The differences between hypotonic, isotonic and hypertonic sports drinks and oral rehydration solutions
Background: The role of sports drinks is to rehydrate the athlete from a dehydrated state to a euhydrated state and replenish carbohydrates and electrolytes. The concentrations of these drinks will impact how quickly you can potentially achieve a euhydrated state. There are two main factors that assist in this process; rate of gastric emptying and absorption through the small intestine. The goal of this review is to explore varying concentrations of sports drinks and its ultimate effect on delivering nutrients and fluid to their intended destinations.
Sports drinks can be convenient for athletes to replace fluid, carbohydrate and electrolyte losses before, during, and after training. However, not all sports drinks are created equally. For example, some are hypertonic, meaning they have a higher concentration of electrolytes and carbohydrates compared to blood. Isotonic, having the same concentration of electrolytes and carbohydrates and lastly, hypotonic, having a lower concentration of electrolytes and carbohydrates to blood. The varying concentrations will impact how quickly the solution leaves the stomach, how well it is absorbed in the lumen of the small intestine, and how quickly the absorbed water enters the fluid compartments. This is important for athletes as it has been well documented in research that >2% dehydration can lead to performance decrements.
When fluids are consumed they are not immediately available for assimilation into the body. They are initially stored in the stomach, where there is little net absorption across the gastric muscoa*. The main regulation of gastric emptying is the volume of which the fluids take up in the stomach. The greater the volume, the quicker the fluids will empty, but only up to a maximum capacity. This maximum capacity is determined by receptors in the duodenum and ileum of the small intestine. It appears the main role of these receptors is to increase or decrease duodenum muscular tone to vary the rate of gastric emptying to prevent the absorptive capacity of the small intestine from being overwhelmed.
Costill and Saltin measured gastric volume 15 minutes after different volumes of a glucose solution were ingested. They found no difference in volume emptied when 600 mL was ingested compared to 800 mL, but found a possible upper limit of gastric emptying at 1000 mL. When a single bolus of a liquid is consumed, there is a fast phase when intragastric volume is at its highest. The emptying rate then becomes progressively slower as the volume in the stomach decreases. They then refilled the stomach in 20-minute intervals with 150 mL, to achieve greater gastric emptying rates than just the initial fluid being consumed alone.
It is clear that plain water empties rapidly from the stomach, while increasing the energy content of ingested solutions slows the rate of gastric emptying. It is well documented that carbohydrates solutions <2.5% empty from the stomach just as fast as plain water with equal volumes, and most studies show that carbohydrate levels >6% slow emptying. Furthermore, glucose concentrations between 4%-5% produce small but significant slowing of gastric emptying. Additionally, some studies have shown no difference in gastric emptying of up to 10% glucose solution, but it is likely due to study design and participant population. Ultimately, increasing the carbohydrate content of solutions slows the rate of emptying in proportion to the energy density, but it results in a faster rates of carbohydrate delivery to the duodenum.
Looking at osmolality in regards to gastric emptying it isn’t quite a clear as energy density. Substitution of glucose polymers for glucose monomers can be used to reduce the osmolality of the solution while maintaining the total carbohydrate content. Several studies that have looked at this have not found consistent findings. Most studies that compare isoenergetic solutions of glucose monomer to polymer found little difference in gastric emptying rates despite the larger differences in osmolality. This can be attributed to the hydrolysis of the polymers occurring before reaching the small intestine osmoreceptors. The authors concluded that, “ the energy density of a solution exerts a greater effect than osmolality in the regulation of gastric emptying and that the substitution of glucose polymer for monomer may slightly increase the rate of gastric emptying, but only at high energy densities”.
Exercise can also be a determinant of the rate of gastric emptying. It appears that steady-state cycle exercise at an intensity below 70% of VO2 max has little effect on the gastric emptying. When intensity is increased above 70% we can see progressive, significant, slowing of the emptying rates of the ingested fluids. For variable intensity exercise, the time spent at lower intensity is not sufficient to improve the rates of emptying. However, the performance benefit of a carbohydrate electrolyte solution is still present as enough carbohydrates can be absorbed and be effective.
Within the small intestine along the brush border, there lies cells called enterocytes. This is where various transport carries are located and have specific solutes they translocate across the intestinal lumen. Some of which include carbohydrate monomers, amino acids, dipeptides, and tripeptides. The transport of these monomers is often linked with sodium uptake, which plays a pivotal role in the absorption of organic and inorganic solutes, and ultimately water absorption. Water is considered passive transport and relies on osmotic, hydrostatic, and filtration pressures. It is the osmotic gradient of solutes that promote the uptake of water from the lumen across the intestinal mucosa.
In the proximal small intestine, more specifically the duodenum, water and electrolytes are most permeable. Moving further down the small intestine to the jejunum, ileum, and colon there is a decrease in permeability of water and electrolytes, respectively. Again, the movement of these solutes across the mucosa is dependent on concentration gradients and the need for specific molecules.
Absorption of water and solutes are absorbed in two proposed ways. One way via aquaporins which penetrate the enterocyte membrane and the other through carrier-mediated transport systems that are embedded in the brush border of the enterocytes within the small intestine. Entry to aquaporins are limited by size as well as electrical charge of molecules. Due to the size of amino acids and monosaccharides they are unable to enter these aquaporins. However, water and electrolytes are able to enter. Carrier mediated transport systems carry specific solutes based on the specific carrier transporter. For example, the sodium-dependent glucose transporter (SLGT1) is responsible for the transport of glucose, but not fructose or lactose. Within the family of carrier systems there are two types of transport, active transport and facilitated transport. Active transport is used when movement is needed across a concentration or electrochemical gradient. This type of transport requires energy and is dependent upon the electrochemical potential gradient of the sodium-potassium ATPase pump located in the basolateral membrane of the enterocytes. Glucose specifically uses this type of transport through SLGT1 while fructose uses facilitated diffusion which is energy and sodium dependent by way of the transporter GLUT5. There are also ion-exchange mechanisms and ion-cotransport systems that are responsible for the transport of solutes such as sodium and chloride. With the transport of the above-mentioned solutes, this ultimately promotes net water uptake caused by osmotic gradients and in turn increased transport of additional solutes from the intestinal lumen.
Carbohydrate Type on Intestinal Absorption
Most sport drinks major nutrient source is in the form of sucrose, glucose monomer, maltodextrins, fructose, or cornstarch. Glucose absorption in the jejunum appears to plateau at about 200 mmol/L (3.6%) and with greater concentrations comes reductions in water absorption. For maltodextrin it appears that with equivalent amounts of maltodextrin to glucose monomer, water uptake is reported as being faster from the maltodextrin solution due to the lower osmolality and more carbohydrates may be absorbed more quickly. It should be noted that the research on glucose absorption from maltodextrin being faster than glucose monomer is not a universal finding.
Sucrose being made up of fructose and glucose seems like it may be an alternative to glucose alone due to glucose saturating the SLGT1 transporter, but in several studies, they found there to be no difference. However, they did find that water absorption from sucrose tended to be slower. This effect is believed to be caused by fructose being absorbed about 2/3 slower than glucose and thus increasing the luminal osmolality. Again, this is not a universal finding as some studies have shown faster water uptake from sucrose solutions than from solution with an equivalent amount of glucose.
Osmolality and Intestinal Absorption
When food and or beverages enter the small intestine from the stomach, the osmolality of the small intestine changes. The small intestine then brings the osmolality back to its baseline of 270 mosmol/kg to 290 mosmol/kg which is approximately isotonic with human serum. This is achieved through bidirectional movement of water and electrolytes across the proximal intestine. The time it takes to reach isotonicity is based on the nutrient load and osmolality of the gastric efflux. Water for example is made isotonic more rapidly by way of influx of electrolytes than a hypertonic solution like fruit juice. Hypertonic solutions require water efflux into the small intestine from the body water pools. This delay in reaching isotonicity and the water efflux into the lumen from circulation makes hypertonic solutions less effective in promoting rapid rehydration than beverages with a lower osmolality.
Water absorption seems to be greater with a carbohydrate electrolyte solution (CES) than from plain water alone. One study found faster rates of absorption from from plain water with an osmolality of about 30 mosmol/kg than from a hypotonic 6% carbohydrate solution entering the duodenum with an osmolality of approximately 200 mosmol/kg, but the rates were similar to those from a 4% carbohydrate solution with an osmolality of 260 mosmol/kg (105). As such it is best to have an osmolality of between 200 and 260 mosmol/kg. This was shown by Leiper et al., where they compared beverages with similar carbohydrate and sodium content but osmolality differed, net water absorption was about twice as fast from a moderately hypotonic (229mosmol/kg) solution than from an isotonic (277 mosmol/kg) solution, which was faster than from a moderately hypertonic (353 mosmol/kg) solution.