What Does BMI Tell Us?

Can we trust the BMI?

Posted Dec 13, 2011

With Christmas approaching the women's magazines sport headlines like ‘10 Easy Tips on How to Control Your Weight Over the Festive Season' or ‘Our Five Minutes a Day Workout Will Keep Your Weight in Check During the Holidays.' Weight is, of course, measured by pounds, but such measures as the BMI (the Body Mass Index) seem to feature more commonly in connection to weight loss, particularly as a measure of obesity. There also seems to be a certain level of skepticism as far as the BMI is concerned. For example, after one my talks an audience member asks: "Do I have to believe in the BMI - I have heard that it does not tell me anything?" There are also several other ways that our bodies are measured and these add to a general confusion regarding the BMI, weight, and fatness. What exactly do these tell us about our bodies?

Most so called ‘body measurements' refer to ‘body composition,' not to body weight or ‘fatness' per se. Body composition is one of the components of health related fitness in addition to cardio-vascular fitness, muscle strength and endurance, and flexibility. To be fit, one has to work to improve each component. The first three components can be improved by engaging in exercise (cardio workouts, resistance training, and stretching) whereas body composition should improve by exercising for the other components. How is this possible?

Body composition refers to the way the body is built from water, protein, minerals, and fat. Currently, body composition measurement is based on a two-component model of a ‘fat component' and ‘fat free component' (bone mineral, muscle protein, water and other chemicals). The body needs both types of components, but a certain ratio is deemed ‘normal' or ‘optimal.' Consequently, a certain percentage of body fat (%BF) is considered healthy and a certain percentage of fat free mass is considered healthy. While there are slight variations on the recommended healthy body composition based on age and physical activity level, as a general rule median or ‘normal' fat percentage for women is 28% and for men 13% while women are considered obese if their fat percentage is 35% or higher and men if their fat percentage is 22% or higher. Women should have at least 20% of their body composed of fat and men 8% of their body composed of fat (Heywood, 2002). Body composition can then be changed by increasing or decreasing the fat free component or vice versa increasing or decreasing the fat component. Most of the time women appear occupied by one of the options: decreasing the fat component.

Merely stepping on a scale to see how much one weighs does not provide the fat component of the body. Consequently, there are several ways to assess one's body composition. Most commonly used in a fitness context are the skin fold method and the bioelectrical impedance analysis (BIA) method. The skin fold method indirectly measures the thickness of the fat tissue located underneath the skin. The thickness of a skin fold is measured in 12 different places of the body using a special ‘caliper' that indicates the thickness of each fold. The thickness is then translated into a reading of one's body fat percentage by using a special ‘nomogram.' While the thickness of the skinfolds is a good measure of fat located directly underneath the skin, the reading depends heavily on the skill level of the technician measuring the skinfold. The BIA method is less dependent on the measurer's skill level as often a hand held analyzer is used. Nevertheless, one should still keep in mind that both of these methods report one's body composition: the relation of fat free component to the fat component. Consequently, they do not indicate the weight of either component. It is possible that a person with low weight (a thin woman, for example), relationally has a high percentage of her body composed of fat and low percentage composed on fat free components. As a matter of fact, fat mass weighs less than non-fat mass. Therefore, a woman with a fat component of 30% does not always need to ‘lose weight,' but might need to increase her non-fat component by building more muscle or bone mass. Likewise, a woman who looks larger does not necessarily have high body fat, but might have a larger percentage of non-fat mass that makes for a bigger sized body. How does BMI fit in all of this?

BMI (the Body Mass Index) indicates the relationship between one's height and weight (weight/height2). Consequently, it does not directly tell us about body composition, but rather refers to the size and proportion of the body. However, as BMI does not measure body composition directly, specific equations have been developed to convert BMI readings into a percentage of body fat. For example, it can be calculated that a BMI of 30 (the cutoff point of obesity) corresponds 37.6% of body fat in women and 27.7% in men. Based on these equations, BMI can been seen to represent an estimate of fatness. Researchers have found, nevertheless, that the BMI is not a particularly accurate estimate of percentage body fat. In their research, Timothy Lohman and Laurie Milliken remind us that the standard error of estimate (SEE) for the BMI is generally 5.4 for women and 5.9 for men. In scientific research a SEE over 5.0 is considered unacceptable and therefore, the BMI should not be recommended as a standard measure of percentage of body fat. Other scientists point out that BMI does not count of differences on body composition based on age, race, or physical training. For example, athletes tend to have high body weight due to high percentage of muscle mass which increases their BMI.

BMI is, nevertheless, commonly used as an estimate of obesity as it is inexpensive and does not require trained personnel to take an accurate measurement. Consequently, BMI does tell us something about the proportions of our bodies, but cannot accurately provide the actual composition of the body. If one is interested in body composition (the ratio of the fat free component to the fat component), one needs to take a skinfold test or bioelectrical impedance analysis or for the most accurate measures engage in laboratory testing such as underwater weighing, air displacement plethysmography, or dual-energy X-ray absorptiometry.

It is important to keep in mind that losing the fat component is not the only way or even the best way of changing one's body composition. One needs to carefully consider whether increasing the fat free component and/or the fat component is necessary for optimal functioning of the body. As the fat component is a necessary aspect of the healthy body, everyone, particularly women, needs a certain amount of it. In addition, it might sometimes be healthier to stop staring at the numbers for weight, body composition, or body proportion and consider how one can function in everyday life in the best possible manner. For most of us, a size 0 body or low body fat percentage just are not useful and can be unhealthy. On the other hand, there can be a problem if one's body size is so large that one can no longer manage daily life. Most of us, however, are somewhere in between and perhaps, that is how it should be. At least, it can save a lot of time and emotional energy to stop obsessing about body fat. One might even think of spending the holiday time more constructively with family and friends, sharing well balanced meals and enjoying a restful season.

Research cited:
Heywood, V. H. (2002). Advanced fitness assessment and exercise prescription (4th edition). Champaign, IL: Human Kinetics.

Lohman, T. G. & Milliken, L. (2003). Body composition assessment in the obese. In R. E. Andersen (Ed.), Obesity, etiology, assessment, treatment and prevention (pp. 73-84). Champaign, IL: Human Kinetics.