UV-B The ultraviolet wavelength that stimulates our bodies to produce vitamin D is UV-B.
The reason it is difficult to get adequate vitamin D from sunlight is that while UV-A is present throughout the day, the amount of UV-B present has to do with the angle of the sun's rays. Thus, at higher latitudes, UV-B is present only during midday hours and only has significant intensity in temperate or tropical latitudes. Only 5 percent of the UV-B light range goes through glass and it does not penetrate clouds, smog or fog.
Sun exposure at higher latitudes before 10 am or after 2 pm will cause burning from UV-A before it will supply adequate vitamin D from UV-B. This finding may surprise you, as it did the researchers. It means that sunning must occur between the hours we have been told to avoid. Only sunning between 10 am and 2 pm during summer months (or winter months in southern latitudes) for 20-120 minutes, depending on skin type and color, will form adequate vitamin D before burning occurs.(9)
It takes about 24 hours for UV-B-stimulated vitamin D to show up as maximum levels of vitamin D in the blood. Cholesterol-containing body oils are critical to this absorption process.(10) Because the body needs 30-60 minutes to absorb these vitamin-D-containing oils, it is best to delay showering or bathing for one hour after exposure. The skin oils in which vitamin D is produced can also be removed by chlorine in swimming pools.
If you have symptoms of vitamin D insufficiency or are unable to spend time in the sun, consider adding a supplement of 2,000 IU daily. Higher levels may be needed but should be recommended and monitored by your health care practitioner.
Adequate calcium and magnesium, as well as other minerals, are critical parts of vitamin D therapy. Without calcium and magnesium in sufficient quantities, vitamin D supplementation will withdraw calcium from the bone and will allow the uptake of toxic minerals. Do not supplement vitamin D and do not sunbathe unless you are sure you have sufficient calcium and magnesium to meet your daily needs. Weston Price suggested a minimum of 1,200-2,400 mg of calcium daily. Research suggests that 1,200-1,500 mg is adequate as a supplement for most adults, both men and women. (Magnesium intake should be half that of calcium.)
In my experience, high, infrequent dosing can lead to problems. In one recent study, blood levels rose from low to extremely high, (more than 300 nmol/l) 24 hours after a 50,000 IU oral dose,65 and then slowly returned to pretreatment suboptimal levels. Clearly this must disrupt normal feedback mechanisms in D and calcium regulation
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