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Most Calcium supplements also contains some vitamin D, it is because vitamin D makes you absorb more Calcium to your body unless you are having a condition like mine with my hyperparathyroidism, the supposed cure for it is some Cinacalcet just to control it and hopefully maybe cure it. It is an expensive medicine for my rare condition.
But soon I will be taking an active form of vitamin D as soon as I received my Ionized calcium and phosphorus test. It had been a year of medication and I have to take some action just to absorb more calcium to aid my body of repairing my bones and halting the progression of my Leontiasis condition.
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Now, if you also want to absorb more iron from your foods, try to take some vitamin C with it. So if you are eating meat or organ meats it is better to take them with vitamin C as well as green leafy vegetables and it will make your blood carry more oxygen for your body's cells and thus giving you energy, better sleep, better appetite, and a better sense of well-being.
On the other hand the tannin compounds that are found in teas, coffee, and cocoa can hinder the absorption of iron so better not to take some teas, coffee, and cocoa if you are taking iron supplements or after eating meats and leafy vegetables so that you can absorb more iron from your food. So it is better to limit the consumption of such drinks otherwise you will get somewhat anemic.
The fat soluble vitamins namely A,D,E, and K can be absorbed more easily by your body if you will combine them with dietary fats.