Body composition: what is it, and why does it matter?

Body composition offers more nuanced information that you can use to obtain a clearer idea of what you need to do to reach your targets, as well as understand with greater insight, what’s actually happening to your body while you’re on your health journey. 

It can also explain things like ‘why am I not losing weight when I’m exercising so much?’ – which is a common frustration we hear as people begin their health journeys.

As we alluded to above, standing on a scale will simply tell you how much you weigh. But it won’t tell you how much of your weight consists of muscle, how much is fat, how much is bone mass, and how much is water. Body composition is exactly this: it breaks down your total weight into its constituent parts. 

We spoke to Chris Zaremba, an Advanced Personal Trainer and founder of Fitness Over Fifty, who specialises in health and fitness for over 50s.  

Zaremba tells us, “Overall weight doesn’t take into account body composition. You don’t have control over the weight of your bones, skin, or organs. You do have control over your body fat, muscle and water. When people say they want to lose weight, they don’t really want to lose muscle; most people want to lose body fat without losing muscle”. 

Why does this matter? Well, if you’re trying to lose weight, traditional scales can’t tell if you are, in fact, losing weight because of a reduction in muscle mass (which we probably don’t want due to the potential impact on our health) or a decrease in body fat.  

This is where measuring your body composition with tools such as smart scales can make a difference; it can help you to see whether any lifestyle changes you’ve made are having the desired effect or not. The most advanced devices, such as the Withings Body Scan, even attempt to measure your muscles mass in different parts of your body, to give you as much detail as possible. You can then make any adjustments you feel are necessary. 

Body Mass Index (BMI) is sometimes used to calculate whether someone is considered a healthy weight for their height, and you might have heard it mentioned or read about as a way of working out if you’re a ‘healthy’ weight. It’s a common metric you’ll find on many smart scales, such as the Eufy P2 Pro, Fitbit Aria Air, or Xiaomi Mi body composition scales 2, among others. But BMI does have some significant limitations. 

As Zaremba explains, “two people with the same weight and same height have the same BMI, but can have totally different body composition”. 

According to the NHS, “Muscle is much denser than fat, so very muscular people, such as heavyweight boxers, weight trainers and athletes, might be a healthy weight even though their BMI is classed as obese.”  

So there is a risk that a BMI score can give people a misleading picture of their health. There are also other factors that BMI doesn’t necessarily consider, such as ethnic background, that mean a more tailored health program is needed. To quote again from the NHS: 

“Your ethnic group can also affect your risk of some health conditions. For example, adults of South Asian origin may have a higher risk of some health problems, such as diabetes, with a BMI of 23, which is usually considered healthy”. 

Body composition goes further than BMI by analysing the percentage of fat, muscle, and bone. These metrics allow for a more detailed picture of your overall health and can be a more effective way of revealing risk factors for potential health problems. 

Body composition is also a better method for tracking changes in your body over time. If, for example, you are losing weight, body composition measurement can tell you whether you are losing fat or muscle, which BMI cannot. 

There are several methods for measuring body composition, including Dual x-ray absorptiometry (DEXA) scan, hydrostatic weighing, bioelectrical impedance analysis (BIA) and skinfold callipers.  

The first two are the most precise, described by some as the “gold standard” in body composition measurement. However, both methods require you to visit a lab or other specialist venue.  

DEXA scans work by sending two low-dose x-rays through your body. One beam calculates soft tissue density, and the second calculates bone density. These then supply an accurate measure of bone, muscle, visceral fat, and subcutaneous fat.

Hydrostatic weighing compares the weight of an individual on land with their weight underwater. The theory is that fat is less dense than water, while bone and muscle are not. So, the two measurements can be used to calculate a person’s body fat percentage. This method can be accurate, but it can’t differentiate between visceral and subcutaneous fat.

BIA is the method employed by body composition scales which you can easily use at home, such as the Withings Body Cardio smart scales shown above. These work by sending a small electrical current through the body and measuring the resistance that the current encounters. Bone, muscle, and fat have different amounts of water. 

The electric current passes more easily through muscle because it has a higher water content than fat. The scale can then calculate the level of impedance (or resistance) and use this to estimate your body composition. Some smart scales, like the Withings Body Comp, or QardioBase X, also estimate visceral fat levels.

This method is far quicker and easier than the two we mentioned above, but it is less accurate. Readings can vary significantly based on factors such as room temperature, how well hydrated you are, and even whether you’ve recently exercised or showered. 

Finally, callipers are the cheapest way to get an idea of your body fat percentage. Usually, measurements are taken from three to seven sites around the body (seven takes longer but tends to be more accurate) by pinching the skin and then performing a calculation to gauge your body fat percentage. Accuracy depends a lot on measuring in the same place on your body each time, which can be difficult. 

So, there are a few of the most common methods for measuring body composition, but you’re probably wondering why it even matters at all? 

According to Zaremba, “As people get older, muscles naturally waste. People don’t want to become frail and weaker, so people should be looking to build and maintain muscle mass.  

“Being muscular means you’re going to be stronger, less frail, and if you have more body fat you’re likely to suffer many more diseases as you get older – metabolic syndrome diseases such as diabetes and heart disease are more likely to happen if your weight comprises of more fat than muscle”.  

Body composition matters then, because it can be an effective way to spot potential health risks and start mitigating these. Too much fat (particularly visceral fat) for example, is linked to an increased risk of problems such as heart disease, stroke, and type 2 diabetes.  

Zaremba says: “Somebody with more muscle than fat is healthier than someone the other way around and much less likely to get…diseases, and much more likely to have a healthy, happy and mobile, and independent, old age”. 

And even in people who look slim on the outside, there may be more fat mass and less muscle than it appears. Reduced muscle mass has been linked to (among other things) reduced mobility, lower quality of life, increased risk of falls, and increased hospitalisations, especially as we age.

Your body composition will change over time and is influenced by several factors. These include gender, age, metabolism, diet, hormones, and even sleep. It’s also important to remember that we all need a certain amount of fat, often described as essential fat. This fat allows our organs to function properly. Women will (in general) have a slightly higher percentage of body fat than men. 

As we age, there is often a decline in muscle mass and a concomitant rise in body fat as a percentage of your body composition. According to a study published in The American Journal of Clinical Nutrition in 2000, depending on age, a healthy body fat percentage in women is between 21 and 36% while for men the figure ranges between 8 and 25%. 

The table below, provided by Tanita, gives us an idea of what “healthy” body fat levels look like for different age groups:

With a clearer understanding of body composition, you can start making lifestyle changes to help support good health. Zaremba tells us: “I would say body composition is more important for the fifty-year-old or sixty-year-old than it is for a 25- or 30-year-old, who can put on muscle very easily. That doesn’t happen when you get to fifty or sixty”.  

With that in mind, whether you adjust your diet, do more exercise, or a combination, taking steps to ensure a healthy body composition can be a significant contributory factor to ageing well.  

If you feel inspired to get more active, our Walking 101 guide is a great place to help you start.