BMI no longer detects dangerous obesity as people age

BMI no longer detects dangerous obesity as people age

A groundbreaking analysis of 120,000 people shows hidden central fat is increasing across the UK and BMI is missing, making waist-to-height ratio the clearer warning sign of the growing risk of obesity.

Study: Long-term trends in central obesity in England: an age-period-cohort approach. Photo credit: Studio Romantic/Shutterstock.com

Various measures are used to track obesity to both understand its prevalence and predict its health effects. A recent study published in theInternational Journal of Obesitysought to track long-term changes in central obesity in the UK by stratifying by birth cohort, age and time.

Gaps in obesity measurement

Body mass index (BMI) has long been the most important measure for detecting obesity and overweight. However, it likely underestimates the prevalence of high-risk obesity compared to measures of central adiposity. These include waist circumference, waist-to-hip ratio, and waist-to-height ratio (WC, WHR, and WHtR, respectively), all of which provide early warning of potential cardiovascular and cardiometabolic health risks.

BMI measures the total weight of the body, regardless of whether it is made up of muscle mass, other muscle mass or fat mass. This is especially true for children and the elderly, whose body composition is different compared to full-grown adults.

Using BMI alone could potentially lead to one in ten of the UK population being incorrectly classified as obese. Conversely, one in four people at high risk of obesity have a “healthy” BMI and are incorrectly classified as low risk. Additionally, childhood obesity is overdiagnosed by BMI.

Again, BMI changes over time, making follow-up testing to determine associated health risks, particularly in older people, essential. Finally, the variation in BMI by age and gender also makes it less easy to interpret at the expert and population level.

These limitations led NICE to suggest using consistent WHtR values ​​in adults and children instead, as these can better predict cardiovascular risk in adults. It also outperforms dual-energy X-ray absorptiometry (DEXA) in measuring trunk and total fat mass in children and correlates better with the prevalence of fatty liver disease and fibrosis in children and adults.

NICE currently recommends the use of BMI and WtHR in people with a non-obese BMI (<35kg/m²). Newer definitions of obesity are being developed to more accurately reflect health risks and identify the need for intervention. The European Association for the Study of Obesity (EASO) has published a new protocol for the diagnosis, classification and treatment of obesity.

Despite extensive knowledge of the changing obesity trajectories over time in the UK, there is a lack of understanding of the direction of key obesity policies over time. The aim of the current study was to examine this variation while analyzing the results for the effects of age, time and generational cohort.

Measuring obesity over time

The researchers used data from the Health Survey for England (2005-2021), which covered 120,024 people aged 11 to 89. Their birth years ranged from 1919 to 2008. The study period spanned 16 years and 17 birth cohorts, including participants born in consecutive five-year periods.

The study used key measures of obesity, namely WC, WHR and WHtR, as well as BMI. High risk thresholds have been set based on the definitions of the World Health Organization and the British National Institute for Health and Care Excellence.

Participants were analyzed for effects of obesity by age, time period, and birth cohort (an APC analysis).

Emerging Obesity Trends

The researchers found a time-dependent increase in high-risk obesity and central obesity, regardless of the obesity measures used. High-risk BMI increased from ~23% in 2005 to ~27% in 2021. Likewise, high-risk WHtR increased from ~24% in 2005 to 33.4% in 2021.

The rate of high-risk obesity also increased at the same rate, from ~38.5% in 2005 to ~49% in 2021. The highest prevalence of high-risk obesity was found in the WHR, from ~46.3% in 2005 to 61% in 2021.

How obesity changes with age

The prevalence of all core adiposity measures except WHtR increased linearly with age until age 65–70 years and slowed thereafter. The WHtR increased from age 11 to age 85 and fluctuated more strongly thereafter.

In contrast, the BMI-age graph showed an inverted U-shape, indicating an early increase in BMI with age. After stabilizing at around 50 years, it began to decline. However, the risk of adverse health effects caused by obesity increases rather than decreases with age. Therefore, the BMI cannot accurately identify this vulnerable group of people.

At ages 85 to 89, women and men had nearly five- and six-fold increased risk of high-risk obesity, respectively, compared to baseline at ages 18 to 19.

While there was a similar increase across all measures in both sexes, the odds of high-risk WHTr increased steadily in men until age 80 to 84 years and decreased over the next five years, in contrast to the linear increase until age 89 in women. Similar differences were observed in high-risk WC, with women showing increasing odds up to ages 80 to 84 years, but men only showing increasing odds up to ages 55 to 59 years. The likelihood of a high-risk BMI also increased until ages 50 to 54 in men but until ages 65 to 69 in women.

The risk of obesity changes over time

When compared over time, the probability of all measures of central and general high-risk obesity was slightly higher for both women and men in 2019 to 2021 than in 2005 to 2006.

Comparison of birth cohorts

Significant differences between birth cohorts were largely absent. Only women born between 2004 and 2008 and men born after 1974 had a slightly lower risk of high-risk central obesity.

Rethinking anti-obesity policies

The study is the first to conduct an APC analysis of general and core adiposity measures. The linear increase in high-risk obesity with age is consistent with the observation that age increases the likelihood of obesity-related disease.

Age plays an important role in developing obesity trends, meaning an aging population could lead to further increases in obesity prevalence.

The results highlight the superiority of core obesity measures, particularly the WtHR, in assessing time-dependent obesity risk. The results suggest that both types of measures identify different risk groups at the population level, but not for individuals.

Currently, WHtR appears to be better than BMI at measuring obesity more accurately and should be used as a standard in clinical situations. This is in line with updated NICE guidelines, which recommend WHtR alongside BMI and not as a complete replacement.

Given these findings, early interventions for children and adolescents are warranted to support healthy aging. Further work should explore other evidence-based obesity thresholds that have the potential to identify true obesity prevalence with comparable accuracy in different groups.

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