Exercise Guidelines: One Trial Does Not Fit All
Doing The Math
The 2008 federal guidelines for physical activity include a definition of exercise intensity using a MET, a metabolic equivalent or a unit useful for describing the energy expenditure of a specific activity. A MET is the ratio of the rate of energy expended during an activity to the rate of energy expended at rest (roughly 1 MET). Accordingly, a 4 MET activity expends 4 times the energy used by the body at rest. If you do a 4 MET activity for 30 minutes, that's the equivalent of 4 METs x 30 minutes, or 120 MET-minutes (or 2.0 MET-hours) of physical activity. You could also achieve 120 MET-minutes by doing an 8 MET activity for half the time (15 minutes). Knowing these units helps you understand the basis behind the guidelines. For example, in Appendix 1 of the guidelines, it states that "a key finding of the Advisory Committee Report is that the health benefits of physical activity depend mainly on total weekly energy expenditure due to physical activity. In scientific terms, this range is 500 to 1,000 MET-minutes per week," or 2 hours and 30 minutes.
Here's the more important part, though, in terms of our discussion: "A range is necessary because the amount of physical activity necessary to produce health benefits cannot yet be identified with a high degree of precision; this amount varies somewhat by the health benefit." [Note this section that I bolded as more evidence of the problem with making these recommendations across the board.] It goes on to say that an activity of 500 MET-minutes a week results in a substantial reduction in the risk of premature death, but activity of more than 500 MET-minutes a week is necessary to achieve a substantial reduction in the risk of breast cancer. So, how much should you be doing? It appears to depend on which health problems you either have or wish to prevent with physical activity.
So How Much Exercise Do You Need To Do?
Admittedly, using guidelines expressed using MET-minutes is not useful for the general public because the concept of METs is difficult to understand and few people are familiar with it. Moreover, it's challenging to know the MET values for all the activities you do. As long as people generally achieve 2 hours and 30 minutes per week (or more), it is likely appropriate to express the guidelines in simpler terms of minutes of moderate-intensity activity and minutes of vigorous-intensity activity. And because not all the benefits of physical activity occur at 500 MET-minutes per week, they're happy if you exceed this amount.
The Advisory Committee indicated that 2 hours and 30 minutes of moderate-intensity activity per week could be regarded as (roughly) equivalent to 500 MET-minutes per week. In fact, doing 3.3 METs for 150 minutes per week is equal to 500 MET-minutes per week. By recommending that adults do at least 150 minutes of moderate-intensity activity per week, adults will achieve 500 to 1,000 MET-minutes per week if the intensity is 3.3 times resting values or greater, which is the equivalent of a "brisk walk" at 3.0 miles per hour or faster. As for recommending at least 75 minutes (1 hour and 15 minutes) per week of vigorous-intensity activity, their rationale was that doing so will also allow you to achieve 500 to 1,000 MET-minutes per week. The lower limit of vigorous–intensity activity (6.0 METs) is twice the lower limit of moderate-intensity activity (3.0 METs). So, 75 minutes of vigorous-intensity activity per week is roughly equivalent to 150 minutes of moderate-intensity activity per week.
These recommendations beget the question as to whether anyone has determined if you can "mix and match" exercise durations and intensities to get a similar physical benefit. The answer is that no one knows for certain. The guidelines address the issue that some people do both moderate-intensity and vigorous-intensity activity in a week. As 150 minutes of moderate-intensity activity and 75 minutes of vigorous-intensity activity are the minimum amounts, the rule of thumb is that 1 minute of vigorous-intensity activity counts the same as 2 minutes of moderate-intensity activity. Has anyone tested this assertion beyond a reasonable doubt? By now you should know that the answer is, of course not.
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