When fuels are metabolized completely to carbon dioxide and water, the resulting carbonic acid formed can be disposed of mostly by the lungs but also by the kidneys with minimal disturbance to the three critical equilibria, the water activity, the acid base balance, and the ion balancing chemistries. Not so with the incomplete combustion of sugars, which produces excess lactic acid. The lungs do not dispose of this. The burden falls on the liver and kidneys to turn the lactic acid back into glucose at a waste of 6 ATP per glucose formed. The burden also falls on the nephrons of the kidneys and they cannot excrete excess lactic acid without perturbing the other molecules in the three key equilibria, the water activity, the acid base balance and the ionic balance. More hurt, more pain will result from the long term execution of this flawed metabolism.
I know there is the background of lactic acid production by red blood cells, by gut bacteria digesting sugars released from fiber via enterohepatic circulation, by feeder cells, by consumption of lactic acid fermented foods, by alcoholic binges (as the liver restores its NAD+ pools), and by overworked muscles.
But there is above this background, lactic acid produced by sugar fermentation in underworked muscles with poor mitochondrial respiration and/or in growing tumors. This does not need to occur if we always (1) fast at least 18 hours and (2) always exercise before we feast, and thus create a real demand for sugar and fatty acids and if we (3) never eat unnatural man-made foods that have sugar and fat added to them.