A successful race day nutrition outcome is influenced to the greatest degree by one factor – sweat rate.
Knowing sweat rate is crucial to determine total fluid and electrolyte requirements on race day, thus making sure the individual reaches the finish line with blood plasma characteristics not significantly different to normal.
This is especially important when racing in hot/humid climates.
Determining sweat rate can be achieved by performing a training session in the acclimatised state at race effort for an extended period of time under expected race environmental conditions and then accounting for fluid loss via body weight changes.
An additional step would be to have a sweat analysis to determine sweat electrolyte content. If I was someone like Bevan Docherty, this is exactly what I’d be doing.
Practically speaking, the process I went through to determine my race nutrition strategy for Hawaii 2011 is outlined below:
My sweat rate is approx 1.5 litres on average for the duration of the cycle leg (lower early when cooler and higher later on when hotter)
Total fluid needs for the bike leg = 1.5 litres x 5 hours = 7.5 litres
Total electrolyte needs for the bike leg (I use the recommendation of approximately 700mg per litre of fluid, but a sweat analysis will give a closer approximation) = 7.5 litres x 700mg = 5250mg of Sodium.
This meant I needed to consume approx 7.5 litres and 5250mg of sodium, along with my carbohydrate requirements (I use the multiple transportable type carbohydrate recommendation of 75-90 grams per hour) over the duration of the cycle leg. I added my carbohydrate and sodium needs to one bottle, topped it up with water and then consumed at equal intervals throughout the cycle leg along with water on race day.
Prior to each aid station, I would take a predetermined proportion of the carb/electrolyte mix and follow with water until the next aid station. I would take a bottle of water at each aid station to make sure I was consuming approximately 500ml between each aid station and fulfilling my 7.5 litre total fluid requirement (15 aid stations in total = 500ml per station).
This even consumption strategy should keep the fluid contents in the osmolality range that allows for gastric emptying.
I went through the same process for the run leg, with sodium requirements met via the use of salt tablets.
Another thing to consider is electrolyte status prior to starting a race. It is in everyone’s best interests to get to the start line with an electrolyte tank that is full, regardless of race day conditions.
On a few occasions in training and at the end of races in hot conditions, I have reached a state where my plasma sodium concentration is in the normal range, but I have been significantly dehydrated. Reaching this condition is due to an inadequate sodium intake, and is totally undesirable.
Recognising this condition is as simple as jumping on a set of scales to determine body weight and hence hydration status. If you are significantly underweight but have no desire to drink, then there is a good chance you are dehydrated with a normal plasma sodium concentration (plasma sodium concentration is the thirst driver).
Consuming water or low sodium fluids to address the situation will likely lead to feelings of nausea and then vomiting. When in this condition, I have found that some sort of salty drink, such as soup, is usually effective in starting the rehydration process. The other alternative is a saline drip bag or 4.