Being fit at 50 seems to improve health related quality of life as people age, a large U.S. study concludes.

Unlike previous research that has looked at physical activity and healthy aging, the study in this week's issue of the Archives of Internal Medicine zeroed in on a measurable fitness level.

Researchers used treadmill tests to assess fitness levels objectively. (Ben Stansall/AFP/Getty)

Dr. Benjamin Willis of the Cooper Institute in Dallas and his colleagues used treadmill tests to assess the cardio-respiratory fitness of 14,726 healthy men and 3,944 healthy woman with an average age of 49 when the study began in 1970.

"In summary, midlife fitness was associated with a lower risk of common chronic health conditions in men and women older than 65 years enrolled in Medicare," the study's authors concluded.

The researchers assessed participants for eight chronic conditions until 2009:

• Congestive heart failure.
• Ischemic heart disease.
• Stroke.
• Diabetes mellitus.
• Chronic obstructive pulmonary disease.
• Chronic kidney disease.
• Alzheimer disease.
• Colon cancer.
• Lung cancer.

Participants were divided into five groups based on their fitness levels.

Among men in the lowest fifth of fitness scores, the rate of chronic disease was 28 per cent compared with nearly 16 per cent among the fittest, the researchers found.

For women, the corresponding figures were 20 per cent versus 11 per cent.

The researchers relied on data from an administrative database rather than clinical diagnoses, which is one of the drawbacks of study.

Genetics matter

The findings were also based on participants who were wealthier, more educated and leaner compared with the average U.S. population.

Investigators took age, body mass index, blood pressure, cholesterol, smoking, blood glucose levels and alcohol intake into account.

In a commentary, Dr. Diane Bild of the National Heart, Lung and Blood Institute in Bethesda called the findings an "excellent addition to the evidence supporting the contention that cardiorespiratory fitness helps people thrive."

Bild also noted that fitness is a function of both exercise and genetics. Genetics likely plays a role in both longevity and disease avoidance, she said.

"Physical activity has a host of well-proven benefits, including effects on weight, cardiovascular disease risk factors, bone health and mental health," Bild wrote.

"Resultant physical fitness may also help individuals maintain health and delay death and get our societal survival curves into the best shape: rectangular."

The study was funded by the Cooper Institute, a nonprofit research group.