Tolerable Upper Limits

 

       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 2½ times the recommended daily requirement.  AI, adequate intake, 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% to 30% 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 FDA (Federal 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. The severity of this risk is not well understood (see below).

 

        The actual mechanism of the toxicity of excessive amounts of several vitamins is known.  Vitamin B6 is inactive as it comes from nature and must be activated in the liver (by addition of an hydroxyl group).  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 (ascorbic acid) acidifies the urine and causes uric acid kidney stones in gouty patients. (We do frequently use vitamin C to acidify the urine).  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 values in my simplified tables which follow are for adults omitting the recommendations for pregnancy, lactation, and for children of various ages.  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 TUL (tolerable upper limit) of each nutrient is from food, water and supplements.  You can find all these details on www.nap.edu

 

 

 

 

                                      Tolerable Upper Limits for Vitamins

                                               All quantities are per day

 

Vitamin

RDA (recommended daily allowance)

TUL (Tolerated upper limit)

A

2300-3000 u or 700-900ug

10,000u or 3000 ug

B1  thiamin

1.1-1.2 mg

ND (not determined)

B2  riboflavin

1.1-1.3 mg

ND

B3  niacin

14-16 mg

35 mg

B6  pyridoxine

1.3 mg

100 mg

pantothenic acid

5 mg

ND

folate

400 mcg

1000 mcg

biotin

30 mcg

ND

choline

425-550 mg

3500mg

B12

2.4 mcg

ND

C  ascorbic acid

75-90 mg

2000 mg

D

200 u or 5 mcg

2000 u or 50 mcg

E

15 mg

1000 mg

K

90-120 mcg

ND

 

u = units 

mg = milligrams  

  ug = micrograms 

   

       1/1,000 grams

       1/1,000,000 grams

 

                  Tolerable Upper Limits for Minerals

 

Mineral

RDA (recommended daily allowance)

TUL  (tolerated upper limit)

calcium

1200 mg

2500 mg

phosphorus

700 mg

4000 mg

magnesium

320-420 mg

750 mg

fluoride

3-4 mg

10 mg

iron

15-10 mg

45 mg

zinc

12-15 mg

40 mg

iodine

150 mcg

1100 mcg

selenium

55-70 mcg

400 mcg *

copper

0.9 mg

10 mg

chromium

25-35 mcg

ND (not determined)

manganese

1.8-2.3 mg

11 mg

molybdenum

76-109 mcg

2000 mcg

arsenic, boron, nickel, silicon, vanadium

ND

ND

Arsenic, although required in tiny amounts, is best avoided as much as possible.  Deficiency has never been reported.

* Selenium is high in some soils, for example, parts of the American Southwest.  People residing in such areas should probably avoid selenium supplements.

You may print these tables from www.frantzmd.info

John A. Frantz, M.D.    

September 14, 2003