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Play Close Attention to Supplement Mix

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Q: I understand that the absorption of some minerals can be impaired when they are taken together. But what if you take a multivitamin-mineral supplement that contains all of them in the same pill?

A: There's no choice in this case, of course. But realize that mineral absorption is only partially impaired and that you also get these minerals from foods.

Having said that, here's an update on interactions among various vitamins and minerals:

Calcium: Calcium may decrease the absorption of iron, zinc and magnesium, though this effect may not be significant.

Dairy foods, due to their high phosphorus content, may reduce calcium absorption from other sources.

Since calcium is an individual supplement, it can be taken apart from other minerals and dairy foods.

Iron: The form of iron present in plant-based foods may decrease zinc absorption; however, the presence of proteins in a meal promotes zinc absorption and can counteract this interaction.

Zinc: Zinc may impair copper absorption, in some cases causing copper deficiency. This is the reason that some zinc supplements also contain copper.

Vitamin C: Vitamin C increases iron absorption from foods and supplements.

Vitamin C may decrease the absorption of dietary copper.

Vitamin E: Vitamin E can increase the absorption of vitamin A.

High doses of omega-6 fatty acids may increase vitamin E requirements.

Vitamin E in higher doses may antagonize the effects of vitamin K. Because vitamin K is necessary for normal blood clotting, this might increase the risk of abnormal bleeding in individuals taking oral anticoagulants (e.g., Coumadin).

Vitamin D: Vitamin D is necessary for the most efficient absorption of calcium.

Vitamin K: Coenzyme Q10, chemically similar to a form of vitamin K, as well as herbs that contain vitamin K (including alfalfa, cabbage, parsley, nettle and plantain), could enhance vitamin K-promoted blood clotting, thereby interfering with the blood-thinning effect of anticoagulants (e.g., Coumadin).

Niacin (vitamin B3): Niacin can be hard on the liver, especially at higher doses, so avoid combining it with herbs that could add to this effect, including chaparral, kava, valerian, borage and uva ursi.

Folic acid (folate): Regular use of folic acid can decrease zinc levels in the body. Zinc supplementation may be helpful.

Because folic acid can mask the signs of vitamin B12 deficiency, you should not take more than 400 mcg of supplemental folic acid unless your doctor has ruled out a B12 deficiency.


(Richard Harkness is a consultant pharmacist who writes on health care topics. You can write him at 1224 King Henry Drive, Ocean Springs, MS 39564. His e-mail address is Volume of mail prohibits individual replies; selected letters will be answered in his column.)


(c) 2003, The Sun Herald (Biloxi, Miss.). Distributed by Knight Ridder/Tribune News Service.

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