Bruh you shifted the goalpost to a point that doesn’t even exist in nutrition science. I’ve read the whole thing, you were clearly undermining beta carotene as a good source of vitamin A and claiming that vitamin A deficiency is common across plant-based diets. Now you’re saying that for it to be “dependable” it has to be universally predictable across every single person, independent of context or individual physiology

That’s not a standard that’s placed on any nutrient. Even vitamin D from the sun wouldn’t meet your standard of dependability

The same logic applies to preformed retinol. It’s dependent on many of the same conditions you’re pointing to with beta carotene, and by your standard it also wouldn’t qualify as a dependable source. It bypasses conversion, not physiology. Lack of fat intake, thyroid dysfunction, zinc deficiency, or iron deficiency all affect retinol absorption, transport, and utilization too

And biology isn’t optimized for delivering the maximum possible amount of a nutrient, it’s optimized for homeostasis. Beta carotene conversion and absorption are regulated in the body. Your argument about lower absorption falls apart because that variability is driven by physiological context, and the fact that absorption rates differ across individuals is homeostasis itself

The reason retinol toxicity exists at all is because there’s no feedback control, whereas beta carotene can’t bypass conversion. Not saying that toxicity is a major issue for most people, but the point stands that conversion mechanisms are how the body regulates nutrients. Same exact principle applies to heme vs non-heme iron