Training for a marathon or triathlon? Thinking of carbing up the night before? Don’t! Glycogen “super-compensation” is an outdated archaic myth, a vestigial artifact left over from the 1980’s. Although it is true that glycogen depletion initiated by an exhaustive fasted aerobic workout of 90-120 minutes at or near anaerobic threshold will deplete approximately 90% of available muscle glycogen stores, doing so will also deplete an athlete’s adrenals and immune function, with little or no benefit on race day compared with simply just going into an event with glycogen stores topped off, achieved healthily via normal carbohydrate consumption just above energy needs in the 2-3 days leading up to competition. As is, topping off glycogen stores is a risky endeavor for any endurance athlete wishing to optimize their body composition by minimizing their body fat. Excess carbs consumed over the maximum amount that can be stored in the liver and muscles will upregulate lipogenesis, i.e. triglycerides stored as body fat.
It is also noteworthy that glycogen stores can never truly be “depleted”, as the body will never allow glycogen to get too low, preserving precious glucose for that most important organ of all, the brain. An additional consideration is that the glycogen depletion that is needed to stimulate the so-called ‘super-compensation’ effect may also result in the catabolism of lean muscle mass in the athlete, which is yet another highly undesirable scenario impacting performance. If you lose weight by losing muscle, you’re not going to go any faster, nor will you delay the dreaded “bonk” or “wall” that sends shivers through the spine of any endurance athlete!
Carb-loading also brings into question the critical issue of fuel partitioning. A highly conditioned endurance athlete such as a distance runner or road cyclist should ideally be burning mostly fat stores as their body’s preferred fuel source for activities involving aerobic respiration. As numerous studies have illuminated over the years, even a very lean athlete has somewhere between 50,000-100,000 calories available to burn as stored body fat, compared with a mere 1500-2000 calories stored as muscle glycogen. In accordance with the “Central Governor Theory” of fatigue proposed by the brilliant Dr. Timothy Noakes a few years back, as long as your brain does not run low on glucose and/or ketones, there is absolutely no physiologically-justifiable need for excessive consumption of glucose during any endurance event lasting longer than 20 minutes, even when one occasionally dips into the glycolytic (sugar burning) realm, such as during sudden surges in a bike race or while drowning in lactic acid on intense hill climbs.
One final consideration that I always bring up to the athletes that I counsel is that aerobic training fueled with sugary gels and high-glycemic carbs generates a tremendous amount of Reactive Oxygen Species (ROS) that can prematurely age you and expose you to a higher danger of cancer cell proliferation, since cancer cells cannot fuel themselves on ketones, i.e. betahydroxybutyrate, nor free fatty acids. Cancer cells thrive on glucose. Virtually anyone can train like a world-class endurance athlete and achieve high levels of aerobic fitness relative to their genetic potential, but can they do so without aging themselves and creating a “cancer-friendly” environment in their body? Yes, they can.
So here’s the practical takeaway. If you want to perform well in an endurance event, train smart, eat smart, avoid extreme low-carb diets, and train fasted at least 2-3 times/week to turn yourself into a fat-burning machine that will never run out of fuel!