Sign in Subscribe

Measuring vitamin D levels

MEASURING VITAMIN D LEVELS IN LABORATORY

Vitamin D (S-D-25 / calcidiol) levels are measured from the blood using a laboratory test. Calcidiol levels fluctuate according to the season and available sunlight (unless a dietary vitamin D supplement is taken). There may be significant individual differences even with identical intake. This is due to genetic variation in vitamin D binding into its carrier protein (DBP). Genetic studies have found DBP crucial in determining the vitamin D levels in any individual's blood serum.


The half-life of S-D-25 is approximately 3 weeks, whereas the half-life of the biologically active form (1,25(OH)2D) is only 4–6 hours. Therefore the calcidiol level is a better indicator of the clinical symptoms of vitamin D deficiency and a better reflection of the stored vitamin D in the body, as well as the vitamin D intake, compared to 1,25(OH)2D or calcitriol (S-D-1,25). The S-D-1,25 specification is usually only used in special cases where illnesses affecting 1-hydroxylation or genetic vitamin D metabolism diseases are studied. Chronic infections may cause high calcitriol levels, possibly indicating that the vitamin D receptor is not fully functioning.


The Finnish reference ranges:

  • Less than 25 nmol/L: Severely deficient
  • Less than 50 nmol/L: Deficient
  • 50–75 nmol/L: Generally considered sufficient
  • 75–130 nmol/L: Ideal level
  • More than 375 nmol/L: Toxicity

The recommended values to prevent cardiovascular diseases, according to a study published by Birmingham University in 2015:

  • Less than 50 nmol/L: Deficient
  • 50–74 nmol/L: Insufficient
  • 75–99 nmol/L: Sufficient
  • 100–150 nmol/L: Optimal
  • More than 375 nmol/L: Toxicity

The sufficient intake of vitamins A and K from food provides protection against the potential toxic effects of excessive vitamin D intake. The optimal level of vitamin D is around 130 nmol/L. This is also reflective of the normal long-term level during human evolution.


Factors that promote vitamin D absorption and levels:

  • The best source of vitamin D is the sun and UV radiation; exposure to the mid-day sun for 15–30 minutes is recommended depending on one’s skin type (N.B.! Many types of sunscreen inhibit the production of vitamin D in the skin)
  • Sufficient vitamin A intake and levels in the blood
  • Sufficient magnesium intake and levels in the blood
  • Transforms vitamin D into an active form
  • Helps carry vitamin D in the body
  • The use of the probiotic Lactobacillus reuteri as a nutritional supplement

Factors that promote the function and levels of the vitamin D receptor (VDR):

  • Physical exercise (not including aerobic exercise)
  • Dopamine (neurotransmitter)
  • Bile and bile acids
  • Omega-3 fatty acids (EPA and DHA)
  • Curcumin
  • Resveratrol
  • Vitamin E
  • Estrogen, testosterone, and phytoestrogens

Factors that inhibit or impair the function of VDR (less active form of vitamin D and fewer effects at the cellular level):

  • Caffeine
  • Reduces the production of VDR
  • Cortisol and glucocorticoids
  • Ketoconazole (antifungal medication)
  • Several (chronic) infections:
  • Helicobacter pylori, Pseudomonas aeruginosa, borrelia, tuberculosis, chlamydia (sexually transmitted infection), mononucleosis (Epstein-Barr virus), HIV, cytomegalovirus (CMV), hepatitis C, Shigella, and some systemic fungal infections