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Overnight Fasting to Increase Speed and Endurance

A new study from France shows that night-time fasting after intense workouts on alternate days helps athletes exercise longer and faster (Med Sci Sports Exerc, April 2016;48(4):663-72).

This is an impeccable study in which 21 trained competitive endurance athletes were randomly divided into two groups and all followed the same training program. They all ate the same foods and the same total amount of carbohydrates per day. The only difference between the two groups was the times of day that they ate their carbohydrates.

The test group ate a low-carbohydrate dinner after their intense afternoon workout and then fasted for 13 hours before their morning recovery workout, then ate larger amounts of carbohydrates after their recovery workouts. The control group ate their meals as they wished, with no fasting requirement.

After just three weeks of the carbohydrate-fasting regimen, the test group was able to pedal at greater than 120 cadence for a longer time (more than 60 seconds), and run 10 kilometers (6.25 miles) faster. Neither group lost a significant amount of weight, but the fasting group lost 8 percent of their body fat, compared to the control group who lost only 2.5 percent. This is very important because when you lose body fat, you lose fat in your liver so you are able to store more sugar there.

Storing Sugar for Speed and Endurance
Your muscles use primarily sugar and fat for energy. You have a virtually unlimited amount of fat in your body but only enough sugar to last about 70 minutes during intense exercise.

When your muscles start to run out of stored sugar, they hurt and burn, and you become short of breath and have to slow down. Runners call this "hitting the wall".

Your liver is the only other place where you store significant amounts of sugar. When your liver starts to run low on sugar, your blood sugar starts to drop and you feel exhausted and can even pass out. Cyclists call this "bonking".

Anything that helps your liver store more sugar helps you to exercise more intensely for longer periods of time. When the test group lost body fat they took fat out of the liver, which allowed them to store more sugar, thus giving them greater speed and endurance.

You get all of your sugar from carbohydrates that are made up of sugars in singles, doubles and long chains (starches and fiber). The most carbohydrate you can take in to increase stored muscle sugar is 90 grams per day. You cannot increase sugar stores in muscles beyond that by eating more carbohydrates, because all extra sugar is converted to fat, which increases weight and just slows you down (Am J Clin Nutr, 1981; 34: 1831-6 and J Appl Physiol, 2005; 95:983-990).

The extra carbohydrates that are converted to fat can end up in your liver. Extra fat in your liver reduces the amount of sugar that your liver can store, so you tire earlier. That is why "carbohydrate loading" has been abandoned by all knowledgeable athletes.

Various ways have been tried to increase the body's ability to store sugar:
1. Low carbohydrate/high fat diets
2. Training without taking in carbohydrates
3. Withholding carbohydrates after you finish intense exercising (delayed recovery)
4. Training twice a day to leave little time for refueling between sessions
5. Training at reduced intensity after an overnight fast

Training when muscles are low in stored sugar teaches them to burn more fat and delays using up their meager stored sugar supplies, but this method interferes with workouts. Nobody has really shown that options #1 and #2 are effective because they interfere with how fast you move during intense training, which is more important than anything else. All athletes hate the early exhaustion and tiredness they feel when they train when their muscles are low on sugar.

The French study is based on options #3, #4 and #5. Success with option #5, overnight fasting, was first reported seven years ago when it was shown to improve endurance in cyclists (J Strength Cond Res, Mar 2009;23(2):560-70). The results from this new study are even more convincing. They suggest that athletes should eat their carbohydrates during the day and then restrict carbohydrate intake after their intense training session in the afternoon and before they go to bed at night. These athletes did their usual morning recovery workout at very low-intensity to promote muscle adaptation for metabolizing more fat.

I hope that the researchers will test their theory for three-month, six-month or longer periods to see if the benefits accumulate. Meanwhile, I recommend that you try their program yourself.

See If This Study's Regimen Makes You a Better Athlete
Most top athletes train twice a day. They know that you have to damage your muscles to make them stronger. They take a very hard workout to damage their muscles in the afternoon on one day and follow that with three less-intense recovery workouts, so they take one hard workout every other day. The other three workouts are to help your muscles recover from their intense alternate-day workout. Restricting carbohydrates after your alternate-day intense workout, followed by an overnight fast, and then exercising muscles with low levels of stored sugar teaches your muscles to preserve their stored sugar by burning more fat and less sugar. This can make you faster and stronger and give you greater endurance.

Obviously, most of us are not top athletes and few of us have the time the pros have to work out twice a day. In the following workout regimen, if you can only work out once a day, just cut out one of the prescribed recovery workouts each day.

On the day of your intense workout:
• Eat a normal breakfast and do an easy workout in the morning. (Cut this workout if you're only able to do one workout a day.)
• Eat a normal lunch.
• Before your intense workout, eat extra fruits, vegetables, cereals, whole grains, seeds and some refined carbohydrates in bakery products and pastas.
• Take your hard workout in the afternoon or early evening.
• After your hard workout, eat a supper that is very low in carbohydrates. Avoid refined carbohydrates such as foods made with flour (bakery products and pastas) and all sugar-added foods. Your meal should be based on vegetables and can include various sources of protein and fat. After supper, no snacks or any other sources of calories for 13 hours. Drink water or other non-calorie fluids as desired.

The day after your intense workout:
• Before your morning recovery workout, drink only water, black coffee or tea (no cream or sugar). If your muscles feel heavy or tired, you can eat a single fruit such as an orange. Do not drink fruit juice.
• After your recovery workout, eat your usual meal for lunch that can include plenty of carbohydrates, protein and fat.
• After your afternoon recovery workout, eat your usual dinner. (Cut this workout if you're only able to do one workout a day.)

My Recommendations
An overnight carbohydrate fast after your alternate-day intense workout keeps your muscles low in their stored sugar. Then exercising muscles that have been depleted of their stored sugar teaches your muscles to burn more fat and less sugar, so you keep sugar in your muscles longer, which can make you faster and stronger and give you greater endurance. If you are training for competition, do two-a-day workouts with only one intense workout every other day. The other three workouts are to help your muscles recover from their intense alternate-day workout.

However, if you're pressed for time and can only do one workout a day, I recommend that you work out intensely once every two days, eat a low-carb dinner, then fast overnight and then do a recovery workout the next day.

If you decide to try this regimen (either the 1x- or 2x-a-day approach), I recommend doing three of the 2-day cycles per week, followed the next day by a long, moderately paced depletion workout. On the day you do the long depletion workout, follow your normal diet. The study only reported the findings after three weeks, so we do not know if the benefits would continue with further cycles. I suggest trying this regimen for three weeks and then gauging your progress. 

April 17th, 2016
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About the Author: Gabe Mirkin, MD

Sports medicine doctor, fitness guru and long-time radio host Gabe Mirkin, M.D., brings you news and tips for your healthful lifestyle. A practicing physician for more than 50 years and a radio talk show host for 25 years, Dr. Mirkin is a graduate of Harvard University and Baylor University College of Medicine. He is board-certified in four specialties: Sports Medicine, Allergy and Immunology, Pediatrics and Pediatric Immunology. The Dr. Mirkin Show, his call-in show on fitness and health, was syndicated in more than 120 cities. Read More
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