Drinking carbohydrate

Solving the problem of how to deliver the most carbohydrate with the maximum amount of fluid

Research on sports drinks continues to move on apace – today’s formulas have changed radically since the first commercial formulations of the 60s and 70s. Much current interest is focused on ways of maximising carbohydrate delivery while not compromising water absorption.

In trying to meet needs for both carbohydrate and fluid intake, there is a conundrum to be solved. Increasing concentrations of carbohydrate tend to decrease the amount of water that can be absorbed from a drink. Both the concentration of carbohydrate by weight, and osmolality (which is defined by the number of particles in solution) appear to have independent effects.

Readers of Sports Performance Bulletin will probably be familiar with the use of glucose polymers (also referred to as maltodextrins) in sports drinks. These are used to decrease the effect of osmolality. Glucose polymers consist of a medium length chain of glucose units strung together to form one molecule. Their use means that more weight of carbohydrate can be used for the same osmolality – this is because to assess osmolality the body just counts the number of particles (solute) in a fluid without being concerned with their size.

Maltodextrin disappointment
In theory, this should allow more carbohydrate to be absorbed for a given rate of water absorption. In practice, the experimental results have not been as spectacular as might have been hoped. Studies have been variable, with one reviewer concluding that ‘gastric emptying of glucose polymer solutions is at least as good as, and perhaps slightly better than, free glucose solutions’. Veteran researcher Tim Noakes has suggested that perhaps maltodextrins do not fulfil their promise because they are rapidly digested in the small intestine, giving rise to a fluid with a high osmolality. There are osmo-receptors present in the small intestine which feed back information to the stomach – if these receptors are registering a high-osmolality fluid, they will instruct the stomach to slow down its emptying.

So, despite extensive investigations into the mechanisms of water absorption over the last 20-30 years, the influence of osmolality, carbohydrate concentration (by weight), and carbohydrate type remain unclear. In a recent study carried out at the University of Iowa, researchers from the department of Exercise Science got together with gastroenterology specialists to look at all these different factors (‘Effects of carbohydrate type and concentration and solution osmolality on water absorption’, X Shi, C Gisolfi et al, Medicine & Science in Sports and Exercise, 1995 pp1607-1615).

They had a hunch that an area that has never been specifically investigated might be of relevance to how sports drinks are absorbed. They suspected that drinks which contained more than one type of sugar would stimulate more water absorption than an equivalent drink composed of just one sugar type. Why should this be likely?

Transport from gut to bloodstream
Once a fluid has made it through the stomach and has arrived in the small intestine, it has to cross the intestine wall (or membrane) to get into the bloodstream. There are different ways of doing this. In the case of water molecules, if the fluid in the intestine is less concentrated than blood, the water can ride down an osmotic gradient into the bloodstream, ie, it has a tendency to move from a less concentrated solution to a more concentrated one. This is why fluids which are ‘hypotonic’ (less concentrated that blood plasma in terms of the numbers of particles dissolved) are absorbed relatively quickly into the bloodstream.

This is not the only way of getting across the intestine wall, however. Various other substances, once they have been digested, need a helping hand to cross the membrane. Let’s take the example of glucose. There is a specific ‘transport protein’ waiting in the intestine wall which picks up glucose molecules and ferries them over to the other side, so they can pass into the bloodstream. This transporter needs sodium to help it perform (this is why salt is added to many sports drinks), and also, while it is sweeping glucose and sodium across, some water gets whisked across too. The amount of water that can hitch-hike over on the back of a transport protein could be significant.

There’s been a lot of focus on the effect of osmolality on the effectiveness of sports drinks. But it’s never been demonstrated whether the effect of osmolality on water flux is modified by introducing multiple transportable substrates into drinks. The Iowa researchers thought that by including substances which used at least two different transport carriers, more water might get swept along, and that this might compensate for having a solution with a higher osmolality.

This is because at a particular concentration of glucose in the small intestine, all the glucose transport proteins will be busy. You could imagine it to be like the tube at rush hour: packing more people onto the platform wouldn’t increase the number of people who could get to the next station if all the carriages were full. But if you provided an additional means of transport such as a bus, even if this was slower than the tube, the net result would be more people getting to their destination in a given time. Thus, the intestine wall has another transport protein which picks up fructose, and also sweeps water across. It’s slower than the glucose transporter, but can still have an additive effect when all the glucose proteins are busy.

What the Iowa scientists did
The team of researchers therefore devised an experiment which compared drinks which had just one type of transportable substance (glucose, or other things that break down to glucose when digested – this includes maltose, and malto-dextrins), and compared these with drinks containing two or more different transportable substances. For this they chose combinations of glucose, fructose, and glycine, an amino acid which uses yet another type of transport protein. The range of carbohydrate provided was between 6 per cent and 8 per cent by weight.

Eight men were fitted with equipment which allowed fluid absorption to be measured in their small intestines. Nine different drinks were given, to cover a range of osmolalities (165 – 477 MOsm/kg; this compares to plasma osmolality of 280 mOsm/kg), different combinations of carbohydrates, plus a solution combining glucose and fructose with the amino acid glycine.

The major finding was that the drinks containing two transportable substances boosted water flux into the bloodstream more than drinks with just one substance, even when the fluids with combinations of substances were of higher osmolality. The scientists therefore concluded that osmolality is not the most important characteristic governing water absorption in the small intestine when multiply transported substances are present.

Maltodextrin solutions might have been expected to perform well because of their low osmolality. However, they did not stimulate as much solute or water transport as equivalent drinks (by weight of carbohydrate) containing two or more transportable substances. For example, a 6 per cent solution of maltodextrin, which was hypotonic, produced less water, electrolyte and solute absorption than 6 per cent carbohydrate solutions containing:
i) glucose 2 per cent and sucrose 4 per cent (sucrose breaks down to yield glucose and fructose) or
ii) glucose 3 per cent sucrose 2 per cent and maltodextrin 1 per cent (maltodextrin breaks down to provide glucose units)

Both latter drinks were slightly more concentrated than isotonic. Even a solution comprising glucose 3 per cent fructose 3 per cent and sucrose 2 per cent which was strongly hypertonic (much more concentrated than blood plasma) produced as much water absorption as the 6 per cent hypotonic maltodextrin solution.

The glycine enigma
Previous studies have come up with conflicting results when adding amino acids to fluid replacement drinks. Evidence for both an enhancement and an inhibition of water transport when the amino acid glycine is added have been found by different researchers. In the present experiment, no enhancement of fluid absorption was found when glycine was added to carbohydrate solutions. The researchers recommended further investigation, because, theoretically, an amino acid should activate yet another transport mechanism, thereby boosting water absorption.

The results of the Iowa research therefore indicate that attention should be given to ‘transport effects’ in sports drinks, rather than purely focusing on osmolality. The study was a small one, with experimental measurements focusing on only one section of the small intestine. Further research is needed to establish whether this is a reproducible effect, and if so, whether this translates into measurable differences for athletic performance. The theoretical reasoning for an effect is sound, and these initial results encouraging. Some commercial sports drinks currently contain fructose and/or sucrose in combination with glucose and/or glucose polymers. These may well offer more benefit for rehydration than drinks containing only glucose or glucose derivatives (maltose, maltodextrins).

The drinks in this study were in the range 6 – 8 per cent carbohydrate. Drinks in this range tend to be targeted at situations where fluid replacement is a high priority. Drinks with a higher carbohydrate content can be useful in situations where exercise continues for longer than 90 minutes, but where sweat losses aren’t expected to be particularly high. It isn’t clear whether the multiple transport effect would continue linearly at higher concentrations of carbohydrate; however, for cases where rehydration is not so critical, the effect would not be so crucial anyway.

Janet Pidcock

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