Fitness is a better predictor of the risk of heart disease and death than weight as defined by body mass index or BMI, a new study says.

Rates of obesity, as estimated based on BMI, are rising in developed countries, even as weight-loss drugs are entering the market.

But a new review of studies, published in the British Journal of Sports Medicine, concludes that heart and lung fitness in particular are stronger predictors of death risk than BMI.

“Fitness, it turns out, is far more important than fatness when it comes to mortality risk,” study co-author Siddhartha Angadi from the University of Virginia in the US says.

The review reveals that fit people, across all BMI categories, have statistically similar risks of death from heart disease. Unfit individuals, in contrast, have a two- to three-fold higher risk of death from heart disease compared with fit people of normal weight.

“Our study found that obese fit individuals had a risk of death that was similar to that of normal weight fit individuals and close to one-half that of normal weight unfit individuals,” Dr Angadi says.

His team reviewed 20 studies covering 398,716 adults in multiple countries, nearly a third of them women. Most of these studies classified people as fit if their exercise test score placed them above the 20th percentile in their age group.

Although obesity is linked to health conditions like heart disease and diabetes, losing weight to counter the risks is often hard for many people.

“Most people who lose weight regain it,” says Glenn Gaesser, another author of the study. “Repetitive cycles of losing and gaining weight is associated with numerous health risks comparable to those of obesity itself.”

Researchers instead recommend improving heart and lung fitness to avoid such adverse health effects.

Current guidelines advise a minimum of 150 minutes per week of moderate-intensity physical activity or 75-minutes of vigorous physical activity for adults along with muscle strengthening two days a week.

For unfit people, in a position similar to the bottom 20th percentile in the new study, starting any aerobic exercise can have a big impact. “This could be achieved with activities such as brisk walking several times per week with the goal of accumulating around 30 minutes per day,” Dr Angadi says.

His team calls for independently verifying the value of a fitness-based approach – rather than a weight-loss approach – in obese individuals to optimise their health.