One of the most significant differences in the new Dietary Guidelines for Americans 2005 from the version released by the federal government five years ago is the recommendation for more exercise - a lot more exercise.

No more just working out for 30 minutes each day. Now, it's double or triple that length of time.

If you don't want to gain extra pounds, the Guidelines advocates an hour of moderate- to vigorous-intensity physical activity on most days. If your goal is to lose weight, that time frame jumps even higher - participation in 60 to 90 minutes of daily moderate-intensity exercise. However, if you want to simply reduce the risk of chronic disease, then 30 minutes is still sufficient, the Guidelines says.

For a nation comprising millions of couch potatoes, can we do it?

Dr. Benjamin Levine, professor of internal medicine at UT Southwestern Medical Center, says your specific goal should determine the time spent exercising. And, don't beat yourself up if it's not up to the Guidelines' lofty standards.

"Ask yourself, what's the point of exercise for me?" Dr. Levine said. "Your answer to that question will determine the dose, or amount, of exercise required to meet your particular objective."

"If your goal is fitness, you may need to exercise at a higher intensity for a shorter period of time. If your goal is cardiovascular health, 30 minutes a day most days a week is pretty universally accepted. If your goal is weight loss, you need to spend more time exercising."

Dr. Levine said the number of calories you daily put into your body and the number of calories you expend also is an indicator of the type of workout a person needs.

"I think recommendations for exercise should be individualized," he said. "Frankly, I think it's difficult to expect anyone to do 90 minutes of exercise a day. I think these new recommendations run the risk of putting people off of exercising entirely.

"My feeling is that you"d be better off to cut down on the number of calories you put in your mouth rather than increase your exercise program just to meet the new recommendations."