BMI: Divides All, Except What it’s Supposed To Divide

By: Mujuthaba

How do I lose my weight? This is a question that is bombarded constantly towards people working in the fitness field. Don’t get me wrong, but this article is not on methods to lose weight. I just want to shed some light on the way of measuring one’s own ‘weight’. The most common of them all is Body Mass Index, or BMI.

What are we really talking about when we talk about gaining ‘weight’ and losing ‘weight’? In layman terms, ‘weight’ is the numerical value which you would see when you stand on a scale. If your ‘weight’ has increased, it’s a bad sign. Well, for some at least. If you turn out to be a body builder, gaining ‘weight’ would bring a smile to the face. In the fitness arena, ‘weight’ loss is the term used for fat loss. ‘Weight’ gain is the term referred to increase in lean body mass or bone and muscle mass.

So, what does BMI measure? This is a very popular measurement used to categorize people into being under-weight, normal, over-weight or classes of obesity. This is used by schools and various commercial businesses and even in the fitness industry. BMI is used to assess weight in relation to height of a person. It is calculated by dividing a persons’ weight in kilograms by height in meters squared (Weight-kg/height-m²). A BMI value less than 18.5 puts a person as underweight, a value between 18.5 and 24.9 is considered to be normal. An over-weight person would have a BMI between 25 and 29.9. Individuals with a BMI beyond 30 are classed under the three classes of obesity.

BMI is a method of measuring body composition, or the measurement of fat and fat-free mass of a body. Although, BMI neither measure a person’s body fat nor does it measure a person’s fat-free mass. I will give you an example. A body builder weighing 80kg with a height of 5’9” (1.75m) would be classified as being over-weight with a BMI of 26.1kg.m^-1. This is similar in cases where the person is of a large body stature. BMI itself doesn’t differentiate between fat and fat-free mass. How would you feel, when you train day and night, pick on the most careful diet for months, still end up being categorized as being over-weight or obese? That’s what BMI does in a number of cases.

BMI may not differentiate between fat or lean mass, but a higher or lower BMI than the normal value, is in fact associated with hypertension, hypercholesterolemia, coronary diseases and mortality rate. Although, these diseases are not associated with increased muscle mass, but rather associated with body fat increase. Trainers should be careful when measuring BMI to assess risk factors, as it may not apply to the population as a whole.

A number of fitness and medical practitioners are wary of what BMI interprets. The prevalence of this measurement has divided a number of practitioners, especially as it is used to classify pupils in schools, which affect some children psychologically. For me personally, a measurement should give valid results, in the case of BMI, an idea of fat mass of a body would be considered valid.

Bibliography:

ACSM (2006) ACSM’s Guidelines for exercise testing and prescription (7^th Ed.) Lippincott Williams & Wilkins, New York

ACSM (2005) ACSM’s Health-related physical fitness assessment manual. Lippincott Williams & Wilkins, New York