The  many  proponents of  low-carbohydrate diets like Atkins and South Beach would have the public believe that carbohydrates are some kind of poison. What an irony it is, then, that carbohydrates are our muscles’ preferred fuel.  Scientists first discovered in the 1960s that the ability to contract muscles for prolonged periods is strongly influenced by the amount of carbohydrate stored in skeletal muscles (glycogen), with muscle glycogen depletion becoming the decisive factor limiting prolonged exercise.

Most runners have enough glycogen to provide energy for only about seventy minutes of running.  Even with the contribution of fat helping to delay the depletion of glycogen, moderate-intensity running can only be sustained for two to three hours.  One of the unique characteristics of the marathon, therefore, is that it is the only race in which you run out of fuel.  Glycogen depletion and the accompanying low blood sugar (hypoglycemia) coincide with hitting the infamous marathon wall.  Unlike shorter races, fatigue in the marathon is due primarily to running out of fuel rather than by-products of metabolism.

At slow running speeds, some of carbohydrate’s metabolic responsibility for energy regeneration is relieved by fat.  With increasing speed, fat use decreases while carbohydrate use increases.  This is why proper early pacing is vital in the marathon—the faster you run the early miles, the more you’ll rely on blood glucose and muscle glycogen, and the sooner you’ll run out of fuel.  The lactate threshold—the fastest sustainable aerobic speed and the fastest speed above which lactate accumulates and acidosis occurs—is the speed at which a fuel change is made, as you go from using a combination of fat and carbohydrates to using only carbohydrates.

Since the pace you’ll average in the marathon is slower than your lactate threshold pace, you’ll use a combination of fat and carbohydrates in the marathon.  Once you run out of carbohydrates, your muscles are forced to rely on fat and, consequently, your pace will slow down because your muscles regenerate energy for muscle contraction slower when using fat compared to when using carbohydrates.  Underscoring the importance of carbohydrates as a fuel, to compensate for a lack of carbohydrates, the liver synthesizes glucose from non-carbohydrate sources, namely amino acids and lactate.

Long Runs

Every marathoner knows he or she must run long almost every weekend.  The main purpose of the long run is to deplete muscle glycogen.  The human body responds rather elegantly to situations that threaten or deplete its supply of fuel.  With no carbohydrates, the muscles are forced to rely on fat as fuel and so become more effective at using fat for energy.  Following the run, more glycogen is synthesized and stored than what was previously present, thus assuaging the threat and increasing endurance for future efforts.  The more your glycogen tank is emptied, the faster and more it’s refilled.  For example, a study published in Medicine and Science in Sports and Exercise in 1991 found that glycogen was synthesized significantly faster when one leg was exercised until glycogen was depleted compared to the opposite leg that was exercised only a little and did not fully deplete glycogen.  The more glycogen you have packed into your muscles, the greater your ability to hold your marathon pace to the finish.  To create the largest muscle glycogen storage possible, you need to deplete muscle glycogen on a regular basis.

Carbs on the Run

With the popularity of marathon running has also come the popularity of carbohydrate drinks, gels, and bars to replenish blood glucose while running.  It seems that everyone now does Sunday long runs with a fuel belt around their waists.  While ingesting carbs during long runs may allow you to feel better since you’ll maintain blood glucose levels, it defeats the purpose of the run, which is to deplete carbohydrates so specific adaptations are achieved.  For example, since ingesting carbohydrates during your long runs provides muscles with a ready fuel, the three adaptations you want to achieve—the muscles’ reliance on fat, the liver’s ability to make new glucose, and the depletion and subsequent resynthesis of glycogen—are blunted.  Therefore, if you want to maximize your physiological adaptations, it’s better to leave the fuel belt at home when you go for a long run.

In the marathon, however, it’s important to maintain blood glucose levels for as long as you can.  Research has shown that supplementation with carbohydrate during prolonged exercise delays fatigue.  Begin ingesting glucose about thirty minutes before you start to feel fatigued so the glucose has time to be absorbed into your blood where it can be used for energy.  Continue ingesting glucose every twenty minutes to maintain blood glucose levels.

Carbs After the Run

Research has shown that the synthesis of glycogen between training sessions occurs most rapidly if carbohydrates are consumed immediately after exercise.  Indeed, delaying carbohydrate ingestion for two hours after a workout can significantly reduce the rate of glycogen synthesis within the first few hours.  My research published in International Journal of Sport Nutrition and Exercise Metabolism in 2006, along with other studies, has shown that chocolate milk, with its high carbohydrate and protein contents, is a great post-workout recovery drink.

The large scientific support for ingesting carbohydrates immediately after a workout has led to the long-standing position of physiologists and sports nutritionists that immediate post-workout carbohydrate ingestion is the best recovery and training strategy for optimal performance.  However, the marathon is different from all other races in that it requires the largest glycogen storage capacity possible, a very efficient capacity to make new glucose, and a very effective use of fat.  Research on the molecular effects of muscle glycogen depletion suggests that “starving” the muscles of carbohydrates may cause an even greater storage adaptation when carbohydrates are finally ingested.  Low muscle glycogen content has been shown to enhance the transcription of genes involved in protein synthesis.  Think of this strategy as creating even a greater threat to the muscles’ survival.

So, if you want to be fully prepared for your next marathon, don’t ingest carbs during long runs and pace yourself wisely during the first half of your race.  And if you train and race smart enough, you’ll surely have the most fuel of all your competitors, perhaps even enough to last until the finish line.

Dr. Jason Karp is a nationally-recognized running coach and founder of Dr. Karp’s Run-Fit Boot Camp in San Diego. He owns RunCoachJason.com, a state-of-the-science running coaching and personal training business.  He holds a PhD in exercise physiology.  He writes for many running, coaching, and fitness magazines, is author of How to Survive Your PhD, 101 Developmental Concepts and Workouts for Cross Country Runners, and the forthcoming Women’s Running Bible, and is a presenter at fitness and coaching conferences. To subscribe to his free newsletter, go to www.runcoachjason.com.

                                        

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Comments (1)

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I'm a personal trainer, triathlete and tri coach. While it's theoretically true that allowing the greatest possible glycogen depletion during long runs would beneficially adapt you, does this really work in practice?

A couple of confouding factors: Many endurance athletes are already frequently low on glycogen, especially while doing 1-3 daily workouts. Especially for under-35 athletes, it becomes a race to consume and absorb enough useful calories just to keep up. By the time of a 2-hour Sunday run, for example, many people bonk if they're not ingesting at least 100 cal/hour...and even then, by the last mile they're redlining on glycogen and energy. If the athlete is tiring and slowing significantly, then he/she's also no longer training at a productive pace. A "bonking" athlete may even have to walk - but if given 100 calories with water and potassium, can make a remarkable recovery and continue at the original pace.

Also, if workouts are very frequent, it's important to recover quickly for the next session. If you were in a glycogen-"redlined" state for too long during the previous workout, then the muscles can't stand up to te next workout, even if enough recovery food and time are taken.

It also may depend on running pace: A runner doing 7:30-min. miles for 2 hours burns more energy than one doing 10:00 miles. Since the most you can absorb and use while running a faster pace is only 80-150 cal/hour, and you're burning several hundred per hour, you're already depleting glycogen quite a bit.

Another way to adapt the body to glycogen depletion, rather than not consuming fuel, is the tempo finish -- increase the pace in the late part of a long run, thereby increasing fuel usage rate, and practicing holidng a faster tempo when you're already tired.

I'd appreciate any thoughts you have - if my understanding isn't quite right, I'd love to understand better!

Thanks!

Diana

Hevvydj , July 22, 2010

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