The Earthen Chalice

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Also known as the “sunlight vitamin”, this little substance has gained a lot of popularity within the last few years. For some, it has even taken a top spot as a “cure-all”, right next to the famed vitamin C. The supposed health benefits of supplementing vitamin D range from stronger bones to enhanced immunity and even cancer prevention. But does it live up to expectations?

Fig. 1: Good ol' sunlight
Fig. 1: Good ol' sunlight

What does it do?

Strengthens disease immunity, improves bone and muscle growth and performance, may possibly help with wound healing, allergies and cancer prevention.

What if I get too little?

Feelings of weakness, fatigue and headaches are common. Back pain may result from weaker bones and muscles. Bone loss may occur long-term. Could possibly result in depression.

How much do I need?

While official recommendations are at 600 IU or 15 µg per day, the dose necessary for optimal vitamin D levels is actually between 2,600-4,000 IU (65-100 µg) per day. Most sources put the safe upper limit at 4,000 IU per day, some go as high as 10,000 IU daily.

Should I supplement?

If you don’t spend a lot of time in the sun, supplementation makes sense. Fifteen minutes a day of sunlight are not enough to reach proper levels, and most foods do not contain any vitamin D. Taking between 2,200-4,000 IU once a day or 18,200-28,000 IU once a week of vitamin D3 will safely raise your serum levels to their optimum.


Health effects

The well-connected hormone

Looking at the scientific literature, it seems like vitamin D is easily going to live up to the hype. One reason why it’s so powerful is probably because it’s secretly a hormone, and an important one at that. In fact, the hormone we call vitamin D can interact with almost all of the 250 different tissue types of the body.[1] And while most of these tissues can live with very little of the vitamin, there are some areas of health that improve a lot as we optimize our blood serum levels. Optimal levels of vitamin D will support our immune function by improving defense against both bacteria and viruses. If we do catch a disease, our symptoms will be reduced. Vitamin D improves both bone and muscle growth, and physical performance. Finally, it could possibly accelerate wound healing and protect us from allergies and certain kinds of cancer.


– tightens connections between cells, improves their communication and defense against microbes[2]

– upregulates general immunity to help stave off bacteria & viruses before they can cause disease[3]

– fine-tunes specific immunity and keeps inflammation low, which makes many disease symptoms milder[4]

possibly keeps your immune system from attacking itself, preventing auto-immune diseases[5]

Musculoskeletal system

– helps muscles to take up nutrients and reduce oxidative stress, increasing performance and muscle growth[1]

– helps ship calcium into your bones and keeps bone-inflammation (yes, that’s a thing!) to a minimum to protect you from osteoporosis[6]

may help initiate the early stages of wound healing[7]


possibly protects you from colon and breast cancer, though this is not well-documented[8]



Craving that sunlight

By the most optimistic approach, about 1 billion people in the world are deficient in vitamin D, or at least have suboptimal levels.[9, 10] In many areas of the world, this number can go up to 80% or more.[11] If the truth lies somewhere in the middle, the chance that you have chronically low levels of vitamin D right now is probably around 50% – the classic coin flip. If you don’t spend a lot of time in the sun, or live in an area of the world where the sunlight just isn’t as intense, a vitamin D deficiency is very likely. Once you have too little vitamin D circulating in your system, the negative effects are basically the opposite of what we find in the previous section. You’ll be more likely to catch infectious diseases, you’ll have them for longer and they’ll feel worse. You may feel weak, fatigued and suffer from headaches. It’s more likely that you will develop back pain, and over longer time spans you may suffer loss of bone mass. Wound healing could be slowed down. And finally, it’s possible that low vitamin D levels can even cause depression, though this isn’t 100% certain yet.


– lacking immune system stimulation makes it more likely that you catch infectious diseases[3]

– lack of anti-inflammatory signalling worsens most symptoms[4]

may make you more likely to suffer from allergies[5]

Musculoskeletal system

– muscle regeneration and performance take a dip, resulting in feelings of weakness and fatigue, and sometimes headaches[13]

– weakened muscles and lack of calcium regulation can combine to cause back pain, bone pain and/or bone loss[14]

– early stages of wound healing could possibly be impaired, slowing the process[7]

Psychological effects

– low levels of vitamin D may lead to depression, more studies are needed[15]



Perfect illumination

There is some confusion around how much vitamin D our body needs every day, and how much we should take up with the food we eat. Since the body can synthesize the vitamin from cholesterol and sunlight, we don’t necessarily have to eat any at all. But seeing that we don’t spend as much time in the sun as our bodies would like us to, it makes sense to think about edible vitamin D-sources.

Official recommendations from the National Institute of Health (NIH) state that our daily dietary need is 600 IU (15µg) of vitamin D at minimum sunlight exposure.[16] These amounts were set to reach blood levels of vitamin D that are enough to “maintain bone health and normal calcium metabolism in healthy people”. That’s nice and all, but it’s not the same thing as having levels that are optimal for our health. But what is optimal? See the collapsible text for more details on how vitamin D status is measured and the official recommendations, or just skip ahead.

Vitamin D status is measured via a direct precursor of vitamin D called “25-hydroxyvitamin D” or 25HVD. The levels of this substance in the serum are simply more stable and therefore more reliable to measure than the active form of vitamin D. For most intents and purposes, it is interchangeable with active vitamin D, and will be referred as such in the rest of the text.

The NIH claims the following about blood levels of this previtamin, with concentrations of 25-hydroxyvitamin D in both nanomoles per liter (nmol/L) and nanograms per milliliter (ng/mL):



Health status



Associated with vitamin D deficiency, leading to rickets
in infants and children and osteomalacia in adults

30 to <50

12 to <20

Generally considered inadequate for bone and overall health
in healthy individuals



Generally considered adequate for bone and overall health
in healthy individuals



Emerging evidence links potential adverse effects to such
high levels, particularly >150 nmol/L (>60 ng/mL)

* 1 nmol/L = 0.4 ng/mL

From the official recommendation table, we can learn two things. First, both very low and very high levels of vitamin D have negative health effects – this is called a “U-shaped curve”. Second, the official recommendation for an optimal level is between 50-125 nmol/L vitamin D in the serum. People from “traditionally living” populations, who still spend a lot of time in direct sunlight, were measured to have around 115 nmol/L.[17] So it seems like the actually optimal level will be around the top end of the NIH recommendation. The only thing left to do now is to find out how we can reach a level of around or slightly over 100 nmol/L of vitamin D in the serum.



Bottled sunrays

Unfortunately, the often recommended 15 minutes a day of sunlight around noon, arms and face exposed, will only work in certain parts of the world and will not necessarily result in optimal levels of circulating vitamin.[18, 19] Notable food sources for vit D are fatty fish, cod liver and animal organs – not very promising. Tanning beds are not effective enough, and are in fact just an avoidable risk factor for skin cancer.[20] What about tanning mushrooms to make them produce vitamin D2 instead? Sounds like fun, but it’s hard to find a proper dosage. Also, you’ll be doomed to eat mushrooms every single day for the rest of your life.

So basically, for everybody who doesn’t spend as much time in the sun as the “traditionally living” populations, taking a vitamin D supplement makes good sense. Since the NIH-recommended dose of 600 IU or 15 µg per day should get us to a serum level of 50 nmol/L, couldn’t we simply double that dose to reach an optimal level of around 100 nmol/L? Not a bad idea, but lucky for us, several studies have actually tested different dosages and blood levels. In one of those studies, a daily dose of 2600 IU was enough to keep serum levels at around 80 nmol/L.[21] Sounds good! In another study, an average dosage of 3440 IU vitamin D per day resulted in serum levels between 70-150 nmol/L. Sounds even better! The authors then calculated a dose of 4600 IU per day to keep our levels optimal.[22] Seeing that 4000 IU is documented to be the safe upper limit for everyone over 8 years of age, this dosage is going to be on our high end.[23]

In conclusion, we can say that a dose of 2600-4000 IU of a vitamin D supplement per day seems suitable to keep us healthy and optimize our vitamin D serum levels long-term. We can always take less or skip supplementation after a day of generous sun exposure. Alternatively, larger doses may be taken once a week or even just once a month (see FAQ).


Frequently asked questions (FAQ)

Since there is a large range of healthy levels of vitamin D, we can allow ourselves an equally large range of our supplement dosage. If you get very little sunlight, 4000 IU is probably a good dose. If you spend some time outside but want to make sure that you have optimal levels, aim for about 2600 IU. Additionally, obese people and people over 71 years of age have increased requirements and should definitely aim for the higher end.[16, 24]

Generally, children need less vitamin D because of their smaller body size, and they have different upper intake limits.[25] According to the EFSA Panel on Dietetic Products, these limits are:

0-1 years: maximum of 1000 IU/day

1-10 years: maximum of 2000 IU/day

Additionally, many foods meant for infants are fortified with vitamin D, which should be included in the calculations. If children play outside a lot, they’re likely to get enough sunlight to produce their own vitamin D, and may not need supplements.

According to a study on elderly people, taking megadoses of vitamin D once a week or even just once a month seems to work well to increase levels of circulating vitamin. In the study, serum levels of the participants all ended up at around 80 nmol/L, no matter whether the supplement was taken daily (1,500 IU once a day), weekly (10,500 IU once in 7 days) or monthly (45,000 IU once in 28 days).[26] If you want an amount of 2,600-4,000 IU daily, your weekly doses would end up at 18,200-28,000 IU, and monthly doses at 78,000-120,000 IU. But is it safe to take that much at once?

To compare, we can look at multiple studies which tested the following doses, all without negative effects:

  • a single loading-dose of 500.000 IU of vitamin D in deficient people[26, 27]
  • 50,000 IU per day for 8 weeks[28]
  • 10,000 IU per day for up to 16 weeks[29]


Vitamin D toxicity generally only happens when serum levels are very high in the long term (well above 250 nmol/L). To reach such levels, one would have to take doses of at least 10,000 IU per day over multiple months, adding up to a regular monthly dose of a whopping 300,000 IU.[27] That’s quite a lot.

In conclusion, even a monthly dose of 78,000-120,000 IU is likely to be perfectly safe. If you don’t feel safe taking so much at once, you can always go for the weekly dose of 15,400-28,000 IU.

There are practically no arguments for vitamin D2, but some against it. It seems to make no difference up to doses of 2000 IU/day, but above that D3 was shown to be superior.[30] So vitamin D3 would probably be better, but in doses between 2000-4000 IU a D2 supplement is unlikely to be much worse.

Originally, official recommendations were based on study data from the year 1997 and were around 400 IU per day for adults. In 2011, they were reworked and are now… wait for it… 600 IU per day for adults.[31] While that’s technically a 50% increase, it’s still not nearly enough to reach optimal levels. In 2011, the study data on optimal dosage presented in this article was already available. The reason why guidelines are so low is probably because they only aim at preventing bone loss, and therefore data on optimal serum levels was not considered.

At least the upper intake levels were increased from 2000 IU to 4000 IU daily, which allows us to reach optimal vitamin D serum levels while staying within official safety guidelines. Hooray!



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  4. Prietl, B., et al., Vitamin D and immune function. Nutrients, 2013. 5(7): p. 2502-2521.
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  7. Oda, Y., et al., Vitamin D and calcium regulation of epidermal wound healing. The Journal of steroid biochemistry and molecular biology, 2016. 164: p. 379-385.
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  13. Knutsen, K.V., et al., Vitamin D status in patients with musculoskeletal pain, fatigue and headache: a cross-sectional descriptive study in a multi-ethnic general practice in Norway. Scandinavian journal of primary health care, 2010. 28(3): p. 166-171.
  14. Heidari, B., et al., Association between nonspecific skeletal pain and vitamin D deficiency. International journal of rheumatic diseases, 2010. 13(4): p. 340-346.
  15. Anglin, R.E., et al., Vitamin D deficiency and depression in adults: systematic review and meta-analysis. The British journal of psychiatry, 2013. 202(2): p. 100-107.
  16. Del Valle, H.B., et al., Dietary reference intakes for calcium and vitamin D. 2011: National Academies Press.
  17. Luxwolda, M.F., et al., Traditionally living populations in East Africa have a mean serum 25-hydroxyvitamin D concentration of 115 nmol/l. British Journal of Nutrition, 2012. 108(9): p. 1557-1561.
  18. Rhodes, L.E., et al., Recommended summer sunlight exposure levels can produce sufficient (≥ 20 ng ml− 1) but not the proposed optimal (≥ 32 ng ml− 1) 25 (OH) D levels at UK latitudes. Journal of investigative dermatology, 2010. 130(5): p. 1411-1418.
  19. Binkley, N., et al., Low vitamin D status despite abundant sun exposure. The Journal of Clinical Endocrinology & Metabolism, 2007. 92(6): p. 2130-2135.
  20. Woo, D.K. and M.J. Eide, Tanning beds, skin cancer, and vitamin D: an examination of the scientific evidence and public health implications. Dermatologic therapy, 2010. 23(1): p. 61-71.
  21. Heaney, R.P., The vitamin D requirement in health and disease. The Journal of steroid biochemistry and molecular biology, 2005. 97(1-2): p. 13-19.
  22. Aloia, J.F., et al., Vitamin D intake to attain a desired serum 25-hydroxyvitamin D concentration. The American journal of clinical nutrition, 2008. 87(6): p. 1952-1958.
  23. Holick, M.F., et al., Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline. The Journal of Clinical Endocrinology & Metabolism, 2011. 96(7): p. 1911-1930.
  24. Wortsman, J., et al., Decreased bioavailability of vitamin D in obesity. The American journal of clinical nutrition, 2000. 72(3): p. 690-693.
  25. EFSA Panel on Dietetic Products, N., et al., Update of the tolerable upper intake level for vitamin D for infants. EFSA Journal, 2018. 16(8): p. e05365.
  26. Bacon, C., et al., High-dose oral vitamin D 3 supplementation in the elderly. Osteoporosis International, 2009. 20(8): p. 1407.
  27. Han, J.E., et al., High dose vitamin D administration in ventilated intensive care unit patients: a pilot double blind randomized controlled trial. Journal of clinical & translational endocrinology, 2016. 4: p. 59-65.
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  29. Heaney, R.P., et al., Human serum 25-hydroxycholecalciferol response to extended oral dosing with cholecalciferol. The American journal of clinical nutrition, 2003. 77(1): p. 204-210.
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