It's the Super Bowl this weekend and NFL fans are likely to be treated to a veritable feast of sporting prowess. But, did you know that the overwhelming majority of elite American Football players are in fact overweight or obese? They’re also, however, highly-trained, professional and evidently very fit athletes – so, how can they be both these things?

The limits of BMI

As you may have guessed, the discrepancy arises from how we classify ‘overweight’ and ‘obesity’. Both these terms actually (to different degrees) refer to the state of having accumulated an excess amount of fat. Nevertheless, we most commonly use total body mass, or more specifically, Body Mass Index (BMI), as a marker of overweight and obese states. The reason for this is clear - BMI is fairly simple to calculate: you simply divide your body weight (in kilograms) by the square of your height (in metres).

According to the World Health Organisation, a BMI of greater than or equal to 25 kg/m2 is considered overweight, while a BMI greater than or equal to 30 kg/m2 is classified as obese.

But, as the World Health Organisation indeed concede, “BMI should be considered a rough guide because it may not correspond to the same degree of fatness in different individuals.” In other words, by merely measuring bodyweight, BMI does not identify or discriminate between what tissue is truly contributing towards that weight – i.e. is it fat, bone, or muscle? One limitation of BMI then, is that it fails to accurately account for body composition.

The BMI of American football players

Using Body Mass Index as a measure, it is perfectly possible to be classified as overweight or obese under BMI, but, in actuality, have a healthy level of body fat. A large study of Division I collegiate football players highlights this point particularly well.

In the study, all 467 American football players were classified using BMI as overweight or obese. As is to be expected given the different athletic demands of the sport, BMI varied according to which position a subject played. Those in the offensive line had an average BMI of 36.4 kg/m2, placing them in the Class II obese category (BMI between 35.0 and 39.9 kg/m2). By contrast, wide-receivers had an average BMI of 25.7, making them slightly overweight.

Body composition of American football players

As established, however, BMI fails to accurately account for body composition. By contrast, the best currently available method, or ‘gold standard’, for assessing body composition is the DEXA (or DXA) scan.

DEXA stands for ‘Dual Energy X-ray Absorptiometry’. As the name suggests, this scan involves using beams of low-dose X-rays with two distinct energy peaks. As bone, fat and muscle are made from different tissues and have distinguishable consistencies, they absorb the two types of X-ray beams to varying degrees. Using this information, it’s possible to calculate how much of a particular body region is composed of fat, lean muscle and bone.

In the study, collegiate football players also underwent DEXA scans to assess their body composition. More precisely, the researchers measured: total body fat percentage, total lean mass (mass that isn’t fat – i.e. bones, muscle, ligaments and tendons), trunk lean mass, trunk fat mass, legs lean mass, legs fat mass, gluteal (buttocks region) lean mass and visceral adipose tissue mass. Of these measurements, total body fat percentage and visceral adipose tissue are particularly important markers of health status.

Body fat percentage is essentially the proportion of your total weight that is composed of fat. Although there is no official consensus, a healthy body fat percentage is generally thought to be somewhere between 8% and 20% for men, and between 13% and 32% for women. Generally speaking, a higher body fat percentage puts you at a greater risk of diabetes, stroke and heart attack.

When it comes to specific regions of fat that confer these health risks, it is fat tissue found deep in the abdomen and around internal organs that is particularly perilous. This fat tissue is known as visceral adipose tissue.

Returning to American football players, the study found that the vast majority of subjects had healthy body fat percentages (13-20%) and low amounts (<500 grams) of visceral adipose tissue. The only exceptions were offensive and defensive line players, who had average body fat percentages of 31% and 23% respectively. Interestingly, the players also had higher bone mineral density measurements than the general population. This reflects an increased bone mass, which is most likely secondary

So, despite ‘unhealthily’ high BMIs, most collegiate football players are relatively lean, carrying a healthy amount of body fat. As such, they are unlikely to be subject to the same health risks ordinarily implied by having a high BMI.

How do I assess my body composition?

In reality, many of us do not have ready access to advanced DEXA scan technology. Fortunately there are simpler, if slightly less accurate, ways to assess your body fat percentage and body composition.

- Skinfold calipers

This involves measuring the thickness of subcutaneous fat found under the skin using calipers. Multiple skinfold measurements are taken at different regions across the body to get an overview of total body fat percentage.

There are lots of online tutorials available to guide you through the technique. For example, this blog explains how to calculate body fat percentage using skinfold measurements at just 4 body regions.

- Body circumference

The circumference of different parts of your body can also be used to calculate body fat percentage. You’re probably already familiar with waist circumference, which can be a good indicator of the amount of visceral adipose tissue you carry. In addition, the circumference of your neck, hips, wrists and forearms can be assessed.

You can then plug these figures into an online calculator, which uses a formula to give you a body fat percentage. An example online calculator is here.

- Bioelectrical Impedance Analysis (BIA)

You may have spotted a BIA machine in your gym. This technique essentially involves passing a weak electrical current through your body and measuring the ‘impedance’ or opposition to the flow of electricity caused by your body tissues. In general, muscle and lean mass creates less impedance (i.e. it’s easier for electric current to flow through) than fat tissue. Using this information, it’s possible to get an idea of how much body fat you carry.

BIA machines are simple to use and can easily be bought and used at home, but their accuracy is known to vary. Readings may also change according to hydration level, bodyweight (BIA is less accurate in obese people) and intensity of exercise before measurement.