Changing the way we measure childhood obesity: in conversation with Andrew Agbaje

Changing the way we measure childhood obesity: in conversation with Andrew Agbaje

Andrew Agbaje MD, MPH, PhD, FESC, a Pediatric Clinical Epidemiologist and Principal Investigator of urFIT-child” research group, University of Eastern Finland (Finland) & University of Exeter (UK) has published a new paper Waist-circumference-to-height-ratio had better longitudinal agreement with DEXA-measured fat mass than BMI in 7237 children in Nature Pediatric Research.

Thank you Andrew, for discussing your very interesting new paper with us on waist-to-height ratio as a better obesity measure in children and adolescents than BMI.

Thank you Sheree for the opportunity to discuss childhood obesity, and one approach to addressing this challenge.

Please briefly explain the motivation behind researching waist circumference-to-height ratio as an alternative to BMI in identifying obesity in children and adolescents.

In our previous research, we discovered that body mass index (BMI) misidentifies and misclassifies children and adolescents as overweight or having obesity even when their weight is largely driven by muscle mass. For example, we found that increased BMI from childhood was positively associated with progressively increased carotid intima-media thickness in nearly 4000 children followed up from ages 9 to 24. We would have interpreted this to be an “obesity problem”, but because we had dual-energy Xray absorptiometry (DEXA) measure of total body fat and muscle mass, we repeated the analyses, and to our surprise, fat mass was not associated with carotid intima-media thickness. Rather, increased muscle mass was associated with increased carotid intima-media thickness suggesting a physiologic adaptation of the smooth muscle wall of blood vessels to skeletal muscle growth in the young population. So, our conclusion for this result was different from that of previous researchers, because we made use of available accurate measures of body composition. Of note, publishing that paper was very difficult, it was rejected twenty-one (21) times by different journals over 2 years because several editors and reviewers felt like it challenged what they had always known for decades.

Given that DEXA, which is recognized as the gold standard measurement of body composition, is not readily available in primary care centers or for at-home use, in December 2022 I began to think of a simple alternative to BMI that people could use that would indicate an increase in fat mass, and would distinguish between fat mass and muscle mass. After completing analyses of various potential alternatives to BMI in January 2023, I noticed the publication of a clinical guideline on managing childhood obesity by the American Academy of Pediatrics (AAP), which was published on 9th January 2023. The AAP specifically listed areas of urgent future research, including “Alternative, accurate measurements of adiposity in primary care”. Thus, I was convinced that this research, which has now been published, is perfectly timed to solve the global problem of defining obesity in children and adolescents.

What are the main limitations of BMI that your study aimed to address?

BMI does not distinguish between fat mass and muscle mass. In childhood the ratio of muscle mass to fat mass is 4:1. It is wrong to ascribe 75% of the weight of a child as expressed by BMI to fat when less than 25% is contributed by fat. In addition, BMI varies significantly with age and sex, and is unreliable. For example, two children with the same BMI will have different fat mass and muscle mass proportions and unfortunately, we could classify them as overweight when they may actually have healthy weight. Having adequate muscle mass protects the cardiovascular system and should be encouraged from childhood.

An important point; the importance of maintaining adequate muscle mass during childhood is noted!

Oh yes, among adults the loss of muscle mass, also called sarcopenia, is a major risk factor for sarcopenic obesity, cancer, type 2 diabetes, cardiovascular disease, musculoskeletal disease, fracture, and premature death. An adequate muscle mass in childhood is promoted by a good diet and exercise. We recently showed in a study published in Nature Communications that engaging in physical activity from childhood through young adulthood increases muscle mass while lowering both total body fat mass and trunk fat mass.

You used the Avon Longitudinal Study of Parents and Children (Children of the 90s data) for this work — what made this an ideal dataset for your research? How did the waist-to-height ratio perform in comparison to BMI in detecting excess fat mass in children and adolescents in your study?

The children of the 90’s cohort from the University of Bristol, UK is the best and only data in the world that has repeatedly used DEXA measures of fat mass during growth from childhood to young adulthood in several thousand children. The project has cost millions, but has produced a remarkable body composition database that will correct the anomalies of BMI which have hampered the true diagnosis of obesity in the young population in our generation. An extraordinary dataset is required for a paradigm shift, and we are extremely grateful to the children of the 90’s and their parents for their continued participation in this project over the past three decades.

BMI performed >15% worse in detecting fat mass measured with DEXA in comparison with waist-to-height ratio. Moreover, BMI measures muscle mass >25% more than waist-to-height ratio. This means BMI measurement reflects both fat mass and muscle mass, making interpretations of the findings difficult. On the other hand, waist-to-height ratio measures fat mass with a maximum agreement of 89% with DEXA-measured fat mass, compared to 65% by BMI. Waist-to-height ratio has a very poor agreement with DEXA-measured muscle mass, sometimes <20%, suggesting that it does not detect muscle mass.

Your study mentions a “very high agreement” of waist-to-height ratio with DEXA-measured total body fat mass and trunk fat mass. Can you elaborate on the significance of these findings?

For the first time, we found an inexpensive tool that could replace BMI, and overcome the limitations of misclassification and overdiagnosis of obesity in children and adolescents. Waist-to-height ratio’s very high agreement with DEXA-measured fat mass and low agreement with muscle mass can become the new tool to redefine the obesity threshold.

Based on your new research, what are the optimal waist-to-height ratio cut points for detecting excess adiposity in males and females, and how were these determined?

It is remarkable that during growth from childhood to young adulthood the average (50th percentile) waist-to-height ratio was 0.45 in more 7000 children.

The following cutpoint has been established in the recent paper.

0.53 in males will detect the 95th percentile category of fat either in the total body or in the trunk.

0.54 in females will detect the 95th percentile category of fat either in the total body or in the trunk.

These waist-to-height ratio values will identify 8 out of 10 males and 7 out of 10 females who truly had excess DEXA-measured adiposity. The cut point also identified 93 out of 100 males and 95 out of 100 females who truly do not have excess fat.

In summary, I will propose a new obesity category based on waist-to-height ratio percentiles among Caucasian youth. This approach could also be adopted in the adult population based on emerging evidence that waist-to-height ratio predicts premature death and cardiovascular diseases better than BMI.

<0.40 in both males and females is very low body fat and/or trunk fat (underweight)

0.40 – <0.50 in males is normal body fat and/or trunk fat

0.40 – <0.51 in females is normal body fat and/or trunk fat

0.50 – <0.53 in males is high body fat and/or trunk fat (overweight)

0.51 – <0.54 in females is high body fat and/or trunk fat (overweight)

>0.53 in males is excess body fat and/or trunk fat (obesity)

>0.54 in females is excess body fat and/or trunk fat (obesity)

The reason for the slight difference between males and females is because nature has endowed females with more fat mass than males, which is perfectly normal. We must also remember that not all fat is bad for health but excess adiposity is. We need a certain amount of fat mass for the normal physiological function of cells and organs. So we should not aim to drain our body completely of fat but ensure that fat content remains within a normal range.

Thank you Andrew, very interesting! Will these findings potentially impact the diagnosis and management of obesity in clinical settings, including in routine pediatric care?

Emerging studies in children and adolescents report that the waist-to-height ratio predicts cardiometabolic risks such as dyslipidemia, blood pressure, metabolic syndrome, and insulin resistance far better than BMI. We cannot ignore such evidence, and pediatric care needs to be updated with this novel evidence so that children are not subjected to unnecessary interventions when they are otherwise healthy. This approach will help us reserve scarce resources for obesity management in an at risk population.

We may need to replace BMI with waist-to-height ratio in the future; however, in the meantime, it MUST be included as a compulsory assessment before obesity is diagnosed in primary health care, where expensive devices like DEXA are lacking. In addition, parents should not be discouraged by the BMI or weight of their children but can inexpensively confirm whether the weight is due to an increase in excess adipose tissue by examining their child’s waist circumference-to-height ratio. If they notice deviation from the normal values they could visit their medical practitioner for a more detailed evaluation of body composition and management suggestions.

Thanks Andrew; this is truly translational science. Do you believe your study’s findings could influence future childhood obesity guidelines and policy statements?

I am convinced that the findings will influence childhood obesity guidelines and policy statements in the future. Several task forces are working across the globe on improving childhood obesity diagnosis, and hopefully this evidence will help contribute to the ongoing policy drafts. Lastly, I would encourage children, adolescents, and parents not to focus on the bathroom scale, but to use your measuring tape.

How to measure waist-to-height ratio at home:

Waist circumference

Acquire a flexible measuring tape or tape rule. Feel for your hip bone and your bottom rib, and place the tape measure evenly around your waist which is halfway between the last rib and hip bone. The waistline may lie directly above your umbilicus. (If needed, use a mirror to guide the tape straight around your waist and parallel to the ground.) Record the values in centimeters.

Height measurement

A tape measure, a flat surface, and a pencil are needed. Stand with your feet without shoes flat on the floor with your heels against the corner where the wall and floor meet. Ensure your head, shoulders, and buttocks touch the wall. Look straight ahead parallel to the floor. Have someone place a book or a ruler over your head and lightly mark the wall with a pencil. Then measure the distance between the marked point and the floor.

Now calculate waist circumference (cm)/height (cm) and check the results against the categories provided above.

Read the full paper here in Nature Pediatric Research:

Read the press release from the University of Eastern Finland: