Vitamin D For Dummies
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Taking a vitamin D supplement correctly is easy. You simply need to have the right dose (usually in the form of a gel capsule), pop it into your mouth, and swallow it with a little water. That’s all there is to it. No advantage is gained by taking a vitamin D supplement several times daily over taking one capsule once a day.

But that vitamin D supplement is best absorbed when taken with food containing fat, and that’s the largest meal of the day for most people. Don’t take it on an empty stomach or in between meal times as you might with some medications.

A study at the Cleveland Clinic showed that if you take your vitamin D with the biggest meal each day, you can increase the level of vitamin D in the blood by an average of 50 percent.

Taking vitamin D once a day may be more reliable than taking seven times the dose once a week. You’ll probably remember it better on a daily basis. If you forget to take the pill one day, just take two the next day.

Drugs that interfere with vitamin D absorption

A number of drugs interfere with the absorption or metabolism of vitamin D. Among those that interfere with absorption are the following:

  • Antacids

    Check with your doctor before taking vitamin D supplements if you have digestive problems. The problem may be more serious than just a lack of vitamin D.

  • Barbiturates

  • Carbamazepine

  • Cholestyramine

  • Colestipol

  • Fosphenytoin

  • H2 blockers: Tagamet, Pepcid, Axid, Zantac

  • Heparin

  • Highly Active Antiretroviral Therapy, a combination of three drugs for AIDS

  • Isoniazid

  • Mineral oil or products containing mineral oil

  • Orlistat

  • Phenobarbital

  • Phenytoin

  • Rifampin

  • St. John’s wort

Steroids such as prednisone and cortisol don’t prevent absorption of vitamin D, but they do affect the metabolism of vitamin D so that less active vitamin D is formed.

Medical conditions that increase your need for vitamin D

Several medical conditions increase your need for vitamin D:

  • Alcoholism

  • Intestinal diseases such as Crohn’s, celiac, cystic fibrosis

  • Kidney disease leading to failure

  • Liver disease

  • Overactivity of the parathyroid glands

  • Pancreatic disease

  • Surgical removal of the stomach

  • Surgical removal of the end of the small bowel (terminal ileum)

Because active vitamin D is formed after vitamin D passes through the liver and the kidneys, diseases of those organs affect the formation of active vitamin D. In that case, it may be necessary to give a form of vitamin D that doesn’t require the liver or the kidneys to intervene, either 25-hydroxyvitamin D if the liver is compromised, and calcitriol (active vitamin D) or 1-alpha-hydroxycholecalciferol (“1-alpha”) if the kidneys aren’t functioning correctly.

However, these aren’t simple supplements — if you have liver or kidney problems, your doctor will prescribe these forms as part of your treatment regimen.

About This Article

This article is from the book:

About the book author:

Alan L. Rubin, MD has been a physician in private practice for more than 30 years. He is the author of several bestselling health titles, including Diabetes For Dummies, High Blood Pressure For Dummies, and Thyroid For Dummies.

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