Is everything we thought we knew about fueling with carbs wrong?
The short answer...no
This last week, a new “study” came out claiming that everything we knew about fueling with carbs as endurance athletes was wrong. I put study in quotation marks because really, it was just a narrative review, which isn’t really a study. It’s just the authors’ opinions on a topic after looking at a bunch of other actual studies. Here’s the link to their paper.
Their primary claims:
Fatigue comes because our blood glucose levels drop and the brain regulates our outputs, not because muscle glycogen levels become depleted.
High carb intake during training and racing is unnecessary. 10-30g/hr is enough.
High carb fueling is damaging for long term health, leading to diabetes in athletes
Carb loading is damaging and unnecessary
A bunch of my current and former athletes messaged me wanting to get my opinion on the piece, so here goes. I think I’ll make 3 main points in my analysis:
Conflicts of interest and study quality: These authors are authors of low-carb, ketogenic diet books. They are also associated with companies promoting and selling ketones. I would like to believe that even so, they are unbiased in their analysis, but I don’t think that’s the case. IMO anyone who is pushing a ketogenic, high fat, low carb diet for the general population or for athletes is going against a massive amount of solid research. So we should probably take their “research” with a grain of salt when it comes to fueling for sport. The study is a narrative review, meaning it is pretty loose-goosy as far as research rigor goes. It is not a controlled trial or meta analysis. It falls much lower on the hierarchy of evidence that we should be paying attention to.
High carb fueling is beneficial for performance: This has been shown over and over (and over) again in sports nutrition research. There is a direct dose-response to more carbs (up to 120g/hr) improving performance in endurance exercise. Of course factors like your own physiology and adaptations, the type of training and racing, and race day conditions all matter, but in general, the more carbs you are able to tolerate per hour, the better outcomes, especially at higher intensities. At high intensities your performance WILL SUFFER if you aren’t taking in enough carbs. The oxygen cost of metabolizing fat is just too high. Carb loading is also hugely important and has been shown to improve performance for endurance athletes. I usually recommend 8-12g carbs/kg/day for 1-2 days before a big event. It doesn’t take any really close analysis to realize that the main thing that has shifted in cycling and running over the last several years is increased carb intake, and that performance has improved as well. Likely not just a coincidence.
High carb fueling has not been shown to lead to poor long-term health outcomes (for athletes): Athletes wonder all the time if all this processed sugar is bad for them. For people who are training frequently, the answer is no. In fact, as dietitians, we see athletes have way more health problems from not eating enough processed carbs for the amount of training they are doing. The body hates storing carbs as fat. It is not energy efficient. If you are consuming anywhere from 3-10g carbs/kg/day as someone who trains regularly, and has normal metabolism at baseline, your body will either use up the carbs during your workout, or will store them as glycogen for your next workout. Sure, resting levels of glucose can be elevated in endurance athletes, but this doesn’t actually turn out to increase risk of chronic illness for them (diabetes, heart disease, etc.). In fact, a couple months ago, I attended a sports nutrition conference where Nicola Guess RD MPH PhD spoke about blood glucose levels in athletes, and why we don’t really need to worry about it. She has a great substack too.
This isn’t a super in-depth analysis, but I wanted to get something out there before everyone looses their minds. So yes, please please please keep fueling with carbs. One narrative review doesn’t change my opinion, and it shouldn’t change yours. I have worked with hundreds of athletes over the years, and you want know the #1 thing that improves performance? (I’m not just saying this)….its MORE CARBS!
Keep trying to hit at least 5g/kg/day in your day to day nutrition, carb loading with at least 8g/kg before big events, and aim for at least 60g/hr during training and racing (and probably closer to 90 or 120g/hr for a lot of you). These are all concepts that I cover in my new online self-paced course, The Aid Station. You can check it out here.
Or if you don’t believe me, go out and do all your long runs and races with 10-30g/hr and see how you feel. Eat a high fat, low carb diet. I hope my athletes race against you.
Tim Noakes as the lead author was enough to let me know that this was going to be biased/impartial against carbs. It should be noted that this “review” was definitively a narrative review, and not a meta-analysis or systematic review, so the inclusion criteria for the review are simply not there. The disclosures on this publication too are a good secondary signal as to the impartiality of the authors.
Appreciate the calm, evidence-based pushback here. It’s refreshing to see performance nutrition grounded in actual trial data and real-world athlete outcomes, not just contrarian takes dressed up as “new science.”