More and more athletes are looking into a combination of glucose and fructose as a way to optimise their recovery after multiple high-intensity efforts. Sparked by a comment from professional cyclist Laurens ten Dam in a recent Live Slow, Ride Fast podcast, we took a look at some of the basic science behind the new enthusiasm for fructose. But be warned: not all glucose and fructose are the same. Our basic motto is that the more natural the source, the better. We’re trying to keep this stupid simple, so apologies to all scientists out there.
Glucose and fructose
Athletes burn fuel in different ways to sedentary citizens. Carbohydrates in the form of the sugars glucose and fructose can adversely affect people who do little or no exercise. (Fructose intake has been linked with diabetes, for example.) However, people whose hobby or profession involves burning a huge amount of calories for hours at a time can benefit from a combination of both fast-burning glucose as well as fructose to refill their fuel tank. This appears to apply both during exercise and for recovery.
When you feed your body carbohydrates in the form of sugars, your body converts them into glycogen, which it then stores in the liver and muscles. Glycogen is the energy source your body burns during exercise and you have a finite stock of it in your body. So just like a motor vehicle has a maximum amount of fuel in its petrol tank, if your tank runs empty you will experience the ‘bonk’ when your body basically grinds to halt.
Endurance exercise burns up the glycogen supplies stored in your muscles and liver. Glycogen will normally be replenished within 24 hours after exercise and before performing heavy exercise again. However, sometimes you don’t have 24 hours in between rounds of exercise. Think multi-day events or multiple-day training sessions, for example.
So, let’s say you have been training really hard on day one. You want to replenish the stores of glycogen in your system, so you take glucose as your chosen carbohydrate. But glucose is absorbed by the body at a fixed rate. Glucose travels through a ‘gateway’ called SGLT1, which is quickly saturated; fructose, on the other hand, travels through a different gateway, called GLUT5. So if you take a combination of fructose and glucose in recovery, your glycogen levels (particularly in your liver) will recover faster.
Dates and figs
To make things more difficult there is ‘bad’ fructose, in the form of industrially-generated corn syrup, and there is ‘good’ fructose, found in fruit and vegetables. Our favourite high-fructose snacks are dried dates (which contain around 35% natural fructose) and dried figs. Readers of the classic Dutch cycling novel The Rider, by Tim Krabbé, may recall he ate quite a few figs during those races. There must have been something in it…
Confused? The guys in this video explain it much better — they are real scientists and conducted a study with 15 well-trained male cyclists to come to their conclusions:
Interested in other aspects of training and nutrition for cycling? There are a lot more articles here.