20-Minute Fat Burning Myth

by Christian Finn
The Facts About Fitness

Back in 1999 when I was studying full-time at university, and working both a full-time and a part-time job, one of the big challenges I faced was making the time to exercise.

Despite my best efforts, I often failed to make it to the gym at all, even for just 20 or 30 minutes. And even if I did manage to summon the enthusiasm to train, by the time I'd finished changing and warming up, it was almost time to leave again.

And I hated it. I felt fat, even though (compared to most people anyway) I still looked in reasonably good shape. I was moody and found it hard to concentrate on anything for more than a few minutes at a time. And a good workout would set me up for the day. Without it, I felt tired and listless.

I felt like an addict in need of my exercise "fix!"

After several weeks of moaning and complaining to anyone who would listen that it was now "impossible" for me to do any exercise at all, I decided to do something about it.

My plan was to cut each workout in half. Driving to work in the morning, I'd stop at the gym and get the first half done. On the way back home at night, I'd do the second half.

However, a few people I spoke with told me that this idea was "silly" and "wouldn't work."

"Your body doesn't start burning fat until you've been exercising for at least 20 minutes," they told me. I was warned that I "wouldn't lose any fat at all" unless I did at least 45 minutes of continuous aerobic exercise.

It's true that your body relies more on carbohydrate and less on fat during the early stages of exercise. It's also true that your body uses more fat and less carbohydrate the longer you spend exercising.

But this ignores what happens to your metabolism in the hours after a workout, when the number of fat calories burned rises significantly.

To lose fat, you need to create a calorie deficit — to consistently burn more calories than you consume. And it doesn't make a great deal of difference whether those calories are burned in one long workout or several shorter ones.

Some evidence for this comes from research carried out at the University of Pittsburgh School of Medicine [1]. For the study, a group of overweight women was assigned to one of two groups. Group one performed a single bout of exercise lasting 20-40 minutes. Group two did the same amount of exercise, but it was split into several smaller bouts lasting just 10 minutes.

And the result?

Twenty weeks later, the women who split their workouts into shorter bouts had lost 20 pounds, compared to just 14 pounds in the single-bout group. The reason for the extra weight loss is simply that women in group two did more exercise (and thus burned more calories) than women in group one, possibly because they found it easier to fit shorter workouts into their day.

Research published in the Journal of the American College of Nutrition shows similar results [3]. There were no significant differences in weight loss with three 10-minute bouts of exercise per day compared with two 15-minute bouts or one 30-minute bout.

Changes in body composition weren't reported in this study, so we don't know how much of the lost weight came from muscle and how much came from fat. I'm guessing that the women lost some muscle, which is fairly common with aerobic-only exercise programs.

That's why it's always a good idea to do some kind of resistance training to help preserve lean muscle when you're losing weight (see How to Fight Fat and Win II for a highly effective whole-body resistance-exercise program).

So, what does all of this mean for you?

Fat is stored energy. To lose it, you have to use more energy (calories) than you get from your diet. And it doesn't really matter whether you burn those calories in one long workout or several shorter ones. As these studies and my own experience show, both approaches work.

References
1. Jakicic, J.M., Wing, R.R., Butler, B.A., & Robertson, R.J. (1995). Prescribing exercise in multiple short bouts versus one continuous bout: effects on adherence, cardiorespiratory fitness, and weight loss in overweight women. International Journal of Obesity and Related Metabolic Disorders, 19, 893-901
2. Kanaley, J.A., Weltman, J.Y., Veldhuis, J.D., Rogol, A.D., Hartman, M.L., & Weltman, A. (1997). Human growth hormone response to repeated bouts of aerobic exercise. Journal of Applied Physiology, 83, 1756-1761
3. Schmidt, W.D., Biwer, C.J., & Kalscheuer, L.K. (2001). Effects of long versus short bout exercise on fitness and weight loss in overweight females. Journal of the American College of Nutrition, 20, 494-501