Digested carbohydrates circulating in your blood or stored in the muscle cells fuel your exercise best, but you tap out of these relatively quickly -- in as little as 20 minutes of all-out rowing or 90 minutes of easy running. That’s why you sometimes sip a sports drink or slurp an energy gel that’s nothing but carbs in the midst of a workout. It’s also why your body breaks down fat as additional fuel during exercise.
Fat to Fuel During Exercise
Stored fat burns more slowly, so your body uses carbohydrates for fuel exclusively when it’s giving a 100 percent effort, like all-out rowing or sprinting. But do a practice run at your half-marathon race pace -- probably between 80 and 85 percent of your maximum heart rate, or MHR -- and your body will use circulating blood fat and stored fat for about 20 percent of the required energy. Slow the run to your typical training pace -- between 70 and 75 percent of your MHR -- and the percentage of fat you burn for fuel increases to about 50 percent. Slow the pace to the point where you can jog and converse in full sentences -- about 65 percent of your MHR -- and your body uses fat for 60 percent of the required energy. To begin to burn fat, your body secretes the hormone glucagon, which activates lipases that break down fat cells.
Role of Glucagon
Think of glucagon as insulin in reverse. Whereas your pancreas secretes insulin to transport digested carbohydrates -- called glucose in your blood and glycogen in your muscles -- to the muscle cells or the fat stores when your blood sugar is too high, your pancreas secretes glucagon when blood sugar goes too low. This secretion does many things chemically, ultimately allowing blood fat and stored fat to be broken down and used to augment your body's preferred fuel -- digested carbohydrates. Glucagon secretion not only occurs when carb fuel is being conserved during exercise, but also at other times when your body is not producing insulin in significant amounts.
Fat to Fuel During Dieting
The dictate of any diet: Consume fewer calories than you ingest, and you’ll lose weight. Low-carb diets follow that dictate, but to increase the odds that the weight loss is fat loss, they severely limit carb consumption because protein and fat produce little change in your insulin level. The first 14 days of the Atkins diet, for example, permit you to ingest no more than 20 grams of carbs a day, the equivalent of 1 cup of lettuce, 1 cup of peppers and 2/3 cup of asparagus. Since the carb fuel your body stores in the muscles and liver only lasts about a half day, glucagon is regularly secreted on a low-carb diet to transform stored fat into energy.
The Longer Your Workout, the More Fat You Burn
Any time you exercise for more than 40 minutes at a time -- even at moderate intensity -- fat replaces digested carbs as the primary fuel source, so three 60-minute workouts trump six 30-minute workouts per week for fat loss. Current advice for the time-crunched hoping to drop weight suggests exercising for 10 minutes at a time three or four different times during the day, but the energy used in such short workouts comes from digested carbohydrates, not fat. Though low-carb diets may work well for fat loss, they have other limitations. Restricting carbs makes it difficult to get the proper amounts of many vitamins, including calcium and potassium, as well as a sufficient amount of fiber. Additionally, cutting carbs impairs recovery from exercise, leading to general malaise and compromised performance.
- Iowa State University Extension: Carbohydrate
- Sports Performance Bulletin: Running at Max Capacity
- Indoor Rowing Training Guide: Training Intensity
- Pathophysiology: Adaptations and Alterations in Functions; Barbara L. Bulllock, R.N., M.S.N. and Pearl Philbrook Rosendahl, R.N., Ed.D.
- Virtual Chembook: Glycogenesis, Glycogenolysis, and Glucogeneogenesis
- Dr. Atkins’ New Diet Revolution; Robert C. Atkins, M.D.
- Oklahoma State University: Carbohydrates in the Diet
- Med Fitness: Timing Is Everything: Why the Duration and Order of Your Exercise Matters
- US News & World Report: Low-Carb Diet: Will It Work for You?
- Sports Coach: Seven Reasons Why a Low Carbohydrate Diet Is Wrong
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