I have a friend who has access to scientific journals and one recently caught my eye, so I asked for a copy. Unfortunately I can’t post it here in its entirety, but I can summarize and quote it for your edification.
This article is a comprehensive overview of recent research on nutrition and race-day performance. It was published in late 2011 and is available for purchase. The article states that the most likely contributors to fatigue in events lasting 30 minutes or more is hydration and carbohydrate depletion, so it focuses on those topics.
Carbohydrate Loading: Muscle glycogen levels can be measured in the lab, and increased glycogen levels have been shown to increase performance in efforts longer than 90 minutes by 2-3%. There is a limit to the benefit, however. One study had participants load with 10g/kg or 13g/kg carbohydrate (easily over 3000 Calories). While the muscle glycogen concentrations were greater in the group with more carbs, the performances were the same. Also, carbohydrate loading causes water retention (~3g/g of glycogen) and over-loading may increase race day weight too much.
Ingestion <60 minutes prior: The authors go into detail regarding the myth of ‘rebound hypoglycemia’ after ingestion. The myth is that consuming carbohydrate just before your event may cause insulin release and leave your blood sugar lower at the start of the event, although this has shown not to be true. Interestingly, they also state that symptoms of hypoglycemia are sometimes seen in athletes, but strangely may not correlate with actual blood sugar concentrations – and that more research is needed in this area.
Fluid Ingestion prior: Again the article goes into great detail and basically reminds us that you should be drinking until your urine is nearly clear, but that hyper-hydration may dilute electrolytes and make you race with a full bladder.
Carbohydrate: It is very well known that intake of carbohydrate during exercise increases performance. What is more interesting is how it works, which is not fully understood. Of course, the availability of more energy helps, but there is more to it. I mentioned on the blog a few months ago the amazing study which during shorter efforts (~60 minutes) showed as much benefit from drinking a carbohydrate drink as only rinsing the mouth and then spitting it out! Even more interesting, a placebo rinse (artificial sweetener without calories) didn’t work, but a drink with tasteless carbohydrate still improved performance. Therefore, in events lasting <2h, large amounts of carbohydrate intake is not necessary.
Carbohydrate in longer events: An important study in 2004 showed that no more than 60g (240 Calories) of carbohydrate could be digested per hour during exercise, and many athletes use this as a ceiling today. However, it appears the gut is trainable and a high carbohydrate diet and greater ingestion during exercise can be achieved. Also, very well controlled studies have shown that in events longer than 2 hours, more is better. Rates of up to 80g/hr have shown to be beneficial, especially with multiple types of transportable carbohydrates in a drink or gel.
Hydration: I’m running short on time as I write this, so I’ll be brief. The authors stick to the well known axiom to limit fluid losses to 2-3% of body weight during competition.
The authors also mention caffeine, but I am going to leave that information for a separate post, as I think it warrants more detail and is a subject near and dear to my heart. As any good cyclist should be, I am a coffee fiend and will share some details soon.