For anyone mildly curious, the weak version (easier to accept) is: Vitamin A (retinol compounds like Retinoic Acid and its precursors like Beta Carotene) are harmful in very low amounts above the daily recommended value. In America where dairy must be fortified with them, and flour and breakfast cereals often are, it's very easy to go way over the daily recommended value, the damage builds up over years and decades, and it's a big contributor to diseases of "old age" and autoimmune diseases. The mechanism of harm is cumulative damage, the damage is retinoic acid's effects on epithelial cells and stem cells causing skin damage, and its effects on DNA mechanisms causing misformed proteins.
The original anecdote is that Grant was diagnosed with chronic kidney disease in around 2007, given five years to live, three or four years later he had chronic fatigue, concentration problems, memory problems, was severely disabled, was trying an "athlete" diet of lots of fruits and vegetables and milk, developed eczema, came up with the idea to try avoiding Vitamin A, and after a three or four weeks of eating lean beef and white rice ... his decade of knee pain disappeared along with the brian fog and memory and concentration problems. So he stuck with it, his gum disease and dry eyes cleared up, his eczema cleared up, his kidneys stopped failing, and he's 16 years on from his 5 year life expectancy.
The strong (harder to accept) version is that retinol compounds should never have been classified as vitamins at all, the 'deficiency' symptoms are not right, the original 1920s studies were not conclusive and the most prominent one saying they are vitamins was bad science which poisoned the rats with retinoic acid overdoses and then took those symptoms as deficiency symptoms. Grant is now stronger at the gym age 62 than he was at 22 despite exercising only weekly, his greying hair has darkened again, his liver spots and skin blemishes and varicose veins have cleared up and his eyesight has improved.
The possible reasons to believe there's something to it is that other people have tried it, and seen their skin conditions improve, their joint pain, posture, vision, memory, insomnia, digestion, body fat, people have found plenty of papers of biological damage evidence, or studies on people contradicting the normal expected outcomes. There is claimed correlation of vast increases in serious health conditions matching the 1970s fortification of dairy with retinol compounds in the USA, and decreases in serious health conditions in Eastern Canada following the 1992 closure of a cod fishing plant, with correlations to oily fish and cod liver oil. Retinol compounds in the form of Isotretinoin were used as chemotherapy agents because they are so harmful to cells, and in the form of Accutane they're used for acne medication which has as its known side effects the symptoms of most autoimmune diseases, which happen in far more people who take it than you'd expect for a common medication.
Possible reasons to believe there's nothing to it is that other people have tried it and found no improvement, sometimes worsening. Many of the people who tried it were seriously ill and trying everything they could think of, including lots of vitamin and mineral adjustments and gut microbiome adjustements and other treatments, so there's too much in play to be sure of cause and effect, and anyway it's all self-selected confirmation bias and anecdotes and correlation != causation and anyway engineers thinking someone else's field is easy and simple is a common trope, besides with so many people researching nutrition wouldn't this be well known by now? The dose makes the poison and of course "too much" is harmful, who would expect otherwise? (See weak version: perhaps "too much" is far lower quantities than commonly thought, but the effects take years to show up?). Or that this study showing diabetes mellitus makes a foetus more vulnerable to retinoic acid damage and birth defects[1] is a study in mice, not humans. There isn't a single cause of disease, no lead bullet (but this isn't claiming it's the only cause).
The crazy (internet kook) version is that chronic Vitamin A toxicity is a significant contributer to all autoimmune diseases, Alzheimers/dementia, type 2 diabetes, autism, ADHD, it's the real cause of Scurvy (but the cause and effect is non-trivial, ascorbic acid is good at calming the symptoms rather than curing the disease), that Scurvy is very close in symptoms to Type 2 Diabetes because they have the same root cause, that retinoic acid is a contributer to male pattern baldness as well as "old age" gray hair, that retinoic acid is documented as affecting 500 gene expressions but that's really a misunderstanding, what it's doing is random chaotic damage, that the documented Vitamin A defficiency eye conditions are all in people with very unbalanced diets with a lot of other defficienies...
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You can see on his blog and forum anecdotes like user YH: https://ggenereux.blog/discussion/topic/nearly-7-months-prog... who says their eyesight is improved, back pain is gone, humpback has gone, chapped lips and dry skin are healed, new hair growing on their balding crown, skin condition is improved, hand-eye coordination in sketching has improved, depression has gone.
Or PuddleDuck who improved from years of chronic fatigue on it: https://ggenereux.blog/discussion/topic/puddleducks-progress... who says "I know this post is going to be so long, but since Grant’s theory is the only one I’ve ever read that MIGHT explain why the heck that happened to me, I feel compelled to share in detail.".
Or Sarabeth whose experiments with diet have improved her children's behaviour, outbursts, bedwetting, food intolerances, and more: https://ggenereux.blog/discussion/topic/our-family-behaviora...
Or El: "it's been six months since i read gareth and grant has been on this diet for almost close to seven months. It changed my life. I had chronic fatigue, dizziness, eye pain, muscle pain, blurred vision. hearing loss" - https://ggenereux.blog/discussion/topic/hi-2/
Or Sand, whose vision has improved to the point they "can manage life now" https://ggenereux.blog/discussion/topic/sands-progress-repor...
Or BodeFofinho who developed Latent Autoimmune Diabetes in Adults in his 30s, and after a year of no/low Vitamin A reversed it ... but then after two years found the diet worsening his health and moved away from it: https://ggenereux.blog/discussion/topic/my-diabetes-type-i-r...
Or if you don't want the rah-rah versions, "Why I don't think this is a valid theory anymore", and in the middle of that pay attention to Johannes-2 posts which are full of biochemistry with study citations and references: https://ggenereux.blog/discussion/topic/why-i-dont-think-thi...
Or if you want more details of mechanisms of action, this https://ggenereux.blog/discussion/topic/biotransformation-of... which I can't read and says "It is now no longer difficult to imagine how retinoic acid could cause auto-immune disease, since the data appears to show that retinoic acid is essentially a pro-inflammatory cytokine that is stored in the liver and, once the liver is saturated, in different types of tissue all over the body, especially in epithelial cells that constitute blood–tissue barriers".
Or if you want reasons to dismiss it as all nonsense, look only for the COVID/vaccine claims, the guy who dismisses vitamin supplements because they're made in Chinese factories, the people who wear Electro-Magnetic Field blocking clothes, the guy who thinks dust in bottled water made him ill but raw ocean water up his nose didn't ... it's really really easy to dismiss this all as crank bullshit.
... Except, the arguments are interesting and pointing from many directions, and the claimed mechanisms of action are far more plausible than - say - claims of how the Pyramids were electrical energy collectors. And there certainly are many health crises around. And it's pretty clear that some food can be harmful, short and long term. And that grouping food by (plant/animal) or (low fat/high fat) or (low carb/high carb) or (processed/unprocessed) isn't enough, and the argument that we need to look at the molecular content of foods rather than where they came from is reasonable.
To my previous comment, how can one ever know?
[1] https://ggenereux.blog/discussion/topic/does-sugar-increase-...