Athletes: do you need to lower your carbohydrate intake?

athlete carbohydrate digestion endurance gi distress gut health low carbohydrate sports nutrition ultramarathon Oct 13, 2023

Gut issues are one of the most common ailments of endurance athletes, with over 85% of us experiencing digestive distress at some point in our training or racing. It is definitely a limiting factor that has massive implications for our ability to execute our training to the best of our ability, therefore changing how we are able to show up on race day.

I read with delight this morning that Camille Herron smashed the record in the Spartathalon, a 153 mile (246 kilometer) race from Athens to Spartan. Amazing. And, in addition to remaining patient in the earlier part of the event, allowing other runners to take the lead, saving herself for the latter part of the race, she credited her reduced carbohydrate approach to enabling her to get to the finish line in the best possible form. She would normally target between 60-75g of carbohydrate an hour during her events, but in this one she kept it below 50g an hour, using a gel an hour combined with squares of honey and salt sandwiches. Her strategy was simple, and it worked, she was able to run without the gut issues that had plagued her in past races.

The reason for the shift was on the back of a metabolic test that determined her fat oxidation rate each hour was extremely high, and she didn’t need the amount of carbohydrate she was taking in previously that led to her digestive distress. This is important intel. I speak to many athletes who target higher carbohydrate amounts during training which tends to go okay, but when the intensity of the race day kicks up, their gut shuts down and they too experience the gut-related issues that can ruin the performance on the day, sort of undoing all of the hard work that has come before the ‘big dance’.

What do we know about the ability to burn fat for fuel as an athlete? Women are naturally better able to burn fat than men, and recent research (though a small study) suggests that there is no difference in fat oxidation rates due to time of day, unlike men who burn more fat in the morning.

Caffeine can also increase fat oxidation. This study found that at 30 and 60% VO2 max, 3mg/kg body weight of caffeine significantly increased fat oxidation rates in trained females during a graded exercise test, and for an ultra endurance run, most of us are going to be in these low intensity ranges for a lot of the time.

We also know training improves fat metabolism. Merely getting out and doing the work is going to enhance your ability to burn fat as a fuel source, regardless of diet. This is in trained and untrained people. But, as Jeff Rothschild’s research suggests, your daily diet matters too. Including more fat in your diet (and lowering carbohydrate) is going to enhance your ability to use fat as a substrate while exercising. This was found to be the biggest driver of fat metabolism, much more than carbohydrate ingested during activity. This has been found by other research groups also. Rothschild also found that, while carbohydrate pre-exercise mattered for fat oxidation for short duration activity, the longer the session the less influence it had on the ability to burn fat as a fuel source.

As a side note (because Kona is upon us and with it comes a massive increase in temperature for a lot of people) heat stress ramps up carbohydrate utilisation, when conducting training that would otherwise be under the same conditions (HR and power targets). The answer could be to increase carbohydrate intake during training, but the aforementioned gut distress doesn’t bode well for this strategy. So, another good reason to get better at using fat as a fuel source. Save those carbohydrate stores in the heat.

One final (and possibly the most important) reason to care about your fat metabolism. Living a healthy life. I interviewed Dr Philip Prins about his research on metabolic health and athletes on Mikkipedia, about his research that compared a high carbohydrate diet to a low carbohydrate diet in middle aged athletes (which, I’m sorry to say, could well be you – it’s me at least, at 46 years old). A shift from to a higher carbohydrate approach (which actually wasn’t that high) caused blood sugar to increase to prediabetic ranges in 30% of participants. Worth considering if you are ramping up carbohydrates because ‘training’. There are other things you can do.

Note, this isn’t suggesting that you don’t need carbs AT ALL. I know there are people who swing the pendulum too far in the opposite direction. Carbohydrate fuel is absolutely key to your performance, however your best bet is the ‘carb appropriate’ approach advocated by Cliff Harvey, or Dan Plew’s ‘fuel for the work required’ (both of which are approaches I advocate.

Let’s get you fat adapted, so you rely LESS on carbohydrate, more on fuelling for the work required. Optimise your ability to burn fat as a fuel source. Lean up a little (if need be) or avoid unwanted fat gain if you’re already lean. AND (as discussed earlier on) avoid the gut problems that occur when you have to rely on carbohydrate as a fuel source but your gut shuts down under conditions of intensity.

Quick and dirty guide to getting fat adapted 

  • Start on your off season, where sessions are low intensity, not high stakes, so the only thing that suffers as you lower your carbohydrate intake (and up your protein and fat) is your ego. It will recover.
  • Train. A lot.
  • Follow Dan Plews EndureIQ course
  • Consume 3mg caffeine per kg body weight pre training. This is a lot of caffeine. Might be a No Doz pill actually
  • Do your easy training sessions fasted from glucose. This means fasted in general (as in, no food or calories) or with no carbohydrate on board prior (so you’re legitimately low carbohydrate). Have your carbs post session in your next meal
  • Do a hard training session in the afternoon. Have a low carbohydrate meal for dinner. Go to bed in that low carbohydrate state and then do an easy session in the morning. This ‘sleep low’ strategy ramps up your ability to burn fat as a fuel source.
  • Touch base with me. I'll sort you out with my athlete plan which has these strategies (carb appropriate) embedded so you can be sure you're doing it 'right'
  • Book a call with me here

PC: Canadian Running Magazine

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