Antioxidants
I have had to take a diuretic for the past ten years or so to control hereditary hypertension. About four years ago, I found that with a pack on, I couldn't go uphill at all. My legs would burn with lactic acid, even though I had been working out at a reasonable level. I got a battery of medical tests that ruled out heart, lungs, and neuromuscular problems. My doctor was stumped.
I found a study (on a college site) that seemed to link diuretics with increased lactic acid, and another paper that linked lactic acid and free radicals. As a result, I tried a pill that contains medium levels of Vitamins A, C, and E, and also Selenium. That seems to have solved the problem with the lactic acid when I ascend with a pack.
I don't know whether this would have the affect of raising lactic "tolerance" in someone who is not affected by medication, but I've also seemed to have had other benefits, like increased resistance to colds, and reduced symptoms when I have caught a cold, over the couple of years that I've been taking this.