Maintaining carbohydrate availability is a key challenge in multi-stage races
A major challenge in multi-stage races such as cycling’s Grand Tours, and the Marathon des Sables, is maintaining adequate carbohydrate availability. This is because we rapidly deplete carbohydrate stores during exercise, and we have a limited carbohydrate storage capacity. Furthermore, when carbohydrate stores a low, we have difficulty maintaining race-pace intensities of exercise. The storage form of carbohydrates in humans is glycogen, which is mainly found in muscle and the liver. The maximum amount of glycogen that an athlete can store is thought to be less than 3500 kcal of energy. This is not enough to support even one full day of racing, and even if athletes are consuming carbohydrates during exercise, they will almost always end up with low glycogen stores at the end of each stage. Therefore, rapidly replenishing these glycogen stores before beginning the next stage is a key nutritional consideration.
Muscle glycogen recovery is mostly dependent of the dose of carbohydrate ingested
Sports nutrition guidelines for recovery from exercise typically state that for rapid refuelling, athletes should aim to consume 1.0-1.2 grams of carbohydrate per kilogram body mass per hour for the first four hours following exercise (1). This is based on the substantial evidence that this rate of carbohydrate ingestion maximises muscle glycogen repletion rates. In terms of the types of carbohydrate to consume, this seems to be less important, as muscle glycogen repletion seems to be similar whether the source of carbohydrates are glucose-based, or glucose-fructose mixtures (2). However, much of the prior work on glycogen repletion has paid little attention to the effects of post-exercise nutrition on liver glycogen recovery. Liver glycogen availability may also be important for the capacity to perform prolonged exercise, and so it is likely to be important to understand the effects of nutrition on both liver and muscle glycogen stores.
Liver glycogen recovery is accelerated by fructose-containing carbohydrates
Compared to muscle glycogen, liver glycogen metabolism seems to be more sensitive to the type of carbohydrates that are ingested. For example, the co-ingestion of fructose (and galactose) with glucose-based carbohydrates potently increases the rate of liver, but not muscle glycogen repletion post-exercise (3; 4). The increased recovery of liver glycogen stores with fructose-glucose co-ingestion is typically double the rate of that seen with glucose alone, even when the total amount of carbohydrate is identical. Whilst these findings were promising, the next logical question to ask, is whether this potential advantage to increasing liver glycogen availability translates into a benefit to endurance performance.
Whilst these findings were promising, the next logical question to ask, is whether this potential advantage to increasing liver glycogen availability translates into a benefit to endurance performance.
Fructose-glucose mixtures during recovery improve subsequent exercise capacity
In a recent study, a group of runners performed two bouts of exhaustive running separated by 4 hours. During the 4-hour recovery period, the runners were provided with carbohydrate drinks containing either glucose-based carbohydrates, or glucose-fructose mixtures. After ingestion of the glucose-fructose mixtures, the athletes were able to run for ~30% longer, compared to the ingestion of equivalent amounts of glucose-based carbohydrates alone (5). This was an exciting finding, suggesting that the type of carbohydrate ingested during recovery from exercise could have an important effect of subsequent exercise capacity. However, it was still unknown whether this is relevant to multi-stage races, as most of these events have recovery durations longer than 4 hours, and include an overnight fast during sleep. This could have important implications for this as a nutritional strategy, as liver glycogen stores are used overnight to supply the brain and other tissues with glucose (6). Therefore, any potential benefit of fructose co-ingestion to liver glycogen stores could be lost overnight. A more recent study addressed this, by asking cyclists to perform exhaustive exercise and then to consume either fructose-glucose mixtures, or glucose-based carbohydrates alone, for 4 hours following exercise (7). Subsequent endurance capacity was then assessed after 15 hours of recovery, after an overnight fast and a low-carbohydrate breakfast. Remarkably, endurance capacity was improved by ~20%, suggesting that fructose-containing carbohydrates in the recovery diet of athletes may improve their capacity to perform exercise the following day.
Remarkably, endurance capacity was improved by ~20%, suggesting that fructose-containing carbohydrates in the recovery diet of athletes may improve their capacity to perform exercise the following day.