Being physically fit may help people to live longer, say researchers who suggest exercise stress tests could be useful for doctors in predicting heart disease risk.

People with high levels of cardiorespiratory fitness had a lower risk of dying from all causes of death, including coronary heart disease and cardiovascular disease, than people with low levels of fitness, Japanese researchers said in Wednesday's issue of JAMA, the Journal of the American Medical Association.

Dr. Satoru Kodama of the University of Tsukuba Institute of Clinical Medicine, in Ibaraki, Japan, and colleagues analyzed data from 33 previous studies including more than 100,000 participants with average ages ranging from 37 to 57 to assess the maximum aerobic capacity of participants and their overall health.

"It is possible that consideration of low CRF as a major coronary risk factor could be put into practical use in the clinical setting through identification of low exercise tolerance by exercise stress testing or in daily life by the speed at which a person can walk before experiencing exhaustion," the study's authors wrote.

The researchers suggested men around 50 years of age should be able to walk continuously at 6.4 kilometres per hour (four miles per hour) and women at 4.8 kilometres per hour (three miles per hour) to prevent coronary heart disease.

Population-based studies have identified a link between poor physical fitness and incidence of cardiovascular disease in healthy people, but clinicians do not routinely consider it when estimating future risk of heart disease, perhaps because the quantitative link is not well established, the researchers said.

The studies included 6,910 deaths from a variety of causes and 84,323 people with coronary heart disease or cardiovascular disease and 4,485 deaths caused by those diseases.
Mortality risk

The risk for all-cause mortality was 1.70 times greater among people with low cardiorespiratory fitness compared with those who scored high on fitness scales, the researchers said.

People who were less fit were also at 1.50 times higher risk for coronary heart disease and cardiovascular disease compared with the fittest participants.

In the study, cardiorespiratory fitness was estimated as maximal aerobic capacity expressed in metabolic equivalent units — defined as the ratio of intensity of physical activity to that of sitting at rest.

Low cardiorespiratory fitness was defined as less than 7.9 MET, while intermediate was 7.9 to 10.8 MET, and high was greater than or equal to 10.9 MET.

Each 1-MET increase in maximal aerobic capacity was associated with a 13 per cent decrease in risk of all-cause mortality and a 15 per cent lowering in risk of both coronary heart disease and cardiovascular disease, the researchers found.

A 1-MET higher level of maximal aerobic capacity is comparable to losing seven centimetres in waist circumference in men.

The review may have misclassified cardiorespiratory fitness since definitions varied between studies, the researchers noted.

They suggested testing whether improving cardiovascular fitness through exercise actually reduces risk of mortality and coronary heart disease in a clinical trial.

The study was supported by the Japan Society for the Promotion of Science and Japan's Ministry of Health, Labor and Welfare.