Thursday, July 8, 2010

Vitamin D: ODA and how to get it

Returning to the graph I have previously posted (click to enlarge)



"Safe" cutoff 
The "safe" cut off of 100 ng/ml is conservative. This is because no harmful effects were noted in a study which kept peoples vitamin D blood levels above 100 ng/ml, therefore it must be ok. However no further studies have been done at levels above this. Hence > 100 ng/ml is termed toxicity, as it is unknown, but potentially not toxic.

Lower cutoff
The lower cutoff of 32 ng/ml is for at least two reasons. Firstly as previous graph shows a minimum of 32 ng/ml is needed to insure healthy bone density in all skin types. I do note that for white and lightly tanned people this number could be considerably larger.

Secondly elderly are at risk of developing secondary hyperparathyroidism. This is where you get kidney failure, pains in joints and bones and possible deformed limbs. The risk for this is dramatically increased with blood vitamin D levels of less than 32ng/ml.

Supplementation with RDI for children

Now when 400 IU vitamin D dose raises the blood vitamin D levels in young children. It raises the blood levels to 32 - 80 ng/ml. This puts then in repletion range of the graph. Hence this is a good level to supplement kids with in winter, and possibly summer as well.

Supplementation for adults
However when 400 IU is taken by an adult, (the adults RDI) there is virtually no change in blood vitamin D levels. Hence this isn't enough. It takes 90 days for blood vitamin D levels to stabilize when taking supplements. The table below shows the increase in vitamin D blood levels are various levels of supplementation:

Supplement - change in blood levels (ng/ml)
     200          decreased(!)
     400            2.8       
  1 000            7.0
  5 000          28
10 000          70
 
Repeating we want to obtain a blood level higher than 32 ng/ml, but less than 100 ng/ml. This corresponds to a theoretically intake of  4 500 - 15 000 IU/day.

However what is our individual blood levels? Well we could pay for a kit to measure them, or next time you visit a doctor you can get your blood tested for vitamin D (in NZ).

However if you don't have access to vitamin D levels in your own blood, one could look at papers studying the population of your area:
  • Christchurch adults are estimated to receive from the sun 1 200 IU / day in summer and only 60 IU / day in winter. This results in theoretical blood levels of 8.4 and  0.42 ng/ml respectively. Therefore supplementation of at least 5 000 IU is needed in summer, and more than this in winter
  • Elderly Dunedin-its approximately 25% and 70% have vitamin D levels below 16 ng/ml. So again high levels of supplementation are needed.   
Hope that this series has been helpful - it sure has been insightful for me!

1 comment:

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