I think almost everyone is quite familiar with the idea that carrots are good for your eyes. This is supposedly due to the beta-carotene contained. Often times beta-carotene is put on the same pedestal as preformed vitamin A, also referred to as retinol, as if they are equivalent. But it’s not that easy of course.
Many “superfoods” get their label partially because they are supposed good sources of vitamin A. Among them are the classic “orange” foods like carrots, sweet potatoes, mangos, pumpkins and cantaloupe, but also lesser known sources like brussels sprouts, kale, spinach and broccoli.
What is vitamin A?
Vitamin A, also called Retinol, is an essential vitamin for animals. It is required for proper embryo development, the immune system and for proper vision. Vitamin A deficiency can lead to:
- Xerophthalmia, a severe dryness of the eyes possibly leading to blindness
- Nyctalopia aka night blindness
- Dry skin & hair
- Infertility
- Infections
What is beta-carotene?
Beta-carotene is a red-orange pigment that can be abundantly found in many different plants and it’s often responsible for the orange color, hence the common association with the name-giving carrots. When consumed it doesn’t have any inherent uses for our bodies, but through enzymes we can convert it into useable vitamin A. It is therefore considered a provitamin.
This is now the part where most websites will stop going into any further details. Go eat your veggies, everything’s great! But not here, we’re going to dig deeper.
Beta-carotene bioavailability
The bioavailability of beta-carotene is extremely variable, between 5-65%. There are a large variety of factors that affect how well your body can utilize it, including:
- Food matrix: Fiber “traps” nutrients
- Food preparation: Cooking increases bioavailability.
- Meal content: Especially fats are needed to absorb beta-carotene, although very little (~5 g) is sufficient.
- Carotenoid content: The presence of other carotenoids and the overall amount of carotenoids can impact bioavailability.
- Current vitamin A status: You might absorb less if your current vitamin A status is good
- Gut integrity: Your gut integrity can impact how much you can absorb
This leads to a huge variability of bioavailability and conversion of beta-carotene. Carrots for example fall somewhere between 7-65%. Spinach is between 5-26% and broccoli (there’s only one study) was found to have 12% bioavailability. The best estimate for overall bioavailability can possibly be inferred by the studies looking at “mixed vegetables and fruits” which puts it at ~14%.
Beta-carotene conversion rates
Up to 2010 the beta-carotene equivalency was assumed to be 6:1 by weight, meaning 6 µg of beta-carotene are equivalent to 1 µg of retinol. This has since been revised to be 12:1 (or 8%) by the Food and Nutrition Board, but it’s still highly dependent on the aforementioned factors and ranges from 10:1 up to 28:1 on average.
Fruits seem to be favorable with conversion rates of 10:1 compared to vegetables with 26:1. But the studies also showed extreme variability from the same food sources. The previous numbers have all been averages, whereas individually measured were conversion rates as high as 3:1 and as low as 84:1.
Genetic variations
There are several genetic variations that can impact how much beta-carotene you can convert which could in part explain the vast ranges observed in the studies. Unless you do genetic testing, you have no idea if you are one of those low-responders. Averaging out the frequency and impact of these variations from the study (see references) ~38-84% of women have a ~53% worse enzyme activity. So all things considered, it’s not unlikely that you could be one of those low-responders.
Averages
The only way to make any kind of conclusive statement from this is to consider averages. On average, the conversion rate is 12:1. This is fine when talking about populations in general, but it’s less suitable when talking about an individual. Your conversion rate might be 28:1, or 84:1 due to genetics. Maybe your choice of food has much less favorable conversion rates.
In other words: It’s quite likely that you get less vitamin A than ideal from these sources. Or at least less than you’d expect, which could fool you into thinking you are getting adequate levels of vitamin A when you’re not.
Carotenosis
Beta-carotene is primarily stored in fat cells. The conversion into retinol happens for the most part in the intestines, which means all that stored beta-carotene will largely not be used for that purpose. This can lead to carotenosis, which is considered benign and reversible. But it will still color your skin orange. If you ask me, I’d rather not have orange skin.
Getting real vitamin A
It is pretty easy to get real vitamin A. It is found preformed and highly absorbable exclusively in animal foods. Liver is very high in retinol, so much so that overconsumption is not advised. 100 g of liver comes with a whopping 21000 µg of retinol. Although, some people in the primal-diet-sphere disagree with that assumption because vitamin A toxicity from natural sources (read: not supplements) has rarely been observed.
Eggs are a little bit more tame, but a great source regardless. 100 g of eggs contains 180 µg of vitamin A. Other great sources include various types of cheese, butter and milk.
Conclusion
On paper, beta-carotene can provide some amount of useable vitamin A for humans. However, bioavailability and conversion rates are highly variable between individuals and food sources, with some getting practically nothing out of it. Of course you can just assume that you’re getting enough based on the average, but keep in mind that the average human has less than two legs. In other words, it is simply an unreliable source of vitamin A.
But why bother with all that guesswork? Preformed vitamin A is right there in a variety of foods. Retinol has bioavailability between 75-100%, and without needing to convert any of it. Also it doesn’t turn your skin orange, which is great.
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