Optimal nutrition & hydration for hot weather races…

Alan Couzens, M.Sc. (Sports Science)

Sept 18th, 2014

Left: Sometimes you have to do what you have to do to get the fluid in but this is probably not 'optimal' :-)

Some of the common themes here at EC are nutrition for health and nutrition for performance. Our editor was right to point out that these two topics are often distinct and perhaps even counter to one another. However, one situation in which they are very much linked is nutrition and hydration for hot weather racing. Get it wrong and not only is your performance going to suffer but you could very well be putting your health at risk!

Hot weather races present a unique nutritional challenge to athletes because the fluid that is so integral to getting nutrition across the gut into the blood stream is also being called upon to prevent the body from overheating. For this reason, nutritional issues tend to be more common in hot weather races and the margin for error in your nutritional choices is much narrower.

Sweat rates for a hot race like Kona can be as high as 1.5-2L/hr, equaling or even exceeding the rate at which fluid can be delivered from the gut. Under more normal circumstances, the body has a fluid reserve that it can draw on to correct nutritional errors that might slow gastric absorption, but when sweat rates are this high, this reserve disappears and the athlete must help the body out as much as possible by providing it with fluid mixes from the get-go that:

  • Can be readily and immediately absorbed.
  • Match the fluid that the body is losing as closely as possible (both in rate and content).

So, what things hasten the rate of fluid absorption? (Costill et al., 1974):

  • Low to moderate intensity of exercise (<70% VO2max / ~80% Max Heart Rate)
  • High levels of gastric volume (>1L)
  • Optimal levels of osmolality (270-300 mosmol/kg)

To put in practical terms:

  1. Do your best to avoid high heart rate "spikes" & wait for heart rate to settle before taking in large amounts of fluid.
  2. Take small sips frequently throughout the bike to keep gastric volume continually high.
  3. Get the bulk of nutrition/hydration from an optimal sports drink.

The third of these recommendations is especially important and demands further investigation as to what is an optimal sports drink for hot weather events.

Osmolality refers to the amount of particles dissolved within a fluid. The body is incredibly determined to keep fluids on either side of its cell walls in balance. This means if you have a whole lot of water in the gut without a lot of particles in it, the body is going to balance this by making you pee a lot and pulling sodium from any reserves it may have to increase the osmolality of said water so it can move across the gut and be absorbed.

Neither of these is particularly appealing to the endurance athlete as we don’t want to be losing a lot of fluids “to the road” and we don’t want to have to wait around for our body to rearrange its sodium stores to get the water across (assuming that we have sufficient stores after sweat losses).

On the flipside, if you have a whole lot of particles in the gut without a lot of water to go with it (whether salt or carbohydrate), the body will pull water from its fluid reserves to normalize the balance in order to move the particles across the intestinal wall. In a hot ironman race, you don’t have a lot of fluid reserves to play with to keep things moving appropriately. Bottom line, we can greatly help the body out by giving it fluids that are at or slightly under the body’s natural osmolality of about 286 mosmol/kg.

There are two basic particles that we can add to water to increase its osmolality: sugar (carbohydrates) and salt (electrolytes). The optimum amount of each to hit that magic 270-300 mosmol/kg number will vary for each athlete.

Obviously, there is clear advantage to getting sugar into the blood stream. However, replacing sodium losses may be even more important. Remember, sodium is the body’s currency in maintaining fluid balance around cells. When it runs short on this currency bad things can happen, namely hyponatremia (“water intoxication”).

While several complex factors are at play in athletes who exhibit hyponatremia, a dilution of the body’s serum sodium levels by inadequate sodium in fluid replacement can be a factor. For example, Baker, Lang & Kenny (2008) showed a drop in serum sodium of 1mmol/L/hr in athletes who maintained body weight/fluid balance over two hours of running in the heat but did not supplement with sodium. Had this rate continued, athletes would have become clinically hyponatremic after about seven hours of exercise. In the same study, athletes who consumed 30mmol/L of sodium in their sports drink (as a group) showed minimal drop in serum sodium over the course of the run.

It should be noted that sodium concentration in sweat exhibits a fairly wide range of variability. Most acclimated athletes will fall in the range of 35-50mmol/L. However, some heavy sweaters will exhibit concentrations of double these numbers, possibly up to 100mmol or 2300mg/L! Given the importance of maintaining sodium balance, these folks are at an especially high risk. If you think you may fall into this category, actual sweat concentration can be assessed in the lab. For athletes experimenting with sodium intake, signs of excessive sodium consumption include excessive thirst and water retention/puffy extremities.

As a general rule, the average sweater in hot conditions should shoot for about 35-50mmol/L (about 700-1200mg/L) of the particles in their drink come from sodium. In an effort to preserve maximal palatability, many sports drinks do not have sodium levels this high. For example, Cytomax has 240mg/L, Accelerade has 500mg/L. Recently, some companies have started to address this by coming up with specific hot ironman mixes with significantly elevated sodium levels (“Ironman Perform”, “Gatorade Endurance”) Or, even better, Infinit nutrition offers custom mixes that allow you to dial in your personal sodium content in line with sweat concentration. Bottom line: check your mix and make sure that sodium levels are appropriate for hot racing. If not, several electrolyte supplement options are available.

Once sodium is fixed, the balance of the particles can come from carbohydrate sources. Maltodextrin offers the greatest calorie bang for its particle size and should therefore make up a good chunk of the calories in a hot weather mix. Most of the maltodextrins used in sports drinks offer more than twice the number of calories per mole than that of sucrose. In other words, a 6% solution of maltodextrin will offer better osmolality than a 3% solution of sucrose! This is why cheaper sports drinks that use sucrose and dextrose as the primary sugar sources, are impractical for long course hot weather racing. And given the interests of business being what they are, it pays to check the sugar type of your sports drink!

In the interests of saving money, some athletes, in fact, buy their own pure maltodextrin in bulk and build their own mix. The only downsides when it comes to pure maltodextrin are that:

  • It is a long chain carbohydrate which can take a longer period of time to be broken down and oxidized. This can leave some athletes who rely solely on maltodextrin feeling a little flat at points during the race.

  • It is transported across the intestinal wall via the same transporters used by glucose. This is not a problem in and of itself, however, some studies have shown that the introduction of multiple CHO sources that use different modes of transportation across the intestinal wall can increase the rate at which these sugars are absorbed and oxidized (e.g. Roy et al., 2004). Therefore, a solution of glucose/glucose polymers and sucrose may be optimal, providing they don’t take osmolality outside the optimal range.

Putting the theory into practice, a mix that falls within the optimal 270-300 mosmol/L range (280 mosmol/L) for an average sweater should have about:

  • 50mosmol/L sodium (1.15g)
  • 50mosmol/L chloride (1.75g)
  • 120mosmol/L maltodextrin (~60g)
  • 60mosmol/L sucrose (~20g)
  • 1L water

Of course, if you are a 'heavy sweater', the electrolyte content will come up and the carbohydrate content will need to come down in order to 'make the math add up' & keep the osmolality within the optimal range.

When I say 'mix', I am not meaning to imply that all of this should be put together in your bottles. While that's an option, I can confirm that, with the salt content of the above mix, it doesn't taste that great! At the least, most athletes will want to dial the sodium-chloride of the liquid down a bit and also use a supplemental source of salt (stick, tabs etc). There are also advantages in having a little flexibility with the fluid mix through the race. That way, if you feel 'sloshy' - like fluid isn't clearing form the gut, you can add some supplemental salt or CHO. If you feel crampy/bloated, like the solute isn't clearing, you can sip on plain fluid. While, I think it's good to have a good, simple core nutrition/hydration plan going in, having access to some 'options' & listening to your body and reacting accordingly is also recommended.

If you’re racing a hot race as your A race this year, pay it the respect it deserves by tailoring your nutrition accordingly. Work out your personal sodium and carbohydrate needs and select a mix that can deliver these essential nutrients to the body in the heat of battle. Your performance, and indeed your health, depend on it!

Train smart,



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