Home / Therapies / Diet & Nutrition / Here We Go Again: The Low-Carb vs. High-Carb and Training Debate

Here We Go Again: The Low-Carb vs. High-Carb and Training Debate

Aug 3, 2021

Author: Sheri R. Colberg, PhD, FACSM

Who knew I’d be talking about carbohydrate intake and being active for the third time this year, but here we go again! Unfortunately, a lot of confusion still exists related to the practice known as “carb loading” as well.  Do you need to do it? Should you? How do you know?

This controversy keeps coming up because of all the low-carb diet followers out there, especially many people with diabetes. For example, at the American Diabetes Association Scientific Sessions this year (held virtually in June 2021), the MOST popular session of the entire conference was the one I set up to debate low-carb and high-carb eating and athletics (discussed in Diabetes In Control in July) with a virtual meeting record of 3,300 views!


As I commented recently, this debate is still ongoing inactive individuals without diabetes. However, some facts are irrefutable, and these can impact the decisions that you make about your dietary plan with diabetes:

  • During more strenuous exercise, your active muscles rely almost exclusively on carbohydrates as fuel. Carbs are converted into energy (ATP) more quickly and with less oxygen required than fat. Carbs act like high-octane gas, while other types give you less energy for the same amount of fuel (and supply it more slowly). You must use carbs to do strenuous exercise.
  • Your body can adapt to a lower carb intake and increase fat use during exercise, at least to a limited extent (see comment above). However, adapting takes weeks, and your performance can be negatively impacted if you go low-carb without time to adapt thoroughly.
  • While you are in the process of adapting, your training may suffer, and you may feel bad during workouts. This may be why many athletes adopt a strategy of periodically doing endurance training with less carb intake but take in free carbs when competing (i.e., train low-carb and compete for high-carb). Following this strategy may help you adapt faster compared to having a low- or a high-carb intake all the time (1).
  • Even if you adapt to lower carbs during training, your body will not necessarily use fewer carbs when active. Instead, it may just shift downward the intensity at which you cross over from less carb to more fat use (2). This shift towards greater fat use occurs naturally whenever your muscles start to run out of stored glycogen; the rate of glycogen use appears essentially unchanged by low-carb training, however.
  • No matter how you eat and train, for extreme events sprinting and powerlifting, a chronic low-carb intake may be detrimental to your performance if your muscle glycogen stores are low. On the other hand, you can usually maintain how well you perform endurance activities after adapting to low-carb eating. Still, chances are your performance may not be better (see first comment above).

When it comes down to it, instead of carb-loading, simply varying your carb intake may be as beneficial to performance. For instance, you may want to endurance train with a lower carb intake to increase your ability to oxidize fat, but take in more carbs leading up to any events and during events to maximize your storage. Of course, people with diabetes have to make sure that they keep their blood glucose levels in check during any high-carb intake.

If you attempt to carb load for even a day (which is usually long enough if you rest or taper) and take insulin, cover the carbs with enough rapid-acting insulin to keep your blood glucose as near normal as possible to maximize muscle glycogen storage. (Glucose cannot get into muscles cells during rest without insulin.) Most people take in plenty of carbs if they eat enough overall and even as low as 40 percent of calories from carbs. Effective carb-loading does not require you to eat a pasta dinner or massive amounts of starchy foods.

If you take any carbs during exercise, which most people do during more extended events and training even without diabetes, you’ll need very little insulin coverage (if any). If you limit your carb intake afterward, though, you may increase your chances of getting nighttime low blood glucose, mainly if you use insulin (3). On the other hand, if you do not use insulin, you are unlikely to need any extra carbs during most shorter activities, regardless of how easy or hard you work out. Again, extra carbs are mainly for more extended events (90 minutes plus) and insulin users.

That said, anyone on a low-carb diet may benefit from supplementing with carbs as needed during endurance activities or training (1). Aim for a maximum of 75 grams per hour during prolonged or multi-day workouts. For example, during intermittent sports like hockey or soccer, 30 to 60 grams of carbs per hour may prevent fatigue or lows near the end of a game. Keep monitoring your blood glucose, though, as going too high or low can negatively affect performance.


Primary reference: Colberg SR, Nutrition and exercise performance in adults with type 1 diabetes. Canadian Journal of Diabetes, 44(8):750-758, 2020 (https://doi.org/10.1016/j.jcjd.2020.05.014)



  1. Impey SG, Hearris MA, Hammond KM, Bartlett JD, Louis J, Close GL, et al. Fuel for the Work Required: A Theoretical Framework for Carbohydrate Periodization and the Glycogen Threshold Hypothesis. Sports Medicine. 2018;48(5):1031-48. doi: 10.1007/s40279-018-0867-7. PubMed PMID: 29453741.
  2. Chang CK, Borer K, Lin PJ. Low-Carbohydrate-High-Fat Diet: Can it Help Exercise Performance? J Hum Kinet. 2017;56:81-92.(doi):10.1515/hukin-2017-0025.
  3. Scott SN, Anderson L, Morton JP, Wagenmakers AJM, Riddell MC. Carbohydrate Restriction in Type 1 Diabetes: A Realistic Therapy for Improved Glycaemic Control and Athletic Performance? Nutrients. 2019;11(5):1022. doi: 10.3390/nu11051022. PubMed PMID: 31067747.


Sheri R. Colberg, PhD, is the author of The Athlete’s Guide to Diabetes: Expert Advice for 165 Sports and Activities (the newest edition of Diabetic Athlete’s Handbook). She is also the author of Diabetes & Keeping Fit for Dummies, co-published by Wiley and the ADA. A professor emerita of exercise science from Old Dominion University and an internationally recognized diabetes motion expert, she is the author of 12 books, 30 book chapters, and over 420 articles. She was honored with the 2016 American Diabetes Association Outstanding Educator in Diabetes Award. Contact her via her websites (SheriColberg.com and DiabetesMotion.com).