Posted by Dave on April 16, 2010 | 12 Comments
For the last 16 days, as a part of this month’s coverage of fitness and nutrition, I’ve been trying to record all of my exercise and food intake. The MyFitnessPal website I use offers a simple way of calculating how many calories a given exercise burns: I just enter the number of minutes I spent doing a given exercise, and it takes factors like my weight into account to calculate the caloric expenditure.
But lately I’ve been noticing a bit of a problem. On flat ground, I can run about a 9-minute mile pace, more or less. Most days recently I’ve been doing the same hilly 4.2 mile loop. Depending on how I’m feeling, some days I do it in 40 minutes, some days 37. It’s harder to run faster, and I’m going the same distance, but MyFitnessPal actually computes a lower calorie expenditure total when I run for 37 minutes compared to 40 minutes. That can’t be right, can it?
Part of the problem is that MyFitnessPal is using minutes, not distance, as its calculation method. If I was running on a treadmill or a track, this would make sense. I can set my pace at exactly 9 minutes per mile, and then ask the site to calculate how many calories I burn for length of time I ran. But if I’m running a fixed distance and my pace varies, the calculation is going to be off a little. MyFitnessPal doesn’t have separate exercise categories for an 8:57 mile versus a 9:15 mile (by the way, I realize that this error doesn’t actually matter much in the greater scheme of things, but it’s still an interesting question).
Still, the question remains: Assuming we’re calculating calorie expenditure for a specific distance ran, does the pace of the run matter? Or would I expend the same number of calories running 8 minute miles or 10 minute miles? Even though it takes less time, I find running an 8 minute mile to be much more exhausting than running a 10 minute mile.
This article on Runner’s World offers a partial answer to the question: Running definitely consumes more calories per mile than walking. It makes some sense: The running motion is much more complicated than walking motion–you’re effectively leaping into the air with every step. It’s obvious (to me, anyways) that hopping on one foot for a mile consumes more calories than walking a mile, so by similar logic, running should too.
The Runner’s World article points to another problem in computing calories consumed. On MyFitnessPal, every calorie “burned” exercising is an extra calorie you can eat and maintain the same weight (or weight-loss plan). But do we really want to count all calories burned? After all, a weight-loss plan assumes some basic resting metabolism–I need to consume some calories, even if I’m just sitting at a computer typing all day. They suggest using “Net calorie burn” to calculate the number of calories used, and offer a simple equation: .63 times weight times number of miles run. So my 4.2-mile run burns 595 net calories — or about 130 calories less than what the MyFitnessPal website calculates if I run at exactly a 9-minute-mile pace. My guess is that MyFitnessPal uses total calories rather than net calories, which would just about explain the difference.
This still doesn’t answer my initial question, though: Does running faster for the same distance consume more calories? Runner’s World seems to assume calorie consumption is about the same no matter the pace (as long as you’re actually running, and not walking!). I’ve done a fair bit of searching around the internet and haven’t found a good answer to this question — do any readers know?
But there’s still one more piece to this puzzle: hills. I think that most runners will agree that running a hilly course is harder than running a flat course, even if it starts and ends at the same elevation. A physicist would say that in an ideal (frictionless) environment, there should be no difference. But empirically, I always run faster on a flat track than a hilly one.
Fortunately, there’s been research on this as well: “Doc 26.2″ blogged about it here. As he reports it, researchers measured the oxygen consumption of runners on treadmills at various inclines and assessed the differences. Previous research has found that oxygen consumption is a reasonable proxy for calorie consumption.
The researchers found that a 20 percent increase in grade on a treadmill required a doubling of energy expenditure. But running downhill compensated for this increase until the grade hit an incline of negative 18 percent. At that point, Doc 26.2 says running downhill is actually harder than running uphill. I find this hard to believe, having recently run both up and down very steep hills in San Francisco. Running down a steep hill might be harder than running on the flat, but it’s not harder than running up the same hill.
Part of the problem may be the fact that this study was done on treadmills. It’s harder to run “uphill” on a treadmill than on a flat treadmill, but it’s still different from running up a real hill. One of the commenters to the post described the problem:
Running on an inclined treadmill is not the same as running up a hill, although they both recruit the same muscles. The calorie expenditure on an inclined treadmill is due to the change in stride/movement. It may be that the movement is less efficient until you build the appropriate muscles. The potential energy gained by running up a hill is never gained on a treadmill.
In other words, on a treadmill, you’re not actually lifting your body weight to the top of the hill. That takes a lot of work. Similarly, running down a real hill allows you to “cash in” on that potential energy in a way treadmills do not.
I still assert that running a hilly route consumes more calories than running on the flat, but at this point it looks like there’s no empirical study to back me up.
So in the end, as I’m tracking my calories expended, should I do anything different? I may start just giving the same “time” for my runs, no matter how long it actually takes — that way the caloric expenditure for a 4.2 mile run will always be the same. I don’t think I’m going to bother to adjust to a “net calorie” method. Since neither Runner’s World nor MyFitnessPal take hills into account, I suspect the difference might just about cancel out.
Overall, not much has changed in the past week. My average daily calorie consumption was 2,807 the first week, and 2834 last week. Exercise-adjusted calories were 2,000 versus 2,040. Exercise didn’t change much either. Next week I’m going to attempt to make some actual changes in my diet, and I’ll give an update at the end of the week to see if I was successful.