health status include:

• Waist-to-hip ratio / waist circumference / Waist-to-height ratio – Each of these methods are cost effective and easy to do. Measuring the waist identifies excess abdominal fat. Weight around our gut can indicate a higher level of visceral fat, the internal type that surrounds your organs. This ‘invisible’ fat is more insulin resistant and can cause more inflammation and is generally more likely to make you poorly, than subcutaneous fat, the type under our skin that is visible. We now know that a higher level of visceral fat can pose potential health risks, such as type 2 diabetes or cardiovascular disease. Recent research shows that we no longer need to solely focus on how much weight we carry but also where the weight is distributed around our body, which is why each of these measurements complement BMI well.
• Hydrostatic weighing – Also known as underwater weighing, hydrostatic weighing is known as the "gold standard" measure when it comes to body composition measurement. It measures body density to estimate body fat percentage with high accuracy. However, this method is not widely accessible or convenient for most people due to the specialised equipment and setup required.
• Skin fold caliper measuring – Using skinfold calipers allows for estimating overall body fat percentage. This method is more widely used in sports nutrition, offering additional insights into body composition alongside BMI. However, accuracy may vary, especially for individuals with higher body fat levels, and some may find this method rather intrusive. Nonetheless, it is a practical method of monitoring overall body fat.
• Body composition analysis (BIA or DEXA) – Body composition analysis techniques like BIA (Bioelectrical Impedance Analysis) or DEXA (Dual-Energy X-ray Absorptiometry) provide in-depth insights into muscle, fat, and bone mass, which can provide a comprehensive view of body composition. However, these methods are a lot more costly and require highly specialised equipment, which largely limits their widespread use compared to BMI measurements.