The Cross-Over Concept and Weight Management
by Scott Murr
originally published in Pace Running Magazine Summer 2016

Many people run to challenge themselves by setting goals (run a certain distance or cover a given distance in a certain time) while some run to help manage their body weight (and some people run to do both). Having an understanding of what fuels your efforts might help you better accomplish your goal.

When you go out for a run (or any sort of workout), carbohydrates and fats fuel your efforts. While you burn both carbs and fat, the primary source for your efforts is determined by many factors; most significantly, your intensity (or pace).

A brisk walk or slow jog is fueled mostly by fat while a 5K race is fueled mostly by carbohydrates. In other words, for low intensity exercise, fat is the primary substrate but as intensity increases, the percent contribution by fat decreases while the percent contribution by carbohydrates increases.

The point at which the fat:carbohydrate fueling ratio changes from mostly fat to mostly carbohydrate is referred to as the “cross-over” point. Runners can increase their aerobic fitness (VO2 MAX) with the right type of training. Runners can also shift their lactate threshold to the right by training at the appropriate pace (more on shifting your lactate threshold in a future issue of PACE). Likewise, runners can shift their cross-over point to the right with their training.

Runners interested in running a given distance in a faster time, especially marathons, would benefit by shifting their cross-over point to the right. Exercise physiologist and author Dr. Jason Karp has written extensively on the cross-over concept and reminds us that our fat reserves are significantly greater than our stored carbohydrates. As Dr. Karp points out in his book Running a Marathon For Dummies, shifting your cross-over point to the right will enable you to maintain a submaximal pace longer, delay “hitting the wall” (glycogen depletion) and run those final 6 miles of the marathon stronger.

One physiological adaptation to distance running is that you become better at burning fat (running at the same submaximal pace). Running at a faster pace before you reach the cross-over point is a good thing if you are a distance runner.

If running performance is your primary concern, then consider what Dr. Dan Benardot, a registered dietician and author of Advanced Sports Nutrition says: “When glucose runs out, the athlete stops performing; therefore understanding how to keep glucose from becoming depleted should become a major focus of an athlete’s nutrition practices.”

Those interested in managing their body weight are often misled by “fat-burning” programs on treadmills or ellipticals or by heart rate charts with a zone labeled as “fat burning zone.”

For weight management, what matters most is the rate of energy expenditure (how many calories you burn) rather than the percentage of energy expenditure derived from fat. Fat is the primary substrate during low intensity exercise, so the “fat-burning” zone is primarily a low intensity workout. The drawback is that in a low intensity workout, the caloric expenditure is lower. For weight management, a focus on caloric expenditure is probably more effective than a focus on whether you are burning fat or carbohydrates.

Dr. Karp suggests that to reduce excess body fat, what matters most is the difference between the number of calories one expends and the number of calories they consume.  High-intensity running (track repeats and short tempo runs) burns lots of calories in a short amount of time, keeps your metabolic rate elevated for hours and will also increase your muscles’ ability to use fat.  (Check out Dr. Karp’s webpage http://run-fit.com/ for more great info on running and performance.)

Unfortunately, there is no such thing as 100 percent fat-burning. One function of carbohydrates is to help burn fat, thus the well-known saying “fat burns in a carbohydrate flame.”  So forget the “fat-burning” zone and increase your pace.

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