Pooling "D" numbers

To examine the potential connections between vitamin D levels in the blood and mortality--death due to any cause--researchers used meta-analysis to combine and analyze data from 14 previous observational studies on this topic.

The study adds important information to the debate about what blood level of vitamin D promotes optimal health, because the researchers identified a point above which having more vitamin D in the body did not provide any additional benefit.

Normal vitamin D levels range from 75 to 185 nanomoles per liter (nm/L). Using a vitamin D blood level of 27.5 nm/L for comparison, the study authors found that an increase in vitamin D levels:

• from 27.5 to 40 resulted in 14% lower risk of death.
• from 27.5 to 52.5 resulted in 23% lower risk of death.
• from 27.5 to 77.5 resulted in 31% lower risk of death.

However, the risk of death did not decrease further if people increased their blood vitamin D levels by more than 87.5 nmol/L above the 27.5 nmol/L reference level (from 27.5 to 115). This suggests that unless a person's vitamin D level is very low--in the deficient range, or below about 75 nmol/L--there isn't a benefit to raising vitamin D levels.

Know your numbers

The results from this large study argue against loading up on a lot of extra vitamin D, unless you've already determined that moderate vitamin D doses are not enough to bring your blood levels into the normal range. Use our tips to find your vitamin D sweet spot.

• Get tested. Many healthcare providers now test vitamin D as part of routine blood work. If your doctor doesn't do this, ask if you can get your vitamin D levels tested.
• Supplement wisely. If your levels are below 75 nmol/L, which is the low end of normal, ask for advice on how much vitamin D you should take. Ask when you should get your blood levels checked again, to make sure the supplements are having the intended effect.
• Know the conversion. Some labs give vitamin D results in nmol/L, while others give results in nanograms per milliliter (ng/mL). If your results are given to you as ng/mL, multiply this number by 2.496 to get nmol/L. For example, if your vitamin D level is 45 ng/mL, this converts to 112.3 nmol/L, which is in the normal range.
• Be sun savvy. If you've tried vitamin D supplements and your levels are still low, consider getting 10 to 15 minutes of sun exposure per day, which may be a more effective way to increase blood levels of vitamin D for some people. Avoid getting sun between 10 am and 2pm, when the sun is strongest and can cause more skin damage.

(Am J Clin Nutr 2012; 95:91-100)