The capacity of the intestine to absorb carbohydrate is dependent on carbohydrate intake in the diet (for complete discussion read the recent review I wrote in Sports Medicine (1)). Similarly, the capacity to absorb is dependent on fat in the diet. If carbohydrate intake is increased, the capacity to absorb carbohydrate is increased. These adaptations can be very specific. For example, if glucose is increased the capacity to absorb glucose is increase, but the capacity to absorb fructose is unaltered. If the fructose content of the diet is increased the absorptive capacity for fructose is increased, but not glucose. Similarly with fat: If fat content of the diet is increased the capacity to absorb fat is increased without affecting the capacity to absorb carbohydrate.
These are interesting findings for athletes, especially because it has been demonstrated that absorption is the limiting factor for carbohydrate use from exogenous sources (ingested carbohydrate).
Glucose is absorbed through a protein that facilitates the transport across the cell wall of the intestine. This protein is called SGLT1: the sodium dependent glucose transporter 1. When the glucose content of the diet is increased for several days, the number of these transporters increases and the absorption of glucose increases in parallel. Fructose is transported by GLUT5 and something similar may happen to GLUT5 transporters.
If carbohydrate intake is reduced the opposite will happen. The number of transporters decreases and les glucose will be absorbed. Thus, any athlete who reduces dietary carbohydrate intake consistently will have a lower capacity to absorb glucose. When this same athlete now enters an endurance event and wants to use carbohydrate in the form of drinks, gels or energy bars, the absorption of the carbohydrate could be poor. This, in turn, could result in increased stomach fullness, bloating and increased gastro-intestinal problems.
This theory seems to be backed up by anecdotal evidence. Athletes who are carbohydrate restricting or athletes with very low energy intake often report more gastro-intestinal problems during events, especially when using these carbohydrate containing products.
Although there is merit in increasing the capacity to oxidise fat by sometimes training with low carbohydrate availability, having a low carbohydrate intake every day is not advised. It is more likely that the combination of low carbohydrate intake on some days, and high carbohydrate intakes might promote adaptations of fat metabolism as well as adaptations to carbohydrate metabolism and absorption. Improving the absorptive capacity of the gut is one of several methods we have available to train the gut. Read more here about training the gut for athletes.
So the periodised approach will help to increase fat oxidation, whilst maintaining the capacity to absorb carbohydrate. This means that on race day, one may burn more fat, spare some carbohydrate and deliver additional carbohydrate without causing gastro-intestinal problems.
These papers are available to download for FREE: