Why would that many people have so much vitamin A ?
From Wikipedia
> "In developing nations, however, 70–90% of vitamin A is obtained from provitamin A carotenoids in plant foods"
and
> "Whereas retinoic acid can be produced from excentric cleavage of β-carotene in humans (31), it is generally considered a minor contributor to circulating concentrations, at least in normal, healthy persons"
Google tells me that "Two pathways have been suggested for the conversion of carotenoids to vitamin A in mammals, central cleavage and excentric cleavage".
Carotene conversion cannot both be "a minor contributor" to body levels of Vitamin A and also the main way billions of people get Vitamin A. So my guess is the Wikipedia page has mistakenly taken 'excentric cleavage' to be the only conversion method. Still, ignoring carotenes entirely, the referenced paper also says:
> "observational studies suggest that more than 75% of people may be routinely ingesting more than the recommended dietary allowance (RDA) for vitamin A"
> "Dietary vitamin A is obtained from preformed vitamin A (ie, retinyl esters from animal foods, fortified foods, and pharmaceutical supplements) as well as from provitamin A carotenoids from plant sources. Preformed vitamin A is efficiently absorbed and utilized by humans at absorption rates of 70–90%. Up to 75% of dietary vitamin A in Europe, the United States, and other industrialized nations is preformed vitamin A (1, 2), which is largely derived from multivitamins, fish liver oil, and the fortification of foods such as milk, butter, margarine, breakfast cereals, and some snack foods. In developing nations, however, 70–90% of vitamin A is obtained from provitamin A carotenoids in plant foods. These are absorbed much less efficiently, at rates of 20–50%, depending on each person’s vitamin A status and other dietary and nondietary factors (3, 4). The cleavage of provitamin A carotenoids to retinal is a highly regulated step, and vitamin A toxicity from provitamin A sources is largely impossible. In contrast, absorption and hepatic storage of preformed vitamin A occur very efficiently until a pathologic condition develops."
If you take "the cleavage of provitamin A carotenoids" and Ctrl+F for 'cleavage' or 'carotene' in that paper, the only other mentions are this:
> "An acute elevation of retinoids other than retinyl esters—eg, retinoic acid—occurs after the ingestion of a large amount of vitamin A, possibly because the intestinal absorptive capacity is overwhelmed, which leads to the oxidation of retinol to retinoic acid by the intestinal enterocytes (30) and to the rapid formation of retinoic acid from retinol in certain cells (5). Whereas retinoic acid can be produced from excentric cleavage of β-carotene in humans (31), it is generally considered a minor contributor to circulating concentrations, at least in normal, healthy persons."
That is, they only seem to be talking about excentric cleavage and not central cleavage. My limited understanding is that beta-carotene is roughly two retinoic acid molecules chained together, and cleaving them in half is what makes two retinoic acid molecules, and that non-central-cleaving would be different. See skeletal formula on right for these two pages:
https://en.wikipedia.org/wiki/%CE%92-Carotene
https://en.wikipedia.org/wiki/Retinoic_acid
(You can also wonder if beta carotene can produce 90% of circulating vitamin A in developing countries, whether getting lots of beta carotene in developed countries can raise the base level in the body, which then leads to easier overdose from preformed vitamin A)
Carotenoids are used as a natural food colouring, beta-carotene is E160a, e.g. I've seen it in Red Leicester cheese, yellow custard powder, red jelly, in glace cherries. American law requires vitamin A fortification in reduced-fat dairy. Companies optionally fortify it in things like wheat flour, or breakfast cereal. Retinoids are in beauty and moisturising skin creams and sun tan lotions, and sometimes used as extreme acne medication.
Things that have been promoted for "good health" for decades, such that someone who wants to avoid it needs to deliberately choose an awkward diet of non-colourful foods like potato, rice, onion, parsnip, coconut, apple, pear, cucumber, zucchini, black beans, white beans, lean meats or carefully chosen meat fats, breads made from un-fortified flour, avoiding many restaurant, take-away, or processed foods and "eat the rainbow" variety of fruit and veg.
Grant Genereux has proposed that overflowing the liver's ability to store and process Vitamin A / retinoids leads to them circulating in the blood, that they can enter cells bypassing the normal methods, can cause damage which makes cells denature and burst, which triggers an immune system response to try and find an attacker, which gets misdiagnosed as the immune system causing the damage. And has proposed this as a mechanism which explains some of the 25 - 50 million Americans having unexplained auto-immune diseases.
Proposing that retinoic acid builds up inside body cells over years and decades even whan taken ordinary 'safe' levels, not enough to trigger hypervitaminosis A or acute symptoms but chronically accumulating until the body has so much of it stored that cell damages releases retinoids into the bloodstream which enter nearby cells, denature some of them, releasing their stored retinoids, causing a domino effect which we know as an auto-immune flare-up. And that it's a matter of luck where the concentration is higher whether that makes a skin rash or a kidney or joint or thyroid inflammation, or just generally high "inflammation".
It's an interesting hypothesis with a lot of specific testable claims and specific proposed mechanisms, mixed with quite a lot of conspiracy theorising and less-testable claims.
- the recommended daily vitamin C is 90mcg for men and 75 for women, the upper limit is 2000mcg. That's a safety margin of 22x, and exceeding the upper limit may cause diarrhoea and gastrointestinal distress.
- the recommended vitamin A is 900mcg for men and 700 for women, and the upper limit is 3000mcg. That's a safety margin of 3.3x, and even that upper limit has "some evidence that this much preformed vitamin A might increase the risk of bone loss, hip fracture, or some birth defects".
- "Sometimes people take cod liver oil for vitamin D but may not be aware it is also very high in vitamin A, beyond the RDA at 192%"
That is, even without any alternative views on it, it's much easier to go above the safe levels, easier to do accidentally, and the risks are more serious.