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Workout Performance vs. Energy Storage | Glycogen Depletion During Exercise (Carb Depletion)…

Phosphorylated glycogen phosphorylase is active and catalyzes breakdown of glycogen to glucose 1-phosphate.
Breakdown of glycogen involves:
1) Release of glucose-1-phosphate (G1P), 2) rearranging the remaining glycogen (as necessary) to permit continued breakdown, and 3) conversion of G1P to G6P for further metabolism.

Remember that G6P can be 1) broken down in glycolysis, 2) converted to glucose by gluconeogenesis, and 3) oxidized (in the pentose phosphate pathway)

*Skeletal muscles mainly express beta2-adrenergic receptors and adrenaline, rather than noradrenaline, stimulates glycogen breakdown*

Glucose is part of this cycle in that it is broken down into pyruvate to feed into the Kreb’s cycle.

IIRC, one molecule of glucose can provide somewhere around 24 net molecules of ATP – it is the glycogen that is broken apart to provide the glucose.

Once your muscle stores of glycogen start being depleted, your body converts blood sugar (glucose) into glycogen in the muscle being depleted.

Your muscles will still get glycogen, just not from other muscles (however, your body is not very good at rapidly converting glucose to glycogen)

It’s been shown in studies, like one published in the journal Frontiers in Physiology, that cross-country skiing mainly depletes glycogen stores in arms, compared to that of the legs.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3248697/

Interestingly, the link between glycogen depletion and impaired muscle function during fatigue is not well understood and a direct cause-and-effect relationship between glycogen and muscle function remains to be established.

Energy consumption at rest is low; oxygen uptake at rest is typically ∼0.25 L O2 and carb oxidation is ∼0.1 g min−1, and the rate of carb oxidation gradually decreases during fasting*

However, humans do not show major decrease in muscle glycogen content during fasting – in contrast, the liver glycogen content decreases rapidly during fasting and the liver glycogen content has decreased by ∼65% after 24 h fasting.

So, why is the majority of glycogen stored in muscles?

It’s believed that the main function of skeletal muscle glycogen, from an evolutionary point of view, is to serve as an energy store in “fight or flight” situations.

In the heart and the brain, glycogen is also the energy substrate that can generate anaerobic energy during short-term oxygen deficiency contributing to survival.

References:
1) The Role of Skeletal Muscle Glycogen Breakdown for Regulation of Insulin Sensitivity by Exercise. (n.d.). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3248697/

2) Ørtenblad N , et al. (n.d.). Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle. – PubMed – NCBI. Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/21135051/

3) Glycogen Biosynthesis; Glycogen Breakdown. (n.d.). Retrieved from https://oregonstate.edu/instruct/bb450/summer09/lecture/glycogennotes.html

4) Muscle glycogen stores and fatigue. (15, September). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3784189/

5) Glycogen Metabolism – Biochemistry – NCBI Bookshelf. (n.d.). Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK21190/

6) Role of glycogen availability in sarcoplasmic reticulum Ca2+ kinetics in human skeletal muscle. (1, February). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3055553/

7) Human skeletal muscle glycogen utilization in exhaustive exercise: role of subcellular localization and fibre type. (1). Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3112561/
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Endurance athletes and those who do a lot of cardio (eg. runners, cyclists, swimmers) have different nutritional needs compared to those who are trying to achieve strength and muscle hypertrophy.

Timestamps
00:00 Intro
00:31 What happens to a muscle during exercise
03:22 Glycogen and fat as ‘protein sparers’
03:42 Comparing a mixed, high carbohydrate and high fat diet for endurance training
05:06 Recovery after an endurance event
05:27 Role of protein during recovery
06:14 Take home points

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Endurance exercise burns through a lot more calories (per unit of time) than resistance training, and the fuel needed to sustain cardio is predominantly glycogen utilised by the aerobic system. This is in contrast with resistance training which relies on mainly the Creatine Phosphate and the Glycogen-Lactic Acid system for short bursts of muscle power.

The body preferentially uses glycogen and fat for energy. While protein can be broken down to make glucose (a process called gluconeogenesis), this process is not as efficient as using glycogen or fat to produce ATP. That’s why glycogen and fat are referred to as ‘protein sparers’.

Athletes on a high carb diet start off with much higer levels of muscle glycogen prior to exercise. This higher levels of glycogen is associated with longer time to exhaustion as compared to a mixed or high fat diet. So for endurance, a high carb diet easily outperforms a mixed or high fat diet.

A high carb diet leads to faster recovery of muscle glycogen compared to a high fat or mixed diet

Studies have shown that a high protein diet does not improve performance, but can improve blood markers of muscle damage and subjective muscle soreness.

Take home points:

1. Endurance athletes should focus on a high carb diet, with one study recommending 6-10g/kg/day of carbs.
2. It takes 48 hours to build up muscle glycogen stores even on a high carb diet. So it is better to avoid strenuous exercise for at least 2 days prior to a endurance event.
3. Protein may assist with recovery. One study recommends an intake of 0.25 g/kg of protein per hours of endurance exercise.
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 #cardio #fitness #docunlock

Full transcript: https://www.docunlock.org/youtube/protein-vs-carbs-for-endurance-and-cardio

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Key References

Guyton and Hall Textbook of Medical Physiology 13ed (2015), chapter 85 (p1085-89)
International Society of Sports Nutrition Position Stand 2017: https://jissn.biomedcentral.com/articles/10.1186/s12970-017-0177-8
Macdermid et al. 2006: https://www.ncbi.nlm.nih.gov/pubmed/16676704?dopt=Abstract
Burke et al. 2011: https://pubmed.ncbi.nlm.nih.gov/21660838-carbohydrates-for-training-and-competition/
Saunders et al. 2007: https://pubmed.ncbi.nlm.nih.gov/17685703-consumption-of-an-oral-carbohydrate-protein-gel-improves-cycling-endurance-and-prevents-postexercise-muscle-damage/
Saunders et al. 2004: https://pubmed.ncbi.nlm.nih.gov/15235331-effects-of-a-carbohydrate-protein-beverage-on-cycling-endurance-and-muscle-damage/

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