Is a lack of B12 causing my breathing issues during sports?

It absolutely can, because think about what B12 does to your red blood cells. You need if you ever heard of pernicious anemia that's a type of anemia. You could have anemia from B12, folic acid, iron. That's probably why you ran out of gas. We need that B12. B12 comes from animal products. However, even if you're consuming animal products and you don't have enough hydrochloric acid to extract that vitamin from your meat protein, you can have a problem with that. So there's many different reasons why you can't absorb it, but definitely B12 could make you feel out of wind, especially if you're climbing up any incline.