Keeping fit when young pays long term dividends by cutting the risk of a heart attack or stroke in middle age, US research suggests.
How to stay healthy
Fit young adults were found to be much less likely to develop conditions such as diabetes and high blood pressure which can put them at risk.
Cardiovascular disease is a major killer, particularly in the West.
The research, by Northwestern University, is published in the Journal of the American Medical Association.
Researcher Dr Mercedes Carnethon said: "If all the young adults in our study had been fit, there would have been nearly a third fewer cases of high blood pressure, diabetes and metabolic syndrome."
Metabolic syndrome is a collection of factors that includes excess abdominal fat, elevated levels of triglycerides and low levels of "good" cholesterol.
The research focussed on over 4,400 men and women aged 18 to 30, who were followed for up to 15 years.
About 2,500 had their cardiopulmonary fitness re-tested after seven years to measure changes in fitness.
The results showed people with low or moderate fitness had twice the risk for high blood pressure, diabetes and metabolic syndrome as those who were highly fit.
As fitness levels dropped, then people put on weight, and their risk increased.
Among those who were obese, 68% had low fitness, while 51% of those who were not obese were rated as highly fit.
However, improving fitness had no significant effect on reducing high blood pressure or low-density lipoprotein, the so-called "bad" cholesterol.
Lead researcher Dr Mercedes Carnethon said: "This may be because low-density lipoprotein levels are affected largely by genetics and diet, and less by fitness.
"The key point is that the development of risk factors for heart disease and stroke isn't just the natural result of aging."
Professor Sir Charles George, Medical Director at the British Heart Foundation said the results echoed those from previous studies.
He said: "Results of the study suggest that adopting healthy lifestyles could avoid or delay the anticipated explosion of type 2 diabetes in middle age and beyond."
Amanda Vezey, of the charity Diabetes UK, said: "This research provides further evidence that taking regular exercise can help people delay the onset of Type 2 diabetes.
"The numbers of people with diabetes are increasing dramatically. Leading healthier lifestyles could definitely help us to avoid this trend."