Alan McCubbin has been an Accredited Sports Dietitian for 15 years. In that time he’s worked with a variety of athletes across a range of sports, from complete beginners to Olympians. For the past 10 years he’s mostly focussed on endurance sports, and after helping a friend out crewing for a 24hr mountain bike race he took up the sport himself. Now with 2 young kids and neck-deep in a PhD there’s sadly not enough time for serious training and racing right now, but he hopes to get back on the bike more in the coming years.
Alan has done a considerable amount of work with trail runners across all distances and race formats, and in 2016 published the first study to describe what competitors take, eat and tolerate in the Marathon des Sables. Another laboratory-based case study is soon to follow, looking at the age-old question – is it better to carry more food and take a weight penalty, or under-fuel but run with a lighter pack?
Currently Alan is in the final six months of a PhD at Monash University in Melbourne, where he also lecturers in sports nutrition.
Can you tell us a little bit about your research at Monash. What is your PhD in?
I’ve spent the last two and a half years investigating how endurance athletes use sodium before and during events compared to day-to-day in training, and the implications for sweat sodium losses during exercise. In particular, we’ve revisited an area of research that has been looked at a couple of times in the past, but generally not with modern endurance athletes in mind: does the amount of salt in your diet influence the amount you lose in sweat?
Is there evidence to suggest that consuming sodium during exercise will improve performance? Explain.
It’s a really interesting question, because when we think about eating and drinking during endurance exercise, we tend to think of the three main things our bodies use or lose – carbohydrate water and sodium. I was intrigued by the sodium aspect because whilst there are very specific guidelines about fluid and carbohydrate during exercise, the guidelines around sodium are quite vague and hard to interpret.
We just published a systematic review on sodium replacement during exercise and endurance performance, meaning we went back and looked for every possible study that has been published on this topic. Interestingly we could only find five studies that have ever looked at this question, and for about half of those it wasn’t actually the main question they were trying to answer. What we found was that of the five studies, only one showed a performance benefit from taking sodium during exercise. But I should point out that none of the five studies were done in a way that would make me completely confident in the result.
Two were done during an actual race (one a half Ironman, the other a full distance), so factors like competitors, a bad transition and so forth can have an influence on the results. One study involved running as far as possible in four hours on an outdoor athletics track, but the temperature varied from 5 to 20 degrees across days, making comparison of performances between the study days almost meaningless.
One was a cycling time trial outdoors, and whilst temperature was similar on each study day, subtle changes in wind speed and direction can have a significant influence on speed on a bike. And the final study was the only one done in a lab, but the performance measure was how long someone could ride at a constantly increasing speed (like in a VO2max test) after two hours of steady=-state riding. That’s a scenario that’s not very relevant to the real world, at least in distance running. So overall, I’d say whilst it doesn’t look like sodium during a race would improve performance directly, the quality of studies to base that conclusion on is not ideal. Clearly there’s more work to be done in this area!
Finally, it’s important especially for ultra-distance events to consider that sodium could indirectly influence performance if it affected other health-related factors. If someone has gut problems, severe cramping or hyponatraemia for example, they’re going to be slower or have to withdraw from a race, which is certainly a performance decrement! So we also have to consider the science around sodium and health outcomes in ultra-runners before deciding whether sodium is important to make you faster or not.
Does this vary depending on the type of exercise, intensity and the duration? So for an ultra endurance trail runner what could be the possible pros and cons?
Possibly, but I don’t think we have enough data to make a call on that yet. Take the carbohydrate and hydration areas for example. The research would suggest that the effect on performance may be there in shorter duration, higher intensity events like a half marathon, or even a full distance marathon for the elite guys, when blood volume may be more important for both oxygen delivery to muscles and cooling the body during exercise. But it’s possibly less important in longer events with less intensity. But of course the million dollar question is where the crossover occurs, and I don’t think we can say that yet with much confidence. Sodium I think will follow the pattern of fluid, because together they influence blood volume. So my take is that the hotter the weather and the higher the intensity of exercise (provided it’s still well over an hour long), the more likely sodium might be important.
What is the importance of sodium for an ultra endurance runner? i.e. EAH
Pure performance effects aside, of course we need to consider whether sodium losses during exercise increase the risk of health issues, be it cramping, gut problems or hyponatraemia. Cramping is a really difficult one to study, because you can’t just make people cramp predictably or on request during exercise in the lab. So most studies are limited to observations – do the people who cramp in a race drink less, consume less sodium, and so forth. This type of research finds no role of sodium in cramping.
Another way to study cramping is to electrically stimulate muscles until they cramp, and measure the threshold, or how much “zapping” you have to give someone before a cramp occurs. Then you alter something like sodium intake, and see if it alters the threshold. This research also finds no effect of sodium. The final angle is just to work athletes really hard but in a controlled lab environment and see if you can make them cramp. There’s less in this area, but I’m aware of one study that hasn’t been published yet (but has been presented at a conference), that suggested a sudden fall in the concentration of sodium in the blood may reduce the cramping threshold. So someone who becomes dehydrated then suddenly chugs down water and drops their blood sodium. But until it’s published, and preferably replicated in other studies, we won’t know for sure.
Sodium probably doesn’t influence gut problems directly, apart from possibly an indirect effect of helping maintain blood volume. This can allow more blood flow to the gut, keeping it functioning well, but the volume of fluid someone drinks is going to be more influential than the sodium content.
And finally, hyponatraemia. Traditionally the viewpoint in scientific circles has been that sodium is unnecessary to prevent hyponatraemia, provided athletes don’t over-drink during exercise. And certainly, the amount of water someone drinks has much more influence on blood sodium concentrations than the amount of sodium consumed. But there are a few published case studies now suggesting that in a small minority of cases, large sodium losses have led to a form of “hypovolaemic hyponatraemia”, where hyponatraemia occurred despite not over-drinking (ie. the athlete lost weight and still got hyponatraemia). This is still being debated in scientific circles, but I think it’s safe to say that the amount of fluid an athlete drinks (avoiding over-drinking) is still by far the most important thing an athlete can do to prevent hyponatraemia.
Finally, there is a small but emerging line of research looking at the impact of sweat sodium losses on bone health. Some research at Western States a few years back suggested that the athletes who had the biggest change in their blood sodium were at greater risk of reduced bone density, which they speculated was due to sweat sodium losses over the 15 or more hours they’re out there racing. Their theory is that sodium losses reduce sodium in the blood, and so sodium is taken from the mineral content in the bones to keep the blood levels in check and prevent hyponatraemia. I believe a research team in the US are working on this, have presented some data at conferences and are working on getting it published. We have some interest in this too and are looking at it from a different angle, but it’s still too early to tell if this is a thing or not.
Are there any specific sports nutrition guidelines for sodium replacement during and post exercise specifically for ultra endurance running?
Currently the general sports nutrition guidelines are non-specific around sodium. They suggest replacing sodium “when large sweat losses occur”, but don’t specify what large sweat losses are, or how much to replace. That’s not a go at those who wrote the guidelines, but reflects the lack of quality research they had to draw on in this area. There’s simply so many unanswered questions in this space.
Looking at ultra-endurance guidelines specifically, there’s even less data to go on, except for the observation studies from races like Western States and Comrades, where in general the amount of sodium someone consumes doesn’t seem to influence finish time, cramping, gut symptoms or hyponatraemia risk.
So the main conclusion at the moment would be that a specific amount of sodium (based on expected losses or a minimum suggested amount) is probably not justified during a race, but I’m still open to the possibility if we see some new and better-designed studies coming through. And even if sodium doesn’t improve performance or prevent health issues during ultra events, that’s not to say that athletes should avoid any sodium. Most people prefer the taste of salty food and drinks during exercise, and it helps retain a bit more of the water consumed so you don’t pee out quite so much. But perhaps at the moment it’s less “test and target”, and more “season to taste”.
In terms of post-exercise, that’s a whole other topic in itself. Our kidneys tend to adjust to what we’ve lost in sweat, so it’s not until the amount you lose through sweat exceeds the amount you consume in a day that things become an issue. And even then it’s likely that the body adjusts to conserve sodium in the following days to get things back to normal. But it’s an area we’ve barely scratched the surface of, so I can’t really comment on how important aggressive sodium replacement is after exercise, except to say that it sometimes helps with rehydration if you need to be in top shape again later that day or the next morning, like in a stage race. But even then, if you’re eating normal food alongside your post-race fluids, the sodium content doesn’t seem to matter so much as the other nutrients, particularly carbohydrate, improves fluid uptake and retention as well.
What influences someone’s sweat sodium losses? For example would my sweat sodium loss be the same in hot versus cold conditions or when I am unfit versus fit and altitude training? Any thoughts on menstrual cycle role here for the female runners?
In short, a lot of things! Sweat is produced in two parts in the sweat gland. Initially the sweat is produced from the fluid surrounding the gland, which has a similar composition to blood in terms of sodium, potassium and so forth. Then, as sweat travels along the gland towards the skin surface, some of the sodium and chloride is reabsorbed back into the body (a bit like what happens in our kidneys), so the sweat that we see on our skin is always a lower sodium concentration than our blood.
But there are numerous factors that influence the sodium loss. Firstly, hydration probably plays a role, because this will alter the sodium concentration of the blood, which in turn influences the sodium in the sweat that’s first produced in the sweat gland.
Increasing core body temperature will increase sweat rate, as you see in hotter and humid weather, with less airflow or at higher exercise intensities. This tends to increase sweat sodium concentration, because the sweat travels too quickly along the sweat duct for all of the sodium to be reabsorbed.
The function of the sweat gland to reabsorb sodium can also be affected. The amount of sodium in your diet is probably one factor, which is what my PhD is investigating. This is what we see in the kidneys, and we expect this to also be the case in sweat glands, although the existing research is not quite as clear cut.
Heat acclimatisation plays a big role. As you acclimatise to a hotter environment, your body makes several adjustments to how the sweat glands function. You tend to produce more sweat to aid in cooling, which you would normally expect to increase sweat sodium concentration. But actually the concentration goes down, because the function of the duct that reabsorbs sodium is also increased to compensate, preventing unreasonably high sodium losses. I’m not aware of research looking at the effect of altitude specifically.
Menstrual cycle is an interesting question, and I’ve heard of some researchers claim that this has an effect. But from my reading the research (and there’s not a lot) it’s a mixed bag, and if there is any effect it’s minor.
If I eat a higher salt diet does that mean that I will just sweat out more salt?
That’s what several studies suggest, although not all studies have found this. It was first looked at in the 1930’s and 40’s, both from a basic physiology perspective but also to investigate the effect of rapid acclimatisation when American soldiers were sent to Pacific Islands to fight the Japanese. It was revisited in the 1980s, often looking at whether the public health messages to eat less salt were going to be a problem for people doing lots of exercise. And now I’m looking at it for my PhD, this time in the context of athlete sweat testing and race preparation. So if someone does a sweat test in training, gets a result and then eats a lot more salt in the three days before a race (either deliberately or inadvertently because you’re eating more to get the carbs), does that mean the result of the sweat test is not valid? Our study is in the final stages, so we’ll see by the end of the year.
Do we know if consuming sodium in different forms may have a varying effect on performance? For example if I were to consume sodium in a capsule versus solution versus salt tablet such as the popular sports nutrition ones on the market?
No idea to be honest! It was a topic I was keen to look at in my PhD but ran out of time – there’s only so much you can cram into three years of research!
When can we look forward to reading more of your work/research results?
Our reviews of the existing studies on diet and sweat sodium and also sodium replacement and performance are already published, and are free to access without a subscription. We’ve also done an online questionnaire about what endurance athletes actually think about sodium and sweat testing, what they do in the days before a race and during it too. A big thanks to any readers who completed that questionnaire, because we had over 360 responses from 17 countries which massively exceeded our expectations. That’s being presented at the Sports Medicine Australia Conference in Perth in October, and hopefully the published paper will also be out around that time. And the final lab studies on diet and sweat sodium losses are just wrapping up now, so we’re looking at conference presentations around December time, and possibly published in late 2018 or early 2019.