Beware new hype for simple solution to fitting in workouts.
The researchers asked 20 healthy men and women (average age, 23 years old) to ride stationary bikes at different intensities. Some were assigned five days a week of biking, doing 40 to 60 minutes of moderate-intensity cycling. Others did four to six sets of 30-second sprints on the cycle, allowing four and a half minutes of recovery time between sets. The total exercise time for this latter, high-intensity group was 15 to 25 minutes three days a week. After six weeks, the study revealed that the intense interval training on the bikes improved the structure and function of arteries as much as the traditional, longer endurance exercise.
While this sounds like good news for those with limited time for daily exercise, there are several concerns about oversimplifying the implications--and overstating the meaning--of this one small study. High-intensity training is not appropriate or even healthy for everyone. For some, high-intensity intervals like sprinting are too demanding and may increase the risk of injury. Second, the study states that while improvements in peripheral vascular structure and function occurred in the high-intensity interval subjects, longer-duration exercise is required for gains in central artery dilation.
Most significantly, remember that metabolically, you burn the most fat at low levels of exertion. While this translates to only around 20 to 30 percent of your maximal effort, if weight loss is your goal, it makes greater sense to work out moderately, at around 40 to 65 percent of maximal effort. This is because, while fat makes up the vast majority of the fuel you burn at levels of 20 to 30 percent, overall fuel needed to sustain such low levels is so low that not much of anything gets burned. You'd have to exercise for hours on end to see fat loss results. At moderate intensity, fat and carbohydrate are burned in nearly equal proportions, but it is this intensity that practically leads to a sufficient number of calories burned overall to see a difference physically. Such a regimen utilizes, say, an hour daily four to five days a week.
This level also retains the readiness-to-run principal, by not overtaxing the musculoskeletal system, leading to overuse injury, and prohibiting the next day's workout. Conversely, at very high levels of exertion, the body switches over to the anaerobic energy system, which burns almost no fat and cannot be sustained very long. This makes relying solely on brief, high-intensity workouts impractical as a means of both caloric expenditure and fat burning.
While high-intensity interval training has its place in an overall regimen, the relationship between the various energy systems and the desired effects of training at certain exertion levels is rather complex. The media tends to offer glib solutions for time-pressed runners that do not take into account the specific needs of a given training program, tailored to a given goal, tailored to a given runner. Proper training is always a highly individuated process that requires speaking with a health care professional before embarking upon, and then continually self-monitoring and learning the nuances of, with regard to the relationships among diet, exercise, weight loss, performance gains, and heart health.
Am. J. Physiol.--Regulatory, Integrative & Comparative Physiol., 2008, Vol. 295, No. 1, pp. R236-R242
New York Times, June 5, 2008, http://well.blogs.nytimes.com/2008/06/05/for-heart-health-sprints-match-endurance-training