Transitioning beyond BMI: Waist-to-height ratio emerges as new technique to determine obesity

Waist-to-height ratio provides a simple and effective way to evaluate whether a person has an unhealthy amount of abdominal fat, which is associated with a higher risk of cardiometabolic diseases.

BySumit Jha

Published Jul 08, 2024 | 7:00 AM Updated Jul 08, 2024 | 7:00 AM

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Obesity is a chronic condition characterised by excessive fat deposits that can impair health. It is a major risk factor for several serious health conditions, including type 2 diabetes, heart disease, certain types of cancer, and premature death.

Body Mass Index (BMI) is the most commonly used measure to determine if a person is underweight, at a healthy weight, overweight, or obese based on their height and weight. For adults, a BMI of 25 to 29.9 is considered overweight, while a BMI of 30 or higher is considered obese.

Despite the wide recognition of obesity as a multifactorial, chronic, relapsing, non-communicable disease marked by an abnormal or excessive accumulation of body fat, the diagnosis of obesity in many settings, is still based solely on BMI cut-off values.

This approach does not reflect the role of adipose tissue distribution and function in the severity of the disease.

However, researchers now state that the system for diagnosing and managing obesity can no longer rely solely on BMI, as this approach excludes many individuals who could benefit from obesity treatment.

More specifically, the accumulation of abdominal fat is associated with an increased risk of developing cardiometabolic complications and is a stronger determinant of disease development than BMI, even in individuals with a BMI level below the standard cut-off values for obesity diagnosis (BMI of 30).

In response, the European Association for the Study of Obesity (EASO) has launched a new framework for the diagnosis, staging, and management of obesity in adults.

Published in the journal Nature Medicine, the framework specifically examines fat accumulation in the abdomen, measured as the ‘waist-to-height ratio.’

According to researchers, an increased waist-to-height ratio is associated with a higher risk of developing cardiometabolic complications.

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What is waist-to-height ratio?

The waist-to-height ratio is a measurement used to assess the distribution of body fat and its potential health risks. It is calculated by dividing the circumference of a person’s waist by their height.

This ratio provides a simple and effective way to evaluate whether a person has an unhealthy amount of abdominal fat, which is associated with a higher risk of cardiometabolic diseases.

To calculate the waist-to-height ratio:

  1. Measure the circumference of the waist, typically at the narrowest point between the rib cage and the hips.
  2. Measure the person’s height.
  3. Divide the waist measurement by the height measurement.

For example, if a person has a waist circumference of 80 cm and a height of 170 cm, their waist-to-height ratio would be 80 / 170 = 0.47.

A waist-to-height ratio higher than 0.5 is considered a risk factor for cardiometabolic diseases.

This measurement is considered superior to waist circumference alone because it accounts for differences in body size and provides a better indication of abdominal fat distribution relative to overall body size.

An “important novelty” of the framework is the inclusion of a waist-to-height ratio higher than 0.5, along with a BMI of 25-30, for diagnosing obesity, according to the authors representing the European Association for the Study of Obesity (EASO).

“The choice of introducing the waist-to-height ratio, instead of waist circumference, in the diagnostic process is due to its superiority as a cardiometabolic disease–such as heart disease and diabetes— risk marker,” the researchers wrote.

They added that accumulation of abdominal fat is a more reliable predictor of health deterioration, compared to BMI, even for individuals not meeting the current standard cut-off value for obesity diagnosis, which is a BMI of 30, the authors said.

Transitioning from BMI to waist-based metrics

“There is no doubt that obesity is a health risk factor because it invites multiple chronic conditions such as diabetes, heart disease, hypertension, stroke, arthritis and cancer. However, all obesity is not the same,” Gastroenterologist and IMA Kerala’s public health advisory panel member, ” Dr Rajeev Jayadevan told South First.

“Fat deposition in the thighs and buttocks is not considered to be as risky as weight deposited around the abdomen area, the metabolic consequences being different in each instance,” Dr Jayadevan said.

The new framework clearly states that belly fat is an important risk factor for health problems, even in people with a low BMI, who don’t show obvious symptoms yet.

It includes people with a lower BMI (25-30) but with more belly fat and any medical, functional, or psychological issues in the definition of obesity. This approach helps ensure that these individuals receive the necessary treatment, unlike the current system that relies only on BMI.

“There is still unnecessary and undeserved emphasis on a parameter called BMI, which is essentially a ratio of height and weight. However BMI does not take into account natural variation in body structure, and does not emphasise abdominal obesity, which is the type of obesity commonly associated with health problems. It also does not take into account muscle bulk or weight of bone,” said Dr Jayadevan.

He added that the parameters such as “waist to-hip-ratio” that detect abdominal obesity are preferred. “We need to move away from total body weight and BMI, and focus on abdominal obesity instead,” he said.

Hyderabad based Neurologist Dr Sudhir Kumar told South First that BMI was becoming increasingly obsolete because it solely considered overall body weight without distinguishing between muscle and fat weight.

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He said that body weight has traditionally been used to calculate the risk of developing cardiometabolic diseases because it is assumed that those with higher body weight also have higher body fat.

“However, we now understand that body weight can be high due to increased muscle mass or increased fat mass. Traditionally, body weight has been correlated with height to arrive at BMI, and almost all past research studies have used BMI to determine obesity and overweight,” Hyderabad-based neurologist Dr. Sudhir Kumar told South First.

He added that there are now two potential inaccuracies with this approach. First, BMI may be normal in a person with less muscle mass and higher body fat because the overall weight may appear normal. Such individuals might be classified as having a normal BMI.

Additionally, as people age, their muscle mass reduces, and particularly in Indians, who are often protein deficient and do not engage in strength training, this muscle mass loss can be pronounced. This loss typically starts after the age of 30 or 40 and is known as sarcopenia.

A person with sarcopenia, even if they have excess body fat, might still have a normal body weight and BMI. Therefore, in the last four or five years, there has been more emphasis on quantifying body adipose tissue.

“This focus aims to assess the actual risk factor, as the risk is due to excess body fat and not excess muscle mass. In fact, excess muscle mass is healthy for us. This is why BMI is not the most accurate method. In addition to total body fat, the most important fat that increases cardiometabolic risk is the fat deposited in the abdomen and around internal organs, known as visceral fat,” said Dr Kumar.

Evolving measuring methods for assessing cardiometabolic risk

Dr Kumar added that abdominal fat and visceral fat are the two critical areas of fat that elevate the risk of cardiovascular diseases. These can be measured by several methods.

“The simplest method is measuring waist circumference, which can be easily done at home or in an office. Other methods include the waist-hip ratio and waist-to-height ratio, with waist circumference being the most straightforward and commonly used,” said Dr Kumar.

For research studies or specific individual assessments, other methods like ultrasound scans, DEXA scans, or even MRI scans can be used to measure body fat. However, these are not routinely used. For routine screening, waist circumference, waist-hip ratio, or waist-to-height ratio are sufficient.

“To quantify the risk of cardiometabolic diseases, we can start with measuring waist circumference. If there is any doubt, more accurate methods like DEXA scans, CT scans, MRI, and ultrasound can be used in a clinical setup to determine the exact amount of abdominal and visceral fat,” said Dr Kumar.

Visceral fat, in particular, cannot be assessed without a scan. For those wanting a more accurate assessment, these direct methods can be employed.

High abdominal fat is a marker for insulin resistance and metabolic syndrome. Individuals with high abdominal adipose tissue are more prone to developing diabetes, hypertension, heart attacks, and strokes.

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How to reduce abdominal fat?

The primary causes of abdominal obesity are often said to be a poor diet — notably excessive consumption of sugar, alcohol, and trans-fats, coupled with insufficient intake of protein.

Kumar said that individuals are advised to reduce the intake of high-carbohydrate foods — particularly those with elevated sugar content, such as soft drinks, sweets, cakes, and chocolates — to combat abdominal obesity.

These items should be prioritised for reduction before considering more staple sources of carbohydrates like rice and roti. Even a small quantity of high-sugar treats can contribute significantly to overall carbohydrate intake.

“Other dietary adjustments include limiting the consumption of packaged fruit juices and ultra-processed foods. Moderation in the consumption of rice and roti is suggested, accompanied by an increase in protein intake. The recommended protein intake is around 1 gram per kilogram of body weight,” said Dr Kumar.

Healthy fats, such as those found in small amounts in butter, cheese, and avocado, are encouraged as part of the diet. These fats contribute to overall health while avoiding excessive intake.

The second key component in addressing abdominal obesity is regular exercise. Aerobic activities like running, cycling, swimming, and strength training are emphasised.

Aerobic exercises contribute to fat loss, while strength training helps build muscle mass, enhancing the metabolism of carbohydrates and preventing their conversion into fat.

Adequate sleep is also identified as a factor influencing obesity. Studies suggest that individuals who consistently get sufficient sleep are less likely to experience obesity compared to those who are sleep-deprived.

The combination of a balanced diet, regular exercise — both aerobic and strength training — and adequate sleep forms a comprehensive approach to combating abdominal obesity.

(Edited by Shauqueen Mizaj)

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