Drinking fortified, reduced-fat milk can be a healthy way to meet the United States Department of Agriculture (USDA) recommended daily intake for calcium, phosphorus, potassium and vitamins A and D. You probably don't want to take your multivitamin (or iron supplement) with a glass of milk, however.
Research has shown that calcium can inhibit the body's ability to absorb the iron in supplements and multivitamins, as described in the October 2010 issue of the International Journal for Vitamin and Nutrition Research. Orange juice would be a better beverage choice, because the vitamin C it contains improves iron absorption, according to the U.S. National Library of Medicine (NLM).
As a tutorial from Washington University in St. Louis notes, vitamins are substances that the body needs in small quantities so that specialized proteins, known as enzymes, can speed up the chemical reactions needed for the body to work properly. Taking vitamin supplements can help you avoid deficiencies — and there is currently no evidence that milk inhibits vitamin absorption.
But you should still be mindful of which vitamins you're taking and when. For example, getting 165 milligrams or more of calcium along with iron can decrease iron absorption by as much as 60 percent, according to an older but frequently cited January 1991 study in the American Journal of Clinical Nutrition. To avoid this interaction, take your multivitamin or iron supplement at least two hours before or after eating or drinking dairy products, per the NLM.
It sounds counterintuitive, but calcium absorption could also be affected. As the National Institutes of Health (NIH) explains, the more calcium you get in one sitting, the less your body absorbs. "Absorption from supplements is highest with doses of 500 milligrams or less," per the NIH. Combining a multivitamin that contains calcium with milk that also contains calcium could cause you to exceed the ideal calcium dosage.
Multis that contain D might pair better with milk. Vitamin D and calcium are one of "nutrition's dynamic duos," according to Harvard Health Publishing; vitamin D actually helps the small intestine absorb calcium.
The U.S. Food & Drug Administration doesn't review multivitamins or other dietary supplements for safety, so talk to your doctor before starting any new multivitamin or supplement. Check the label of your multivitamin to see how much of each vitamin is included, and avoid products with values well over your needs for the day. If you take too much of certain vitamins and minerals — such as vitamins A and D or calcium, which are all found in fortified milk — you could develop the symptoms of a vitamin toxicity.
According to the NLM, for example, "Vitamin A toxicity can occur from the ... oral form of vitamin A." The Mayo Clinic notes that vitamin D toxicity — aka hypervitaminosis D — "is usually caused by large doses of vitamin D supplements."
Per the USDA, one cup of fortified 2 percent milk contains 203 micrograms of vitamin A and 111 international units of vitamin D. The NIH's recommended daily allowances for most adults are 700 to 900 micrograms of vitamin A and 600 international units of vitamin D. Make sure that your morning milk-and-multivitamin combo doesn't put you in the danger zone.
- USDA National Nutrient Database: 'Milk, Reduced Fat, Fluid, 2% Milkfat, With Added Vitamin A and Vitamin D'
- National Institutes of Health Office of Dietary Supplements: 'Calcium'
- U.S. National Library of Medicine: 'Taking Iron Supplements'
- U.S. National Library of Medicine: 'Calcium and Iron Absorption — Mechanisms and Public Health Relevance'
- American Journal of Clinical Nutrition: 'Calcium — Effect of Different Amounts on Nonheme- and Heme-Iron Absorption in Humans'
- Washington University in St. Louis: 'Vitamins and Minerals as Essential Dietary Components'
- Harvard Medical School: 'Nutrition's Dynamic Duos —
- FDA: 'What You Need to Know about Dietary Supplements'
- U.S. National Library of Medicine: 'Vitamin A Toxicity'
- Mayo Clinic: 'What Is Vitamin D Toxicity?'
- National Institutes of Health: 'Vitamin A'
- National Institutes of Health: 'Vitamin D'