Vitamins are necessary to your diet because they provide your body with nutrients for basic biological functions. They work together with your existing enzymes to support metabolism and tissue repair. The majority of your vitamin intake is supplied by the various foods you eat. Taking more than the daily recommended dosage for most vitamins does not generally cause adverse effects. Supplemental forms of vitamins that exceed the tolerable upper limits can increase the risk of toxicity, however.
Tolerable Upper Intake Levels
The Food and Nutrition Board serves as the regulating body of vitamin intakes and upper limits. The board sets clear guidelines for you to follow to prevent you from exceeding the average dose of vitamins per day. The maximum dose you can take to prevent adverse effects is referred to as the tolerable upper intake level, or UL. In most instances, consumption of vitamins from foods that may exceed the recommended dietary allowance, or RDA, does not cause toxic effects but this is not the case with supplemental use of vitamins. The Institute of Medicine and U.S. Department of Agriculture websites publish the ULs online and in chart form to help you safely consume adequate amounts of vitamins. Vitamin supplements are not necessary if you are generally healthy and consume a well-balanced diet. Some conditions, however, warrant taking supplements to prevent specific vitamin deficiencies, such as in the case of niacin deficiency due to poor digestive absorption.
UL for Vitamin A
Vitamin A is fat-soluble, meaning it accumulates and stores in your body for later use. Vitamin A is a general term referring to several related compounds like retinol or retinal, which are preformed versions of vitamin A. In your body, these compounds convert to other chemical structures used to facilitate various functions. Beta-carotene and the carotenoid compounds are grouped as provitamin A carotenoids and these come from dietary sources like squash or spinach. The preformed compounds in supplements are more likely to cause toxicity if you exceed the ULs. The UL for preformed vitamin A in adults is 3,000 micrograms per day. Liver toxicity, severe headaches, skin roughness, hair loss and bone pain are associated with excess doses of preformed vitamin A.
UL for Vitamin D
Vitamin D is the other fat-soluble vitamin that can become highly toxic if you take excess supplemental doses. Vitamin D is synthesized in your skin as cholecalciferol, or D-3, upon exposure to ultraviolet-B radiation from sunlight. Some foods like milk are fortified with vitamin D but the range of foods is limited, which makes deficiency of this vitamin increase in those who do not get regular sun exposure. The risk of skin cancer is also an issue for obtaining natural sources of vitamin D. The UL for vitamin D in adults is 100 micrograms. Vitamin D toxicity is not common from sun exposure but excess intake of supplemental form induces high serum calcium levels that can result in bone loss, kidney stones or organ calcification. Pain in the joints, nausea and alternating constipation with diarrhea are symptoms of vitamin D toxicity.
UL for Vitamin C
Vitamin C is water-soluble, so you need to consume the recommended allowance daily because it is quickly used and eliminated from your body. This vitamin is associated with high dose usage for improving the immune system and warding off the common cold. Exceeding the UL for vitamin C can become toxic, however, especially if you have ever experienced kidney or gallbladder stones. Your body metabolizes a small portion of vitamin C to produce oxalate, which is a substance that induces stone formation in already compromised kidneys. Excess use of vitamin C increases your risk of stone formation and it can also cause temporary headaches or diarrhea. The UL for vitamin C in adults is 2,000 mg per day.
Additional Vitamin ULs
The the majority of the B vitamins have no upper limit restrictions. Vitamin B-6 in very high doses can cause nerve damage, however. Niacin, or vitamin B-3, is associated with adverse effects from high doses like flushing, blood pressure changes and gastrointestinal disturbance. The UL for vitamin B-6 in adults is 100 mg per day and for niacin it is 35 mg. Folate is the other B vitamin with an UL of 1,000 micrograms per day but adverse effects are associated with supplemental forms rather than dietary folate. Vitamin E, an important antioxidant that also protects your cells and skin, has a UL of 1,000 mg per day. Toxicity of supplemental vitamin E is associated with excessive bleeding.
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