Vitamin D and sunlight
Why we are deficient in vitamin D
How much vitamin D?
Sunlight and vitamin D
Food sources of vitamin D
Vitamin D toxicity
References
Why we are deficient in vitamin D
Getting sufficient vitamin D is one of the keys to staying healthy and long-lived. Hundreds of studies show that most people in the Western world are deficient in vitamin D. Even the most conventional journals now acknowledge that a lack of vitamin D in the population is responsible for a wide variety of ailments (1, 2) including autoimmune diseases, infections, cardiovascular disease, and cancer.
Humans evolved in equatorial Africa, where there is constant and regular sun throughout the year. With all that sunlight, the black skin of our African ancestors synthesised much higher levels of vitamin D than most humans obtain today. About 50,000 years ago a single mutation that occurred was responsible for the appearance of white skin in humans. White skin, with less melanin, synthesizes vitamin D in sunlight six times faster than dark skin (3). These humans were able to successfully migrate to higher latitudes around the world because they could synthesise sufficient vitamin D to survive in the lower levels of sunlight there.
The majority of the world's population now lives above latitude 35 North. Most people are unable to synthesise vitamin D from sunlight for some of the winter months, depending on the angle of the sun and the amount of skin exposure they get. For example, in Seattle (47 N) and London (52 N), there is insufficient sun for skin to make vitamin D from October to April. People living at high latitudes have an increased risk of diseases caused by a lack of vitamin D (8, 9).
The half-life of vitamin D circulating in the body is approximately one month. By the end of winter in the high latitudes most people are deficient in vitamin D unless they have a vitamin-rich diet.
How much vitamin D?
The level of vitamin D needed for good health is a lot higher than the U.S. Food and Nutrition Board set as the Recommended Dietary Allowance (RDA). The RDA recommends 200 international units (IU) per day for children and adults less than 50 years old, 400 IU for adults age 50-70, and 800 IU for adults over the age of 70. The RDA was set at a level that prevents rickets but does not prevent numerous other diseases.
An optimal level of vitamin D can prevent and heal cancers (4), multiple sclerosis (5), cardiovascular disease, autoimmune diseases, and infections such as influenza (6, 7). Here is the most recent thinking on blood vitamin D levels:
| ng/ml | nmol/L | ||
| <10 | <25 | severely deficient | |
| 10-19 | 25-49 | deficient | |
| 20-29 | 50-74 | insufficient for good health | |
| 30-49 | 75-124 | sufficient for good health | |
| 50-99 | 125-249 | optimal for good health, healing and disease prevention | |
| 100-150 | 250-375 | excessive | |
| >150 | >375 | potentially toxic |
Without sun exposure, the optimal level of vitamin D of 50-99 ng/ml requires 5,000 IU per day in food or as a supplement. Most multivitamin tablets contain 400 IU of vitamin D, which is insufficient.
Sunlight and vitamin D
A light-skinned adult human will synthesise 5,000 IU of vitamin D in 5 minutes of naked sunbathing at midday on the equator. Five minutes per day is all you need to get your optimal daily supply. In a little over half an hour, you'll get your entire week's allowance (assuming you get no vitamin D from your food, which is unlikely). Further from the equator, and further from mid-summer and midday, the level of vitamin D synthesised will fall, but at a temperate latitude it is still easy to get your full daily requirement just from the sun. A black-skinned person will only synthesise one sixth as much; these people are much more at risk of vitamin D deficiency at temperate and cooler latitudes (3).
In my book Grow Youthful, I strongly recommend getting sufficient sunlight if possible. Sunlight has many beneficial effects in addition to creating vitamin D. The light regulates sleep cycles and helps create melatonin, and the far infrared warms the body and helps with other systems.
Sunlight has two types of ultraviolet radiation. Ultraviolet-B radiation is the light that helps your skin make vitamin D. It is the short wavelength light that that causes sunburn and does not penetrate the skin deeply. UV-B does not penetrate glass. If you get your sunlight behind a window, while driving etc, you will not get vitamin D. Sunscreens also prevent the formation of vitamin D.
Ultraviolet-A radiation has a longer wavelength, and penetrates through the outer skin deep down to the elanocytes, the cells that become cancerous in melanoma cases. Most sunscreens do not block UV-A, which can also pass through glass, cloud and polluted air. Do not sunbathe behind a glass window - the damaging UV-A radiation gets through the glass, but the beneficial UV-B radiation is blocked by the glass.
Cloud cover blocks ultraviolet light, particularly UV-B. However, some UV-B still gets through, and of course more of the damaging UV-A. It is easy to get severely burned through over-exposure to the sun in cloudy weather.
When you sunbathe, vitamin D is created by the action of the light on cholesterol close to the skin surface. For good health, you need sufficient cholesterol in your diet. It is one of the reasons why coconut oil, a saturated fat, is an excellent sunbathing oil. The newly created vitamin D on the oily skin surface needs 48 hours to be absorbed. Do not wash off the oil for 48 hours after sunbathing. If you have a shower after sunbathing, do not wash all your skin with soap, or you will wash away the newly synthesised vitamin D. In my book Grow Youthful, I suggest that you do not use soap for washing unless it is the only way to get off grime or a smell.
Food sources of vitamin D
The best food sources of vitamin D are wild (not farmed) oily fish like salmon, mackerel, and sardines and cod liver oil. Other sources include free-range eggs and free-range meat, and fresh or sun-dried mushrooms. Very little vitamin D is naturally present in our food, especially for those who consume a modern, processed food diet or people who avoid animal-based foods.
Supplement D2 or D3?
There are two kinds of vitamin D supplements: vitamin D3 (cholecalciferol), the kind our skin makes, and vitamin D2 (ergocalciferol), an inferior synthetic variant. Vitamin D2 is only 10-30% as effective in raising 25-hydroxyvitamin D blood levels compared to vitamin D3. A recent study concluded the "Vitamin D2 should not be regarded as a nutrient suitable for supplementation or fortification" (10).
Vitamin D toxicity
You can take a daily 10,000 IU vitamin D supplement for many months with no evidence of adverse effect (11).
Daily consumption of 50,000 IU for several months will cause hypercalcemia (elevated calcium level in the blood), which is the first manifestation of vitamin D toxicity. In contrast, the optimal level of 5,000 IU per day prevents the build up of calcium in blood vessels (12).
References
1. American Journal of Public Health 2006;96:252-261
2. New England Journal of Medicine 2007;357:266-281
3. Science 2005;310:1782-1786
4. American Journal of Clinical Nutrition 2007;85:1568-1591
5. Neurology 2007;69:381-388
6. Science 2006;311:1770-1773
7. Epidemiology Infection 2006;134:1129-1140
8. Toxicology 2002;181-182:71-78
9. European Journal of Clinical Nutrition 2004;58:1095-1109
10. American Journal of Clinical Nutrition 2006;84:694-697
11. American Journal of Clinical Nutrition 1999;69:842-856
12. Circulation 1997;96:1755-1760