The BMI and Obesity Paradox

What is body mass index (BMI)?

Body mass index (BMI) is an important anthropometric calculation that provides an estimate of the amount of fat in your body. It is based on two factors – your weight and your height. It is also known as Quetelet’s Index.

It is generally assumed that a higher BMI correlates with a higher amount of fat in your body. This is true to a certain extent but has certain limitations, as we will see later.

How is BMI calculated?

BMI is calculated by dividing your weight by the square of your height.

There are slight differences in the formulas for calculating BMI, based on the units of weight and height that are used in calculations. However, the result obtained is the same with all formulas.

Below mentioned are equations for two commonly used systems – the SI system and the UCS system.

As an example, the BMI has been calculated by both methods for a 70 kg (154.32 lbs) man with a height of 5 feet and 6 inches (66 inches or 1.67 meters). By each method, BMI comes out to be 25.

• In the SI system (Systeme International system) of units, weight measured in kilogram (kg) is divided by the square of height in meters (m). This is the most commonly used system worldwide.
• The other system is the American Unit system or the United States Customary (UCS) system. In this, the weight measured in pounds is multiplied by 703 and then divided by square of height in inches.
• BMI (SI) = Weight (kg)/ height² (m) = (70/1.67×1.67) = 25.0 kg/m²
• BMI (USC)= 703 x Weight (lbs)/ height² (in) = (703×154.32/66×66)= 25.0 lbs/in²

What is the normal range of BMI?

The normal range of BMI varies from 18.5 to 24.9

What does abnormal BMI mean?

In adults, a BMI of 18.5 – 24.9 is considered as normal BMI.

Adults with BMI of < 18.5 are considered Underweight and a BMI of > 30 are considered Obese.

The body weight is related to BMI as follows:

• BMI 18.5 – 24.9 – Normal weight
• BMI 25 – 29.9 – Overweight
• BMI > 30 are classified as Obese
• BMI 30 – 34.5 – Obesity Class I
• BMI 34.5 – 39.5 – Obesity Class II
• BMI > 40 – Obesity Class III (Extreme obesity)

Uses of BMI

• BMI indicates whether you have a healthy body weight for your height.
• BMI gives the simplest and easiest estimate of the fat content in your body.
• BMI helps in identifying people at higher risk for many conditions: People with higher BMI, especially the obese category, are at an increased risk of developing conditions like:
  • Insulin resistance
  • Type 2 diabetes
  • High blood pressure
  • Cardiovascular diseases like angina, heart attack (MI), sudden cardiac death
  • Cerebrovascular diseases like stroke
  • Dyslipidemia
  • Cholesterol gallstones
  • Arthritis
  • Certain cancers like breast, endometrial and gall bladder cancer
  • And an overall increased risk of death

However, a higher BMI may not always mean more fat, as we will see below in limitations of BMI.

Limitations of BMI

BMI is widely used in healthcare settings for the assessment of body fat and for predicting the risk of development of obesity and its associated diseases. However, BMI does not consider factors like age, sex, genetics, race, ethnicity, muscle mass and body fat distribution. These factors have a significant impact on the overall predisposition to the development of various metabolic and cardiovascular diseases like insulin resistance, type 2 diabetes, stroke, myocardial infarction, and sudden death, making BMI an imperfect and inaccurate measure for assessment of obesity and related diseases. We will consider all these factors one by one below:

• Age: With advancing age, muscle mass decreases and fat starts accumulating. Thus, a 65-year-old man with a BMI of 24 may be healthy in terms of BMI, but may have excess body fat, predisposing him to cardiovascular diseases.
• Sex: For the same BMI, there are significant differences between men and women with regard to their overall body fat, distribution of body fat, and the associated risk of cardiovascular and metabolic diseases related to obesity.
  • First of all, women have higher fat content and lesser muscle mass than men of same weight. So a higher BMI in a woman is more significant than in a male with same BMI. BMI calculations do not factor sex in their calculation.
  • Secondly, women have more gluteofemoral fat (fat in the lower part of their body) as compared to men, who have more abdominal fat. This is important as abdominal obesity is much more significantly linked to the risk of cardiovascular or metabolic diseases than gluteofemoral obesity. BMI does not consider the distribution of fat and so the risk of disease prediction based on BMI is inaccurate.
  • Thirdly, males are genetically more predisposed to cardiovascular and metabolic diseases than women. This again is also not accounted for by BMI.
• Race, genetics, and ethnicity: BMI does not consider the racial, genetic, and ethnic differences in different individuals having the same weight and height, in terms of their overall body fat, muscle mass, bone mass, and their inherent risk of developing a particular disease. Certain races may be inherently more susceptible to a particular disease, even with a lower BMI whereas certain races may be resistant to the same disease, even at a higher BMI. For example, Asian women have been found to have a higher risk of developing metabolic diseases like diabetes and cardiovascular diseases at lower BMI’s than women of other regions.
• Muscle mass: BMI does not factor in the muscle mass in its calculation. So its assessment of body fat is erroneous in many cases as all the excess weight is supposed to be because of fat. For instance, athletes and bodybuilders with higher muscle mass, may fall into the overweight or obese category in BMI calculation despite being in the best of their health and certainly being low risk for metabolic diseases. Whereas some people with lower muscle mass may have a normal BMI even if their fat content is high. Such people would be erroneously labeled as normal weight and low-risk individuals whereas, they might be high-risk for cardiovascular diseases. For example old age, muscular dystrophies etc.
• Waist to hip ratio: BMI does not take into account the distribution of fat in the body. It is well known that people with abdominal obesity (central obesity or high waist-to-hip ratio) have a far higher risk of insulin resistance, type 2 diabetes, cardiovascular diseases, metabolic diseases and sudden death than the ones who have obesity in their lower body (more gluteofemoral fat). Thus distribution of body fat is a very important predictor of associated diseases, which is not considered by BMI.

In a nutshell, BMI does not differentiate between muscle and fat, does not consider fat distribution (abdominal or gluteofemoral) and certainly can not be used as a sole indicator for predicting the risk of metabolic, cardiovascular or cerebrovascular diseases. However, it is still the most convenient, quickest and easiest method of a broad weight classification and knowing how much an individual deviates from normal. Further risk assessment is done by healthcare professionals using other much more refined indices.

Carry home message

BMI has been since a long time to assess whether a person’s weight is appropriate for his height and also to classify him as healthy, overweight or obese. As we have seen above, BMI alone does not indicate good or bad health, and the overall risk of developing cardiovascular, metabolic, and cerebrovascular diseases is influenced by many other factors like your age, sex, genetics, race, level of activity, concurrent medical conditions, smoking, alcohol, fat distribution etc.

However, the negative health outcomes of being overweight are well documented and BMI is still one of the easiest and best tools to identify if you are overweight or not. If you fall in the overweight category, you should consult your doctor. Your doctor will do a further risk assessment, considering all factors like your age, sex, muscle mass, body fat distribution, to determine whether you need lifestyle changes to reduce your health risks. Stay healthy and maintain your BMI below 25!

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