Good news for chocolate milk lovers.  A recent study published in the February 2009 issue of Applied Physiology, Nutrition, and Metabolism concluded that chocolate milk is an acceptable option to replenish energy stores (glycogen) depleted during endurance exercise.  Researchers investigated the effectiveness of chocolate milk (Mars Refuel) compared to two commercially available sport drinks, one a carbohydrate replacement drink (Endurox R4) and the other a fluid replacement drink (Gatorade), on endurance performance.  Endurance performance is affected by levels of glycogen stores in the body.

     The best source of energy for muscles is glucose (a simple sugar).  Glucose is stored in your muscles and liver in the form of glycogen.  Glycogen is the primary source of glucose for your muscles during prolonged bouts of endurance exercise.  An enzyme in your body, known as glycogen synthetase, forms glycogen.  High levels of this enzyme circulate in your body during the first two hours post-exercise.  It is within this time frame that health professionals recommend consuming carbohydrates to replenish depleted stores.

     The researchers of this study wanted to determine if chocolate milk would be a good source of carbohydrate to aid in the resynthesis of glycogen.  The subjects tested were trained athletes (cyclists).  The participants underwent experimental trials on three separate occasions, each at least one week apart from the previous trial.  A testing session included an initial bout of exercise that resulted in depletion of muscle glycogen stores, a 4 hour recovery period (during which the subjects ingested either chocolate milk, a carbohydrate replacement drink, or a fluid replacement drink one minute and then again 2 hours post-exercise), and an endurance exercise session to exhaustion.  The subjects were allowed to drink as much water as they desired throughout the testing sessions to ensure proper hydration.

     The results of the study revealed that when the participants drank chocolate milk they were able to exercise 51% longer than when the carbohydrate replacement drink was ingested and 43% longer than when the fluid replacement drink was consumed.  Although the investigators suggest that further research is needed to clarify the means by which chocolate milk enhances performance during endurance exercise, they offer some possibilities.

      The researchers suggest that one of the types of carbohydrate (sucrose) present in the milk, but not in the carbohydrate replacement drink, could be a factor in improved performance.  This is because research has shown that sucrose ingestion results in a greater storage of glycogen in the liver when compared to glucose consumption.  

     The investigators also attribute the higher fat content in the chocolate milk to play a role.  They believe that it may lead to a greater amount of free fatty acids in the blood, which in turn, may delay the depletion of glycogen. 

     A final factor that may have led to enhanced performance was that the athletes consumed more water during the exercise session when they drank chocolate milk than when they consumed the sport drinks.  

     Regardless of whether it was the type of carbohydrate, the fat content, or the increased water consumption associated with the ingestion of chocolate milk, I think I will pour myself a glass upon completion of my exercise session tomorrow.

Resources:

Appl Physiol Nutr Metab. 2009 Feb; 34(1):78-82. Improved endurance capacity following chocolate milk consumption compared with 2 commercially available sport drinks. Thomas, K., Morris, P., Stevenson, E.

Int J Sport Nutr Exerc  Metab. 2006 Feb; 16(1):78-91. Chocolate milk as a post-exercise aid. Karp, JR. Johnston, JD, Tecklenburg S., Mickleborough, TD., Fly, AD., Stager, JM.