Peak Fat Burning - Good Short Summary

I have often heard that low-intensity workouts burn more fats. I think this is a great explanation of that and some context that really makes sense as to why this may not be the optimal way to workout for fat burning. I copied this from Runner's World website.

--Carter.

Jan. 24: FACTfiction–Is There a "Fat Burning Zone?" And Where Is Yours?
01/24/2011 8:21 AM

Recently someone asked me: What pace should I run for maximum calorie burning? At last, an easy one!

Answer: You burn the most calories/minute when you run your fastest. Calorie burn is directly related to effort. Whether you're running, biking, swimming or doing pushups, you burn the most calories (for that activity) when you're doing it as fast as you can. (Of course you probably can't keep going for more than 10 seconds or so, but that's another issue.)

The fat-burning question is more complex and confusing. So confusing that wrong answers lurk in many places: at the water cooler, in the locker room, and on endless Internet pages.

Here are the facts and fiction.

1) You often hear/read that fat-burning is greatest when you do low-intensity exercise like walking. This is partially true if the question being asked is: What percent of my total calorie burn is coming from fat? (Let's call this measure PTCBFF–percent total calorie burn from fat.) But it's the wrong question to ask. Sleeping produces an even higher PTCBFF than walking, and I doubt anyone thinks sleeping is a great exercise or fat-burner.

2) No, you shouldn't be interested in PTCBFF. You want to know your maxFat burning zone–the intensity of exercise where you burn the most total fat in a minute, an hour, or whatever.

3) For trained athletes, maxFat occurs at about 70 to 75 percent of your max heart rate. If you want to remember just one number, the 70 will do fine. When you exercise within 5 to 10 beats of 70 percent of your max HR you're still very close to peak fat burning. (However, once you go about 20 beats higher, fat burn drops precipitously. That's because your muscles "prefer" carbohydrates-glycogen when you are going hard.)

4) What I find interesting about this 70 percent of max HR is that it almost perfectly matches the pace you naturally choose for EZ and Long Run days. Many runners do up to 80 percent of their training at this pace. Why are runners thin? Because their typical running pace puts them squarely in the maxFat zone.

5) Your maxFat pace also burns a lot of total calories because you can maintain it for hours on end. Unlike your maxCal pace, which you can only maintain for 10 seconds, as noted above.

6) If you're relatively fit, exercising at 70 percent of your max HR feels quite comfortable. However, an exercise physiologist would consider this "vigorous" exercise. In other words, you get a lot of the big fitness benefits from running at 70 percent of your max HR.

7) If you've got a friend who's a walker, and is looking to burn more calories and fat, try to get this friend to become a walk-runner. A few short running intervals during a walk will boost total calorie burn, total fat burn (edging the workout up toward the 70 percent max HR level), and total fitness benefits. (Or your friend could just walk faster; many fitness walkers go too slow to reap maximum benefits from their activity.)

[Note: Much of the above information came from studies published by sport nutritionist and triathlete Asker Jeukendrup, from the University of Birmingham, England. On his web site, he recently published a more indepth version of the info I've summarized above.]