The Myth About the Myth About Exercise

I never meant to write about exercise in this blog, but I wanted to comment on a recent TIME magazine article.

TIME magazine's cover article of its August 17, 2009 issue, titled "Why Exercise Won't Make You Thin", concludes that exercise is not an effective way to lose weight. Based to a large extent on personal experience, the author, John Cloud, argues that most folks will simply eat more and compensate or even overcompensate for the calories burnt during exercise. Cloud concludes that you're much better off simply eating less.

Cloud's article does not appear to be thoroughly researched and may cause lots of people to let their exercise regimen slide or, worse, may discourage many from starting an aerobic exercise program.

Personal Experience

I bike 11 miles to work a typical 5 days a week and ride at what most people would call a reasonably fast pace. This amounts to about 10 hours of aerobic activity each week. Based on past experience, I'm probably generating about 200 to 250 Watts when pedaling (i.e. when not waiting at a traffic light). If I do this for 20 min each way, I am expending about 229 to 287 dietary calories each way (assuming that I am 25% efficient, see below for the details of this calculation). This is roughly 500 calories a day, or about 2500 each week. That's a fair bit of aerobic exercise, and yet, I can still gain a lot of weight if it's the only thing I do.

This agrees with Cloud's thesis and this experience is probably shared by many who embark on an aerobic exercise-centric weight-loss program. Should we all just scrap our exercise plans? If it doesn't make us look good, why bother with it? Is Cloud's conclusion correct?

Burning Fat

The TIME magazine's fundamental shortcoming is that it did not consider how the duration and intensity of aerobic exercise influence weight loss. Endurance athletes know very well that aerobic activity does lead to weight loss provided it forces you to switch on your fat-burning machinery.

If you're going to do 30 minute cardio workouts, you're likely to be tapping into glycogen, a short-term energy source stored in your muscles and liver (the sugar tank). You're also likely to get hungry after the workout since your body would like to top off these energy stores as soon as possible.

It gets different if you teach your body to burn fat (use the fat tank). The only way to do this is to do exercise beyond the point of glycogen depletion. In other words, you need to run or bike far. In that sense, you're better off lumping a big fraction of your cardio work together in one long run or ride each week. I find that if I do one long (i.e. more than 2 hours) run each week, I get skinnier. The same thing happens when I make one of my bike commutes longer by taking a longer route one day a week.

For most of us, this means you have to get off the threadmill (or elliptical trainer, or whatever it is you use) and get outdoors. Very few people are going to be able to exercise regularly for over 2 hours on a stationary machine watching TV.

I typically head out for a 3 to 4 hour, 30 to 40 km trail run in the hills each Sunday morning. I got to those distances by gradually adding distance (about 5 km) each weekend. I've been doing this since May 31, 2009 (a little over 2 months) and got 8 kg lighter. This is about 1 kg per long run. This happened automatically, without paying attention to what I eat.

1 kg of fat is roughly 9000 dietary calories. Do I burn 9000 calories during a 3 hour run? No. Between 2000 and 3000 is more likely. Teaching your body to use fat stores by running far enough apparently allows you to use this energy source during your normal daily activities too. 9000 for the price of 3000, that's what I call an ROI!

I frequently "bonk" near the end of a long run. Bonking is the term endurance athletes use to describe the feeling associated with glycogen depletion. When this happens, speed drops, muscles ache, perhaps you feel cold, and your thinking gets foggy. Many endurance exercise gurus will advise that bonking is a bad idea since it takes longer to recover from a workout with a bonk. I think this is just plain wrong. I've always felt I my fitness improve most dramatically after having recovered from a tragic bonk since it teaches my body to use fats a little faster or longer.

In conclusion: if you find that your exercise regimen is not leading to the expected weight loss, try lumping workouts together (e.g. one long session instead of 5 short ones) to get your internal engine use the slow-burning fats instead of the high-octane glycogen stores.

DISCLAIMER: I know nearly nothing about exercise physiology. So take what you read with a grain of salt.

NOTE: To calculate the dietary calories expended based on mechanical Watts generated:

Dietary calories = P * t / ( e * 4184 Joules/cal)

Here, P is the mechanical power generated (e.g. 200 Watts), t is time period in seconds (e.g. 20 min would be 1200 seconds), e is your efficiency (typically 25% or 0.25). Using these numbers, you get 200*1200/(0.25*4184) = 229 calories.

Note that these are dietary calories. One dietary calorie equals 1000 physical calories with a physical calorie being the amount of energy needed to heat one gram of water by 1 degree Celsius.