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Book excerpt: The myth of ‘The Fat-Burning Zone’

  • By Matt Fitzgerald
  • Published Aug. 16, 2010
  • Updated Dec. 14, 2012 at 3:25 PM EDT
Racing Weight: How to get lean for peak performance

Editor’s Note: The following section is adapted with the publisher’s permission from Racing Weight: How to Get Lean for Peak Performance by Matt Fitzgerald. The book is the first weight-loss book written for endurance athletes and is available in bookstores, bike shops, and online at VeloPress.com. Matt Fitzgerald is a prolific endurance sports journalist who has written more than 20 books on cycling, running, and triathlon. He is a certified sports nutritionist and featured coach for TrainingPeaks.

How to Ride Yourself Lean

Your power-to-weight ratio is one of the best predictors of your cycling performance. Understanding the negative effect of weight on performance, many cyclists are fanatical about minimizing the weight of their bodies and bikes alike.

But the wrong approach to minimizing body weight — namely, severe calorie restriction or endless moderate-intensity riding or a combination of both — will sap power even as it annihilates excess body-fat stores. So the greatest weight management challenge for cyclists is to train and nourish themselves in a way that increases sustainable power output while also minimizing body weight.

To increase your power capacity requires that you consistently perform a small amount of training at very high power-output levels. This type of training sends a message to your body that it needs to let the muscles adapt in ways that will enable them to meet the stress imposed by maximal and near-maximal efforts. A little high-power training goes a long way.

High-intensity training will only increase your power capacity if you support it with a diet that allows your muscles to fully adapt to it. If you’re currently above your racing weight, this objective is best achieved with a slight caloric deficit. Anything more than a slight deficit will deprive your muscles of adequate fat and protein to maintain themselves and adapt to training and adequate carbohydrate to fuel optimal performance. A slight deficit will reduce your body-fat percentage and perhaps also your body weight without affecting your average power output in performance tests such as a 40K time trial. A daily caloric deficit of 100 to 300 calories is most likely to yield these results. My book Racing Weight shows readers how to track calories in and calories out to achieve this deficit. You should also track your body weight, fat percentage, and performance to ensure that this deficit is in fact yielding the desired result of making you leaner without making you less powerful.

All Zones Burn Fat

For many years a debate has raged between two factions of what we might loosely call the exercise community. The debate concerns the best way to exercise to get leaner. Some argue that prolonged, moderate-intensity exercise in the “fat-burning zone” is best. Others argue that high-intensity interval training is the best way to shed excess body fat. The truth is, both types of exercise are effective for fat-burning, and a program that combines the two is likely to be more effective than one based on either type alone.

There are three changes you can make to your training to become leaner and yield better performance: increase the volume of moderate-intensity workouts, add more high-intensity training, and do more strength training.

Let’s cover moderate-intensity workouts, aka the “fat-burning zone.”

The so-called fat-burning zone of exercise intensity is a concept that has spread rapidly throughout all levels of exercise culture. Suppose you were to perform an incremental exercise test on a stationary bicycle in which you started pedaling very slowly in a low gear and then pedaled progressively faster in higher and higher gears until you were sprinting all out. At the beginning of the test your muscles would burn fat almost exclusively, and not much of it. As your intensity level increased, your rate of fat burning would steadily increase, and your muscles would also enlist more and more carbohydrate. At a still fairly moderate exercise intensity the rate of fat burning would peak and eventually begin to decrease as the rate of carbohydrate burning spiked. By the time you reached an all-out sprint your muscles would be burning carbohydrate at an extremely high rate and no fat at all. The intensity zone surrounding the point at which the rate of fat burning peaks is your fat-burning zone. Typically it falls at roughly 59 to 64 percent of VOmax in trained endurance athletes, which corresponds to a comfortable but not dawdling pace in cycling.

Exercising within the fat-burning zone is indeed an effective way to burn off excess body fat, but it is not necessarily more effective than exercising at higher intensities, where carbohydrate burning is greater and fat burning is less. The reason has to do with what happens after moderate-intensity and high-intensity workouts are performed.

Your body replaces burned calories in a specific order. In short, if you burn mostly fat during a workout, you will store mostly fat afterward. And if you burn mostly carbohydrate during a workout, you will store mostly carbohydrate afterward. If you want to get leaner, it doesn’t really matter which kind of calories your muscles use predominately during exercise.

Total calories burned is key

What matters is the total number of calories used. The more calories your muscles use during a workout, the more likely it is that you will consume fewer total calories than your body uses over 24 hours, and if this is the case, then you are likely to experience a net loss of body fat. This will happen even if you burned mostly carbohydrate during your workout, because the body always replenishes muscle glycogen preferentially and it doesn’t take a heck of a lot of calories to do it. Thus, unless your diet is carbohydrate-deficient, any exercise-induced caloric deficit will ultimately take the form of body fat loss instead of muscle glycogen loss. What matters from a fat-loss perspective is not the type but the total number of calories burned during a workout. Because high-intensity exercise burns calories faster than moderate-intensity exercise, high-intensity exercise is, in the big picture, the more efficient way to shed body fat. However, a person can do a lot more moderate-intensity exercise than high-intensity exercise, so it’s moderate-intensity exercise that ultimately has the greatest potential to reduce body fat.

Some endurance coaches promote training in the fat-burning zone to increase an athlete’s fat-burning capacity and ultimately increase fat-reliance in racing. Research has shown that training in the fat-burning zone does improve fat-burning capacity. However, it only improves fat-burning capacity within the fat-burning zone itself—that is, at lower exercise intensities. No matter how fit they are or in what manner they’ve trained, all endurance athletes rely on carbohydrate when racing at intensities that are near or above the lactate threshold.

That said, workouts that serve primarily to enhance fat-burning capacity certainly have their place in any endurance athlete’s training regimen. The workouts that have the greatest effect on fat-burning capacity are those that most deplete your muscle-glycogen stores—namely, very long workouts that you finish cross-eyed and drooling. If you do not currently drive yourself this deep into the pit of fatigue in your longest workouts, you might want to consider extending them for the sake of possibly increasing your fat-burning capacity.

But simply doing a high overall volume of moderate-intensity training will stimulate more or less the same benefits as doing very long workouts. Like a long individual workout, a daily succession of moderately long workouts or morning and afternoon workouts will challenge your muscles to perform in a glycogen-depleted state. Even with adequate carbohydrate intake, your muscles will not be able to fully replenish their glycogen supplies between workouts, and as a result your fat-burning and glycogen storage capacities will increase (provided you periodically give your muscles a chance to fully recover).

In fact, epic rides can be counterproductive. That’s because very long training sessions are extremely taxing and create a significant recovery demand. Once your long endurance workouts exceed a certain critical duration, they begin to limit your overall training volume because they require you to take it easy for a day or two afterward. In most cases you’re better off making high training volume a greater priority than single-session duration and limiting the duration of your longest endurance workouts to that which is strictly needed to ensure you can “go the distance” in races.

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