It's always been my understanding that vit C is helps in the absorption of iron. (e.g. tomato sauce (vitC) helps absorb the in-home iron in pasta. Vit D Vit k are helpful with calcium absorption along with some minerals . Sun is great but if you have sunscreen on not sure how much is being absorbed. Also if you live in northern climes, no way to get enough d from the sun. Fatty fish like salmon the yolks in eggs and cooked mushrooms are good sources of Vit d. But hard to get enough from food.