Monday November 30 2015
Some sports drinks contain only glucose
Adding a spoonful of table sugar to a glass of water could be just as good as – or better than – a sports drink, several media outlets have reported. The news comes from a study that compared whether a group of long-distance cyclists performed better when they had a glucose or sucrose mix drink.
Fourteen experienced male cyclists were randomly given a drink of sucrose or glucose stirred into water before and during a three-hour cycling stint.
Both drinks maintained the body's glucose stores, which are broken down to provide energy during physical activity if there's not enough glucose available in the bloodstream. However, British researchers found the cyclists performed better on the sucrose drink.
Many sports drinks designed to provide energy during exercise use sucrose or mixes of glucose and fructose – but many still rely on glucose alone. Sucrose is made up of glucose and fructose, whereas glucose is available in a form ready to be used by the body.
The researchers suggest glucose-only drinks could produce gut discomfort, and sucrose-based alternatives, or simply sugar in water, could make exercise easier.
While the findings are interesting, this is a small study involving just 14 male endurance cyclists. The results can't inform us of the effects in women, less experienced exercisers, or people performing different types of exercise. Even for male cyclists, a much larger sample may give different results.
This study does inform us about how the body may use sucrose and glucose differently during exercise, but limited firm conclusions can be drawn about the best form of nutrition before, during or after exercise based on its results alone.
Where did the story come from?
The study was carried out by researchers from the University of Bath, Northumbria University, Newcastle University, and Maastricht University.
It was funded by Sugar Nutrition UK and Suikerstichting Nederland, and was published in the peer-reviewed American Journal of Physiology – Endocrinology and Metabolism.
The news reporting is generally representative of the study's main findings, but would benefit from acknowledging that this research has limited implications because it used such a small, select sample group.
What kind of research was this?
This small randomised crossover trial aimed to compare the effects of glucose and sucrose (table sugar) drinks on the body during endurance exercise. A crossover trial means the participants acted as their own controls, drinking both drinks on two separate occasions.
Carbohydrate – which includes sugar – and fat are the main energy sources used during moderate-intensity endurance exercises. The carbohydrate source comes from glucose in the blood, which is continuously being topped up from the liver by the breakdown of glycogen, the stored form of glucose.
The body's glycogen stores therefore become depleted during exercise, unless carbohydrate is taken in the form of food or drink to provide a fresh source of glucose.
The researchers aimed to better understand the effect that drinking different types of sugary drinks has on the depletion of glycogen stores during exercise.
What did the research involve?
This study involved cyclists who performed endurance exercise while drinking either glucose or sucrose drinks. Researchers compared the cyclists' glycogen stores before and after exercise.
14 healthy endurance cyclists (all male) were involved in the study. They were randomised to either a glucose or sucrose (granulated sugar) drink before an exercise test. One to two weeks later they performed a repeat test after drinking the other drink.
On each occasion, participants arrived at the test centre after fasting for 12 hours and having avoided strenuous exercise for the previous 24 hours. The cyclists' last meal was standardised by the researchers, so they all had the same energy intake.
The carbohydrate test drinks were made up of 108g of either glucose or sucrose mixed with 750ml of water to give a 7% carbohydrate solution. Participants were given 600ml of the drink (86.4g carbohydrate) immediately before exercise, with a further 15ml (21.6g carbohydrate) given every 15 minutes during exercise.
The exercise involved a five-minute warm-up at 100 watts, after which power was increased up to 50% of the individual's peak power output (established during preliminary tests) for the remaining three hours.
A special imaging technique called magnetic resonance spectroscopy (MRS) was used to examine the breakdown of glycogen in liver and muscle tissue before and after exercise.
The researchers took blood samples to look at glucose and lactate levels, as well as expired breath samples to look at oxygen and carbon dioxide levels. They also questioned the participants about abdominal discomfort and how tired they felt during exercise.
Four of the participants also attended on another occasion to perform a control exercise test, where they drank only water.
What were the basic results?
Liver glycogen stores did not decrease significantly after the exercise tests, and did not differ between the two drinks. Muscle glycogen stores did significantly decrease after the tests, but again were not significantly different between the two drinks. Comparatively, both liver and muscle stores declined when only water was consumed during exercise.
Carbohydrate use was estimated by a calculation looking at the difference between the carbon dioxide expired and the oxygen used up during exercise. This was significantly greater with sucrose than glucose, suggesting the sucrose drink was being better used to provide energy.
Participants also reported their perceived exertion increased to a lesser extent during exercise when they had sucrose compared with glucose. Gut discomfort was also less with the sucrose drink.
How did the researchers interpret the results?
The researchers concluded that, "Both glucose and sucrose ingestion prevent liver glycogen depletion during prolonged endurance exercise".
They say sucrose ingestion does not preserve liver glycogen concentrations any better than glucose, but sucrose does increase whole-body carbohydrate utilisation compared with glucose.
This study aimed to see whether having a sugary drink available during endurance exercise preserves the body's glucose stores in the form of glycogen. They also wanted to see whether there was any difference between sucrose or glucose in terms of performance.
As may be expected, the researchers found drinking both sucrose and glucose drinks during exercise provided an energy source, thereby preserving the body's glycogen stores.
However, the body seemed to make better use of the carbohydrates to provide energy when it was given in the form of sucrose rather than glucose, and participants felt they were getting less exhausted.
The findings suggest both sucrose and glucose are a good energy source during exercise, though plain sugar (sucrose) in water had the slight edge in this study.
These tests involved just 14 male endurance cyclists, which is an important limitation of this study. This means we should take care before applying the results to all groups – for example, women, less experienced sportspeople, or people performing different types of exercise. Even for male cyclists, a much larger sample could have given different results.
There are also many different aspects related to sports nutrition that could be examined, such as the effects of eating food and drink containing different nutrient sources an hour or two before exercise, or the effects of eating after exercise in replenishing energy stores.
Overall, this study informs us about how the body may use sucrose and glucose differently during exercise, but limited firm conclusions can be drawn about the best form of sustenance before, during or after exercise.