The UV index is a measure of the intensity of ultraviolet radiation B (UVB) from the sun. It is expressed as a number on a scale of 0-11. A higher number indicates a higher sunlight intensity. Exposure to the ultraviolet B (UVB) portion of sunlight is necessary to cause the skin to produce vitamin D. Nevertheless, unless the body is deeply tanned or naturally very dark, a very high UV index can cause sunburn, so caution is necessary.
A low UV index is also problematic.
However, if the UV index is very low, it cannot stimulate the production of vitamin D in the skin. Vitamin D production is essential for human health. Thus, lack of vitamin D may be a major reason that flu-like diseases occur primarily during winter, when there is little or no vitamin D production. This, of course varies according to latitude.
A List of vitamin D benefits from Business Insider.
A recent article from Business Insider (a Philippines online paper) discussed various health effects of vitamin D and sun exposure. Here are some of their salient points, not all of which are correct.
- The primary cause of vitamin D deficiency is sun deprivation (correct).
- Vitamin D helps keep the immune system, so deficiency could be the reason for frequent flu (correct).
- You can get more vitamin D by spending at least 5 to 10 minutes outside 3 times a week without sunscreen (correct).
How much vitamin D does a person need?
- The most incorrect statement of the article is this. The average adult needs around 600 international units (IU) of vitamin D a day (not correct). For reference, a serving of salmon contains roughly 400 IU (not correct). 20 minutes of full-body, unobstructed sunlight can cause the skin to produce up to 20,000 IU of vitamin D. If 600 IU were the only source of vitamin D, one would produce a level of six ng/ml, which would be woefully inadequate. 600 IU is only slightly better than nothing.
- Fifteen minutes of non-burning midday sun (without sunscreen) would optimize vitamin D levels in a few days. Dark-skinned individuals would need much more exposure, up to an hour.
Getting more frequent colds or respiratory infections could be a symptom of vitamin D deficiency (correct). However, it is even more likely to be a sign of sunlight deprivation.
- Colds and flu nearly disappear in the summer when the sun is direct. During this time of the coronavirus epidemic, everyone on earth needs daily direct or indirect sun exposure. Those who do not tan can obtain plenty of sunlight from being outdoors under an umbrella or even in the shade near where the sun is shining.
Bone diseases, vitamin D and sunlight.
- Vitamin D deficiency can cause Osteomalacia and Osteoporosis, conditions where your bones become less dense (correct). Sun exposure produces about 90% of the body’s serum levels of vitamin D. In addition, not all vitamin D supplement studies have produced stronger bones.
Sunlight is king for increasing bone strength.
- However, sun exposure, or exposure to sunbeds (tanning beds) produces marvelous results. For example, a Spanish study showed that women in Spain—those who regularly enjoyed sun exposure—had about one-eleventh the risk of hip fractures as women who had little exposure. Women who use sunbeds also have profoundly higher bone mass than women who do not use them. Their vitamin D levels are also remarkably higher.
Mental problems due to a low UV index and subsequent vitamin D deficiency.
- Vitamin D has some links to depression (correct). Nevertheless, that link in my opinion is 100% due to to sun deprivation. Dr. Gavin Lambert and his colleagues in Australia measured serotonin levels in response to varying degrees of bright light. To do so, they drew blood samples from the internal jugular veins of 101 men and compared the serotonin concentrations of the blood to weather conditions and seasons. The results were remarkable: Men measured on a very bright day produced eight times more serotonin than those measured on a cloudy, dismal day.
More on the UV index and “solar noon.”
When meteorologists report UV index, the emphasis usually warns against the maximum UV level. That level generally occurs around “solar noon.” Solar noon is a four-hour period stretching from 10:00 AM to 2:00 PM. As you have probably noticed, forecasters constantly warn us to avoid the 10:00 to 2:00 hours. Avoiding intense UV radiation is their watchword. Many of them say, “Get sun exposure in the early morning or late afternoon to produce vitamin D.” They should know that the UV index must be over 3 to produce vitamin D.
These reporters do not understand how low UV index hinders vitamin D production.
They do not comprehend that UV index is so weak in the morning that it produces absolutely no vitamin D. The same is true in the evening. Therefore, the closer to solar noon you plan your outdoor time, the more vitamin D you will produce. Vitamin D produced in the skin from solar UVB exposure does not lead to vitamin D toxicity and is safe, as the body limits its own production.
Another method to measure the efficacy of sunlight for vitamin D is to stand in the sun at any time of day and observe your shadow. If your shadow is shorter than your height, you are producing vitamin D. This, the shorter the shadow, better.
Caveats regarding the UV index and vitamin D. Morning sun is still very healthful.
Early morning sun exposure, when the UV index is very low, (under 3) has remarkable health effects. These include resetting the circadian rhythm, increasing production of serotonin, lowering blood pressure through nitric-oxide production and assisting weight loss. Nevertheless, vitamin D stimulation in the skin is not one of the health effects of early morning light.
So safely soak up the sun at any time of day and reap the health benefits. Never burn.
Sun exposure and health… by Dr. Marc Sorenson, Sunlight Institute…
Lack of vitamin D, which is produced by sun exposure, leads to rickets, osteoporosis, osteomalacia and other bone diseases. In addition, research well after the first discovery of vitamin D has shown that vitamin D deficiency and sunlight deprivation also lead to many cancers, heart disease and multiple additional maladies. Now, as the world has modernized, the population is moving indoors, and even in the areas that are sunny throughout the year, sunlight exposure and vitamin D deficiency is increasing, both in rural and urban populations. The bones become so weakened without regular sun exposure, that the slightest movement may cause a fracture. As an example, the mother of an acquaintance of mine—a woman who avoided the sun—turned over in bed one night and broke her hip. Osteoporosis often destroys all quality or life for those who suffer it.
The importance of the sun in maintaining and producing strong bones has been known since antiquity. Dr. Richard Hobday, author of The Healing Sun, writes the following comments along with a history in an online article. “Traditionally, sunlight deprivation has been linked with weak or brittle bones. One of the earliest references to this was made more than two thousand years ago by the Greek historian Herodotus (480-425 BC), who noted a marked difference between the remains of the Egyptian and Persian casualties at the site of battle of Pelusium which took place in 525 BC:
‘At the place where this battle was fought I saw a very odd thing, which the natives had told me about. The bones still lay there, those of the Persian dead separate from those of the Egyptian, just as they were originally divided, and I noticed that the skulls of the Persians were so thin that the merest touch with a pebble will pierce them, but those of the Egyptians, on the other hand, are so tough that it is hardly possible to break them with a blow from a stone. I was told, very credibly, that the reason was that the Egyptians shave their heads from childhood, so that the bone of the skull is indurated by the action of the sun — this is why they hardly ever go bald, baldness being rarer in Egypt than anywhere else. This, then, explains the thickness of their skulls; and the thinness of the Persian’s skulls rests upon a similar principle: namely that they have always worn felt skull -caps, to guard their heads from the sun.’
Herodotus, ‘The Histories’”
And here is a perhaps the transcendent study on hip fracture and sun exposure: research in Spain showed that women who were sun seekers had only about one-eleventh the risk of hip fracture as those who stayed indoors (See the chart below).
That is very powerful evidence of the efficacy of sun in preventing weak bones. In stark contrast to this research are studies done on women who completely avoid the sun and suffer from osteomalacia. Osteomalacia is a soft-bone disease known as adult rickets, resulting from severe vitamin D deficiency, which deficiency prevents bone from properly mineralizing. Women who seldom go outdoors, or who are nearly always fully covered with clothing, have an extremely high incidence of osteomalacia at a very young age, even if they live in geographical areas with abundant sunlight.  If one is never exposed to the available sun, the sun will not be able to produce its beneficial effects on the body, so one may as well live at the North Pole.
Sunbed use also is associated with stronger bones and higher vitamin D levels. An excellent study compared 50 people who used sunbeds regularly with 106 who did not. The sunbed group had 90% higher vitamin D, significantly higher bone density and lower PTH levels (high PTH levels are associated with lower bone mass). The users had healthful vitamin D levels of 46 ng/ml [115 nmol/L] compared to only 24 ng/ml [60 nmol/L] for those who did not regularly use sunbeds.
Scientists at one time believed that sunlight and vitamin D were good only for preventing rickets, osteoporosis and other bone weaknesses. That belief has been supplanted by myriad research studies that show the efficacy of both sun exposure and vitamin D repletion on protection against numerous additional diseases. Nevertheless, we should never forget the extraordinary, never-changing value of sun exposure to maintaining a strong skeleton well into old age.
Richard Hobday. The Healing sun: Sunlight, Brittle Bones, and Osteoporosis. http://sunlightenment.com/the-healing-sun-sunlight-brittle-bones-and-osteoporosis/. (accessed February 5, 2016)
 Larrosa M, Casado E, Gómez A, Moreno M, Berlanga E, Ramón J, Gratacós J. Vitamin D deficiency and related factors in patients with osteoporotic hip fracture. Med Clin (BARC) 2008;130:6-9.
 Sahibzada AS, Khan MS, Javed M. Presentation of osteomalacia in Kohistani women. J Ayub Med Coll Abbottabad 2004;16:63-5
 Al-Jurayyan NA, El-Desouki ME, Al-Herbish AS, Al-Mazyad AS, Al-Qhtani MM. Nutritional rickets and osteomalacia in school children and adolescents. Saudi Med J 2002;23:182-85.
 Tangpricha V, Turner A, Spina C, Decastro S, Chen TC, Holick MF. Tanning is associated with optimal vitamin D status (serum 25-hydroxyvitamin D concentration) and higher bone mineral density. Am J Clin Nutr. 2004 Dec;80(6):1645-9.