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  • Writer's pictureSheri Colberg, PhD

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

Who knew I’d be talking about carbohydrate intake and being active for the third time this year, but here we go again! 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. 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 in active individuals without diabetes. However, some facts are irrefutable, and these can impact the decisions that you make about your dietary plan with diabetes:

  • During harder exercise, your active muscles rely almost exclusively on carbohydrate as a fuel. Carbs are converted into energy (ATP) more quickly and with less oxygen required than fat. Carbs acts 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 hard 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 fully adapt.

  • 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 unrestricted carbs when competing (i.e., train low-carb and compete 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 do adapt to lower carbs during training, your body will not necessarily use fewer carbs when active. 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 largely unchanged by low-carb training, however.

  • No matter how you eat and train, for intense events sprinting and power lifting, a chronic low-carb intake may be detrimental to your performance if your muscle glycogen stores are low. On the other hand, for endurance activities, you can usually at least maintain how well you perform after adapting to low-carb eating, but 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. People with diabetes just have to make sure that they are keeping their blood glucose levels in check during any high-carb intake.

If attempting to carb load for even a day (which is usually long enough if you rest or taper) and you 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 are eating 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 in during exercise, which most people do during longer 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 a nighttime low blood glucose, particularly if you use insulin (3). 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. Supplemental carbs are mainly for longer events (90 minutes plus) and for 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 activities. 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 have negative effects on performance.

Primary reference: Colberg SR, Nutrition and exercise performance in adults with type 1 diabetes. Canadian Journal of Diabetes, 44(8):750-758, 2020 (


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.

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