When I made a preliminary report here on William Haskell's presentation on exercise last month, I missed the main point. This Stanford University professor of medicine spoke at the first day of the American Diabetes Association's scientific sessions in San Francisco.

This was one of the talks that I most wanted to hear. Exercise along with diet are the best tools we have to control our diabetes.

But my plane was late, and I missed most of what he had to say. In fact, most people who wanted to hear him speak couldn't get into the full conference room.

The conference organizers apologized to those of us who weren't able to get in. To make up for it, the ADA now has the full text of his talk with accompanying slides -- along with the two other presentations on exercise --  available online.

This panel presentation, "Current Issues in Exercise," is one of only eight webcasts that the ADA offers free. They also offer lots more, but they don't come cheap.

I just reviewed these exercise presentations. Professor Haskell's was in fact a sneak preview. This is essentially a summary of the preliminary report to the Physical Activity Guidelines for Americans - 2008, which the U.S. Secretary of Health and Human Services plans to release in October.

The new physical activity guidelines will parallel the Dietary Guidelines for Americans. That document is the government's key advice to us on what to eat.

The preliminary 683-page report on the physical activity guidelines is also online. The country's top experts on exercise prepared it. Professor Haskell was perhaps too modest, but he didn't mention in his ADA presentation that he chaired the physical activity guidelines advisory committee.

His presentation zeroed in on the amount of exercise we need. In other words, how little we can get away with and how much is too much.

For most people, the next point is by far the most important one. "We need to be very careful not to discourage the least active people," Professor Haskell says. "Some exercise is much better than none."

Even very inactive people who get just a little exercise "are headed in the right direction," he says. Only recently have we had that data to support this conclusion.

If you are generally healthy, ambulatory, and physically independent, just adding10 minutes of moderate activity per day -- which works out to 70 minutes per week -- seems to provide some fitness and health benefits.

Some in this context means limited. Professor Haskell says that we do have reasonably strong evidence that more than 150 minutes of moderate to vigorous exercise per week will provide greater health benefits that this minimal amount. So the advisory committee set the target at 500 to 1,000 MET-minutes per week.

MET-minutes is the one technical concept that we need to understand. MET is a unit of metabolic equivalent.

The committee's report provides a concrete example. "An adult can achieve a target of 500 MET-minutes per week by walking at about 3.0 miles per hour for approximately 150 minutes per week (7.5 miles), walking faster at 4.0 miles per hour for 100 minutes (6.6 miles), or jogging or running at 6 miles per hour for about 50 minutes per week (5.0 miles)," it says. "To achieve 1,000 MET-minutes per week, these amounts of activity would need to be doubled."