If you have ever embarked on a journey to lose, or gain, weight, chances are you might have come across something called BMI. Short for body mass index, BMI is a simple mathematical formula – a person’s weight divided by her height squared, telling if she is underweight, overweight, obese, or has normal weight. A BMI between 18.5–24.9 is considered normal, that less than 18.5 term you as underweight, 25–29.9 as overweight and above 30 as obese. “BMI is commonly used as a screening tool to identify people who are overweight or obese, which are known risk factors for a number of health conditions such as diabetes, heart disease, and certain cancers. It is also used as a tool to monitor weight changes over time,” explains Dr Muffazal Lakdawala, director – department of general surgery and minimal access surgical sciences at Sir HN Reliance Foundation Hospital.
Simple. Or is it?
The metric dates back to the 19th century and was designed by a Belgian mathematician, not physician, Lambert Adolphe Jacques Quetelet. He came up with the formula as a quick and easy way to assess obesity in the population, thus assisting the government to allocate resources. But, for such ease, it leaves out some crucial markers.
It lies often
“BMI can only be used to categorise people into broad categories – underweight, normal weight, overweight, or obese, and is primarily used to see trends in communities. However, it has a lot of limitations while measuring an individual’s health,” says Dr Sukhvinder Singh Saggu, dept of GI, minimal access and bariatric surgery at the CK Birla Hospital (R) in Delhi.
“BMI is not a perfect measure of body fatness, as it does not take into account factors such as muscle mass, bone density, and distribution of fat in the body. It means that someone with a high muscle mass, such as an athlete, may have a high BMI but be in excellent health. Conversely, someone with a low muscle mass and high body fat may have a normal BMI but be at risk for health problems,” explains Dr Lakdawala. “Hence, it is not considered a diagnostic tool but a screening tool, and it should be interpreted in the context of other health indicators such as blood pressure, cholesterol, and blood sugar levels,” the expert adds.
A study involving 40,000 people found that over 30% of those who had a normal BMI were cardio-metabolically unhealthy, based on health markers such as blood pressure readings, cholesterol, etc. Additionally, almost half of those overweight and 29% obese were found to be healthy based on their health markers. The study went on to estimate that a whopping 74 million people, considered unhealthy based on BMI, are in fact healthy based on health markers.
It also does not tell if the weight is due to muscle mass or fat, causing BMI inaccuracy as muscle is denser than fat. Hence, muscular people would qualify as obese on the BMI scale despite not having a lot of fat in their body, notes the Harvard TH Chan School of Public Health.
In a study, almost 50% of women and a quarter of the male participants were found to be obese as per their fat percentage but did not qualify as such on BMI.
“Additionally, BMI is unable to predict the body mass distribution in the body (apple-shaped or pear-shaped body) and fails to identify the health risk. Often, patients with an apple-shaped body have more fat deposition in the abdomen or around the vital organs like the liver, pancreas, and kidneys, and they are more likely to have metabolic syndrome or other diseases like diabetes, high blood pressure, and other health risks that BMI is unable to predict,” explains Dr Saggu.
The metric is especially flawed in the case of pregnant women who “typically gain weight during pregnancy, causing their BMI to rise. However, this weight gain is primarily due to the growing foetus and associated maternal tissues, and is not necessarily an indicator of excessive body fat. Additionally, pregnant women may experience hormonal changes that can affect body composition, making it difficult to accurately assess their body fat percentage using BMI. Therefore, other measures such as weight gain tracking and measurements of abdominal circumference may be more appropriate for assessing the health of pregnant women,” says Dr Lakdawala.
“Other groups of people for whom BMI may not be accurate include athletes and older adults, as they may have higher muscle mass and lower body fat, which can result in a higher BMI but be in good health. It is also worth noting that the use of BMI cut-offs to classify individuals as underweight, normal weight, overweight, and obese is not recommended for people with certain conditions such as muscle wasting diseases or eating disorders,” he elaborated. “Therefore, it is essential to use other methods such as body composition analysis, bioelectrical impedance, skinfold thickness measurements, and waist-to-hip ratio along with clinical evaluation and history to have a better understanding of a person’s health status,” Dr Lakdawala adds.
Then there is error caused by misreported height. As per a 2017 study published in Menopause, it was found that 29% of participants had underestimated their height and almost 15% had overestimated.
Can it trigger body image issues?
“Yes, as many start focusing solely on their weight and not the overall health,” says Dr Bela Sharma, additional director of internal medicine at Fortis Memorial Research Institute, Gurugram.
Seconding that, Dr Lakdawala adds that BMI is a “single number that is used to classify a person’s weight status and it is not an accurate measure of health. This can lead people to believe that they are not healthy or attractive based on their BMI, even if they are actually healthy and fit. This can cause feelings of shame, guilt, and low self-esteem, which can lead to negative body image. Additionally, because BMI is based on weight and height, it does not consider individual differences in body composition or muscle mass, which can lead to misclassification of an individual’s weight status. This can cause confusion, frustration, and dissatisfaction with one’s body, which can lead to negative body image.”
However, BMI alone cannot be seen as the determinator of a person’s body image as other factors such as societal norms, media representation, personal and family beliefs and experiences, and self-esteem, too, play a role here, the experts point out.
Better metrics
“The body fat mass, the distribution of the body mass, and lean muscle mass can be considered when assessing an individual’s health. These indicators can be calculated using body composition, which includes total body mass, lean muscle mass, bone mass, fat mass, and mineral and protein weight,” says Dr Saggu. “Body mass distribution can be assessed by calculating the body’s composition and examining the amount of fat in the body. The fat should be less than 50% of their height. For example, if a person is 180 cm tall, his waist should be less than 90 cm. However, if the waste or fat levels are higher, he or she is more likely to develop diabetes, hypertension, metabolic syndrome, and cardiovascular diseases, such as high blood pressure, stroke, diabetes mellitus, high cholesterol, and high blood glucose levels,” he explains.
But it continues to be used.
Despite its inadequacies, BMI as a health metric continues to be used. Being a straightforward mathematical formula, it is easy to be calculated and can be calculated on a wider population to compare and assess the general state of health. “Additionally, it has been used for many years, and is familiar to most health professionals and researchers, which makes it easy to compare data across different studies. It is also a relatively inexpensive and non-invasive method to estimate body fat, which makes it an attractive option for large population studies and health screenings,” Dr Lakdawala points out.
However, it should be pointed out here that despite the flaws, BMI can be a useful tool to assess the population at risk of obesity, and hence related diseases. However, it should not be the sole indicator and other methods such as body composition analysis, bioelectrical impedance, skinfold thickness measurements, and waist-to-hip ratio along with clinical evaluation and history should also be employed to assess the overall health of the individual. Diet, exercise, mental health and family history should also be factored in.
BMI: Pros & cons
- BMI is not a perfect measure of body fatness, as it does not take into account factors such as muscle mass, bone density, and distribution of fat in the body
- A study involving 40,000 people found that over 30% of those who had a normal BMI were cardio-metabolically unhealthy, based on health markers such as blood pressure readings, cholesterol, etc
- Despite flaws, BMI can be a useful tool to assess the population at risk of obesity, and hence related diseases
- In a study, almost 50% of women and a quarter of the male participants were found to be obese as per their fat percentage but did not qualify as such on BMI
- The metric is especially flawed in the case of pregnant women who typically gain weight during pregnancy, causing their BMI to rise
