Is BMI Accurate To Measure Health

Introduction

Defining the concept of body mass index

The body mass index (BMI) is an anthropometric measurement that serves to quantify the body’s fat content. It is calculated from your height (in metres) and weight (in kilograms), expressed in units of kilograms per metre squared (kg/m²). It is used as a health measurement or screening tool for categorising individuals into various weight groups that may be indicative of their body fat (adipose tissue) content.¹ A higher BMI value means that you are more likely to be obese, whilst a very low BMI value is a sign of malnutrition. The BMI is proportional to body weight, indicating whether an individual may be too heavy or too light for their given height. 

Though widely applied, the method of using BMI to measure the health of patients is a contentious subject because it is viewed as a crude and therefore inaccurate marker of a person’s state of health.² Despite BMI also being used as a diagnostic tool for underweight or malnourished individuals, a greater focus is placed on excess weight and obesity. This is because obesity or being overweight is more prevalent across the globe, particularly in some regions of sub-Saharan Africa. Obesity and excess body weight are also responsible for more deaths compared to malnourishment.³ 

This article gives an in-depth analysis of the concept of BMI as an instrument for assessing health, taking into consideration its weaknesses. Alternatives to BMI measurements are also discussed, taking into context the criticisms around BMI being an inadequate health assessment tool.

BMI categories and calculation method

The kg/m² units of BMI measurements are derived by dividing weight by the square of the individual’s height. BMI is categorised into 4 numerical ranges, each range being an indicator of one of the following bodyweight classifications: malnourishment, normal weight, excess body weight and obesity (Table 1).¹

Table 1: Categories of BMI and the associated health statuses.

In children aged 2 to 19, the BMI calculation is a bit more complex as it also takes age and gender assigned at birth into account. This is because children, especially adolescents, are still growing. For children, the BMI is categorised according to percentiles, whereby the BMI of the child is compared to the BMI of other children of the same age and gender.¹ For example, if the BMI of the child is greater than that of more than 95 per cent of their mates of the same age and gender, they are considered to be obese (Table 2). Since the BMI tool is very ‘sketchy’ when it comes to children, the expertise of a paediatrician may be required to set things straight.

Table 2: Categories of BMI in children and the associated health statuses.

A commonality of BMI assessments in both adults and children is that BMI values greater than 30 are indicators of obesity. Additionally, a BMI value of greater than 35 and 40 suggests extreme obesity in children and adults, respectively. On the other end, a very low BMI value of less than 17.5 could be a signal for eating disorders such as anorexia. A BMI of less than 15 is accepted as a marker of starvation.⁴

The importance of accurate health measurements given the risks associated with obesity

Health measurement tools, such as BMI, may serve as a gauge for measuring the levels of risk an individual may be facing due to being overweight or obese. Obesity or excess body weight place individuals at risk of developing non-communicable comorbidities such as hypertension, diabetes, cardiovascular diseases, respiratory disorders as well as some types of cancers.⁵ These comorbidities are caused by the high fat content found in people with obesity or excess body weight. The most detrimental form of body fat in this case is visceral fat, which is found wrapped around vital organs of the midsection.⁶ The comorbidities of obesity are increasingly being viewed as public health threats around the globe.

Obesity, as a medical condition, has been declared to be a global epidemic by the World Health Organisation (WHO).⁷ The extent of the problem has prompted some countries to employ various strategies to combat this scourge. For instance, the tax regime of some countries includes sin taxes, to collect extra revenue that funds healthcare due to the contribution of products such as alcohol and sugar in the prevalence of obesity. It is therefore vital to adopt body fat screening and measurement methods that may help in alerting patients of the health risks that they may be facing. Awareness of their health risks could prompt individuals to make healthier lifestyle choices as a way of mitigating disease development risks.

Historical account of how BMI came to be adopted as an indicator of health status

The adaptation of this numerical method as a measure of health status

The BMI measure, though first conceptualised in 1832 by Adolphe Quetele,⁸ was adopted as a measure of body fat after World War II. At the time, insurance companies were recording an increase in claims culminating from the death of obese policyholders, necessitating the development of a risk calculation method.⁸ To that effect, insurance companies implemented a method of calculating risk by comparing the weight of policyholders with the average weight of people of the same height.^8,9 Unimpressed by this method, Ancel Keys, a physiologist, devised a study to determine alternative ways in which health risks could be assessed. Keys' study concluded that BMI (referred to as the Quetelet Index at the time) was the best way to calculate risk.⁹ Due to Keys’ study, BMI was adopted as a standard by the life insurance industry in calculating risk for premium charges, and the healthcare fraternity in assessing health status.

Why the BMI method may be flawed, given its history

• When developing this numerical assessment, Adolphe Quetelet was not focused on obesity⁸
• Quetelet’s study only assessed Scottish and French people assigned male at birth, making it only suited for Western European people assigned male at birth. The same holds for Keys’ study which only included European people assigned male at birth.
• Keys did not address BMI in people assigned female at birth, children and the elderly¹⁰
• Keys himself, in his study, acknowledged that this method of measuring body fat is not the most accurate¹⁰

Shortfalls of BMI in achieving an accurate health assessment 

Whilst BMI does give an idea of a person’s relative body weight, it is not accurate across the board as numerous variables such as body shape, muscle mass and bone density also need to be accounted for. This is because these factors may also contribute to patients having a higher body mass. Since BMI is only derived from height and body weight, it is possible to come across individuals who have a high BMI, but a low fat content. Conversely, people with a BMI that indicates normal weight may be facing the same risks as those considered to be obese.² BMI measurements are also unable to specifically pinpoint visceral fat content, and this happens to be the form of fat that brings about the most harm. Herein the shortfalls of BMI are discussed:

• BMI simply relies on total body weight as determined by a scale, so it does not account for the respective contributions of fat mass and muscle mass to the total body weight. The consequences of this are seen in people with an increased muscle mass as they will be recognised to have a high-fat content due to a high BMI score which has been driven by an increased muscle mass. An example of such individuals is athletes.^4,11
• BMI does not account for bone density. People with a higher BMI, in osteoporosis studies, have been found to have a higher bone density which reduces the risk of osteoporosis.¹² This means that bone density could contribute to BMI by adding to the total body weight.⁴
• In athletes, particularly those who engage in weight-bearing and high-impact sports, bone density is elevated and is very unlikely to have a high fat content. In weight-bearing athletes, bone density can be up to 10% higher when compared to non-athletes.¹³
• BMI does not account for fat distribution as it relates to body shape. Visceral fat is the most dangerous form of fat as it wraps around your vital organs increasing the likelihood of developing various non-communicable diseases. So a person with a larger midriff is more likely to have more visceral fat, but someone with a pear-shaped body may have more fat around their thighs and hips. Since BMI is not appreciative of body shape, people with pear-shaped bodies could be misclassified as being at a higher risk of developing comorbidities associated with increased visceral fat content.
• BMI does not accurately apply to people of all ethnicities. A prime example is South Asians who have been documented to have a higher likelihood of bearing high levels of visceral fat, despite having a BMI that is indicative of normal weight.⁴ This visceral fat deposition places South Asians at a higher risk of developing diabetes⁴
• Africans and Polynesians, on the other hand, usually have lower body fat compared to Europeans of the same BMI, and may therefore be misclassified as having a high fat content according to BMI measurements¹⁴
• Due to BMI not being specifically adapted for fat content, BMI calculations may be inaccurate for people assigned female at birth and the elderly as they tend to have a higher fat content than men and younger people of the same BMI, respectively.
• BMI as an indicator of obesity, has been argued to be disconnected from cardiovascular and metabolic health. Some studies of BMI have shown that some individuals with a high BMI and fat content exhibit cardiovascular and metabolic fitness. Fitness has been reported to be a risk-reducing factor in individuals who are classified as obese for diseases such as diabetes, some cancers and cardiovascular ailments.¹⁵ People who are fit but medically obese according to the BMI measure have also been shown to display metabolic markers that indicate minimal risk of disease development. This phenomenon is referred to as “fat but fit” and is currently the subject of much debate¹⁵
• For the non-communicable diseases which are comorbidities of excess body weight and obesity, there is an element of genetics when considering the risks of developing these conditions. However, BMI does not factor in genetic predisposition

Alternative anthropometric measurements

The BMI method of assessing health is flawed. As such, there should be considerations of alternative or supplementary health measurement methods to enhance accuracy. This section discusses other anthropometric measurements that may be applied in place of BMI. These anthropometric measurements are also better suited for assessing visceral fat.

Waist-to-hip ratio

The waist-to-hip ratio is calculated by dividing your waist circumference by the circumference of your hips around the buttocks. A higher ratio indicates a higher fat deposition around your waist. This is a better indicator of whether an individual may have a higher visceral fat content that can put them at risk of developing associated non-communicable diseases. Waist-to-hip ratios greater than 0.90 and 0.85 in men and women are indicators of obesity.¹⁶

Waist-to-height ratio

The waist-to-height ratio is calculated by dividing your height by the waist circumference. Together with the waist-to-hip ratio, this measure has been reported to be more accurate than BMI in assessing obesity. The recommended waist-to-height ratio is between 0.4 and 0.5. Waist-to-height ratios above this range are indicators of obesity and increased risk of developing the associated non-communicable disease. The higher the waist-to-height ratio, the more at risk the individual might be.¹⁷

Waist circumference

Waist circumference is determined by simply measuring your waist using a tape measure. This measure is also recommended for measuring visceral fat and assessing the risk of developing the associated non-communicable diseases. The downside to this measurement is that it does not apply to children and cannot be applied uniformly across people of different ethnicities.¹⁷

Body composition analysis 

Body composition analysis entails the measurement of fat in the entire body using several techniques. This type of analysis can determine your fat and lean muscle composition, for calculating or estimating your exact body fat percentage. Body composition analysis can be achieved using many methods which include the skinfold test, bioelectrical impedance analysis (BIA), hydrostatic weighing and dual-energy x-ray absorptiometry (DEXA). These measurement techniques can assess fat deposition across the entire body. The downside to some of these techniques is that they require specialised equipment and a certain level of skill in application.¹⁸

Disadvantages of applying the BMI measure in healthcare

BMI is widely used as a screening tool for patients, despite the large body of evidence that disproves it as an accurate measure of health. The BMI measurement is preferred for assessing health and is applied in population surveys due to its simplicity.¹⁹ The WHO guidelines on obesity are also based on the BMI metric. The implications are that patients may be misdiagnosed as obese or misclassified as being at risk of developing certain non-communicable diseases. Furthermore, some healthcare guidelines are based on the concept of BMI. A prime example is how patients who are classified as being obese according to BMI may be denied surgery until they lose weight. 

BMI also plays into societal perceptions around weight and obesity, and may negatively affect the healthcare experience for high BMI individuals. People considered to be obese according to BMI may avoid consulting healthcare practitioners due to the stigmatisation of obesity. The BMI metric may also affect access to healthcare insurance as healthcare insurance companies may deny coverage for individuals with high BMI scores.

Summary

The BMI measure is a simplistic tool that is widely applied in defining obesity, though it may also be used to detect malnutrition. It is a ratio of weight (in kilograms) and a square of height (in metres). Due to the increasing burden of obesity globally, defining obesity and assessing the disease risks individuals may be facing is vital. BMI, in this regard, is a cheap and easily applicable tool. 

However, despite the wide application of BMI, due to its simplicity various bodies of work have demonstrated that it is an imperfect measure as it is not applicable across the board. BMI does not take into account differences in body shape (fat distribution), muscle mass or bone density. This metric also fails to consider the BMI dynamics that come into play when comparing fat content in people assigned male or female at birth, different ethnic groups as well as the elderly. More accurate obesity measurement tools are available, these include the waist-to-hip ratio, the waist-to-height ratio and various body composition analysis techniques.

Nevertheless, the widespread use of BMI, despite the availability of more accurate tools, is driven by policy, with the WHO recommending the metric for use in defining obesity in patients. The application of BMI in healthcare has some disadvantages as it has led to discrimination of patients considered to be obese, even affecting access to healthcare insurance.