Vitamin D

LCRH FERTILITY DICTIONARY
Written By Sam

What is Vitamin D?

Vitamin D is technically a hormone rather than a vitamin. Your skin produces it when exposed to UVB sunlight, and you can also get smaller amounts from food (oily fish, egg yolks, fortified products) or supplements. The liver and kidneys then convert it into its active form, which acts on receptors found in tissues all over the body.

It's best known for regulating calcium and bone health, but vitamin D receptors are present in the ovaries, testes, uterus, placenta, and pituitary — which is why it's relevant in fertility care.

Blood levels are measured as 25-hydroxyvitamin D, usually in nmol/L. For LCRH fertility patients, we consider:

  • Deficient: below 25 nmol/L 

  • Insufficient: 25–50 nmol/L 

  • Sufficient (for fertility): above 75 – 150 nmol/L 

Why might it matter for fertility?

The evidence here is suggestive rather than conclusive, but several associations keep showing up in the research:

  • Ovulation and cycle regularity. Deficiency has been linked to PCOS and ovulatory dysfunction. Supplementation in deficient women with PCOS sometimes improves cycle regularity and insulin sensitivity.

  • Egg quality and IVF outcomes. Some studies show higher clinical pregnancy and live birth rates in women with sufficient vitamin D undergoing IVF, though other studies find no effect. The picture isn't fully settled.

  • Endometrial receptivity. Vitamin D appears to influence the uterine lining and immune environment around implantation.

  • Sperm parameters. In men, deficiency has been associated with lower sperm motility and morphology, and supplementation may help when baseline levels are low.

  • Miscarriage and pregnancy complications. Low levels are associated with higher rates of miscarriage, pre-eclampsia, gestational diabetes, and preterm birth.

  • Conditions that affect fertility. Endometriosis and PCOS both show associations with low vitamin D.

What are LCRH’s management options? 

In the UK, where sunlight is limited from October to March, the NHS recommends everyone consider a daily supplement of 10 micrograms (400 IU) during autumn and winter. 

LCRH advises our patients to test their level and supplement to bring it into the sufficient range — typical additional doses range from 1,000 to 4,000 IU daily depending on the individual. Higher loading doses sometimes prescribed for deficiency. 

Vitamin D is fat-soluble, so it's best taken with a meal containing some fat, and it's possible to take too much (toxicity is rare but real at very high sustained doses).

Sam
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