Scientists measured the glycogen levels, or stored carbohydrates, in the body and how they can be sustained during a workout.
While a number of sports-performance drinks now use sucrose - or a combination of glucose and fructose - some still use glucose alone.
But researchers warn that such glucose-only drinks may produce gut discomfort. They suggest that sucrose-based alternatives, or sugar in water, could help make exercise easier.
Effectiveness of sports drinks?
The study found that adding 30g to 90g of sugar per hour, diluted to 8g sugar per 100ml, can help athletes have 30% more endurance during exercise compared to plain water.
Lead researcher Dr. Javier Gonzalez, from the University of Bath in the UK, told BeverageDaily this level will vary from person to person.
“Some can have 90g an hour and feel fine, others can struggle to consume that much,” he said.
The study, titled Ingestion of Glucose or Sucrose Prevents Liver but not Muscle Glycogen Depletion During Prolonged Endurance-type Exercise in Trained Cyclists, has been published in the American Journal of Physiology - Endocrinology & Metabolism.
What researchers wanted to learn
While the science and exercise communities know a lot about what happens with muscles during exercise, Gonzalez said the liver has been far more difficult to figure out.
“In the study, we were able to have access to a scanner where you can measure what happens in the liver in response to exercise and nutrition,” he said.
“The liver is important because it contains and stores carbohydrates to be used as fuel during exercise. If you exercise for a long period of time, stores of carbohydrates can become quite low. When they reach critically low levels, that’s linked to early fatigue.”
For this study, researchers had 14 cyclists complete two three-hour bouts of cycling at 50% of peak power output while taking glucose or sucrose at 1.7g per minute, or 102g per hour. Four cyclists performed the test where only water was consumed.
In the end, the study found both glucose and sucrose prevent liver glycogen depletion during endurance exercises. However, sucrose ingestion increases whole-body carbohydrate utilization.
Ingesting sugar during the workout can have a “really profound effect on the liver,” Gonzalez said. Glucose and sucrose both similarly affected levels of energy, but he said sucrose produced less discomfort. Consuming glucose during a workout had side effects, including stomach problems during exercise.
“We think that glucose can only be absorbed by stomach and intestines at a certain rate,” he said. “Drinking lots and lots of sugary drinks, the stomach and intestines can’t absorb it fast enough, so it sloshes around in the stomach and intestines [during exercise].”
Comparing ingesting a drink with sugar versus water, Gonzalez said there is the potential for 30% more energy. This means someone who can exercise for three hours on water alone can add about an hour when ingesting sugar while they work out.
“It’ll have some benefit if you ingest before, but it’s more potent if you ingest during [exercise],” Gonzalez said.
Gonzalez said sports drinks are the market vary in their effectiveness, but advises looking for those that only have sucrose instead of the drinks that work as a mixture of glucose and sucrose. Even putting table sugar in water may help make exercise easier.
Source: American Journal of Physiology - Endocrinology & Metabolism
Ingestion of Glucose or Sucrose Prevents Liver but not Muscle Glycogen Depletion During Prolonged Endurance-type Exercise in Trained Cyclists
Authors J. Gonzalez, C. Fuchs, F. Smith, P. Thelwall, R. Taylor, E. Stevenson, M. Trenell, N. Cermak, L. van Loon