Ditching some sports drinks for a spoonful of sugar could make exercise easier for weary marathon runners and long distance athletes, scientists say.
Researchers at the University of Bath say that stirring table sugar in to a water bottle before a big physical event could be the difference between success and failure.
In a new study, published in the American Journal of Physiology - Endocrinology & Metabolism, they assessed the impact of endurance exercise on liver glycogen levels.
The team tested various drinks to see how different carbohydrates could help avert the decline of liver glycogen levels and tiredness.
Their experiment, conducted on long-distance cyclists, showed that ingesting carbohydrates in the form of either glucose or sucrose can achieve this.
Both sucrose - in the form of table sugar - and glucose are important carbohydrates often referred to as simple sugars.
The major difference between them is that each sucrose molecule is made up of one glucose and one fructose molecule linked together.
Researchers found that combining different sources of sugars improves the rate at which people can absorb them from the gut.
An increasing number of sports drinks designed to provide energy during exercise use sucrose or mixes of glucose and fructose - but many still rely on glucose alone.
Scientists warn that such glucose-only drinks could produce gut discomfort and suggest sucrose-based alternatives - or simply sugar in water - can help make exercise easier.
Dr Javier Gonzalez, the lead researcher, said: "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.
"Our study showed that ingesting carbohydrates during exercise can prevent the depletion of carbohydrate stores in the liver but not in muscle.
"This may be one of the ways in which carbohydrate ingestion improves endurance performance.
"We also 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 recommend up to 90g of sugar per hour - diluted to 8g sugar per 100ml - for optimal performance during exercise lasting over two and a half hours.