Sun exposure and vitamin D

Christopher Smith Archive

The link between vitamin D and good bone health has been explored quite a bit in the literature (see here, herehere, here, and here just to name a few). Vitamin D is important in bone health because it helps regulate how your body uses calcium and phosphorous.

Once metabolized by your body, vitamin D becomes calcitriol, which attaches itself to proteins in your cells to be released into your bloodstream. The proteins in your intestine help transport calcium into your bloodstream, affecting the absorption rate of calcium from your diet.

A person with a vitamin D deficiency may only absorb 10 to 15% of the calcium in their diet, whereas someone with normal levels of vitamin D can expect 30 to 40% of their dietary calcium to be absorbed.

Calcium is so essential for neuromuscular and cardiac function that the body will leech the calcium from bones to ensure that levels of blood calcium do not fall.As a result, vitamin D deficiency causes a double-whammy on bone health: first by reducing the overall absorption of calcium, and second by forcing the body to make “calcium withdrawals” from the skeleton to compensate for the lower levels of calcium.

The primary source of vitamin D comes from your skin, which converts UV rays into vitamin D3. The only other sources of vitamin D are a small number of foods (eggs, fatty fish, and fortified dairy products) and supplements. A common reason that people don’t include vitamin D supplements in their diet is the belief that exposure to the sun provides all the vitamin D needed by the body.

However, in response to concerns about skin cancer, Canadians have reduced their sun exposure and embraced sunscreen (which blocks UV rays) in a big way. Add to that the often depressingly short Canadian summer, and it isn’t hard to see why Statistics Canada estimates that 1 in 10 Canadians are vitamin D deficient.

Now, a recently published study from Australia has shown that the amount of vitamin D absorbed by your skin can vary widely as a result of your behaviour toward sun exposure. Time spent outdoors was, surprisingly, far less important than clothing cover in explaining variability in concentrations of active vitamin D. Moreover, BMI was negatively associated with active vitamin D concentrations (i.e. people with higher BMI had lower levels of active vitamin D in their bloodstream).

The take home message for Canadians is that unless you spend most of your day outdoors in your bathing suit all year long, you should almost certainly be taking vitamin D supplements. The benefits of maintaining healthy levels of vitamin D are well documented, and contribute to not just bone health, but cardiovascular and neuromuscular health, as well.

The Dietary Recommended Intake of vitamin D from Health Canada:

* Adequate Intake rather than Recommended Dietary Allowance.
Age group Recommended Dietary Allowance (RDA) per day Tolerable Upper Intake Level (UL) per day
Infants 0-6 months 400 IU  (10 mcg)* 1000 IU (25 mcg)
Infants 7-12 months 400 IU  (10 mcg) * 1500 IU (38 mcg)
Children 1-3 years 600 IU (15 mcg) 2500 IU (63 mcg)
Children 4-8 years 600 IU (15 mcg) 3000 IU (75 mcg)
Children and Adults9-70 years 600 IU (15 mcg) 4000 IU (100 mcg)
Adults > 70 years 800 IU (20 mcg) 4000 IU (100 mcg)
Pregnancy & Lactation 600 IU (15 mcg) 4000 IU (100 mcg)


DISCLAIMER: Do not rely on the information in this website posting as an alternative to medical advice from your doctor or other professional health care provider.   If you have any specific questions about any medical matter, you should consult your doctor or other professional healthcare provider.