I have repeatedly warned about vitamin A. I am not the first. As early as 1933, Professor Alfred Hess, who discovered that sunlight both prevented and cured rickets, warned about vitamin A consumption, writing in JAMA,
“…as to a requirement of thousands of micrograms of vitamin A daily, the unquestionable answer is that this constitutes therapeutic absurdity, which, happily, will prove to be only a passing fad.”
He was wrong about it being a passing fad. Four years ago, I got 16 nutrition experts together, including Professor Walter Willett at Harvard, to warn about vitamin A once again:
Cannell JJ, Vieth R, Willett W, Zasloff M, Hathcock JN, White JH, Tanumihardjo SA, Larson-Meyer DE, Bischoff-Ferrari HA, Lamberg-Allardt CJ, Lappe JM, Norman AW, Zittermann A, Whiting SJ, Grant WB, Hollis BW, Giovannucci E. Cod liver oil, vitamin A toxicity, frequent respiratory infections, and the vitamin D deficiency epidemic. Ann Otol Rhinol Laryngol. 2008 Nov;117(11):864-70. Review.
“Although activated vitamin D and vitamin A signal through common cofactors, they compete for each other’s function. Retinoic acid antagonizes the action of vitamin D and its active metabolite. In humans, even the vitamin A in a single serving of liver impairs vitamin D’s rapid intestinal calcium response. In a dietary intake study, Oh et al found that a high retinol intake completely thwarted vitamin D’s otherwise protective effect on distal colorectal adenoma, and they found a clear relationship between vitamin D and vitamin A intakes, as the women in the highest quintile of vitamin D intake ingested around 10,000 IU/d of retinol.”
We also stated,
“Furthermore, the consumption of preformed retinol — even in amounts consumed by many Americans in both multivitamins and cod liver oil — may cause bone toxicity in individuals with inadequate vitamin D status. Women in the highest quintile of total vitamin A intake have a 1.5-times elevated risk of hip fracture. Indeed, a recent Cochrane Review found that vitamin A supplements increased the total mortality rate by 16%, perhaps through antagonism of vitamin D. Another recent Cochrane Review concluded that although vitamin A significantly reduced the incidence of acute lower respiratory tract infections in children with low intake of retinol, as occurs in the Third World, it appears to increase the risk and/or worsen the clinical course in children in developed countries.”
Now a brand new study confirms the detrimental effects of vitamin A in the USA and certainly in cod liver oil countries in Scandinavia. Dr Ting-Yuan Cheng and Marian Neuhouser of the University of Washington looked at the interaction of vitamin D with vitamin A in lung cancer in the USA.
Cheng TY, Neuhouser ML. Serum 25-hydroxyvitamin D, vitamin A, and lung cancer mortality in the US population: a potential nutrient-nutrient interaction. Cancer Causes Control. 2012 Sep;23(9):1557-65. Epub 2012 Jul 25.
Expectedly, they found that “vitamin D sufficiency was associated with a 50-70% decrease in lung cancer mortality among nonsmokers.” Unexpectedly, to most, they found, “These beneficial associations were not observed among those with high concentrations of circulating vitamin A or users of vitamin A dietary supplements.” However, the statistical evidence of effect modification was limited by the few lung cancer deaths in their cohort.
They also found that the majority of Americans had elevated levels of retinyl esters and that 29% of Americans have potentially toxic levels. Vitamin A is found in so many foods that the thousands of ugs per day of vitamin A that Professor Alfred Hess warned about many years ago is easily obtained from diet alone. However, Americans may ingest as many as 5,000 ug/day due to diet, cod liver oil, and multivitamins containing retinyl acetate or retinyl palmitate. Some people even take pure vitamin A supplements.
In the undeveloped world, vitamin A food fortification, varied diets, and colorful vegetables are rare among people subsisting on a single carbohydrate source (rice, wheat, etc.), and vitamin A deficiency is common. However, in developed countries, as Drs. Cheng and Neuhouser showed, the problem is sub-clinical vitamin A toxicity, probably working by countering the effects of vitamin D.
As I have hypothesized repeatedly, I believe the vitamin A toxicity in developed countries explains the U-shaped curves we sometimes see in vitamin D studies, especially in the cod liver oil countries. See some of my other writings that cover this topic: