http://www.themonroetimes.com/h1009pfr.htm Dr. John A. Frantz
Because of the large number of people taking dietary supplements in varying amounts, the tables of recommended daily allowances (RDA) are now supplemented with a new value, "tolerable upper limit," the largest daily intake which is unlikely to cause harm.
The concept of tolerable upper limit for vitamins is pertinent only for supplementary vitamins because toxic amounts of vitamins are not available from natural food sources. The only exception: lethal quantities of vitamin A in the livers of polar bears and seals and presumably in the livers of certain fish eaten by polar bears and seals, but none of us would be eating the livers of many such fish at one sitting. In the case of nutrients where the maximum tolerable dose is unknown, a good rule of thumb is to limit intake to 2 1/2 times the recommended daily requirement. Adequate intake (AI) is specified in some tables where the data were less extensive than required for an RDA. In general RDAs and AIs are interchangeable.
Even though it is not difficult to get all the necessary nutrients from a varied diet, we physicians frequently recommend supplements. In addition to overt cases of mal-absorption such as Crohn's disease or short bowel syndromes, 10 percent to 30 percent of the elderly do not absorb vitamin B12 adequately from dietary sources. These people need extra vitamin B12. Also the prevalent softening of the bones in the elderly makes vitamin D supplementation desirable for many, especially in locales where exposure of the skin to the sun is limited. Conversely those taking the blood thinner, Coumadin, should avoid large amounts of vitamin K because it counteracts the drug. Beta carotene is no longer recommended by the Food and Drug Administration as a supplement because it is a common occurrence for many of us to exceed the toxic level of vitamin A. This could happen, if all our dietary beta carotene were to be converted to active vitamin A. Vitamin B6 is inactive as it comes from nature and must be activated in the liver. When the liver's ability to make the modification is overwhelmed, the inactive form circulates and inactivates the very enzyme it is intended to assist. There are three known mechanisms for vitamin C toxicity. Vitamin C acidifies the urine and causes uric acid kidney stones in gouty patients. In extreme excess vitamin C is not only excreted but also destroyed with oxalic acid as the residue. Calcium oxalate kidney stones may result. Vitamin C also enhances the absorption of iron even when such absorption is not desirable. Vitamin D increases calcium absorption and in excess increases calcium levels in the blood producing abnormal calcium deposits in various tissues.
Finally a related example of nature's wisdom: Beta carotene is a major precursor of vitamin A in a normal, varied diet and is found in many fruits and vegetables. Beta carotene absorption is limited in the presence of ample amounts of vitamin A already present in the body. Absorption of pure vitamin A is not limited in this way.
The table lists values for adults. Where a range is given, the first number is for women, the second for men (and, in the case of iron, women after the menopause). The tolerable upper limit of each nutrient is from food, water and supplements. You can find all these details on www.nap.edu.