Even textbooks are sometimes confusing when it comes to pre-exercise meals. Some books will tell you to avoid carbohydrate in the hour before exercise and some will tell you that you need it to improve performance. The reason for these different views stems from a couple of early studies. In these studies in the 1970s, it was observed that eating carbohydrate in the hour before exercise resulted in high blood glucose and insulin concentrations 45 min after ingestion. At that point exercise was started. As exercise started, a rapid drop of the blood glucose levels was observed because of a combined effect of high insulin levels and exercise on glucose uptake (Figure 1). Blood glucose concentrations dropped rapidly and they dropped so much that hypoglycemia occurred. This is often called rebound hypoglycemia or reactive hypoglycemia and is associated with symptoms of weakness, nausea and dizziness. It is also often assumed that hypoglycaemia will have a negative impact on performance. One of the early studies reported that performance was reduced when carbohydrate was ingested before exercise compared with placebo (water) ingestion.
Since then, however, numerous studies have been performed with slightly different experimental designs. Some of these studies investigated carbohydrates that do not result in a large insulin response (low glycemic index carbohydrates) such as fructose.
Others used glucose (higher insulin response) or other high glycemic index carbohydrates. In addition to different types of carbohydrates, these studies used a variety of modes and intensities of exercise, and participants had different training backgrounds; some trained, some untrained were used. This makes it very difficult to compare the results and very difficult to find out exactly what caused the different effects. However, these studies (over a hundred from different research groups all over the world) showed almost all either no effect of carbohydrate feeding on performance or a positive effect.
Unfortunately it was difficult to extract any meaningful advice from the roughly 100 studies with respect to the optimal amount and type of carbohydrate as well as the timing of intake. Therefore, we performed a series of studies in which we investigated the effects of pre-exercise carbohydrate feedings very systematically. All studies had a similar design and we only changed one variable at a time. The overall conclusion of these studies was that there was no effect on performance despite the fact that in some cases hypoglycemia did develop. There was no relation at all between the blood glucose levels and performance. Hypoglycemia occurred more often when smaller amounts of carbohydrate were ingested (25g) compared with larger amounts (75g or 200g) 45 min before the start of exercise. Hypoglycemia was less prevalent when it is ingested just (15 min) before exercise compared with 45 and 75 min before. Low glycemic index carbohydrates did not cause hypoglycemia.
An interesting finding was that some individuals developed hypoglycemia in all conditions whereas others never developed it. This observation is in line with some athletes reporting to be very sensitive to carbohydrate feedings and others who can eat whatever they want and never get symptoms of hypoglycaemia. Somewhat surprisingly this was not linked to insulin sensitivity of the individual.
In practical terms this means that it is ok to consume carbohydrate before exercise as there do not seem to be any detrimental effects on performance. Individuals prone of developing reactive hypoglycemia can find solutions to avoid it. These solutions could include choosing low glycemic index carbohydrates, ingesting carbohydrate just before exercise or during a warm up or avoiding carbohydrate in the 90 min before.
Every individual is different and therefore every athlete will need to develop their own pre-exercise routine that works best for him/her.
Jeukendrup AE, Killer SC. The myths surrounding pre-exercise carbohydrate feeding. Ann Nutr Metab. 2010;57 Suppl 2:18-25.
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