|Under optimal conditions, humans can indeed convert carotenes to vitamin A. This occurs in the upper intestinal tract by the action of bile salts and fat-splitting enzymes. Of the entire family of carotenes, beta-carotene is most easily converted to vitamin A. Early studies indicated an equivalency of 4:1 of beta-carotene to retinol. In other words, four units of beta-carotene were needed to produce one unit of vitamin A. This ratio was later revised to 6:1 and recent research suggests an even higher ratio.5 This means that you have to eat an awful lot of vegetables and fruits to obtain even the daily minimal requirements of vitamin A, assuming optimal conversion...
Even worse than vitamin-A vagary is vitamin-A knavery in the form of concerns that vitamin A may be toxic in more than the minuscule RDA-recommended amounts. In fact, so great is the propaganda against the vitamin that obstetricians and pediatricians are now warning patients to avoid foods containing vitamin A!
Recently an "expert" panel recommended lowering the RDA (recommended daily allowance) for vitamin A from 5000 IU daily to about 2500 IU and has set an upper limit of about 10,000 IUs for women. The panel was headed by Dr. Robert Russell of Tufts University, who warned that intake over the "upper limit" may cause irreversible liver damage and birth defects—a ridiculous statement in view of the fact that just a few decades ago pregnant women were routinely advised to take cod liver oil daily and eat liver several times per week. One tablespoon of cod liver oil contains at least 15,000 IU and one serving of liver can contain up to 40,000 IU vitamin A. Russell epitomizes the establishment view when he insists that vitamin-A requirements can be met with one-half cup of carrots daily.
The anti-vitamin-A campaign began in 1995 with the publication of a Boston University School of Medicine study published in the New England Journal of Medicine.15 "Teratogenicity of High Vitamin A Intake," by Kenneth J. Rothman and his colleagues, correlates vitamin-A consumption among more than 22,000 pregnant women with birth defects occurring in subsequent offspring. The study received extensive press coverage in the same publications that had earlier extolled the benefits of vitamin A. "Study Links Excess Vitamin A and Birth Defects" by Jane Brody appeared on the front page of the New York Times on October 7, 1995; on November 24, 1995, the Washington Times reported: "High doses of vitamin A linked to babies' brain defects."
When a single study receives front-page coverage, it's important to take a closer look, especially as earlier research discovered the importance of vitamin A in preventing birth defects. In fact, the defects listed as increasing with increased vitamin A dosage—cleft lip, cleft palate, hydrocephalus and major heart malformations—are also defects of vitamin A deficiency.
In the study, researchers asked over 22,000 women to respond to questionnaires about their eating habits and supplement intake before and during pregnancy. Their responses were used to determine vitamin-A status. As reported in the newspapers, researchers found that cranial-neural-crest defects increased with increased dosages of vitamin A; what the papers did not report was the fact that neural tube defects decreased with increased vitamin A consumption, and that no trend was apparent with musculoskeletal, urogenital or other defects. The trend was much less pronounced, and less statistically significant, when cranial-neural-crest defects were correlated with vitamin-A consumption from food alone.
The study is compromised by a number of flaws. Vitamin-A status was assessed by the inaccurate method of recall and questionnaires; and no blood tests were taken to determine the actual usable vitamin-A status of the mothers. Researchers did not weight birth defects according to severity; thus we do not know whether the defects of babies born to mothers taking high doses of vitamin A were serious or minor compared to those of mothers taking lower amounts.
The most serious flaw was that researchers failed to distinguish between manufactured vitamin A in the form of retinol, found in supplements and added to fabricated foods, from natural vitamin-A complex, present with numerous co-factors, from vitamin-A-containing foods. It is well known that synthetic vitamins are less biologically active, hence less effective, than naturally occurring vitamins. This is especially true of the fat-soluble vitamins like vitamin A, because these tend to be more complex molecules, with numerous double bonds and a multiplicity of forms. Natural vitamin A occurs as a mixture of various isomers, aldehydes, esters, acids and alcohols. Pure retinoic acid, a metabolite of vitamin A used to treat adult acne, is well known to cause birth defects. Apparently pure retinol has teratogenic properties in high amounts as well.
Researchers found that cranial-neural-crest defects increased in proportion to the amount of retinol from supplements consumed during the first trimester of pregnancy (although the total number of defects remained stable up to 15,000 IU daily). Research into vitamin A has indicated that many factors interfere with its absorption and utilization. Inadequate fat in the diet, poor production of bile salts, low enzyme status, and compromised liver function can all interfere with the uptake and usage of vitamin A, especially when given as a supplement in the form of retinol, rather than as a component of whole foods. It may be that the teratogenic effects of commercial vitamin-A preparations are exacerbated in women whose dietary practices and general health status are poor. Some researchers believe that synthetic vitamin A interferes with the proper utilization of natural vitamin A from foods.
Pure retinol is added to many fabricated foods like margarine, breakfast cereals and pizza. The study made no distinction between those women whose vitamin A was supplied by whole animal foods and those who ingested retinol added to margarine, white flour and extruded breakfast cereals—foods which contain many other factors that can cause birth defects. Natural vitamin A provided by liver, eggs, butter, cream and cod liver oil is well recognized as providing excellent protection against birth defects.
Distinctions between synthetic and natural vitamin A have been absent in the extensive media coverage of this study—on the contrary, the newspaper reports contain implied warnings against pregnant women eating liver, dairy products, meat and eggs, but none against eating fabricated foods like margarine and breakfast cereals to which synthetic vitamin A is added. And there has been no media coverage for subsequent studies, which found that high levels of vitamin A did not increase the risk of birth defects. A study carried out in Rome, Italy found no congenital malformations among 120 infants exposed to more than 50,000 IU of vitamin A per day.16 A study from Switzerland looked at blood levels of vitamin A in pregnant women and found that a dose of 30,000 IU per day resulted in blood levels that had no association with birth defects.17
Maybe MT will chime in about this as well.
This information describes how there is NO RISK for natural vitamin A supplementation at all, and that the studies showing problems were done on synthetic vitamin A which is an entirely different animal.
I think the myth that natural vitamin A is toxic has done huge damage to the health of mothers and children. For example studies were done on rats showing lack of vitamin A causes extremely long labors and inability to push their babies out.
As someone who had a 40 hr. labor (and thankfully pushed all 9lbs 13oz of this baby out) and was told NOT to take cod liver oil during pg by my OB b/c of supposed toxicity, I'm pretty p.o.'d about that.
Again, native diets contained something like 30-40 times more vitamin A than the standard diet in the 1930's when Weston Price did his work (so probably even greater disparity now) and were much healthier for it.