Body mass index, or BMI, is a common and convenient way to measure a person’s weight. However, it’s not always the most accurate depiction of someone’s overall health.
While the BMI takes into account a person’s height and weight, it does not consider other factors, such as medical conditions that make them retain water or how much of the weight they carry is from water, fat or muscle.
“BMI is not always an accurate measure for weight and health, because it does not account for muscle mass, gender, ethnicity or menopausal status,” said Dr. Peter Jian, weight management specialist and assistant professor of family and community medicine at Baylor College of Medicine. “For instance, athletes tend to have less body fat than non-athletes at the same BMI.”
BMI is calculated by dividing a person’s weight in kilograms by the square of their height in meters. A BMI greater than 25 is considered overweight, while greater than 30 is considered obese.
While BMI has its limitations, Jian said it could help identify potential health risks. For example, a BMI of 30 or greater is associated with higher risks of type 2 diabetes, high cholesterol, high blood pressure, obstructive sleep apnea, fatty liver, osteoarthritis and even certain cancers.
“BMI can be used to measure health trends over time,” Jian said. “A decrease of BMI that corresponds to 5% of total body weight has shown to improve health conditions such as blood pressure, cholesterol, osteoarthritis and blood sugars.”
While BMI has some advantages, he said there are other ways to classify weight, such as measuring waist circumference and body fat percentage.
“To better define, characterize and treat obesity as a medical condition, one should include BMI and other data points such as related medical conditions,” Jian said.
BMI and bariatric surgery
Once a person’s BMI passes a certain threshold, they are eligible for weight loss procedures or bariatric surgery.
According to Dr. Samer Mattar, professor and chief of the Division of Metabolic and Bariatric Surgery in the Michael E. DeBakey Department of Surgery at Baylor, there are limitations to the BMI measurement when considering a patient for bariatric surgery. He suggests going beyond the BMI to look at the patient as a whole.
Insurance usually covers bariatric surgery for patients whose BMI is above 40. For patients whose BMI is between 35 and 40, insurance may cover their bariatric surgery if they have at least one or two associated medical diseases, such as diabetes, hypertension or sleep apnea.
However, Mattar said that there is no evidence to show that BMI alone is how one should classify a patient.
“For example, we have body builders who have a lot of muscle and they can have the same BMI as someone who has very little muscle mass and a lot of fatty tissue,” he said. “In addition, patients with heart disease and kidney disease who retain a lot of water will have high BMI levels without necessarily having high fatty tissue.”
A person does not need to reach a certain BMI to develop serious, life-threatening diseases. Mattar said that it is possible for patients who have a BMI of less than 35 to have severe diabetes, high blood pressure and other metabolic diseases. His team has found that if they operate on these patients, they get the same benefits as those with a higher BMI.
“The number does not determine the diseases. You can have a lower BMI and still have health issues that can be addressed through bariatric surgery,” Mattar said.
In addition, it’s possible to be overweight and not have any documented metabolic diseases, but Mattar said that it’s only a matter of time until they do appear. His team carefully evaluates patients who fall into this category and can still offer bariatric surgery as a preventative procedure.
In his practice, Mattar emphasizes looking at the patient as a whole by assessing their physical weight and associated diseases. His team assesses whether the weight is limiting their activities, such as their ability to walk, exercise, work or play with their kids. He also considers a patient’s mental health, noting that anxiety and depression are common in bariatric surgery patients.
“This is why the modern management of patients with obesity is now a multidisciplinary approach,” he said. “As a surgeon, I can review a patient’s medical and surgical history and come up with a surgical plan for them, but I also rely heavily on listening to what my partners say, and these partners include experts in psychology, nutrition and exercise physiology.”
Mattar aspires to develop a better scoring system to determine eligibility for bariatric surgery based on data collected from the scientific literature and from expert opinion.
The components of this scoring system should include a patient’s weight, the number of associated diseases they have, how advanced the diseases are, what medications they are taking and how their day-to-day activities are impacted.
“It’s time for a new way of evaluating patients and classifying them according to more scientific methods rather than BMI alone.”