Prologue:

I put this essay together after doing a carbohydrate/fat fuel test with one of my athletes at the Boulder Center for Sports medicine.  Fat utilization was a major concern with the athlete being tested.  This athlete burned a very high amount of carbohydrates in relation to a very small amount of fat.  After the test and my friend and colleague Neal Henderson, MS, CSCS from Boulder Center for Sports Medicine was able to provide us some unique and progressive information which you will find summarized below. 

Many cyclists are trying to boost fat burning by just plain training longer without consuming carbohydrates and pushing back the event of the proverbial bonk. The methods outlined in below might offer more effective method of achieving the same result with less overall stress and at the same time create potential for more quality during long rides.

Training low on Carbohydrates to stimulate fat utilization:

Typically athletes are encouraged to train and race on high carbohydrate diets.  While the racing on high carbohydrates still stands, there is some recent evidence that suggest select workouts could be more effective to our overall aerobic capacity when carbohydrate deprived.

A study done by Bente Pederson of the University of Copenhagen used a really interesting model.  They had subjects serve as there own controls.  One leg (the control) trained every day for an hour while the other leg trained twice a day for an hour every other day.  Both legs increased intensity equally and did the same total volume.  The study lasted 10 weeks.

The results were the most significant in the time to exhaustion while exercising at 90% of maximum power.  The two-a-day group (leg) lasted nearly twice (190%) as long as the one hour a day group (leg).  An interesting note is that resting muscle glycogen content in the two-a-day group was much lower before the start of the 2^nd session.  There was also significantly increases in mitochondrial and metabolic enzyme levels in the twice a day group leading the authors to conclude that training with low muscle glycogen content increased both responses to endurance training and performance in a sub-maximal endurance task.  (Pederson et al, 2005)

In summary, the study supports that aerobic training done on low muscle glycogen content shows a greater propensity to enhance aerobic metabolism than aerobic training on a high carbohydrate diet.  Possible explanation could be that athletes learn to metabolize fat and conserve glycogen stores.  This is a highly beneficial adaptation for all endurance athletes.

Many elite athletes like pro-cyclists Miguel Induran, Tom Danielson and Kenyon runners have offered anecdotal support for this practice.

Putting this information to use: 

All high intensity workouts should be done on a high carbohydrate diet (60-70% carb, 15-25% protein, 15% fat) from either the night before or during the day before intense workouts.  After your workout you will want to replenish your carbohydrates with an appropriate carbohydrate and protein recovery drink.  However your subsequent meals for the rest of your day up until your next workout should be low in carbohydrate (40-50% carb) even if a night of sleep falls between.  This follow-up workout should be a lower intensity workout at 55-70% of your maximal aerobic capacity and be completed consuming less than 40g of carbohydrate per hour.  After this second workout you will go back to the normal 60-70%carb diet.  Ideally you can follow a two-a-day approach so that way you can start more workouts with higher carbohydrates and more workouts with lower carbohydrates. 

As a triathlete you definitely want to have one swim, bike and run each week and one weekend brick with fully loaded glycogen stores.  If you only exercise once a day during the week you would still have a couple days during the week and maybe one weekend day to try this out.  For those multiple workout per day, this becomes easier and it gives merit to separating your workouts and not always doing back to back brick workouts. 

With single sport athletes, those competing in lower impact sports like cycling will have the easiest time implementing these concepts into training.  Runners can certainly benefit but will have to be much more careful with the added stress.  You will need to gradually build into this as single sports athletes will most likely end up with a bit more volume. 

So if you’re looking to get a little more out of your easy workouts and your 1-2 hour aerobic endurance workouts, play a little with your nutrition a little. 

Essay written by Jared Berg CSCS.  Based off information provided by and discussed with Neal Henderson, MS, CSCS.

Date modified: 2/10/2010