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Comment by funnybeam | Hacker News Reader

Sorry, I was unclear.

High intensity exercise is generally anaerobic yes, not a third type of exercise.

I was really querying your point about high intensity exercise using lactic acid as an energy source. My understanding is that during any exercise, the muscles will first use creatine phosphate as an energy source and will then use blood glucose and stored glycogen for energy, and it is the breakdown of glucose that creates the energy - lactic acid is a by product of this reaction and is not a source of energy so I don't think this would cause blood sugar levels to increase?

Any increase in blood sugar levels following short duration exercise would be due to the body reacting to the fall in glucose levels and so releasing more from the liver to compensate.

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Blood glucose would rise because youre unable to provide enough oxygen to break it down/convert it into energy (ie more unused glucose accumulates in the blood).

Lactate/Lactic acid is a byproduct of broken down glucose, but also temporarily picks up the slack in energy when the body can’t supply enough oxygen to breakdown glucose for energy requirements. Lactic acid is used in the mitochondria of cells and keeps the heart pumping during these intense phases of exercise where the oxygen/glucose can’t do the job...again as lactic acid is temporarily being used as fuel it would follow glucose builds up as the body can’t break it down fast enough and is now using an alternate fuel source.

Creatine as I understand it isn’t a fuel/energy directly, but does facilitate recycling of energy. Moreover, as ATP (energy) is produced in the mitochondria and used by the cell it is converted into ADP, but the body/muscles will use creatine to recycle ADP back into ATP. On its own though I don’t think creatine is fuel/converted into ATP at any point like glucose/ketones.

Creatine Phosphate is a fuel - it is converted very quickly and the muscles only store tiny amounts (enough for a few seconds only), and creatine is used in recycling ATP as you say.

I didn't realise lactic acid was used to provide energy so thanks for mentioning this - having looked it up though, this produces tiny amounts during anaerobic respiration at which time glucose is still the main energy provider.

Blood glucose may rise because your body has released stored glucose from the liver, but this takes time - there is a delay between blood sugar levels dropping (due to the intense exercise) and the liver releasing stores and it also takes time for the full aerobic cycle to produce significant quantities of energy and convert that glucose

I am most definitely not an expert here so details may be incorrect, but I looked into this when I started running. I was focused on how to maintain energy levels over long distances so may have a different perspective from someone more interested in weight training or other high intensity/short-duration exercise.

I found [1] useful as a simple overview of the different energy producing mechanisms, and [2] goes into far more detail than I can readily understand...

[1] https://www.acefitness.org/fitness-certifications/ace-answer...

[2] https://en.wikipedia.org/wiki/Bioenergetic_systems

I would love to see better explanations of how this all works

I’m a runner also...depends what you mean by long distances, but I’d promote becoming fat adapted and running in ketosis.

I’ve done a full marathon with no fuel (hydration only) in ketosis, pretty awesome and much better than my glucose/gu runs...of course I’ve had some amazing long runs out of ketosis but at certain distances the fueling is a real battle and a battle I’ve lost from time to time. I don’t think there is a good answer for fueling long distances that will work for everyone it’s all about personal experimentation and adaptation.

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