DEAR DOCTOR K:

I believe you when you say that regular exercise protects my health. But I’m just 20. For me the question is, when should I begin regular exercise? The diseases that exercise protects against usually don’t develop until a person is older than 60. I’d like not to have to worry about exercise for a few decades.

DEAR READER:

I wish I could rid you of that worry. But, in fact, if you are not already exercising regularly, you are probably putting your health at risk 30 years from now. Many of the diseases that exercise protects against actually start in young adulthood and take decades to become severe enough to cause symptoms. For example, autopsy studies have been performed on soldiers killed in war — people your age. They reveal that many already have early signs of atherosclerosis, the cause of heart attacks, strokes and premature death.

Many studies have shown that people who exercise regularly have lower rates of heart disease, strokes, diabetes and several types of cancer. It is true that those studies have usually been done in adults over age 50. However, a new study of young adults, like you, comes to the same conclusion.

The study was performed at Johns Hopkins and was published in the medical journal JAMA Internal Medicine. Nearly 5,000 young adults, ages 18-30, were included. Then they were followed for nearly 27 years. The study participants came from different geographic areas in the United States. There were both males and females and people of different racial and ethnic groups.

The study did not measure how much the study participants exercised. Instead, it measured the physical fitness of these young adults — a reflection of how much they exercised. Fitness was assessed by how many minutes study participants could exercise on a treadmill test before becoming too tired to continue. The test was first performed when a person enrolled in the study. It was repeated in most of the study participants seven years later.

The people who were the most fit when the study began had a lower risk of developing heart disease and of dying prematurely. For each extra minute that people were fit enough to exercise on the treadmill, their risk of developing heart disease was lowered by 12 percent, and their risk of death was lowered by 15 percent.

The people who stayed fit when retested seven years later also had lower risks of heart disease and death than those who did not. The least-fit young adults were most likely to develop thickened heart muscle that was straining to do its job.

It’s hoped the study will continue to follow its participants for another 15 to 20 years. If so, we could learn for sure whether those who stayed fit all of their adult lives did better than those who started regular exercise in their 50s or later. That would give a better answer to your question. However, based on everything we know, my advice is that you start a program of regular exercise now.