Body Mass Index (BMI), often used as an indicator of overall health, is not an accurate indicator of survivability in heart failure patients, according to a new University of Alberta study.
The BMI is a measure of body weight based on height and weight. Individuals can input personal statistics into a BMI calculator to determine where their BMI falls relative to the 'healthy' weight standards.
"In the general population, the higher your BMI the greater your risk of disease and dying early. We were the first ones to measure body composition in [heart failure] patients," said Antigone Oreopoulos of the U of A's School of Public Health.
Oreopoulos led a study to test the "obesity paradox": why heart failure patients with higher BMI have greater rates of survival.
"We asked what component of increased weight — is it fat, is it muscle, or is it both — that's protecting these patients from death?" she said.
The study recorded the BMI of 140 patients, along with five "surrogates of death," which are known prognostic factors for heart failure. These factors included nutritional status, quality of life, inflammation, exercise capacity, and severity of heart failure. Body composition was then measured using a Dual Energy X-ray Absorptiometry (DEXA) scan to get an accurate measure of muscle mass and body fat percentage. The results revealed that BMI is not a good predictor of positive prognostic factors for heart failure patients, as it doesn't discern between fat and muscle.
"It suggests there are differing effects of muscle and fat in heart failure, and that's a really key finding. It shows that really, clinicians need to be measuring both to track changes, and we need to be measuring both in research to understand this phenomenon."
In the meantime, according to Oreopoulos, doctors need alternatives to BMI to assess heart failure patients. A DEXA scanner costs approximately $100,000 and requires a specialized technician, so is impractical for routine use. Oreopoulos also tested other methods of estimating body fat, such as waist circumference and waist-to-height ratio, as well as cheaper instrumental methods such as bio-electrical impedance, used in home body fat scales, and near infrared interactance.
"[The near infrared interactance] uses near infrared light. It has a sensor which you apply to the biceps and it measures your percent fat and your muscle," Oreopoulos said.
Most of the alternatives had similar issues to the BMI measurement and were not reliable. However, near infrared interactance performed within acceptable limits, and at around only $5,000 per unit, is a much more feasible alternative.
Discerning between fat and muscle is only the first step in understanding the obesity paradox, explained Oreopoulos. While muscle is a greater indicator of health in the general public, it may be that fat acts as an energy store for heart failure patients in the final stages of the disease.
"At later stages of the disease, [patients] can get 'wasting.' They get inflammation and their metabolic rate is very high, so they're losing weight uncontrollably. So it's possible that fat may still play a protective role." Oreopoulos said.
The next stage of Oreopoulos' research is to test the effect of intentional weight loss in very obese heart failure patients. However, if muscle is the driving factor, weight loss programs that do not strive to maintain muscle may be detrimental. Muscle is often hard to gain or maintain, as exercise is difficult for heart failure patients. Until longer-term studies on the effects of muscle versus fat are completed, definite recommendations cannot be made.