Fluids & Carbs – More or Less? A Practical Race Day Summary

It’s really hard to get it right and it’s equally as hard to give people direct advice to get it right. But if you can just understand how the system works, you’ll be a lot closer to almost getting it right. If you could jump on the scales at the end of each hour, the job would be so much easier. So how much fluid with how much carbohydrate and electrolyte should you have? I’m going to try and make this clear in physiological and practical terms.

What type of fuel to put in?

Imagine your gut is a filter. It is essentially a water filter, then secondarily an electrolyte and glucose filter. Then, and only then is it a fat and protein filter. The latter items require the most time and remain the most complex items to filter. Indeed they are not required on race day, because you don’t burn much protein (it is not a significant contributor to metabolic requirements) and you have more stored fat than you can use in a day (try a week!). Fat and protein clog the filter, so do not use them on race day (unless you’re racing for fun, not performance).

Water never truly clogs the filter. You can drink water almost as much as you like. Water even helps when the filter is blocked, by flushing carb and electrolyte through the system. It helps because it dilutes the contents of your gut. The advice on your gel packet reads: ‘take with water’ for this very reason. However, water in excess can be a problem with causing bloating of the gut, or the other extreme, water intoxication (which makes you wee and bleeds sodium from your body – otherwise known as hyponetremia). But on the whole, water is helpful.

Carbohydrate and electrolyte do clog the filter, but not as much as fat and protein. There are specific transporters for these substances in your gut membrane. In fact, there are co-transporters which take all three (water, sodium and glucose) in one hit. This is another reason why water should be added to your gut contents when you take a gel. In addition, glucose and electrolyte are vital to maintaining the metabolic output of your muscles. Neither are stored in huge amounts in your body, like fats. This discrete supply means you should focus on their replacement (with water) on race day.