The recommended dietary allowance of 400 IU (International Units) of vitamin D is, according to the research1 of Professor Vieth of the University of Toronto, woefully inadequate for optimal health. "One full-body exposure to sunlight can be equivalent to an oral vitamin D intake of 10,000 IU. . . . Humans evolved as naked apes in tropical Africa. The full body surface of our ancestors was exposed to the sun almost daily. In contrast, we modern humans usually cover all except about 5% of our skin surface and it is rare for us to spend time in unshielded sunlight. Our evolution has effectively designed us to live in the presence of far more vitamin D (calciferol) than what most of us get now." In other words, we evolved to get our vitamin D produced in our skin by ultraviolet sunlight and now, when many of us live in cold climates, cover our bodies, and are seldom out-of-doors, we need to get vitamin D orally.

Interestingly, if our ultraviolet exposure goes beyond the beginning of skin redness (sunburn), vitamin D production does not increase. "The ultraviolet-induced production of vitamin D precursors is counterbalanced by degradation of vitamin D and its precursors. The concentration of pre-vitamin D in the skin reaches an equilibrium in white skin within 20 minutes of ultraviolet exposure. Although it can take 3-6 times longer for pigmented skin to reach the equilibrium concentration of dermal previtamin D, skin pigmentation does not affect the amount of vitamin D that can be obtained through sunshine exposure. However, age does lower the amount of [an essential form of cholesterol] in the skin and lowers substantially the capacity for vitamin D production." Vieth gathered data from many studies on blood vitamin D (25(OH)D; calcidiol) levels in people exposed to the sun (farmers in the tropics, lifeguards); those with no exposure (in submarines); those receiving ultraviolet treatment; and those receiving various doses of vitamin D orally.

"Humans evolved as naked apes in tropical Africa. The full body surface of our ancestors was exposed to the sun almost daily. In contrast, we modern humans usually cover all except about 5% of our skin surface and it is rare for us to spend time in unshielded sunlight."

"Apparently, some people require more vitamin D than others to reach a given concentration of 25(OH)D in serum. . . . Ultraviolet exposure and time spent outdoors are better predictors of 25(OH)D concentration than is dietary vitamin D intake."

On the basis of all the research, Vieth recommends a desirable blood level of 25(OH)D to be above 100 nmol/L (nanamoles per liter). To attain this level the total supply of vitamin D from dietary and environmental sources must be 4,000 IU per day—ten times the 400 IU, which is the Recommended Dietary Allowance (RDA) for most adults.

Vieth deals at length with extreme toxic levels of vitamin D—cases where the body cannot deal with excessive vitamin D and then there is too much calcium in the blood. He postulates "homeostatic control systems . . . to buffer against variability in vitamin D supply." Ordinarily, 25(OH)D in our blood is "maintained within a narrow range . . . across vitamin D supplies from 800 IU to the physiologic limit of 10,000-20,000 IU per day." Vieth cites many studies when the intake (often for just one person) was far higher. In one case a vitamin D concentrate was mistaken for cooking oil. In Europe vitamin D is often given in one large dose to last for a season. He describes one case of toxicity. On the other hand, in Finland women given one dose of 150,000-300,000 IU in the autumn lowered their probability of fractures by about 25%. These women’s blood concentrations of 25(OH)D were in the range for osteomalacia (softening of the bones). Other studies have shown that people with especially low levels of 25(OH)D are helped by increased vitamin D much more dramatically than people with better blood levels—another example of our bodies seeking balance.

Finally, a few more excellent reasons for having adequate vitamin D:

1) In the elderly when there is insufficient circulating vitamin D, the parathyroid gland becomes overactive (hyperparathyroidism), which can result in renal failure. Vieth points out that circulating levels of 25(OH)D greater than 100 nmol/L can suppress this over activity.

2) "The prevalence of hypertension in a population increases with distance north or south of the equator and it was reported recently that hypertension becomes less severe in subjects whose 25(OH)D concentrations are increased to more than100 nmol/L."

3) "The probability that established osteoarthritis will progress to a more severe stage is reduced with better vitamin D nutritional status."

4) "Vitamin D deficiency impairs immune function in animals and in children there is a strong association between pneumonia and nutritional rickets."

5) "Epidemiologic studies show that higher serum 25(OH)D concentrations or environmental ultraviolet light exposure are associated with lower rates of breast, ovarian, prostate, and colorectal cancers."

6) "There is impressive circumstantial evidence that multiple sclerosis is more prevalent in populations having lower concentrations of vitamin D or ultraviolet exposure."

When we consider all these factors, such as heart disease, cancer, renal failure, osteoporosis, we will want to check out our vitamin D intake and even perhaps ask to have our blood levels measured.


1Vieth, Reinhold, "Vitamin D supplementation, 25-hydroxyvitamin D concentrations and safety," American Journal of Clinical Nutrition, 1999, 69:842-56.

*We were alerted to this research by The Felix Letter, A Commentary on Nutrition, P. O. Box 7094, Berkeley CA 94707, an excellent publication by Clara Felix, whose writings are both humorous and most carefully documented. She has much more material than covered here on "The Sunshine Vitamin" and "Catching the Good Rays" in two double issues of The Felix Letter, Nos. 103 & 104 and 105 & 106. (Six issues cost $12.)

Article from NOHA NEWS, Vol. XXV, No. 4, Fall 2000, pages 5-6.