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Nutrition FAQ: Endurance Fueling for 2+ Hour Events
When and what you eat during an endurance event is critical to performance.
Body storage of glycogen
Muscles can hold only about 1.5-2 hours of glycogen before stores are exhausted. The liver can store about 600 calories of glycogen, a bit less than an hour’s worth for most athletes.
When your body runs out of glycogen, you’ll bonk— you’ll quite literally start to feel stupid (your brain runs entirely on glucose). Your ability to do all sorts of things becomes impaired, and you also risk physical injury (e.g., a crash from impaired brain function). Your author knows firsthand how scary this can be. Do not bonk— it’s very unpleasant, and can take 30-60 minutes to recover. Always have an energy gel on hand just in case.
Done wrong, carbo-loading is bad news: overeating paradoxically floods the system with too much sugar at once, which forces it to be stored of fat, and directs it away from muscle tissue. My advice is to eat properly, and avoid carbo-loading, unless you really understand it technically.
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Caloric intake for 2+ hour events
Dr Clyde Wilson’s Endurance Nutrition book states that the body can absorb at most 250-300 calories of glucose per hour and deliver it to muscles (varies by individual and body size), and that the liver can process another 60-90 calories of fructose per hour into usable glycogen (sucrose is digested as half glucose and half fructose). Eating more than those amounts can result in various undesirable side effects: too much and energy levels will be seriously impaired as the body tries to deal with excess blood sugar.
Assuming an athlete is contemplating events longer than 2 hours, perhaps as many as 8-10 hours, the optimum strategy for caloric intake is critical— not just any food will do, since the “burn rate” for most athletes greatly exceeds what can be consumed each hour. It is critical to consume primarily calories that can be put to immediate use in the muscles— food with glucose.
The calorie deficit during events
Your site editor (Lloyd) burns at least 750 calories per hour on moderate training rides, and 1100 calories per hour on more strenuous climbs (as determined using the SRM power meter).
For Lloyd, that means a deficit of between 450 and 800 calories per hour, a gap that must be filled via metabolism of body fat (and a small amount of protein). Which implies that training for endurance vents must maximize aerobic capacity.
On the Mt Hamilton Challenge (8300 vertical feet, 131 actual miles), Lloyd’s energy usage was 5825 KJ, or about 6050 Calories. Food consumption during the ride was about 2000 Calories, leaving a deficit of 4000 Calories.
What kind of food (calories) to consume?
Consume 200-250 calories of a maltodextrin-based energy gel or drink every hour, with enough water to replenish that lost from exertion. Reduce calorie intake if dehydration is occurring. Your author’s favorite sports gel is Gu, which is 80% maltodextrin and 20% fructose.
Sports gels or drinks may also include up to 50 calories of sucrose or fructose for additional calories, but do not exceed 90 calories of fructose per hour.
Low-fiber and easily digestible solid food can also be consumed, but do not exceed similar calorie counts (eg. ~ 250 calories) of solid food per hour, and consume it in small portions to avoid stomach upset. The body cannot digest most foods effectively with much of the blood supply diverted to muscular effort, and dehydration further worsens the situation.