Is BMI an Accurate Measure of Health?
It doesn’t matter whether you are reading an article about an exercise or diet plan on the web, searching for effective weight loss strategies, or attending a doctor appointment, you will most likely come across the abbreviation – BMI.
Body mass index has long been used worldwide to assess body weight and associated health risks. Apart from that, WHO and various state health institutions are using BMI calculators to track obesity rates in populations.
Despite its widespread application, BMI is not universal and has some flaws. In the article below, we will discuss the accuracy, history, downsides of BMI, compare it to alternative options and try to answer the main question – Is it an accurate measure of our health status?
What Does Your BMI Mean?
Body mass index is a value that allows you to assess the degree of correspondence between a person’s mass and his height and thereby judge whether the mass is insufficient, normal, or excess.
It is based on a mathematical formula that divides a person’s weight in kilograms by their height in meters squared.
BMI = weight (kg) / height (m2)
If you are more comfortable with the imperial system, you can use the alternative formula:
BMI = (weight (lbs) / height (in2)) x 703
Or, you can simply use an online calculator that does the job for you, such as the one provided by CDC.
According to the results, you will fall into one of these categories:
|<16.5||Severely underweight||Very high|
|18.5 - 24.9||Normal weight||Low|
|25- 29.9||Overweight||Low to moderate|
|30 - 34.9||Obesity class I||High|
|35 - 39.9||Obesity class II||Very high|
|>40||Obesity class III or extreme obesity||Extremely high|
History of BMI
To better understand why BMI sometimes fails to assess individual health, we need to look into the history of its development.
Around 200 years ago being overweight was not generally seen as a significant health issue, since by that time low body weight was the leading concern of medical practitioners.
But with the rise of science, the correlation between excess weight and morbidity became more clear, and already at the beginning of the 20th century, the insurance companies began to use weight charts to facilitate the standardization of the medical selection process.
The need for a more practical index became more apparent in the 1950s when numerous actuaries reported the increased mortality of their overweight policyholders.
The modern term “body mass index” was coined in 1972, when Ancel Keys, Ph.D. published a paper titled “Indices of Relative Weight and Obesity” in the Journal of Chronic Diseases.
In his study, the author used the data of more than 7,000 men from European countries and offered a weight-to-height index (also known as the Quetelet index), created in 1832 by Adolphe Quetelet, as a way to evaluate bodyweight about height.
Quetelet was a mathematician, not a physicist, so he created the formula following statistics, concentrating on the average human body.
Even Dr. Keys himself judged BMI as appropriate for population studies and inappropriate for individual evaluation. Still, due to its simplicity, BMI has come to be widely used for preliminary diagnoses.
Downsides of BMI and When it is Inaccurate
The BMI was not originally designed to measure health
After reviewing the history of BMI creation in the previous section, this should not come as a surprise. Adolph Quetelet worked on the conception of “Homme Moyen” or average man.
All his works were concentrated on population average or in other words, his formula just reflects how much people weigh on average. It does not intend to give medical advice on how much people should weigh.
BMI can not differentiate between fat and muscle
Even though BMI is used to measure body fat composition, in the case of individuals with decent muscular mass, it fails to depict the truth.
The formula assumes all weights have been equal, but in reality, 1kg of muscle takes up less space than 1kg of fat.
As a result, two people with the same weight and height, who look completely different: one is lean, with a muscular build and the other has high-fat mass, will have the same BMI.
If we take the iconic boxer, Mike Tyson at his prime, with his 5 ft 10 in (178 cm) height and 218 pounds (99 kg) weight, he would have a BMI of 31.6 – obesity class 1, with high health risks. Does not make sense, right?
The same problem arises not only with athletes but with the elderly who tend to have less muscle mass, lower bone density, and may have false “normal” BMI.
So it is important to consider a person’s muscle, fat, and bone mass and not rely alone on BMI.
Does not consider the fat distribution
In general, high fat mass is linked to poor health outcomes, but the location of fat makes a difference.
People with pear-shaped bodies and fat stored around the stomach possess a greater risk of chronic diseases.
A 2020 meta-analysis of 72 studies, with more than 2 million participants concluded that central obesity “independent of overall adiposity, was positive and significantly associated with higher all-cause mortality risk.
Larger hip circumference and thigh circumference were associated with a lower risk. The results suggest that measures of central adiposity could be used with body mass index as a supplementary approach to determine the risk of premature death.”
Does not consider other factors of health
We have already mentioned that BMI does not take into account fat distribution, muscle mass, and age.
Multiple other factors determine our health, including genetics, gender, race, lifestyle, past medical and family history, or lab values. Cholesterol, blood glucose, and arterial pressure are as important measurements of health as BMI.
So relying solely on BMI to determine cardiovascular, endocrine, or overall health status is not always a good idea.
May lead to weight bias and even stigma about the weight
Sometimes overconcentration on BMI, especially when a patient presents with non-weight-related problems, may divert attention from more serious health issues and lead to poor healthcare quality.
In addition, a study about weight bias shows that they negatively affect a patient’s engagement and cause a delay of health service, since people with high BMI tend to miss their regular checkups, because of previous negative experiences and fear of being judged.
Once we have listed the downsides of BMI application, we can name the people groups for which BMI may be inaccurate:
Due to high muscle mass.
The study of BMI and all-cause mortality in older adults shows that BMI less than 23 in people over 65 is associated with higher health risks and it’s better to have a BMI of 27.
Asian and black people
Numerous studies have confirmed that Asians have more body fat at any given BMI compared to Europeans. Asians are also more likely to accumulate fat around the waistline.
We have already mentioned that abdominal fat is the most dangerous.
As a result of these differences, Asians have an increased risk of chronic diseases at lower BMI values, so WHO developed Asian-Pacific BMI guidelines, which provide alternative BMI cut-off points.
The situation is the opposite in black people, who in general have lower fat and higher muscle mass, so a BMI scale with higher cut-off points should be applied, especially for Black women.
Pregnant or nursing women
Fat percentage increases due to changes during and after pregnancy. These physiological effects are reversible and necessary for baby nourishment. They do not correlate with long-term health risks.
Very tall or very short people
Many mathematicians point out that squaring of height in BMI formula, leads to the division of weight by too large a number, hence short people are misled into considering themselves thin and tall people, vise versa, think they are fatter.
Is BMI a Good Indicator of Health?
Although BMI might not be accurate in certain individuals or groups, most studies show that overall it is quite accurate in identifying chronic disease and premature death risks, which are especially high for people with BMI lower than 18.5 and higher than 30.
Obesity significantly increases the risk of type 2 diabetes, heart, kidney, liver, and lung diseases.
Furthermore, studies have demonstrated that the reduction of BMI is associated with decreased rates of the above-listed diseases. There are no questions about the reliability of the data.
A study published in 2014, describes the correlation between BMI and mortality risk in 17,000 adults.
“Underweight” people tend to live an average of 6.7 years less and “extremely obese” people 3.7 years less than people who have BMI in a normal range.
The risks are not attributed to cardiovascular or endocrine diseases alone, instead, sociodemographic and behavioral factors are also involved.
Other, 2017 retrospective study of more than 100,000 deaths, revealed that people with BMI >30 had on average two times greater risk of death.
Alternatives to BMI
Body fat percentage assessment
This can be done using tools like skinfold measurement or bioelectrical impedance analysis. Both have a high risk of error.
Alternatively, a dual-energy X-ray absorptiometry will be more effective but quite expensive. For individual purposes, it offers a more detailed picture, but it can not be used as a massive screening tool.
Reflects your central obesity and some studies found it more useful for determining the risk of developing type 2 diabetes and other obesity-related disorders. For men: waist measurement above 40 inches (102cm) indicates a higher risk.
For women, the limit is 35 inches (85cm). The main benefit of the use of waist circumference is that it requires only a measuring tape, but as with BMI, it does not take into account different body types.
The waist-to-hip or waist-to-height ratio
These methods are also very simple, you will need a measuring tape and calculator.
Waist-to-hip ratios more than 0.95 in men and 0.8 in women are considered to be high, indicating the presence of visceral fat. In turn, an ideal waist-to-height ratio should not exceed 0.5. These methods, as the previous one, do not consider differences in body types.
BMI is a simple, inexpensive, and non-invasive way to measure body fat. This accounts for its wide use to define obesity in general populations and multiple studies prove its efficacy in this field.
But as a single measure, it is not perfect and health practitioners can not rely on BMI values only and should always be aware of the above-listed downsides.