Should Endurance Athletes Switch to a Low Carbohydrate Diet?

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High carbohydrate pasta with tomatoes
Pasta – a typical meal for endurance athletes.

Should endurance athletes trade in their high carbohydrate gels, gummies, and pasta for fatty steak and butter?  A recent study found elite ultra-marathoners and iron distance triathletes on a low carbohydrate diet  burned significantly more fat while running than  their counterparts on a typical higher carbohydrate diet. There was no difference in the level of glycogen depletion between groups after a 3-hour run.

Why Carbohydrates Matter

For several decades endurance athletes have relied on a carbohydrate rich diet to fuel their training and performance. Carbohydrates are the primary source of energy used during activity. They’re also a fast fuel – your body can use gels, gummies and sports drinks very quickly while also accessing the carbohydrates stored in your muscle when your energy needs outpace how quickly you can squirt more gel in your mouth. Regular intake of carbohydrates during  prolonged activity provides an important source of energy for working muscles and helps spare dipping into your reserves in muscle tissue (in the form of glycogen). Once glycogen levels start getting too low, your performance will subsequently decline.

If carbohydrates are important for performance why would anyone go on a low carbohydrate diet?

The longer you run, bike, swim or exercise in general, the more carbohydrates you need to keep up with energy demands. There are three main reasons athletes (particularly ultra endurance athletes) want an approach that doesn’t require carbohydrate during long bouts of exercise are:

  1. Your taste buds get tired –  Eat any food over and over again and you will get sick of it eventually. Now imagine running 30, 50 or 100 miles and eating a gel every 30 minutes. The consistency, sweetness and flavors will make your taste buds revolt.
  2. Your stomach might get upset. Exercise + eating (even seemingly easy to digest carbohydrate products) can cause stomach upset in some people.
  3. You are trying to lose body fat. If you are exercising for long periods of time it may sound counterintuitive to consume 30-60 grams of carbohydrate (or 90+ depending on the type of carbohydrate, your stomach’s tolerance and the type of exercise you’re doing) each hour while training.

If any of these apply to you, a diet that doesn’t force your body to rely on carbohydrates for energy may sound very appealing.

The Study & the Low Carb Diets for Endurance Athletes

The body has amazing ability to adapt to changes in the macronutrient composition of your diet.  In other words, if you eat more fat you’ll burn more fat. If you are adapted to a low carbohydrate diet, you will rely on your body fat for fuel and will not need to consume gels, gummies or any other carbohydrates while running, biking or swimming. However, there is an adaptation period.  It takes time for your body to switch over from relying on carbohydrate to fuel activity to using primarily fat. The study subjects included elite male ultra-endurance athletes who habitually consumed a high carbohydrate diet (> 55% of calories from carbohydrate) and a separate group of those habitually consuming  a low carbohydrate diet (< 20% of calories from carbohydrate and > 60% from fat though the average was 70% from fat) for at least 9 months. Both groups slept, reported to the lab fasted and then drank a 343 calorie shake (the shake contained 4.3 grams of carbohydrate for the low carbohydrate group and  42.7 g of carbohydrate for the high carbohydrate group). Ninety minutes later they ran on a treadmill.

Results

As expected, the low carbohydrate high-fat diet group used a lot more fat when jogging then the high carbohydrate group (88% of calories from fat vs. 56% in the high carbohydrate group). They also used more fat at a higher intensity than the high carbohydrate diet group. They were able to use fat at a good rate – fat is typically a slow source of energy but the rate of fat use in this fat-adapted group was pretty compatible to the typical rate (but not the maximum) at which an athlete can use carbohydrates. Glycogen levels at rest, glycogen breakdown during exercise and re-synthesis after exercise was the same in both groups. * There was no difference in the amount of calories burned between the two groups.

Is This Diet Right for You?

Ultra endurance athletes can adapt to and train on a higher fat diet.  They can also do this without glycogen depletion – glycogen depletion can come with other negative consequences including potential suppression of immune system functioning.  At this time, we do not know if regularly following a lower carbohydrate diet = better endurance performance.

What you need to consider:

  • According to this study you will not burn more calories during exercise when on a low carbohydrate, high fat diet. ** See note below.
  • Your body needs at least 1 month to adjust. The first week will probably suck (you’ll feel terrible and have low energy).
  • You might not improve performance (we don’t know).
  • Can you stay on a low carbohydrate, high fat diet? Do milkshakes made of  heavy cream, olive oil, walnut oil and whey protein sound yummy? Is this diet practical for your lifestyle? If you answer yes to those 2 questions,  then it might be worth a shot. Work with a nutrition expert to ensure you are getting all of the fiber, vitamins, and minerals you need for performance and health.

* Keep in mind the results from this study are specific to endurance athletes.

** If weight loss is your goal, it makes no difference if you burn more fat during exercise if you aren’t burning more total calories in that exercise session. The only caveat here is if a low carb diet means you consume few to no calories during exercise. In this scenario, a low-carb diet may help you consume fewer total daily calories.

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