D3 is the most active form of vitamin D.This is the form that we take in supplements.
In our body precursors of vitamin D (7-dehydrocholesterol) is converted to vitamin D3 in part with the help of sunlight on the skin.
vitamin D3 (what we take as a supplement) is converted finally to its final and most active form within the body to 1,25-dihyrodroxycholecalciferol.
With you having osteopenia it would be recommended for you to take 1000 to 1200mg per day of calcium. You may take this as calcium carbonate or as calcium citrate. The latter would be somewhat better absorbed. you don't need to make it chewable this is your choice.
The vitamin D you should have at least 400 IU per day. This is often found in combination with calcium because of the frequent need for this combination. You may take 400IU up to 800IU/day and this would help.
As for the kidney stones the calcium by itself is not usually the problem. Drinking PLENY of fluids helps. the most common type of stone is calcium oxalate and reducing this from diet may help. Tea is a strong source of oxalate in the diet.
Some people excrete a high amount of calcium in their kidneys. If a 24 hour urine showed that this was comnnected to your kidney stone problem then HCTZ a mild diuretic helps to decrease the urinary excretion of calcium and thus stone formation is reduced.
I hope this information helps. Further questions are welcome if you have others.
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Thank you and Best Regards,
Anthony Bray MD