The best heart rate for burning fat differs for each individual and often does not align with the "fat-burning zone" on commercial exercise machines, according to a small study.

Instead, clinical exercise testing -- a diagnostic procedure to measure a person's physiological response to exercise -- may be a more useful tool to help individuals achieve intended fat loss goals, said researchers from the Icahn School of Medicine at Mount Sinai.

The study, which used a machine learning-based modelling approach, was published online in Nutrition, Metabolism and Cardiovascular Disease.  

"People with a goal of weight or fat loss may be interested in exercising at the intensity which allows for the maximal rate of fat burning. Most commercial exercise machines offer a 'fat-burning zone' option, depending upon age, sex, and heart rate," said lead author Hannah Kittrell, a doctoral candidate at Icahn Mount Sinai. 

"However, the typically recommended fat-burning zone has not been validated, thus individuals may be exercising at intensities that are not aligned with their personalised weight loss goals," she added.

The term FATmax is sometimes used to represent the exercise intensity and associated heart rate at which the body reaches its highest fat-burning rate during aerobic exercise. 

At this point, fat is a significant fuel source and therefore this intensity may be of interest to those seeking to optimise fat loss during workouts.

As part of the study, the researchers compared heart rate at FATmax, as measured during a clinical exercise test, to predicted heart rate at percentages of maximal effort within the typically recommended "fat-burning zone".

In a sample of 26 individuals, the researchers found that there was poor agreement between measured and predicted heart rates, with a mean difference of 23 beats per minute between the two measures. 

This suggests that general recommendations for a "fat-burning zone" may not provide accurate guidance.

Next, the researchers plan to study whether individuals who receive a more personalised exercise prescription demonstrate more weight and fat loss, as well as improvement of metabolic health markers that identify health risks like Type-2 diabetes, obesity, and heart disease.