Sunday, August 23, 2015

Hypervitaminosis A and sweet potatoes

Can consumption of sweet potatoes cause hypervitaminosis A? The answer is “no”, even if you eat ten or more sweet potatoes per day. Sweet potatoes do have high vitamin A content, more than almost any other food. However, most of it is in the form of β-carotene, which is used by the body to produce the active form of vitamin A, retinal (yes, with an “a”), only if the body’s vitamin A status is low.

The graph below shows the vitamin A content of different foods, together with the recommended daily allowance. It was prepared with information from (), with the horizontal axis in international units (). The graph also takes into consideration some key research findings related to the bioavailability of vitamin A. For example, the sweet potato is assumed to be taken with some fat to facilitate the absorption of vitamin A.

Primarily, vitamin A is available either as retinol, from animal foods; or β-carotene, from plant foods. There are other carotenes available from plant foods, but their vitamin A contribution is relatively small compared with β-carotene. High β-carotene content is “advertised” by plant foods to animals via a characteristic orange color. The main sources of β-carotene throughout human evolution have probably been fruits, which plants “want” animals to eat so that the plants’ seeds are dispersed.

Retinol also needs to be converted by the body to retinal, and when consumed in excess it tends to be stored in body fat reserves – hence lean individuals tend to store less retinol than fat ones. It seems that intake of retinol from sources like beef liver is naturally controlled via satiety. In the case of plant sources, like sweet potatoes, a key control mechanism is limited internal production of retinal. My impression is that most people, if given the chance, would prefer to eat a lot of sweet potato than a lot of beef liver.

Like all of the fat-soluble vitamins, the bioavailability of vitamin A from foods is dependent on whether they are consumed together with fat. For example, a lot more vitamin A will be absorbed from a sweet potato if it is eaten with butter than if it is eaten by itself (again, if the body’s vitamin A status is low). I should note that butter is itself a good source of vitamin A, in addition to providing the fat needed for absorption. Beef liver is low in fat, which means that the vitamin A content in the graph above may be an overestimation.

Hypervitaminosis is a fat-soluble vitamin phenomenon, and it is usually associated with consumption of supplements (e.g., cod liver oil). Generally speaking, one does not develop noticeable hypervitaminosis symptoms from consumption of natural food sources. This is probably due to a combination of satiety and internal regulation of the production of the active forms of the vitamins.


THEGAP said...

Why didn't you mention red Palm oil as a source of vitamin a and a potential cause of hypervitaminosis A?

js290 said...

Satiety and internal regulation... no need to outsmart Nature.

Nick said...

Hi Ned,

Do you recommend that one not supplement with cod liver oil? Or is there an amount you think if okay, say a tablespoon once or twice a week?

Thank you

Ned Kock said...

The “red” and minimally processed variety of palm oil appears to be indeed a good source of vitamin A, and has in the past at various times been considered a “miracle food”. Unfortunately, and unlike butter, it carries a load of omega-6 fats, which are generally pro-inflammatory.

Ned Kock said...

Generally I would not recommend supplementation of any kind, unless closely monitored by a specialized health professional based on regular targeted tests. Supplements tend to bypass our body’s mechanisms to avoid overdosing on nutrients, such as satiety and regulated endogenous production.

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Ugur Gundomgus said...

Hi Ned.

You say "You would not recommend supplementation of any kind, unless closely monitored by a specialized health professional based on regular targeted tests".

Are you really against all kind of supplements? For example, what about vitamin D supplementation for someone who lives in country where there is no sunlight in the winter?

Thanks in advance.

P.S. I believe you already know. But, I just want to say it again: Spammers write stupid posts on your blog. It may be a good idea to get rid of them.

Ned Kock said...

Hi Ugur. I will split my answer in three parts.

Ned Kock said...

Vitamin D is usually perceived as a good candidate for supplementation. However, we produce about 1,000 IU per minute of sun exposure – that is a lot, and fast. This assumes a level of skin exposure comparable to that of someone wearing a bathing suit. (See links below.)

Given the patterns of equinox and solstice that cause the seasons, vitamin D production from the sun does not cease at high latitudes. Vitamin D that is stored in fat in spring and summer will be released during fall and winter – as in a battery:

Ned Kock said...

Also, some evidence exists of a relatively uniform occurrence of low vitamin D levels in Indians living in India and Chinese in China. This goes somewhat against the idea that latitude is a major factor in defining vitamin D levels (see links below).

Low animal fat diets may be partly to blame in these cases, as animals also store fats-soluble vitamins in their fat tissues. Whether skin pigmentation is to blame is a matter of some debate. For example, the evidence in the post linked below suggests that skin pigmentation has little effect on the amount of vitamin D produced from sun exposure. Interestingly, low total cholesterol seems to be associated with impaired ability to produce vitamin D from sun exposure.

Ned Kock said...

Additionally, it seems that vitamin D acts in concert with other nutrients, particular vitamins A and K. Thus raising one’s vitamin D levels without increases in vitamins A and K may not be very effective. Exceptions would be diseases stemming from severe vitamin D deficiency, where supplementation seems to make a big difference for the better.

Finally, the active form of vitamin D is rarely measured. What is usually measured is the prehormone calcidiol; a.k.a. 25-hydroxycholecalciferol, or 25-hydroxyvitamin D, and abbreviated as: 25(OH)D. There is some evidence that the active form of vitamin D does not vary nearly as much as the measured one.

Ned Kock said...

Btw Ugur, I haven’t found an efficient way of getting rid of spam. I agree it is a nuisance. If you know of a low cost way of blocking spammers, please let me know. I don’t consider low cost having to moderate every single comment.

Skyler said...


Great post. I remember from one of my nutrition courses (my former professor is a leading researcher on Vitamin A) that it took 20 units of beta carotene to convert into 1 unit of retinol. So divide that sweet potato amount by 20 and you get a much smaller number even before your point about how it's used.

Great post!

Ugur Gundogmus said...

Thank you very much for your detailed response, Ned.

2 years ago, when I read your blog post about Paul Jaminet's book PHD, I had bought it and read it. Plus, had I started some of the supplements Paul recommended. Because his arguments made sense.

As far as I understand, Paul and you share similar approaches to diet and nutrition in general. For some reason, I thought you agreed with him on his supplementation list as well. For this reason, I was surprised to learn that you're not a big fan of supplements.

In that sense, I think you're little bit closer to Stephen Guyenet (He only recommends vitamin D in the winter months if one doesn't get enough sunshine. That's why I mentioned vitamin D in my question).

Anyway... It seems that I need to think more carefully about my supplementation intake. Having said that, since I started following PHD for 2 years, all my blood tests have been really good. Maybe, this has nothing to do with my supplement intake. I could be just my diet. But, it's something I wanted to mention.

Thanks again for your response and especially for this wonderful blog. I always check it to see if you post something new.

As for the spammers, unfortunately, the best way is moderation and I understand that this is kind of time consuming for you. This means we'll be seeing these spam posts in the future:) No big deal at all.

Obat Herbal Green World said...
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Rahasia Cantik Alami said...
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Ned Kock said...

Spam comments above deleted.

Ned Kock said...

I’ll give moderation a try. The volume of spam is getting ridiculous.

Anonymous said...
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ben said...

Is there a typo regarding pre-formed animal-based retinol and plant-based pro-vitamin A (carotenoids)?

Because, the post mentions that 'Retinol also needs to be converted by the body to retinal..' , but isn't it the other way around?

The Wiki page for Retinal for example, says 'The other main forms of vitamin A, retinol, and a partially active form, retinoic acid, may both be produced from retinal.'

Ned Kock said...

Hi Ben. It is not a typo, but contrasting retinol with retinal is not easy. They have similar properties and functions. See this attempt:

Retinol is used to produce retinal, but the other way around is also possible – presumably for better storage, if retinal needs are satisfied.