Not entirely true. You should stick to the links I provide on my posts.

B12 is complexed with intrinsic factor in the stomach and absorbed in the intestine. Without intrinsic factor (which may be broken), it is passively absorbed mostly in the intestine but in smaller amounts, hence larger doses are needed. The sublingual is minimally absorbed, no matter what you read!If your B12 is good, and you don't take supplements, then you probably have intact intrinsic factor.

The kidney does not absorb vitamins. It excretes them when they are no longer needed into the urine. Same with minerals. The kidney has a double filter...the first one lets most things thru into the urine, the second loop reabsorbs the critical ones. The kidneys are our filtering system to remove drugs, toxins and other by products of metabolism. Your GI tract absorbs your nutrients from your food or supplements.

If I were you I'd google some general biology sites and read them so you will understand this better. Did you have biology in school? I think most high schools cover this subject these days.

You can be low in magnesium if you eat the wrong foods... it is estimated that 75% of Americans are low in magnesium just from diet.
Please read that link I gave you which explains that.