HAMS: hydration hype or hydration heaven?

Here in the northern hemisphere, the prospect of summer heat and sunshine is still at least three months away. However, if that event you’re training for now takes place in the summer months, you’ll need to think about how to prepare for any potential heat. Of course, the human body can be very well adapted to coping with hot weather exercise thanks to the ability to perspire through the skin. Due to the physical properties of water - notably its very high ‘latent heat of evaporation’ – evaporating water from sweat on the skin can remove large amounts of heat, thereby helping to regulate the body’s core temperature. However, the sweating mechanism also means that water losses from the body can be greatly increased, which in turn means that the role of hydration before, during and after exercise becomes critically important.

Hydration matters

In an early article on this topic, we highlighted some of the key research on hydration for athletes. Some of the findings we reported were as follows:

·        A US study that looked in to the effects of 5% (ie quite severe) dehydration on running economy (biochemical efficiency of muscles during sub-maximal running) during a 10-minute run⁽¹⁾. It found no significant differences in running economy between fully hydrated and 5% dehydrated states at any workload. Likewise, there were no differences in perceived rates of exertion or in post-exercise lactate concentration;

·        A study on basketball players performing 2-hour exercise sessions in hot conditions⁽²⁾. Compared with full hydration, a dehydration level of just 2% adversely affected the players’ shooting skills and performance in other maneuvers requiring high levels of motor skills;

·        A study on uphill cycling that compared the performance of well hydrated and 3.5% dehydrated cyclists during an uphill ride to exhaustion trial⁽³⁾. Although the dehydrated cyclists had better power-to-weight ratios, their core temperatures and heart rates at a given workload were higher, and their times to exhaustion were 13.8% poorer;

·        A study on runners that looked at the effects of 5% dehydration vs. full hydration on the hormonal responses to training – specifically the levels of testosterone, cortisol and the testosterone/cortisol ratio⁽⁴⁾. This showed that while a 5% dehydrated state didn’t seem to measurably increase physiological stress during exercise or affect testosterone concentrations before, during, or after exercise, pre and post-exercise cortisol concentrations were increased in the dehydrated runners, indicating an increased likelihood of muscle tissue breakdown (a bad thing!).

In summary, while athletes may get away with up to 2% fluid loss, higher dehydration levels could result in reduced performance, even allowing for a potential gain in power to weight ratio. By contrast, athletes whose sports involve a large skill component should aim for minimal dehydration – even a 2% fluid loss may be too much to maintain optimum performance.

Maintaining hydration

One key recommendation given to athletes is to ensure that they start their event thoroughly hydrated, especially in endurance events such as marathons and triathlons, where carrying extra fluid is difficult or where drink stations are not available. Because of this need, some athletes have experimented with taking onboard extra fluid before an event in order to start the race ‘hyper-hydrated’. A temporary state of hyper-hydration can easily be achieved by drinking lots of water in excess of the body needs. However this water-induced hyper-hydration is very transitory; the drop in osmolarity (blood concentration) that occurs stimulates the kidneys to remove most of the excess water within an hour, and this in turn means frequent trips to the loo – hardly conducive to fast race times!

Back in the 2000s, the use of a compound called ‘glycerol’ to achieve hyper-hydration became quite popular for a while. This practice gained traction following the 2004 Athens Olympic Women’s Marathon. Those who are old enough may remember the almost unbearable image of Paula Radcliffe sitting in a very hot (39C/102F) and sunny Greek gutter, sobbing her heart out as her Olympic dreams lay in tatters. Although she had been the lead most of the way, Radcliffe collapsed at the 36km mark after having just been passed by the American athlete Deena Kastor, who according to some sources had used glycerol in her pre-race preparation to enhance and maintain hydration.

Glycerol itself is a sweet tasting clear syrupy liquid, but it also possess another property; namely that when mixed with water solutions, it is able to increase their concentration or more technically, osmolarity. Because the human body requires the osmolarity of body fluids to remain fairly constant, ingesting glycerol stimulates the absorption and retention of water. Importantly, ingesting a solution of glycerol and water doesn’t cause the dilution and drop in osmolarity that drinking pure water alone would produce, which means the ingested water is retained by the body and only excreted when the extra glycerol is either removed by the kidneys or broken down by the body⁽⁵⁾.

Research had previously shown that adding glycerol to water ingested before an event can prolong the period of hyper-hydration for up to four hours, which also explains the use of glycerol by athletes seeking to enhance endurance performance in hot weather conditions⁽⁶⁾. Despite this however, the use of glycerol largely fell out of favour as the research on its performance benefits was far from convincing. Yes, hyper-hydration with water and glycerol did seem to improve fluid retention and markers of hydration for longer during an exercise bout, but in a number of studies, this didn’t seem to translate into actual performance benefits^(7-9). Overall, the evidence suggested that the use of glycerol for hyper-hydration was only of potential value for very long events held in very high levels of heat and humidity, and even then it was debateable. To further complicate matters, glycerol ingestion can come with side effects such as stomach upsets, together with headaches and blurred vision at higher doses.

HAMS: a hyper-hydration alternative?

In recent years, alternative products claimed to aid hydration have appeared on the market including drinks containing a substance known as ‘high-amylose maize starch’ (or HAMS for short). HAMS consists of long-chain carbohydrates but with a high proportion of ‘resistant starch’. Unlike most carbohydrates, which are readily digested in the small intestine, resistant starch travels undigested down to the large intestine where it is fermented by the naturally occurring gut bacteria in this part of the intestine. This fermentation process produces short-chain fatty acids such as acetate, propionate, and butyrate⁽¹⁰⁾. These short-chain fatty acids have the ability significantly enhance the absorption of sodium and water across the colonic wall thereby improving hydration. This explains why HAMS ingestion has been studied as a potential treatment method for cholera and chronic diarrhoea^(11-12), where dehydration may become serious or even lethal.

This ability to enhance hydration via the large intestine route has prompted sports scientists to ask whether pre-loading with HAMS before exercise might help hydration during exercise by creating an ‘internal reservoir’ of water, which is then slowly released back into the body during the later stages of an event. Although this sounds like a plausible theory and one that could practically applied, there’s been almost no research into HAMS and hydration in a sports setting.

In fact, the only previous study in the literature was carried out in 2018, where researchers compared the use of a conventional pre-exercise sports hydration drink to pre-exercise HAMS in Australian Rules soccer players⁽¹³⁾. It found that pre-exercise HAMS ingestion was able to significantly improve key markers of hydration like hematocrit concentration and body weight during intense/hot summer training compared to a conventional hydration drink. These better hydration markers were observed not only at the end of training, but also following a subsequent recovery period and also prior to the start of training. However, while very encouraging from a hydration standpoint, the researchers didn’t investigate whether HAMS ingestion resulted in actual performance gains.