Body Mass Index (BMI) – is it the best measure of obesity?

Introduction

          In my post “What is Obesity – is it merely about BMI? What is obesity? I had defined obesity as an abnormal accumulation of fat such that health is impaired. Body Mass Index is the most widely used measure of identifying obesity. However, in my post, I had cautioned that contrary to the general perception, obesity is not about BMI. In this post, I will be discussing what is Body Mass Index, its advantages and limitations. In addition to describing the method for calculation of BMI, for the ease of readers, I will be briefly discussing ‘BMI calculator tool’ and ‘BMI charts’.

What is Body Mass Index

Body Mass Index is a simple index of weight-for-height that is commonly used to assess ‘normalcy’ for body weight. It is defined as the body mass divided by the square of the body height. It is universally expressed in units of kg/m2, resulting from body mass in kilograms and height in meters. If pounds and inches are used as a measure of body mass and height respectively, a conversion factor of 703 is used. The formula for calculation of Body Mass Index  is as under:

BMI = Body weight (kg)/(Height in meters)2 or

= Body weight (lb)/(height in inches)2 X 703

Body Mass Index does not measure body fat directly, but research has shown that it gives a fair estimate of body fat in relation to body fat estimated using more direct measures such as skinfold thickness, densitometry (underwater weighing), bioelectrical impedance analysis (BIA), dual-energy x-ray absorptiometry (DEXA), and imaging techniques such as CT and MRI scan. Also, it appears to indicate the risk for various obesity-related diseases on par with these direct measures of body fat. In general, BMI is an inexpensive and easy-to-perform method to assess how much an individual’s body weight departs from what is considered normal or desirable for a person’s height. Commonly it is used to screen individuals for weight categories such as underweight, normal or healthy weight, overweight and obesity.

Depiction of individuals according to weight categories based on BMI
Depiction of individuals according to weight categories based on BMI

How is Body Mass Index interpreted in adults?

For adults 18 years and older, Body Mass Index is interpreted using standard weight status categories viz. underweight, normal or healthy weight, overweight and obesity. These categories are the same for men and women of all body types and ages. World Health Organisation (WHO) has laid down a classification based on BMI, as under:

WHO International classification of adult underweight, overweight and obesity according to BMI
Table. The International classification of adult underweight, overweight and obesity according to BMI.

How is BMI used in clinical practice?

A high Body Mass Index can be an indicator of high body fatness. However, it is not diagnostic of body fatness or health of an individual. To determine if a high Body Mass Index is a health risk, a further detailed evaluation would be needed. In brief, BMI is not a diagnostic tool, instead, it is used to screen for potential weight and health-related issues.

Advantages of Body Mass Index

Body Mass Index provides a simple, yet very useful population-level measure of overweight and obesity as it is the same for both sexes and for all ages of adults. It is considered a population level measure as due to various limitations, which will be discussed in the subsequent section, it is considered a crude guide for predicting obesity-related diseases risk at the individual level. Carl Lavie, an American Cardiologist has aptly written 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.”

Limitations of Body Mass Index

  • It does not differentiate body fat from lean (fat-free) body mass – The weight excess or deficiency may, in part, be accounted for by body fat (adipose tissue), but in some individuals, other factors such as muscularity also affect Body Mass Index significantly. As BMI fails to differentiate between these different body constituents, athletes or individuals engaged in regular physical activity may be misclassified as overweight or obese.
  • It does not differentiate central (intra-abdominal or visceral) from peripheral fat – In my post ‘What is Obesity – is it merely about BMI?”, referred to above, I had described abdominal obesity and its ill-effects on health. In my post “Normal Weight Obesity – a myth or a reality?” Normal Weight Obesity I had described that even in individuals with normal Body Mass Index, increasing abdominal obesity is linked to higher mortality in adults; intra-abdominal (visceral) fat is a risk factor for the development of various lifestyle diseases regardless of the total body fat. However, even though BMI gives a better estimate of body fat and disease risk compared to other estimates based simply on stature and body mass, such as height-weight tables, it fails to identify where this fat is stored.
  • The relationship between BMI and adiposity (degree of fatness) varies with ethnicity – With the emergence of various techniques to measure body composition (i.e. specific determination of body fat as distinct from lean tissue such as bone and muscle), many studies were published in which the association between Body Mass Index and the percentage of body fat was investigated. Most of these studies showed that the relation between BMI and body fat depends on age and sex, and differs across ethnic groups. The variation in the relative percentage of body fat at different BMIs within populations depends on environmental factors, such as the amount of physical activity, as well as physiological factors.

Body Mass Index classification for obesity in Asians

In my post “Normal Weight Obesity – a myth or a reality?” referred to above, I had discussed that excessive body fat is now a well-established major risk factor for the development of various lifestyle diseases (chronic or non-communicable diseases) as it results in impaired regulation of metabolic processes. I had also highlighted there that there is convincing evidence to prove that intra-abdominal (visceral) fat is a risk factor for the development of various lifestyle diseases regardless of the total body fat content. In my post “What is Obesity – is it merely about BMI?” referred to above, I had described that ethnicity has an important correlation with body composition and regional distribution of body fat. I had highlighted that various studies have reported a higher percentage of body fat in Asians at lower BMI as well as increased prevalence of truncal fat, compared to Caucasians. Some Pacific populations also have a lower percentage of body fat at a given BMI than do white or European populations. In view of the association between body fat levels and disease risk, the current WHO BMI cut-off points will underestimate obesity-related risks in Asian populations.

In the year 2000, a report titled “The Asia-Pacific perspective: Redefining obesity and its treatment” was published. The Report. The document was coordinated by the International Diabetes Institute. It was co-sponsored jointly by the Regional Office for the Western Pacific (WPRO), World Health Organisation, the International Association for the Study of Obesity and the International Obesity Task Force. The document recommended different BMI ranges for the Asia-Pacific regions based on risk factors and morbidities. In Asians, the cut-off for overweight (> 23.0 kg/m2) and obesity (> 25.0 kg/m2) are lower than the WHO criteria.

Proposed classification of weight by BMI in adult Asians
Table: Proposed classification of weight by BMI in adult Asians

However, these cut-offs do not apply to Pacific Islanders. In these populations, higher BMI cut-offs of > 26 kg/m2 and > 32 kg/m2 are required to define overweight and obesity respectively. However, sparse data exist at present to make definitive recommendations.

Another issue is that the term Asian characterises a vast and diverse portion of the world’s population. Diversity in Asian countries is based on ethnic and cultural subgroups, degrees of urbanisation, social and economic conditions, and nutrition transition. As ethnic-specific cut-off points for BMI were thought to increase confusion in health promotion, disease prevention and management in the increasingly multi-cultural society, WHO convened another expert consultation on BMI in the Asian population, which met in Singapore from 8-11 Jul 2002.

On the basis of the available data in Asia, the WHO expert consultation concluded that Asians generally have a higher percentage of body fat than white people of the same age, sex, and BMI. Also, the proportion of Asian people with risk factors for type 2 diabetes and cardiovascular diseases is substantial even below the existing WHO cut-off point of 25 kg/m2. Thus, current WHO cut-off points do not provide an adequate basis for taking action on risks related to overweight and obesity in many populations in Asia. According to the expert consultation, the available data do not necessarily indicate one clear BMI cut-off point for all Asians for defining overweight or obesity. The BMI cut-off point for observed risk in different Asian populations varies from 22 kg/m2 to 25 kg/m2; for the high risk, it varies from 26 kg/m2 to 31 kg/m2.

In view of the above, and major implications of lowering the BMI cut-off points on their prevalence rates overnight which would entail changes in public health policies, clinical management guidelines and increased costs for governments, the expert consultation made no attempt to redefine BMI cut-off points for each population separately. The consultation also agreed that the WHO BMI cut-off points should be retained as international classification. Rather they identified further potential public health action points (23.0, 27.5, 32.5, and 37.5 kg/m2) along the continuum of BMI and proposed methods by which countries could make decisions about the definition of increased risk for their population. After all, the increased risk is a continuum with increasing BMI and that cut-off points are merely a convenience for public health and clinical use.

For many Asian populations, additional trigger points for public health action were identified as 23 kg/m2 or higher, representing an increased risk, and 27.5 kg/m2 or higher as representing a high risk. The suggested categories are as follows:

Additional trigger points, according to BMI, for public health action in Asian populations
Additional trigger points for public health action in Asian populations

Another important recommendation of expert consultation relates to the incorporation of waist circumference to refine action levels on the basis of BMI, in populations with a predisposition to central obesity and related increased risk of developing metabolic syndrome. However, this being an important aspect, it requires detailed deliberation and so will be discussed in a subsequent post.

Body Mass Index in children

The worldwide epidemic of childhood obesity is a public health problem and will be discussed in a subsequent post. The adult BMI cut-offs are not considered appropriate for measurement of overweight and obesity in children and adolescents because their bodies undergo a number of physiological changes as they grow. The American Academy of Paediatrics recommends the use of Body Mass Index to screen for overweight and obesity in children beginning at 2 years old.

Calculation of BMI in children

BMI in children is calculated the same way as in adults, using the same BMI formula. However, the interpretation of BMI in children and teens is entirely different compared to adults.

Interpretation of BMI in children

For adults, Body Mass Index is interpreted as weight status categories, viz. underweight, normal or healthy weight, overweight, and obese. However, it is not appropriate to use the BMI categories for adults to interpret the Body Mass Index of children and teens. As children grow, there are changes in weight, height, and body fat levels with age. Therefore, instead of comparing against fixed thresholds for underweight and overweight, BMI levels among children and teens are expressed relative to other children of the same sex and age. After Body Mass Index is calculated for children and teens, it is plotted on the BMI-for-age percentile growth charts to obtain a percentile. BMI Percentile calculators for children and teens are available online.

Definition of Obesity in children

In children and teens, overweight and obesity are defined as Body Mass Index above a selected percentile. However, as with adults, different percentile cut-offs need to be considered for different ethnic groups. Accordingly, many countries have developed their own BMI-for-age percentile growth charts, and have also selected different percentiles for the cut-offs. This has resulted in a plethora of definitions of childhood overweight and obesity. For e.g. in the USA overweight and obesity are defined as BMI exceeding 85th and 95th  centiles of the US CDC 2000 reference, while in the UK the definitions use the 91st and 98th centiles of the British 1990 reference, respectively. A child in a particular percentile (say 95th) means that his/her BMI is greater than the BMI of that much per cent (in this case 95) of the children of his age and sex, in the reference population. It is difficult to provide healthy weight ranges for children and teens because the interpretation of BMI depends on weight, height, age and sex.

BMI-for-age weight status categories and the corresponding percentiles, in the USA, are as under:

BMI-for-age weight status categories and the corresponding percentiles
BMI-for-age weight status categories and the corresponding percentiles

For the ease of understanding, an example of how sample Body Mass Index numbers would be interpreted, according to the above definition of obesity, for three 10-year-old boys with different BMIs, is given below:

Interpretation of sample BMI numbers for a 10 year old boy
Interpretation of sample BMI numbers for a 10 year old boy

In the year 2000, a paper titled “Establishing a standard definition for child overweight and obesity worldwide: international survey” was published in the British Medical Journal. The paper aimed to develop an internationally acceptable definition of child overweight and obesity, based on an international survey of six large nationally representative cross-sectional growth studies.

Body Mass Index calculator

A large number of BMI calculators are available online for the ease of calculation of Body Mass Index.

Can adult BMI calculator be used in children?

The adult BMI calculator provides only the Body Mass Index value and not the Body Mass Index percentile that is needed to interpret Body Mass Index among children and teens.

BMI Charts

A number of BMI charts, both for kg and meter and lb and inches are available online for easy reference. Again, it is not appropriate to use Body Mass Index categories for adults to interpret the Body Mass Index of children and teens.

BMI chart based on weight in kg and height in meters
BMI chart based on weight in kg and height in meters

Conclusion

Accurate diagnosis of obesity is important both at the individual and population level. At the individual level, misdiagnosis can lead to undertreatment or potential stigma. On the other hand, at the population level, inaccurate measurements could mislead our interpretation of the incidence and prevalence of obesity; this would have a direct bearing on the planning of services for the prevention and management of obesity and related risks. Accurate, direct measures of the amount and distribution of body fat, such as dual-energy x-ray absorptiometry (DEXA) and imaging techniques such as CT and MRI scans, despite their accuracy are cumbersome and expensive, and hence less suited for routine use. On the other hand, BMI is an inexpensive and easy-to-perform tool for the assessment of obesity. Therefore, despite its limitations, the Body Mass Index remains the most commonly used, widely accepted and practical measure of obesity in both children and adults.  However, it is not diagnostic of body fatness or health of an individual. To determine if a high Body Mass Index is a health risk, a further detailed evaluation would be needed.

 

 

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