20. 120 VS 90 g/h OF CARBS TO SPARE MUSCLE GLYCOGEN?

Photo by Tuvalum on Unsplash

We have three main energy sources during exercise:

1. Endogenous carbs (prevalently muscle glycogen)

2. Fats

3. Exogenous carbs (gels, maltodextrin, bars or whatever you eat during your ride)

ENDOGENOUS CARBS (MUSCLE GLYCOGEN)

Maintaining muscle glycogen (carbs) level is crucial to optimize durability (fatigue resistance during prolonged rides). For physiology geeks, this because low glycogen levels disrupt the ionic mechanisms that permits muscle contraction: calcium release, sodium and potassium regulation. (1) Unfortunately, muscle glycogen stores are a very limited energy resource: about 2500 kcal. (2) So, using as less as possible this energy source is crucial to optimize durability. Here the importance of fats and exogenous carbs.

FATS

On the other hand, fats stored in the human body are a virtually unljmited energy source. Even a 70kg man with 10% of fat mass (lean individual) have ~68000 kcal stored as fats. (2) So, ideally we would like to use this energy source as much as possible. While everyone can aim to increase peak fat oxidation, fatmax and generally fat oxidation rates during exercise, when exercise intensity increases, fat oxidation rates decrease while carbs consumption exponentially increase. Here an example from a lab test:

CHO, carbohydrates; PFO, Peak Fat Oxidation; VT1 and VT2, first and second ventilatory thresholds. g/min is grams per minute; W is watts.

Even if there can be variations in how much and when fat oxidation rates decrease, the huge increase in carbs consumption when exercise intensity increases is observed universally in all individuals (with no genetic variations). Furthermore a relevant amount of carbs is used even at low intensity (zone 2). So, we still need another way to spare muscle glycogen. Here enter the importance of exogenous carbs.

EXOGENOUS CARBS (INGESTED DURING EXERCISE)

The main rational behind exogenous carbs ingestion during exercise is to substitute muscle glycogen use delaying fatigue and improving durability. While it has been noted already a century ago that ingesting carbs during prolonged exercise improves performance (3), in the last years the focus shifted on the optimal amount of carbs to ingest during exercise. The main limiting factor seems to be how much carbs we can ingest and absorb while pedalling. While initially it was thought only 60 g/h of exogenous carbs can be absorbed and used, then it has been shown that even ~90 g/h can be used when combining glucose/maltodextrin with fructose, as fructose uses another gut transporter than glucose/maltodextrin to enter into the blood and the glucose transporter is saturated at around 60 g/h.

Then, in the last 3-5 years we saw many pro cyclists ingesting higher and higher carbohydrates amounts arriving up to 120 and even 140 (!) g/h during the longest and most demanding races. More is better?

Does 120 vs 90 g/h of carbs intake during exercise make spare muscle glycogen (endogenous carbohydrates)?

A study by Tim Podlogar (nutritionist of Team Red Bull-Bora-Hansgroe) and Colleagues published on European Journal of Applied Physiology in 2022 tried to answer this question. (4)