Measuring heart disease and obesity risk in Indians: Why BMI (body mass index) should change to WHtR (waist to height) ratio

BMI DOES NOT FACTOR IN RACE DIFFERENCES

However, these ‘universal’ cut-offs, which were developed in Caucasian populations, proved to be inappropriate for other populations. Recognising that Asian populations manifested a rising risk of diabetes and cardiovascular disease at lower levels of BMI, the World Health Organisation (WHO) recommended that public health and clinical action thresholds should be reset for those populations at BMIs 23 and 27.5, with the ideal range being between 18.5 and 23. While these are interpreted as Asian cut-offs for defining overweight and obesity respectively, international comparisons across all countries mostly use cut-offs of 25 and 30. Lower BMI cut-offs are being recommended for Black African and Black Caribbean populations too.

The problem with BMI is that it does not differentiate between weight contributed by fat, muscle and bone. A very muscular individual may have a BMI over 30, inviting the label of being obese, even if the body fat content is not high. Similarly, a person with low muscle mass or low bone weight may have a BMI of 22 but still be at a high risk of diabetes, hypertension and heart attacks. While BMI broadly correlates with the prevalence of cardio-metabolic diseases at the population level, it does not distinguish well at the individual level.

This problem of misclassification of risk has been particularly prominent in South Asians of the Indian sub-continent. It was initially noted among Indian migrants studied in different continents. Their increased propensity for diabetes and excess risk of heart attacks was not explained by a high BMI, as many had values within the ’normal’ range. They, however, had high amounts of body fat, especially concentrated around the abdominal organs. Such ‘visceral adiposity’ or ‘abdominal obesity’ poses a high risk of diabetes and heart disease. Some international algorithms for predicting the risk of a future cardiovascular event now use ‘South Asian ethnicity’ as an independent risk marker.

THE WAIST TO HEIGHT RATIO HOLDS GOOD FOR ALL RACES

These findings have also been corroborated by several studies of Indians residing in different parts of India. More than BMI, the waist to hip ratio (WHR) has emerged as a better predictor of diabetes and cardiovascular disease than BMI. This index, which compares the waist circumference to that around the hips, measures the degree of abdominal obesity. Since it is not always easy to measure the hip circumference, especially when clothing is interposed between the tape and the body, an alternate measure has emerged in recent years. This is the waist to height ratio (both expressed in centimetres). This ratio has been shown to be a very good predictor of cardio-metabolic risk, certainly better than BMI and in some studies even better than WHR. Initially developed, validated and applied in children, it is being increasingly used in adults.

In December 2022, UK’s National Institute for Health and Care Excellence (NICE) recommended that waist-to-height ratio be used instead of BMI. A ratio higher than 0.5 carries an increased risk of cardio-metabolic disease. This relationship does not appear to be altered by differences in race, age or gender.

UNDERSTANDING THE OBESITY PARADOX

One of the consequences of using a flawed measure like BMI has been the reporting of an ‘obesity paradox’ where obesity appeared to be protective in some health conditions. It was reported that it may protect against deaths from heart failure, stroke and diabetes. These studies have been criticised on the ground that the contributions of body fat, muscle, bone and fluids to the excess weight have not been assessed. Similarly, the high prevalence of ‘thin diabetes’ in India will appear puzzling if the body fat content and visceral adiposity are not taken into account.

A recent study, however, showed that the fallacy around ‘protective obesity’ arose because BMI was measuring body weight rather than body fat, which is mainly responsible for inflammation and disease. Published on March 22, in the ‘European Heart Journal,’ the study looked at the clinical outcomes in persons with chronic heart failure and reduced ejection fraction (pumping ability) of the heart. Scientists analysed data from 6,567 men and 1,832 women participating in the PARADIGM-HF heart failure trial, which was conducted in 47 countries. While unadjusted BMI-based analysis appeared to support the idea of the ‘obesity paradox’, analysis with adjustment for other variables removed that effect. More important, the use of waist to height ratio showed a positive correlation with higher mortality risk.

Fluid retention occurs in heart failure, adding to body weight but not reflecting the adipose tissue content of the body. Waist to height ratio is unaffected by that altered weight. After adjustment, both BMI and waist-to-height ratio showed that higher levels were associated with a greater risk of death or hospitalisation for heart failure, but this was more evident for the waist-to-height ratio.

WAIST TO HEIGHT (WHtR) RATIO OVER 0.5 = BELLY FAT

Race or ethnicity related differences in BMI’s gradient of cardio-metabolic risk have implications for screening of persons at high risk in a population. A recent study, published on March 24, in the American Journal of Preventive Medicine, examined the impact of racial differences on population screening for diabetes in the US. The existing guidelines of US Preventive Services Task Force (USPTF) recommend screening for pre-diabetes and diabetes in people aged 35-70 years, if they are overweight or obese by conventional BMI standards.

The authors of the new study reported that such screening underestimated the numbers with diabetes or pre-diabetes in Asian, Hispanic and non-Hispanic Black adults. The percentage of cases identified among persons of Asian ethnicity was significantly lower among the Asians residing in America, compared to other racial groups, when the age cum BMI criteria were used, compared to measured blood glucose values. They recommend that the BMI based screening criteria be dropped and all adults in the age group of 35-70 be screened. Matthew O’Brien, lead author of the study avers that such a revised strategy would promote greater health equity, by reducing the effect of racial differences. Perhaps the USPTF should look at waist-to-height ratio too!

From this welter of information, a clear message emerges for us to use in India. Waist to height ratio over 0.5 = visceral adiposity = raised risk of obesity related diseases. Works for children, women and men.

(Prof K. Srinath Reddy is a cardiologist, epidemiologist and Distinguished Professor of Public Health, PHFI)