Swapping some specialist sports drinks in favour of water mixed with a spoonful of sugar could boost the performance of long-distance athletes, a study finds.

Researchers at the University of Bath brought in a team of club cyclists and used an adapted MRI scanner to assess the impact of prolonged exercise on the levels of glycogen – stored carbohydrate – in the liver.

They tested various sucrose and glucose-based drinks and established that ingesting carbohydrates in the form of either sucrose or glucose prevents the decline in liver glycogen.

But they also discovered that the cyclists found exercise easier – and their gut felt better – when they ingested sucrose rather than glucose.

Both sucrose, in the form of table sugar, and glucose are important carbohydrates often referred to as “simple sugars”. The big difference between them is that each sucrose molecule is made up of one glucose and one fructose molecule linked together. It appears that combining different sources of sugars improves the rate at which we can absorb these from the gut.

Although an increasing number of sports-performance drinks designed to provide energy during exercise now use sucrose, or mixtures of glucose and fructose, many still rely on glucose alone. The researchers claim that such glucose-only drinks could produce gut discomfort and suggest that sucrose-based alternatives, or sugar in water, can help make exercise easier.

Dr Javier Gonzalez, the lead researcher, explained: “The carbohydrate stores in our liver are vitally important when it comes to endurance exercise as they help us to maintain a stable blood sugar level. However, whilst we have a relatively good understanding of the changes in our muscle carbohydrate stores with exercise and nutrition, we know very little about optimising liver carbohydrate stores during and after exercise.

“We found that the exercise felt easier, and the gut comfort of the cyclists was better, when they ingested sucrose compared to glucose. This suggests that, when your goal is to maximise carbohydrate availability, sucrose is probably a better source of carbohydrate to ingest than glucose.”

The scientists behind the new study recommend that if your goal is optimal performance during exercise lasting more than two and half hours, then consume up to 90g of sugar per hour – diluted to 8g sugar per 100ml. The research is being published in the international journal the American Journal of Physiology – Endocrinology and Metabolism.