This is taken from my subsection on carotenoids in the vitamin A chapter.            

    This chapter is focused on dietary and supplemental active vitamin A. However, as you are likely aware, vitamin A can be synthesized in the body from carotenoids within plant matter, the most familiar source being carrots. Other yellow, orange, and even green leafy fruits and vegetables such as sweet potatoes, spinach, and papaya all contain appreciable amounts of carotenoids. The main consideration is whether or not the body is able to make enough vitamin A from these food sources for ideal health. If not, then dietary/supplemental vitamin A may be necessary, especially in certain conditions to maximize ones nutrient status.

                You will frequently hear many “experts” claim that you can get all the vitamin A you need from carotenoid conversion. This message goes hand in hand with the message of avoiding active vitamin A. In the short term, it is most likely true that most people can get by just fine on carotenoids as a primary or exclusive source of vitamin A. However, for optimal health, for pregnancy and child development, and for a host of medical conditions, active vitamin A is preferable, if not flat out required.

                If you think you’re supplying yourself with enough carotenoids, then you may want to reconsider. The body has to absorb and convert dietary carotenoids into active vitamin A as needed. The steps of absorption and conversion leave a great deal of variability. The type of carotenoid source, the way the food is prepared, the fat content of the meal, stomach acidity, and many other nutritional factors affect absorption, and ultimately conversion to vitamin A. For example, the absorption from raw carrots is virtually non-existent, whereas the absorption from carrot juice is up to 60%. (8) The studies show a wide range from a factor of 4:1 all the way up to a factor of 28:1. In other words, you have to consume anywhere from 4 to 28 units of carotenoids to get one unit of vitamin A.

                Here’s what a study looks like that determines these things. In Boston, 14 subjects under strict dietary controls were given active vitamin A, and organically labeled carrots and spinach. The amount of retinol (vitamin A) and carotenoids in the blood was measured. It was found that in this healthy, well-nourished adult group, where pureed carrots and spinach were given with fat, that the conversion for carrots was about 15:1 whereas the conversion for spinach was about 21:1. (9) The point is as one set of researchers puts it, “foods containing preformed vitamin A (carotenoids) need to be consumed in atypically large amounts in order to meet the vitamin A requirements.” (10)

                This previous lab experiment was done with healthy adults. What about many others. Pregnant women, children, and people with certain conditions should all most certainly consider their vitamin A status. In addition, as the topic of genes keeps coming up, according to one researcher, “It should be remembered that human subjects may have different abilities to convert provitamin A carotenoids to vitamin A. These differences in conversion efficiency may be due to the genetic variability in beta-carotene metabolism of individual human subjects. Therefore, provitamin A carotenoids might not be a good vitamin A source for those subjects of the poor converter phenotype” (11) The only caveat here is that we simply don’t know who these people are at this moment in time.

                So let’s take a look at a number of instances where vitamin A intake may very well be warranted. As you read through the rest of this chapter, keep a couple of items in mind. The only realistic source of vitamin A is from liver, although shellfish and egg yolks are a reasonable source as well. As our Paleolithic ancestors evolved over millennia, they at the whole “free range” game. This would include the muscle meat we eat today, but the brains, marrow, liver, and other organs as well. I know it sounds repulsive, but it’s as close to the truth as we can verify. Likewise, consider that our current standard American fare does not include liver, as we all know it’s quite distasteful to put it kindly. Lastly, reconsider a few of the afformentioned properties of vitamin A, immune strength and regulation, mucus membrane integrity to include the airways and gastrointestinal tract, growth, and embryonic development. If you pair this up against  IBD, asthma, infection, autoimmune disease, and birth defects, I would strongly argue that we are running relative vitamin A deficiencies in this country in the absence of rampant night blindness.