Have you ever googled the word BMI to understand what it actually means? Have you ever used a BMI calculator yourself to figure out your BMI or which BMI category you fall into? And then tried to figure out how to increase your BMI? Or have you been asked about your BMI at health clinics or in social events and parties very often?
Answered ‘yes’ to any one of these? Well, you have already fallen into the trap of BMI then.
Let’s understand what BMI is, why it is not the whole truth, and how it is used as a trap.
Body Mass Index (BMI) refers to a measure that relates body weight to height. Basically, it consists of 2 major components – weight measure in kilograms and height measured in meter square. The BMI is a convenient thumb rule used to broadly categorize an individual as underweight, normal weight, overweight or obese.
The formula for BMI is: BMI = Weight (in kgs)/ Height (in m2)
Example, Weight = 80 kgs, Height = 1.71 m. Thus BMI = 80 ÷ (1.71 x 1.71) = 27.35 kg/m2
According to Asian Indian classification of BMI cut offs, the different categories of BMI are as follows -
Classification | Cut offs |
Underweight | < 18.5 |
Severe Underweight | < 16 |
Moderate Underweight | 16 – 16.9 |
Mild Underweight | 17 – 18.4 |
Normal weight | 18.5 – 22.9 |
Overweight | 23 – 24.9 |
Obese | > 25 |
Obese Grade I | 25 – 29.9 |
Obese Grade II | 30 – 34.9 |
Obese Grade III | > 35 |
Based on whether an individual falls into the normal weight or underweight or overweight category, predictions can be made about the individual’s risks of developing diseases in future. BMI does not measure body fat however it is moderately related with body fat. Besides, BMI seems to be closely correlated with various metabolic diseases and the disease outcomes.
Is BMI the correct and authentic parameter to indicate good health?
Physicians and scientists disagree about multiple factors of BMI due to its simplicity. BMI is a measurement which does not take into account a lot of factors like the age of a person, their fat mass, bone weight, overall body composition, gender and built of the person, racial differences, physiological conditions like pregnancy and lactation, physical activity and exercise regime.
Hence, it cannot be considered as the whole and sole marker of good health. It cannot be the only parameter to be considered while deciding the total caloric requirements of individuals and it certainly cannot be used as a diagnostic parameter for obesity, underweight or normal weight categories solely.
Age
As we get older, our lean body mass tends to drop. As a result, BMI of such individuals will fall under normal weight category, but their composition has higher percentage of fat as compared to lean muscles which is unhealthy. This information is not captured when only BMI is considered as a sole marker of good health.
BMI correlates proportionally to weight and is inversely proportional to the height of a person. A tall individual with a slim frame will have lesser body fat as compared to an individual of the same body weight but shorter height. Thus, the BMI of tall people will be higher due to natural scalability. Such a tall individual may be placed at the normal category of BMI or at higher end of the BMI categories and thus it does not denote the actual characteristic of leanness or lesser body fat.
For an individual with tall stature, the BMI overestimates by roughly 10% while it underestimates roughly by 10% for a smaller frame or an individual with a short stature.
The biggest flaw with BMI is that it does not take into account the person’s body fat versus muscle (lean tissue) content. When we talk about athletes, who have a higher percentage of lean body mass, subsequently their total body weight is higher too. Hence, when compared to the Asian cut offs standards for BMI classification, such an athlete would fall under the category of over-weight or obese. However, in reality athletes are generally the fittest lot amongst the entire population of a state or country.
Women tend to have lesser lean body mass as compared to their male counterparts. Thus, their fat mass is inherently more as compared to men. However, there is no representation of this aspect in the formula for BMI. In a study by Romero-Corral et al. 13,601 subjects from the USA were examined and found that BMI-defined obesity (BMI ≥ 30) was prevalent only in 21% of men and 31% of women. However, Body fat-defined obesity was prevalent in 50% of men and 62% of women.
Dr. Carl Lavie, an American Cardiologist states that ‘The BMI tables are excellent for identifying obesity and body fat in large populations, but they are far less reliable for determining fatness in individuals’
In the July edition of Mayo Clinic Proceedings, Dr. Levie pointed out that while it has been correlated that having a higher BMI or being overweight can put one at risk of heart ailments, recent medical advances prove that patients with heart failure have lower mortality rates (lower death rates) if they are obese.
BMI is a measurement which does not take into account a lot of factors like the age of a person, their fat mass, bone weight, overall body composition, gender and built of the person, racial differences, physiological conditions like pregnancy and lactation, physical activity and exercise regime.
BMI levels vary drastically based on the race and ethnicity of individuals and even amongst subpopulations within ethnicities due to environmental and lifestyle factors. For example, compared to urban Chinese, rural Chinese have lower average BMI levels.
Regular calculations for BMI cannot accurately indicate the state of a person’s health at some stages in life like pregnancy, lactation, childhood and adolescence. For this reason, BMI calculations for children and teens varies.
BMI is only a general guideline and it does not take into account the physical activity of an individual. Hence, an individual who exercises and has good body composition might be categorised under the section of obese or overweight incorrectly.
After every few weeks, statements such as ‘BMI is incorrect’, ‘BMI is flawed’, ‘BMI is not the whole truth’ make for the headlines. Facts are presented and these statements are established as true. However, a few days later, all goes quiet and the practise of using BMI is continued.
So the question arises - Why was BMI invented in the first place?
BMI was invented by Lambert Adolphe Jacques Quetelet, who was a mathematician, in the 19th century. He devised this formula to understand and measure the degree of obesity. This formula was given by Quetelet some 200 years ago. He was not a physician and had not studied medicine. It will be surprising to know that BMI was originally called the Quetelet index. With advancements in technology and science, we now have far better and more reliable ways to measure body fat. We have also understood the limitations of BMI and, hence, there must be a radical shift in our approach to defining and measuring fitness.
Where can BMI calculations be used?
BMI is extremely useful in public health set up where one needs to collate data for comparision purposes. BMI is suitable for recognizing trends within sedentary or overweight individuals because there is a smaller margin of error.
A general correlation of height and weight, as done while calculating the BMI, benefits particularly for consensus data regarding obesity or various other conditions. It can be used to build a semi-accurate representation of the masses. Using this representation, a solution can be stipulated, or the RDA for a group can be calculated.
Article by - Dt. Niti Dhulla, RD | PGD | BSc (FND)
(Registered Dietitian, Gold Medallist in Food, Nutrition and Dietetics from University of Mumbai, Certified Yoga Instructor and Internationally Certified Fitness Expert)
Owner and Founder at Online Nutrition Consultancy “NutriDietbyNitiDhulla”
Website – www.nutridietbynitidhulla.com | Instagram - @NutriDietByNitiDhulla
Facebook - NutriDietByNitiDhulla | Email - nutridietbynitidhulla@gmail.com
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