Vitamin D levels: Sunlight, age, and toxicity

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This article by “The Health Correlator” examines the effects of sunlight and vitamin D has on age, body toxicity, and disease. Most interestingly, natural sunlight vitamin D does not show signs of toxicity at any level.  However artificial vitamin D in the form of dietary supplements does shows sign of toxicity at high dosages.

Vitamin D levels: Sunlight, age, and toxicity

Ned Kock
Saturday, February 6, 2010

Calcidiol is a pre-hormone that is produced based on vitamin D in the liver. The term “vitamin D” is used here as synonymous with “vitamin D3”. This is the over-the-counter form, which is different from vitamin D2. The latter is the one often prescribed by doctors in spite of being clearly inferior to D3 in terms of positive health effects.

Blood concentration of calcidiol is considered to be a reliable indicator of vitamin D status. In the research literature, calcidiol is usually referred to as 25-Hydroxyvitamin or 25(OH)D. Calcidiol is converted in the kidneys into calcitriol, which is the active form of vitamin D.

The table below (from: Vieth, 1999; full reference at the end of this post; click on it to enlarge), shows the average blood vitamin D levels of people living or working in sun-rich environments. To convert from nmol/L to ng/mL, divide by 2.496. For example, 100 nmol/L = 100 / 2.496 ng/mL = 40.1 ng/mL. At the time of this writing, Vieth (1999) had 692 citations on Google Scholar, and probably more than that on Web of Science. This article has had, and continues having, a high impact among researchers.


The maximum average level of blood (or serum) vitamin D shown in the table is 163 nmol/L (65 ng/mL). Given that the human body produces vitamin D naturally from sunlight, it is reasonable to assume that those blood vitamin D levels are not yet at the toxic range. In fact, one of the individuals, a farmer in Puerto Rico, had a level of 225 nmol/L (90 ng/mL). That individual had no signs of toxicity.

Several studies show that pre-sunburn full-body exposure to sunlight is equivalent to an oral vitamin D intake of approximately 250 µg (10,000 IU).

In spite of claims to the contrary, vitamin D production based on sunlight does not cease after 40 years of age or so. Studies reviewed by Vieth suggest that among the elderly (i.e., those aged 65 or above) pre-sunburn full-body exposure to sunlight is equivalent to an oral vitamin D intake of 218 µg (8,700 IU).

Sunlight-induced vitamin D production does seem to decrease with age, but not dramatically.

Post-sunburn sunlight exposure does not increase vitamin D production. Since each person is different, a good rule of thumb to estimate the number of minutes of sunlight exposure needed to maximize vitamin D production is the number of minutes preceding sunburn. For a light-skinned person, this can be as little as 7 minutes.

Vitamin D accumulation in the body follows a battery-like pattern, increasing and decreasing gradually. The figure below, from Vieth’s article, shows the gradual increase in blood vitamin D concentrations following the start of daily supplementation. This suggests that levels start to plateau at around 1 month, with higher levels reaching a plateau after 2 months.


While sunlight exposure does not lead to toxic levels of vitamin D, oral intake may. Below is a figure, also from Vieth’s article, that plots blood levels of vitamin D against oral intake amounts. The X’s indicate points at which intoxication symptoms were observed. While typically intoxication starts at the 50,000 IU intake level, one individual displayed signs of intoxication at 10,000 IU. That individual received a megadose that was supposed to provide vitamin D for an extended period of time.


Non-toxic levels of 10,000 IU are achieved naturally through sunlight exposure. This applies to modern humans and probably our Paleolithic ancestors. Yet, modern humans normally limit their sun exposure and intake of vitamin D to levels (400 IU) that are only effective to avoid osteomalacia, the softening of the bones due to poor mineralization.

Very likely the natural production of 10,000 IU based on sunlight was adaptive in our evolutionary past, and also necessary for good health today. This is consistent with the many reports of diseases associated with chronic vitamin D deficiency, even at levels that avoid osteomalacia. Among those diseases are: hypertension, tuberculosis, various types of cancer, gingivitis, multiple sclerosis, chronic inflammation, seasonal affective disorder, and premature senescence.


Reinhold Vieth (May 1999). Vitamin D supplementation, 25-hydroxyvitamin D concentrations, and safety. American Journal of Clinical Nutrition, Vol. 69, No. 5, 842-856.



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ryan @ vitamin d benefits said:

on December 24th, 2010

When it comes to the how vitamin D benefits your overall health in a lot more ways than is commonly known. The ‘sunshine’ vitamin, as it is sometimes called, is commonly known for its effect on strengthening the bones and joints in the human body by aiding the absorption of calcium. It also helps the body build up immunity. Our bodies can produce vitamin D on it’s own, if we have sufficient amounts of sun (at least an hour a week). Alternatively, it is often added to certain foods such as milk, egg yolks, fatty fish, sardines, mackerel, tuna and salmon. These vitamin d rich foods are known to be a great source of omega 3 fatty acids. Cod liver oil capsules are one of the most common nutrient and Vitamin D dense supplements. I’ve found that adequate vitamin D levels are absolutely essential for a healthy body. I hope it helps you as much as it did for me.


Michael Light said:

on January 25th, 2017

What if the question in this world was how much shadow is bad for u? Where as a child did u feel the most fear, in the Dark or in the Light (and where does fear lead people) ? Science is missing something Dark in u that it will never find because it is part of y u r here. Night provides the shadow for rest, there is no such thing as too much sunlight (once u have adapted). There r essentially 4 types of people when it comes to the Sun – those that don’t want to be in it (&spread the gospel), those in it for the wrong reasons (eg. for vanity), those deceived into not being in it & those who like to be in it. The question is if u r deceived or like to be in it, how do u use it. What happens to free radical behavior when u r in the Sun – it comes to the surface – maybe u should wash it off. If something is hidden in you to TEST u, maybe it’s in the animals as well so don’t eat the meat. The Sun is the only source of Balanced Light in the world, 2 remove Dark Energy(if you have a summer) -b Vegan,b fit,don’t eat b4/during,no lotions,reduce skin shadow,wash in sea/1-2hr. Outside of that it’s important 2 let the Sun into your mind. As most are scared 2 look at the Sun, go to a place where the Light reflects of water or even snow, and allow it to enter your mind, u may notice a minor difference in your state of feeling at the end of the day


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