Vegetable vs. Animal Sources of Vitamin A: Why Eating Carrots Isn't Enough

Think you're getting enough vitamin A from carrots? Think again. (Photo by Ben Hussman)

In the comment section of my previous post on genes, diet and cavities, a couple of readers commented that vitamin A is necessary for proper dental health. I replied that I get plenty of beta-carotene (the vitamin A precursor found in vegetables and fruits) from red palm oil, which I use for most of my cooking these days.

Today, however, I came across a couple of papers looking at how well humans actually absorb beta-carotene and convert it to vitamin A (link, link). To my surprise, the conversion rate was much poorer than I'd previously thought. The first paper, which looked at beta-carotene absorption in 11 men, had this to say:

The vitamin A activity of ß-carotene is variable. The carotene in fruit, grains, and oils seems to be more effective as a source of vitamin A than that in dark-green leafy vegetables.

So, not all sources of beta-carotene are equally good in terms of absorption. The conversion rate also depends on the amount of beta-carotene (the higher the amount, the lower the rate) and whether fat is included or not.

Strikingly, only 6 of the 11 men included in the study absorbed and converted the beta-carotene they were given. The remaining 5 were classified as non-responders. The authors conclude that the vitamin A activity of beta-carotene can be "surprisingly low and variable". Even in those who did respond to supplementation, mean absorption was only ~4% and the conversion ratio was ~0.05.

In a similar study on 11 women, the same thing happened: only 6 of the women absorbed and converted beta-carotene enough to be measurable, while 5 women were non-responders. In those who did respond, mean absorption was ~6% and the conversion ratio ~0.1. This confirms earlier findings reporting that women absorb and convert beta-carotene more efficiently than men; the same also appears to be true in rats.

These figures suggest that the commonly accepted conversion rates of beta-carotene from plant sources may be too optimistic. Even red palm oil might not be up to par with animal sources. In one study, switching from green leafy vegetables to red palm oil did increase retinol levels, but only in subjects who were vitamin A deficient to begin with (link). And for many Westerners who don't eat palm oil or organ meats, the reality is even worse:

Also, it seems that the vitamin A activity of ß-carotene that is not dissolved in oil and emulsified is low and variable. Most ß-carotene in the American diet is not consumed in an emulsified form with fat. Our intent was to replicate a typical diet to develop better leads for how the body utilizes its given resources. The fat content of the meal that accompanied the doses in our study was the recommended amount, 30%. Many professionals recommend lower-fat diets.

Now there's something for the low-fat raw food vegetarians to chew on. On the other hand, the absorption of retinol, found in animal sources such as liver and eggs, appears to be much higher. Most of the estimates I've seen on various websites are between 60-90%, but even that may be too conservative, as the only actual study I could find showed that absorption was >99% (link).

Since my vitamin D levels are already good, and I take vitamin K2 supplements, the missing link in the trinity of dental health could indeed be vitamin A in my case. While I do eat eggs every now and then, their retinol content is only about 10% of that of liver. I guess it's time to put organs on the menu.

For more information dental health, see these posts:

Genes, Diet and Oral Health: Why Do Some People Get Cavities and Others Don't?
Tea, Coffee and Cocoa: All Good for Your Teeth
Dental Health Effects of Green and Black Tea
Preventing Mouth Ulcers with Tea Tree Oil Toothpaste - Results after Two Months