Measuring the body – Part 2: Body fat percentage
Overweight and obesity are associated with chronic diseases such as diabetes, heart disease, stroke and cancer. However, research has shown that this risk is caused not so much by the exact weight as by the proportion of fat in the body. Because of this, the risk of chronic disease can be elevated even in someone who is only slightly overweight or perhaps even of normal weight but who has excess fat; this situation can develop in some people with a very sedentary lifestyle or with a nutritionally poor diet.
We now routinely use methods to measure the percentage of body weight that is made up by fat. These methods include skinfold measurement or body composition analysis by DEXA or bioelectrical impedance analysis. These measurements are very useful when deciding on the need for treatment, and subsequently to guide therapy.
You will notice that women have a higher percentage body fat than men. This is normal and has its explanation in evolution. The healthy range is the percentage of fat that is considered not to increase our risk of disease.
Another important factor that must be taken into account is where that fat is located. You can read about that here. In this article, suffice it to say that visceral adipose tissue (the fat within the abdomen, around the organs) is more problematic than subcutaneous fat (under the skin).
Weight management programmes must be designed to get the percentage body fat into the healthy range. If muscle volume is low, the programme must also include measures to ensure an improvement. As we all know, muscle volume depends on exercise (the type of exercise and its frequency are very important); but that is not the only factor – diet composition is critical. Hence the need for a multidisciplinary team, with medical control, dietitian input, a personal trainer, and coaching to avoid dropouts, as weight loss in this situation can be slower, and motivation can falter.
Please read on if you want a more detailed explanation of this subject.
Why is it essential to measure body fat percentage and not just weight and body mass index?
Up to now, most doctors have based our assessment of overweight and obesity on calculation of the body mass index (BMI). However, BMI tells us nothing about body composition, i.e. how much fat, how much muscle, etc. More and more studies are showing us that it is not the absolute weight that determines health risk, but rather the proportion of fat in the body. Look at these two men who have the same BMI of 31 and consider which one you believe is healthier:
Both of these men are in the obese range of BMI, but the one on the right clearly does not fit our idea of obese. Just to remind those of you unsure of BMI, it is weight in kilograms divided by the square of the height in metres. Here are the ranges we use:
So, what about body fat percentage?
Body fat percentage is the proportion of body fat expressed as a percentage of body weight.
Many clinical studies have shown that body fat percentage is more useful than BMI from a health perspective, and there is now a movement in medical circles to change terminology from obesity or overweight to overfat. But, while the term “overfat” may be used in medical circles, it sounds distinctly pejorative in lay English and we need to choose our language carefully to ensure respect of the individual. This point will no doubt be discussed at length by all affected groups and cannot be resolved in this article; I plan to use “excess body fat percentage” (shortened to excess BF%).
Ideal body fat percentage varies according to race, age and sex. At the moment, many ranges have been published and, though not identical, most of them are in general agreement. The 2016 position statement from the Spanish Society for the Study of Obesity gives us the following figures:
Tanita, a company specialised in the evaluation of body composition analysis, has produced a graph that adjusts for age and sex:
How is body fat percentage measured?
Clinical and technological methods are available.
- Skinfold callipers: Formulae using the thickness of 4 or more skinfolds give us an approximation of BF%. This method becomes inaccurate as body fat increases (limit usually given as BMI ≥30).
- Neck, waist and hip circumferences: Simple and quick but subject to considerable variability depending on measuring technique. Waist circumference is an important and clinically useful indicator of abdominal adiposity.
- Waist-to-hip ratio and Waist-to-height ratio: The waist-to-hip ratio (WHR) gives an idea of body shape (android or gynoid fat distribution); an android distribution, so named because it is more common in men, is more closely linked to chronic disease. The waist-to-height ratio (WHtR) is recognised as a very valuable measure of central adiposity, even in the absence of significant overweight (see my article on body fat distribution here.) The commonly accepted cut-off is 0.5, meaning that an individual whose waist circumference is more than half his or her height is considered to be at risk of chronic disease.
2. Technological (only those typically accessible)
DEXA: Dual energy x-ray absorptiometry. DEXA is the gold standard for body composition and its accuracy has been validated for the measurement of fat, lean tissue and bone. DEXA can inform you about bone density (osteopenia/osteoporosis) as well as muscle and fat. It is reasonably widely available but is expensive. The test uses low-dose x-rays to measure body tissues and takes about 10 minutes.
Bioelectrical impedance analysis: Good accuracy compared to the gold standard DEXA method, though results can be affected by recent food or water ingestion or exercise. Domestic appliances can be acquired, but more advanced devices are often available in weight management clinics and tests are economic. These devices provide total and regional values for muscle and fat, as well as body water, bone mineral mass and visceral adipose tissue (see bioimpedance article here.) The test takes less than a minute.
Air displacement plethysmography: Commonly known as the Bod Pod, a trademark of this device. For the test, you sit in an egg-shaped chamber and body composition is based on body density calculated from an analysis of body weight and air displacement. No radiation is required. It is usually only available at specialist and research facilities. The test takes less than 5 minutes.
Is body fat percentage related to BMI?
A very interesting study published a few months ago (November 2019) looked at the correlation between BMI or WHtR and excess body fat. They found that over 33% of adult men and 52% of adult women with a BMI between 20 and 25 kg/m2 (normal range) had excess body fat. And this occurred despite the fact that the authors of the study chose a relatively high cut-off to define obesity by BF% (25% in men and 35% in women). This result highlights that fact that when we use BMI to determine whether we should treat overweight, a large number of individuals who are at increased risk of disease are actually escaping our attention.
Monitoring weight loss
When a patient starts a weight-loss programme, it is important to know that we are achieving efficacy, not just by seeing the weight fall but also by monitoring the percentage body fat. Ideally, only fat would be lost; maintenance of muscle mass will facilitate weight stability once the target weight is reached because it raises the basal metabolic rate more than fat. We can only gain this information by monitoring body composition
Although BMI is a useful and applicable method in most people, other validated methods that assess body fat rather than just body weight should also be used to ensure detection of all individuals at increased risk of chronic disease. Assessment using only BMI will leave some individuals unaware that they may need to make changes to their diet and lifestyle to achieve a healthy body composition. Body composition analysis should form a regular part of the monitoring plan of any weight loss programme.