lactic acid
"When you are exercising anaerobically, your muscles don't have enough oxygen to fuel them. That is why you have lactic acid, lactic acid is like a substitute for oxygen. It is just another way to produce energy for your muscles. But this lactic acid is only used when your exercising anaerobically because it causes fatigue, and makes you tired faster."
"In simple terms, lactic acid fermentation is like one thing of food (a peice of bread, fruit, etc.), while respiration is a full meal. If were dying of hunger, you would probably not take the time to make the full meal, you'd just scarf down a peice of bread or something. Certain muscles may start "dying of hunger" at any point, especially if your not using efficient bio-mechanics. You probably wouldn't have every-muscle involved "dying of hunger" at the same time until the end of a long run, but periodically individual muscles, or even just groups of cells would get too "hungry" and switch to fermentation, until they're "full" enough to take the time to make a whole meal. You can actually do fermentation and repiration in the same cell at the same time, but lets not get into that."
"Consuming foods prepared by lactic acid fermentation is like taking a probiotic supplement, only better. All you need is a little with each meal. These cultured foods are both delicious and easy to learn how to make."
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