Taking dietary supplements has both benefits and risks, especially if you're taking large amounts. The Institutes of Medicine, which is part of the National Academies, determines how much of each nutrient you need every day, as well the tolerable upper intake level (UL), which is the largest intake of each nutrient known to be safe. You typically don't have to worry about going over the ULs, unless you're taking large doses of dietary supplements -- it's difficult to go over the ULs from food sources alone.
Vitamin AVitamin A is necessary for normal vision, reproduction, immune system function and cell development (here are the symptoms of vitamin A deficiency). The recommended daily allowance is 700 micrograms for women and 900 micrograms per men. The UL is 3,000 micrograms per day. Regularly consuming more than this amount may cause hair loss, cracked lips, dry skin, headaches and weakened bones. Continued overuse of vitamin A supplements may lead to liver damage, especially if you also have high intake of alcohol, or have liver disease or high levels of cholesterol and triglycerides. Pregnant women who consume too much vitamin A have increased risk of birth defects.
Vitamin DVitamin D helps your body absorb more calcium, so it's important for strong bones and teeth. The recommended daily allowance ranges from 5 to 10 micrograms per day. The UL is 50 micrograms (or 2,000 International Units) per day. Going over this amount on a daily basis may cause a condition called hypercalcemia (elevated level of calcium in the blood). Early symptoms include nausea, vomiting and diarrhea, and a continued high vitamin D intake may eventually lead to dehydration, kidney stones and kidney damage.
Vitamin EVitamin E is works as an antioxidant in your body. The recommended daily allowance is 15 micrograms per day, and the UL is 1,000 micrograms per day. Consuming more than the UL every day could lead to bleeding problems, especially for people who are also taking blood thinning medications. Large amounts of vitamin E can also cause muscle weakness, nausea and diarrhea.
Vitamin KVitamin K is necessary for normal blood clotting and is important for healthy bones. The recommended daily allowance is 90 micrograms per day for women and 120 micrograms per day for men. The Institute of Medicine hasn't determined a UL for vitamin K intake.
Taking Dietary SupplementsMost multi-vitamins are designed to help you reach your daily recomended intake for several nutrients, so they usually contain levels of vitamins and minerals that are well below the ULs. Some individual vitamin supplements, and even some formulations, contain large amounts of vitamins and you could reach the ULs if you take mega-doses. Dietary supplements aren't well regulated so choose high quality supplements (I've got more tips for safe use of dietary supplements.) To avoid reaching the UL for any nutrient, follow the directions on the dietary supplement label and don't take more than the recommended amount without speaking to your health care provider first.
Institutes of Medicine of the Institutes of Health. "Dietary Reference Intakes." Accessed December 17, 2012. http://iom.edu/~/media/Files/Activity%20Files/Nutrition/DRIs/DRI_Vitamins.pdf.
The Merck Manual Home Health Handbook. "Vitamin A." Accessed December 17, 2012. http://www.merckmanuals.com/home/disorders_of_nutrition/vitamins/vitamin_a.html.
The Merck Manual Home Health Handbook. "Vitamin D." Accessed December 17, 2012. http://www.merckmanuals.com/home/disorders_of_nutrition/vitamins/vitamin_d.html.
The Merck Manual Home Health Handbook. "Vitamin E." Accessed December 17, 2012. http://www.merckmanuals.com/home/disorders_of_nutrition/vitamins/vitamin_e.html .