Being a nephrologist, I deal with bone disease related to chronic kidney disease on a daily basis and vitamin D is a major player in these patients. Also, I have been frequently asked about this by many people, so I thought it might be a good idea to put some relevant information together for a quick reference.
In June 2011, the endocrine society published recommendations for daily intake and optimum levels, based on extensive literature review. However, these are not based on
large randomized controlled trials (which haven’t been conducted, so far)
With whatever evidence we have, these are the take home points:
Main source for Vit D is diet, but very little present, except for oily fish. Another important source is UVB radiation, which converts 7 dehydrocholesterol to pre-vitamin D3 and then D3
Plants have vit D2 more like a pre hormone than a vitamin.
Vit D (either 2 or 3) gets hydroxylated at the position 25 in the liver (cyp2r1 & cyp27a1), stable and half life is 2 wks.
Then, hydroxylated at position 1 in the kidney cyp27b1 (also present in other tissues)
This is the active form of vitamin D.
When serum levels are measured the 25 hydroxy form is measured since it is more stable, concentration about 1000 times more than the 25 hydroxy form
The hydroxylation at position 1 depends on PTH (parathyroid hormone), FGF-23 (fibroblast growth factor-23), Calcium, Phosphorus
Recommended daily doses of Vit D:
Age upto one year: At least 400 IU/day
1 year and older: at least 600 IU/day However, at least 1000 IU/day of vitamin D may be needed to raise the blood level of 25(OH)D consistently above 30 ng/mL
19 to 70 years: at least 600 IU/day
>70 years: at least 800 IU/day of vitamin D.
In adults (>19 yrs) at least 1500 to 2000 IU/day of supplemental vitamin D may be needed to keep 25(OH)D levels above 30 ng/mL.
Pregnant and lactating women need a minimum of 600 IU/day of vitamin D; 1500 IU/day may be needed to maintain blood levels of 25(OH)D higher than 30 ng/mL.
“Obese children and adults and children and adults on anticonvulsant medications, glucocorticoids, antifungals such as ketoconazole, and medications for AIDS need at least 2 to 3 times more vitamin D for their age group to satisfy their body’s vitamin D requirement,”
Tolerable upper limits of vitamin D, which “should not be exceeded without medical supervision,” include the following:
- 1000 IU/day for infants aged up to 6 months,
- 1500 IU/day for infants aged 6 months to 1 year old,
- 2500 IU/day for children aged 1 to 3 years,
- 3000 IU/day for children aged 4 to 8 years, and
- 4000 IU/day for everyone older than 8 years.
Of note all recommendations are only for bone health. No recommendations for ‘purported’ benefits of Vit D (for cancer, cardiovascular health) since they are mostly association studies.
Exposure to sunlight:
In fair skinned people: 10 minutes in mid day sun with no sunscreen (when UV index is < 3) is enough to produce about 10000 units of Vit D. This holds good provided the clothing is minimal exposing about 50% of the body. which essentially comes down shorts and tank top
In darker skinned individuals: the time required may be more.
Also note: darker individuals have lower risk for than fair skinned for skin cancer, so they probably could expose themselves longer in order to get the benefits of the natural vitamin D
New FDA rules regarding sunscreen:
Sunscreens will carry a broad spectrum label- to indicate that it covers both UVA & UVB radiations
The SPF indicates protection against UVB only which is responsible for sunburn, skin cancer, and vitamin d production in the skin with sun exposure
While UVB affects only the outer layer of the skin, UVA enters deeper layers, 30-40 times more prevalent than the UVB, and linked to tanning and skin aging and skin cancer as well
Now, the highest SPF products can claim is 50+ since value above that are not meaningful in terms of protection
They can carry labels claiming to prevent cancer, only if SPF is higher than 15
Water resistant sunscreens will have to undergo testing, and labels have to specify time of protection in minutes for swimming/sweating
Manufacturers have one year to comply with these rules