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      VITAMIN A, BETA-CAROTENE CANCER

Abstract:  Vitamin A and Beta-carotene are antioxidants that used in moderate amounts can reduce the risk of most causes of cancer somewhat for those that have inadequate Vitamin A in their present diets.  Vitamin A is less effective in reducing risk of cancer than is Vitamin E, and the use of amounts of Vitamin A above 5,000 IU or Beta Carotene much above 15,000 IU may be counterproductive and should be avoided.  But because Vitamin A also probably will reduce risk of heart disease somewhat, its supplemental use in moderate amounts usually should be beneficial.

As described more completely for Vitamin A and Coronary Heart disease, Vitamin A designates a family of vitamins that are fat soluble. Most tables of Vitamin values in foods are expressed in IU units of Vitamin A.  But most research is based on values of Beta-carotene, and most users take Vitamin A supplements as Beta-carotene. The body converts Beta-carotene to Vitamin A.  But it is claimed by some experts that the body converts Beta-carotene to Vitamin A only in amounts needed.  Typical conversion ratios from IU of Beta-carotene to IU equivalents in Vitamin A vary widely, and typically from from 1 to 2 to up to 1 to 6.  Life Ahead assumes a conversion value of 3 Beta-carotene to 1 Vitamin A for both the research results in Table A following and for user values in the program.  But it should be kept mind that Life Ahead values Beta Carotene IU's from supplements as of only 1/3rd the value of IU's of Vitamin A cited in usual tables food nutrient values. Values of Beta-carotene are sometimes expressed in mg, and 1 mg of Beta-carotene equals about 1600 IU of Beta-carotene.

The research results found for Vitamin A and Beta Carotene are summarized in Table A following.  Differences in amounts of vitamin used are expressed in IU's of Vitamin A.  Risk ratios for cancer are not nearly as consistent or extensive as are those for Vitamin E.  An average value for all observation studies but eliminating study #5 as inconsistent is 0.71.  With some  favoring of  the results of the larger studies a better average risk value could be about 0.75 for a difference of about 4000 IU of Vitamin A.  The 11 observation study values are near statistically consistent with this average.  Thus is seems likely that that Vitamin A and Beta-carotene in the amount used in these studies probably does reduce risk of cancer. This is expected from its known antioxidant property.  Because nearly any cause of cancer will be duration dependent and a typical duration of use in the research to date probably was about 10 years, Vitamin A in amounts of about 4,000 IU probably will slow the progress of cancer at a risk ratio of 0.97 or 3% per year. 

No clear pattern exists for the risk of Vitamin A on different types of cancer from the observation type studies.  The average risk of the 3 studies for all cancer types was about the same as the average risk of the other studies by cancer type.  Insufficient research is now available to identify a statistically different risk for the differing cancer types.  Thus tentatively Life Ahead assigns the same risk to all cancer types.

But the results of the clinical studies #13 to #16 show no effect of Beta-carotene on risk of cancer, and this is confirmed with quite high significance.  Although the short duration of 3 of the clinical studies could have been factor, this did not explain the negative result from study #13.  As for the results of Vitamin A on heart disease, a possible reason for this lack of measured benefit is that use of the very high amounts of Vitamin A used in these studies - and that averaged 3 times the levels used in the observation studies - became counterproductive.  The amounts of Vitamin A used in these clinical studies are far above the 4-5,000 maximum for the RDA, and above a level that can provide other health negatives such as bone loss. 

Data confirming this hypothesis comes from a Table 4 in Hunter's study #2. Adding supplements of Vitamin A for those participants that had very low Vitamin A from food provided a quite significant benefit.  But supplements added to those having higher amounts of Vitamin A in in food provided no further benefit.  A maximum value of Beta-carotene benefit appears obtained at about 15,000 IU, or the equivalent of 5,000 IU of preformed Vitamin A as used herein. Risks for a total of Vitamin A beyond 10,000 IU actually reversed to higher values in this table of results.  This same maximum amount of Vitamin A also appeared indicated useful for heart disease.  The research to date shows that use of the very large amounts of Beta-carotene employed in the clinical studies probably not only was of no value but produced reverse or negative health benefit.

Life Ahead now values Vitamin A for both heart disease and cancer only for amounts up a total of 5,000 IU equivalent of preformed Vitamin A (or 15,000 IU of Beta-carotene) in a total of food plus supplements.  It thus provides a value for a Beta-carotene or Vitamin A supplement only when amounts in food are deficient vs. the present RDA values.  Again, Life Ahead values Vitamin A as one of 4 antioxidants, and will give no credit to any further amount of any of these antioxidants when a total limiting value of all antioxidants is attained.  The benefit of Vitamin A is less well confirmed than is true for the other antioxidants. Life Ahead values this benefit that is small only in combination with the other antioxidants, and users should take some of each of the included antioxidants preferably in a multivitamin for best assurance of a good antioxidant benefit.

There has been widespread speculation by researchers from the observation study results in Table A that much larger amounts of Beta-carotene might reduce risk of both heart disease and cancer more substantially.  Research to date has suggested that this hypothesis is probably false.

                                                                                          Table A

EFFECT of VITAMIN A and BETA-CAROTENE on CANCER