Some preamble:

Before we begin, we need to define an adequate dose, or our end goal in dosing. This end goal is sufficient vitamin D blood levels, generally defined as over 15 nmol/L or 10ng/ml (those are equivalent, just different units). Levels higher than 250nmol/L (100ng/ml) are considered excessive. “Optimal” are 75nmol/L and 30ng/ml (1).

You should also know, for comparison to the doses I talk about below, that the dose recommended to adults, infants, pregnant and nursing individuals, ranges from 400-600 IU (2). Yes that’s right. A 10lb infant and a 200lb adult supposedly need almost the same amount of vitamin D.
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Adults:
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I will talk about some special circumstances down below, but for most adults, a good dose of vitamin D is 4000 IU to 8,000 IU.

That’s a big range I know. So let me walk you through how I came up with that number, and then you can decide what dose you are comfortable with for yourself.

First of all, after my last post on toxicity, you shouldn’t be overly concerned with any dose of 10,000 IU or less. But just because 10,000 IU won’t kill you, doesn’t mean that’s how much you should take.

The RDA as I stated above is 400-600 IU. A few papers have come out since that recommendation was made and claim that there was a statistical error in the calculation that came up with that 400IU number. The papers recalculated the RDA and one estimates the true RDA to be 8895 IU and the other estimates 7000 IU (3, 4). However those assume you get no vitamin D from the sun and food sources. Therefore, the amount to supplement would be a bit lower than 8895 IU or 7000 IU a day.

So why not just dose at 6,000 IU for all adults or higher? Two reasons.

First, the studies I have found that dose at 6,000 IU or higher did not study people for a long period of time (5, 6). Therefore I am a bit wary of recommending people take doses of 6,000 or higher for a long time when the dose hasn’t been adequately studied. However, these doses are of course unlikely to be studied any time soon because they are so much higher than the current RDA. So we won’t have those data for a long time.

Secondly, there are a few papers that suggest a maintenance dose of about 4,000 IU is sufficient (7, 8, 9) to maintain blood levels at 80-90 nmol/L (which is in the optimal range), with no evidence of anyone becoming toxic at those doses.

So I am left with the large range of 4,000-8,000 IU for most adults. To determine what is right for you, I would think about where you live, how much time you spend tanning in the warm months, and what your own comfort level is. Of course, the best way to tell what dose is best for you personally (since everyone is a bit different) is to have your blood levels monitored. If your levels are in the low range of normal, increase your dose a bit and if they are getting too high, decrease a bit until you reach your goldilocks dose.

Racial differences:

What is considered “normal” is not different across different groups of skin color. What is different is the prevalence of deficiency. Meaning the darker your skin, the higher the risk of deficiency. So the issue is not so much “what dose should dark-skinned or light skinned people take” but “how do we get to optimal levels”.

The reason there is a deficiency in darker individuals is because making vitamin D is harder when your skin in darker – melanin is basically sunscreen. Add modern day clothing to that and it becomes hard to manufacture enough vitamin D no matter where you live. Therefore, I suggest taking vitamin D year-round if you have dark skin. If you live in a northern area and have dark skin, I would also dose at the higher end of the dosing spectrum – perhaps closer to 6000-8000 IU because you are not going to be manufacturing much yourself at high latitudes.

You may hear about a paradox with vitamin D in dark skinned populations. This “paradox” is that vitamin D deficiency doesn’t result in osteoporosis often in dark skinned people, even though it does in light skinned people. This has led some people to think that dark skinned people don’t need vitamin D. But that only makes sense if you think that vitamin D is only useful for bone health. Also, Dr. Vieth points out that the question should not be “why are black people fine (since humanoids evolved with dark skin and lost pigment as a response to lack of vitamin D at northern latitudes)” but instead “why do white people have such weak bones?”. You will find if you look at bone density by skin color, Caucasians have less bone than blacks at all ages. By looking at it in this light, it makes more sense that light skinned people lost bone mass as they went north because it allowed them to survive in conditions with low vitamin D. With less vitamin D up north, whites evolved to have small (weak) bones, because they needed less vitamin D to mineralize them properly. And regardless, we need vitamin D for immune and hormonal function, not just bones, so dark people should have normal levels of vitamin D too. (10, 11).

Pregnancy:

Vitamin D sufficiency in the person carrying the baby helps prevent low birth weight, decreases the size of the fontanelles and helps with normal weight gain and growth rates for both the carrier and baby. Interestingly for the person carrying the baby, there is some evidence that vitamin D supplementation improves the carrier’s protein nutrition, as opposed to the people only receiving placebo (12). Vitamin D deficiency is associated with slowed cardiovascular and brain development in the baby (13).

During pregnancy, doses of 4,000 IU have been studied without side effects and have shown to reverse deficiency if it exists in all races (14). This is important because pregnant persons with darker skin have higher risks of babies with rickets and other deformities due to low vitamin D levels (15). One study found that “minority” (I am assuming that means “not white”) pregnant women in the UK were given 800-1600 IU during pregnancy and were still deficient by the end of their pregnancy if they started pregnancy deficient. This is also true for all adults (16). So I would hazard a guess that pregnant people are just like other people. Shocking, I know. But there it is. If we consider the normal dose for adults to be 4000-8000, then I would guess that pregnant women need the same amount based on this data.

Lactation and the neonate:

I find this to be the most complicated of all populations to understand, partly because we just don’t have all the answers yet. But I will summarize what I have found as simply as I can, so that you can make your own decision.

Some history to put things in perspective:

In the eighties, hypoparathyroidism was treated by high dose vitamin D. Parathyroid hormone and vitamin D are essentially opposites and work against each other. So in hypoparathyroidism where the parathyroid hormone is absent, you essentially have too little calcium. Vitamin D increases your calcium, so they gave high doses of vitamin D to treat the hypoparathyroidism. There are cases from this time period of women given 100,000 IU a day who became pregnant, continued taking 100,000 IU a day and had normal babies (17). The women breastfed their babies, which meant the babies were getting the equivalent of 7,000 IU a day and had blood levels above the toxic dose of vitamin D and yet seemed healthy.

In Finland, infants used to be given 4000-5000 IU a day (18). This policy was in place for 15 years before it was decreased to 2000 IU for 10 years, and then reduced to 1000 IU and finally, today, to 400 IU. There were seemingly no ill effects during the years of high supplementation and the rates of diabetes type I (an autoimmune disease linked to vitamin D deficiency) has increased by 350% in children under four since the supplementation has decreased (19).

Discussion on toxicity in infants:

The lowest dose I found evidence for toxicity in an infant was 5,400 IU a day (20). There are many cases in the literature of toxicity in infants when the dose was 20,000-600,000 IU a day (21), but at those doses an adult could become toxic as well.

However note the difference between the hypoparathyroid group, the Finnish history and toxic cases: in the hypoparathyroid group, the mothers got high doses of vitamin D, and in the last two cases, the infants were given the higher doses of vitamin D. Now even the high dose in Finland (4000-5000 IU) was lower than the lowest toxic dose (5400 IU), which may be why no issues were seen in Finland. But notice the key sentence in the hypoparathyroid study: “[the babies] had blood levels above the toxic dose of vitamin D and yet seemed healthy”. So what’s going on?

If infants who are not supplemented and are exclusively breastfed are tested for vitamin D, they are found to have a specific, active, metabolite of vitamin D present, but no vitamin D itself (22). Meaning they don’t have any of the substance present that is in a vitamin D pill (called cholecalciferol), but they have a substance your body makes from the vitamin D pill (called 25-hydroxyvitamin D or 25(OH)D). Therefore, before supplements were invented, the vitamin D an infant would get is a different form than what we give them today, and the infant would be completely reliant on how much vitamin D the lactating mother has. Researchers say the amount of 25-hydroxyvitamin D (25(OH)D) present in breastmilk is a linearly related to the amount the lactating mother has circulating in her blood. Therefore the more vitamin D the mother has, the more the babe gets.

Perhaps then (and this is just my theory based on the above evidence), infants only become toxic when supplemented with cholecalciferol at levels akin to 5400 IU or over.

Now, I do not mean to imply that if you are breastfeeding, you shouldn’t also give vitamin D to your baby. 400 IU is recommended for infants being breastfed, and for once, the data seem to indicate that’s an appropriate dose (23, 24). Additionally, the vast majority of ricketic (ie severely vitamin D deficient) infants are breastfed infants (25, 26).

However, the question remains that at what dose of vitamin D for the lactating person, will there be enough vitamin D in the breast milk for the baby? As far as I can tell, only one study looked at this, and the dose was 6400 IU (27). So in other words, a lactating person taking 6400 IU a day supplied the equivalent of 400 IU to the infant in her breastmilk. This was only one study however, and a small one, so I wouldn’t take the number to be definitive. There was another study that looked at something similar with the lactating person taking 4000 IU instead, but they only reported that the infant’s levels were “normal” when the mother was taking this dose (28), they did not report whether the infant reached the same amount it would have if supplemented with vitamin D.

Pediatrics (kids from 1-18):

There is very little data on pediatric populations over 1 or 2 years old, which makes this a difficult population to make recommendations for. Additionally, as opposed to the 100ng/ml toxic range mentioned at the beginning of the article, out of caution the range is lowered in pediatric cases to 50ng/ml (29). Therefore, dosing is even more cautious than in the adult population (not that I want to make kids sick with vitamin D of course, it’s just hard to know how to dose appropriately without good data).
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Below is a chart that shows the various recommendations for aged children (30). Now there have been a few studies at the upper limit level (UL) of 1000-2000 IU/d that show no harm and a healthy increase in vitamin D blood levels (31). I would guess then, that the appropriate dose is closer to this “upper limit”, but again, it’s hard to know for sure.

Dosing by bodyweight:
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A final way to calculate the appropriate dose, regardless of age, is by adjusting the dose to your weight. There was a study that tried to quantify the increase in blood level with different dosing schemes (32). In that paper, there is a graph that shows the dose of vitamin D by body weight and the corresponding blood level (I apologize for the academic look of the graph! It's a very statistics-heavy paper; I don't recommend reading it unless you have insomnia!)

The green bar I added at the side shows what the optimal blood level is. The blue lines show what the dose per kg would be to get to that blood level. I put the highest blood level a bit lower than the top of optimal just out of an abundance of caution.

​This chart shows a dose of 1.5-3.5mcg/kg to get to optimal blood levels. Let’s calculate a dose in the middle – 2.5 mcg/kg, which is equivalent to 100 IU/kg. So if we calculate the infant, pediatric and adult doses it looks like this:

7.5lb baby = 3.4 kg = 340 IU/d
90lb child = 40.8 kg = 4000 IU/d
154 lb adult = 70 kg adult = 7000 IU/d
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These results are remarkably like the doses discussed throughout this post. So perhaps for pediatrics, this is a great guidance tool, as children grow quickly and at individual rates. There have been other papers that look at the dose per kg bodyweight, but I haven’t found specific numbers in any of them (33).

Conclusion:
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So after all this, I do want to stress that the only way to be absolutely sure the dose you are taking is safe and effective is to have your blood levels of vitamin D monitored by a health professional. This is especially true if you start using higher doses of vitamin D in the 6000-10,000 IU range because these doses haven’t been studied frequently. The point of this article is of course just to educate you on the data available for different dosing strategies so you can make an informed decision with your health care provider. After all, knowledge is power. ​

More on vitamin D: If you are interested in the benefits of vitamin D, you can read more here, or more about toxicity and toxic doses here.