Vitamin D and Fertility: The Sunshine Vitamin You Might Be Missing

The supplement I took before any other

When I started building my fertility protocol, vitamin D was the first thing I added. Not because it's glamorous — it's not. It doesn't have the buzz of CoQ10 or the reputation of folic acid. But the research was clear, my levels were low, and living in Cornwall meant I wasn't getting enough from sunlight for roughly eight months of the year.

I took 10 micrograms daily — the NHS recommended amount — throughout my entire fertility journey. It was one of the simplest, cheapest things I did, and the evidence for it has only grown stronger since.

Why vitamin D matters for fertility

Vitamin D isn't really a vitamin — it's a hormone. Your body produces it when UVB sunlight hits your skin, and it has receptors in almost every tissue, including the ovaries, uterus, placenta, and testes. That's important, because it means vitamin D isn't just doing one thing. It's involved in multiple processes that directly affect your ability to conceive and sustain a pregnancy.

Here's what the research tells us:

Egg quality and ovarian function. Vitamin D receptors are present in ovarian granulosa cells — the cells that surround and nurture your developing eggs. A 2024 study in Reproductive Sciences found a dose-response relationship between vitamin D levels and IVF outcomes: the higher the levels (within the sufficient range), the better the clinical pregnancy rates (Zhao et al., 2024). A separate 2024 study confirmed that vitamin D supplementation improved embryo quality in IVF programmes regardless of baseline status (Ferraro et al., 2024).

Implantation and the endometrium. Vitamin D plays a role in immune regulation at the endometrium — the uterine lining where an embryo implants. It helps modulate the balance between inflammation and tolerance that's needed for successful implantation. Deficiency has been associated with endometriosis and recurrent implantation failure in IVF (Chu et al., 2018).

Pregnancy maintenance. Once you're pregnant, vitamin D continues to matter. Deficiency has been linked to increased risk of pre-eclampsia, gestational diabetes, and preterm birth. A UK study found that a third of pregnant women in Britain are vitamin D deficient (Sherrell et al., 2021).

PCOS. Vitamin D deficiency is particularly common in women with PCOS — and supplementation has been shown to improve insulin resistance, menstrual regularity, and ovulation rates. A 2025 systematic review in Frontiers in Medicine found that baseline vitamin D levels significantly influenced IVF outcomes in PCOS patients specifically (Li et al., 2025).

The IVF evidence is hard to ignore

If there's one area where the vitamin D research is strongest, it's IVF. A landmark 2018 meta-analysis in Human Reproduction — analysing 11 studies and over 2,700 women — found that women with sufficient vitamin D levels had significantly higher clinical pregnancy rates and live birth rates than those who were deficient (Chu et al., 2018).

That finding has been confirmed repeatedly since. The weight of evidence now suggests that:

• Vitamin D sufficiency (>75 nmol/L) is associated with better IVF outcomes across multiple measures — fertilisation rate, embryo quality, implantation, clinical pregnancy, and live birth
• Deficiency (<50 nmol/L) is associated with reduced live birth rates
• Supplementation improves embryo quality even in women who weren't deficient to begin with

This doesn't mean vitamin D guarantees IVF success — nothing does. But it does mean that going into treatment with adequate levels gives you a measurably better chance. And given how cheap and safe supplementation is, there's very little reason not to check your levels before starting any fertility treatment.

What about men?

Vitamin D receptors are also found in the testes and on sperm cells themselves. A 2023 systematic review and meta-analysis in Andrologia found that vitamin D is positively associated with sperm motility — particularly progressive motility, which is the type that matters most for reaching and fertilising an egg (Miao et al., 2023).

The testosterone connection is less clear. Some observational studies found a positive link between vitamin D and testosterone levels, but most randomised trials haven't shown a significant direct effect. The current thinking is that vitamin D may influence testosterone indirectly through sex hormone-binding globulin (Yan et al., 2023).

Bottom line for partners: get your vitamin D tested alongside your semen analysis. Deficiency is easy to fix and the potential benefits for sperm quality are real.

The UK problem: we're almost all deficient

This is where it gets personal for anyone living in Britain. The UK sits between latitudes 50°N and 60°N. Between October and March, the sun is too low in the sky for our skin to produce any meaningful vitamin D at all — regardless of how much time you spend outside.

The numbers are sobering:

• A third of UK pregnant women are vitamin D deficient (Sherrell et al., 2021)
• 1 in 5 adults in the UK have serum levels below 25 nmol/L — classified as deficient by the NHS
• People with darker skin, those who cover up for religious or cultural reasons, and those who spend most of the day indoors are at even higher risk
• Even in summer, factors like sunscreen, cloud cover, and limited outdoor time mean many women still don't produce enough

NICE recommends that all adults take a 10 microgram (400 IU) supplement daily, especially during autumn and winter. For pregnant women, 10 micrograms daily is recommended from preconception through breastfeeding (NICE PH56).

Many fertility specialists recommend higher doses — typically 1,000–2,000 IU daily — for women with confirmed deficiency (below 50 nmol/L). If you're genuinely deficient, your GP can prescribe a loading dose to bring your levels up quickly before switching to maintenance.

How to check your levels

A simple blood test measuring serum 25-hydroxyvitamin D [25(OH)D] is all you need. You can request this through your GP, or order a home finger-prick test from services like Thriva or Medichecks for around £30–£40.

Sources: NHS, NICE PH56, Endocrine Society Clinical Practice Guidelines. Fertility-specific thresholds from Chu et al. (2018).

If you're actively trying to conceive or about to start IVF, aim for a level above 75 nmol/L. If you're below 50, talk to your GP about a loading protocol before starting treatment.

Food sources — real but limited

You can get some vitamin D from food, but it's almost impossible to reach sufficient levels through diet alone:

• Oily fish (salmon, mackerel, sardines) — the richest dietary source. A 100g portion of salmon provides roughly 10–15 micrograms
• Egg yolks — about 1.5 micrograms per yolk
• Mushrooms exposed to UV light — some supermarkets now sell these, providing up to 10 micrograms per portion
• Fortified foods — some milks, cereals, and plant-based alternatives have added vitamin D, but amounts vary widely
• Red meat and liver — small amounts, but liver should be avoided in pregnancy due to high vitamin A content

I eat salmon at least twice a week and have eggs most mornings, but I still supplement. For most people in the UK, supplementation isn't optional — it's the only reliable way to maintain sufficient levels year-round.

Which supplement to choose

Vitamin D supplements come in two forms:

• Vitamin D3 (cholecalciferol) — the form your body produces naturally from sunlight. This is the form recommended for supplementation. It raises serum levels more effectively than D2
• Vitamin D2 (ergocalciferol) — plant-derived. Less effective at raising blood levels, but suitable for vegans

For most people trying to conceive:

• Minimum: 10 micrograms (400 IU) daily — the NHS baseline
• Commonly recommended for fertility: 25–50 micrograms (1,000–2,000 IU) daily if levels are below 75 nmol/L
• Upper safe limit: 100 micrograms (4,000 IU) daily unless under medical supervision

Vitamin D is fat-soluble, so take it with a meal containing some fat for better absorption. It's included in most prenatal multivitamins, but check the dose — many contain only 10 micrograms, which may not be enough if you're already deficient.

The bottom line

Vitamin D is one of the most affordable, well-researched, and straightforward things you can do for your fertility. The evidence linking sufficient levels to better IVF outcomes, improved egg quality, and healthier pregnancies is strong and growing. The UK's latitude makes supplementation essentially non-negotiable for most of us.

Get tested. If you're below 75 nmol/L, start supplementing. If you're significantly deficient, see your GP for a loading dose. And if you're about to start IVF, make this one of the first conversations you have with your clinic — because going in with sufficient vitamin D is one of the simplest ways to give yourself the best possible chance.

References

1. Chu, J. et al. (2018). Vitamin D and assisted reproductive treatment outcome: a systematic review and meta-analysis. Human Reproduction, 33(1), 65–80. Oxford Academic
2. Ferraro, S. et al. (2024). Supplementation with vitamin D improves embryo quality in IVF programs, independently of baseline vitamin D status. Nutrients, 16(11). PMC
3. Zhao, Y. et al. (2024). Association Between Vitamin D Level and Clinical Outcomes of Assisted Reproductive Treatment: A Dose-Response Meta-Analysis. Reproductive Sciences. Springer
4. Sherrell, H. et al. (2021). Level of adherence to vitamin D supplementation guidelines in an antenatal centre in Birmingham, UK. BMC Pregnancy and Childbirth. PubMed
5. Li, Y. et al. (2025). The relationship between initial vitamin D levels and IVF outcomes in PCOS patients: a systematic review. Frontiers in Medicine. Frontiers
6. Miao, Y. et al. (2023). The Association between Vitamin D and the Components of Male Fertility: A Systematic Review. Biomedicines, 11(1). PMC
7. Yan, L. et al. (2023). The Association between Serum Vitamin D Levels and Male Fertility: A Meta-Analysis. Andrologia. Wiley
8. NICE. (2014). Vitamin D: supplement use in specific population groups (PH56). NICE

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