Vitamin D Is Not Just a Vitamin: It's a Hormone

The research on vitamin D has moved well beyond bones and sunlight. Here is what women need to know about its wider role in the body.

Most women know the basics: not enough sun means not enough vitamin D, and not enough vitamin D is bad for bones. That part is true, and it matters. But it is also a fraction of the picture. Vitamin D is now one of the most researched nutrients in medicine, and the emerging science places it at the centre of everything from immune function to cancer biology to how the body processes pain. For women in particular, the implications are significant.

Vitamin D Is Actually a Hormone

The first thing worth understanding is that vitamin D is not quite a vitamin in the conventional sense. Once it enters the body, whether through sunlight, food or a supplement, it is converted into a hormone. That hormone, called calcitriol, then travels through the bloodstream and binds to receptors found in almost every tissue in the body: the brain, the immune system, the gut, the ovaries, the skin, the heart and more.

This matters because it explains why vitamin D deficiency does not just affect bones. When a nutrient that regulates dozens of biological processes is in short supply, the effects can be felt across multiple systems at once. Persistent fatigue, low mood, muscle aches and a tendency to catch every illness going around are all associated with low vitamin D. None of those symptoms are unique to deficiency, but they are common enough that checking vitamin D levels is often a useful starting point.

Its Role in Inflammation and Chronic Pain

Inflammation is the body's first line of defence, but in many chronic conditions it becomes the problem rather than the solution. Vitamin D helps regulate this process. Research shows that it reduces the production of prostaglandin E2, a compound that drives both inflammation and pain signalling, and that it moderates the same inflammatory pathways implicated in conditions like endometriosis, PMOS / PCOS and autoimmune disease.

A 2025 systematic review published in Nutrients examined vitamin D supplementation and pain in fibromyalgia, a condition affecting women at three times the rate it affects men. The review found a meaningful association between low vitamin D levels and symptom severity, with supplementation showing benefit in reducing pain scores in several of the included studies. A separate 2025 review in PLOS ONE looked specifically at vitamin D for period pain and endometriosis-related pain, concluding that the evidence base for its role in pain modulation warrants further clinical trial investment.

Studies have also found consistently lower vitamin D levels in women with endometriosis compared to those without the condition. Whether this is cause or consequence remains under investigation, but vitamin D's anti-inflammatory and anti-proliferative properties make it biologically plausible as a factor in the condition's development and progression.

What the Cancer Research Is Showing

This is an area where the science is genuinely interesting but requires careful reading. Researchers have been studying the relationship between vitamin D and cancer for several decades, prompted by early observations that cancer rates were higher in populations with less sun exposure.

A 2025 umbrella review published in Biochimica et Biophysica Acta, which analysed 71 separate systematic reviews, found strong evidence that vitamin D3 supplementation reduced overall cancer mortality, and identified suggestive evidence for a potential preventive effect on breast, colorectal, lung and renal cell cancers. A 2024 integrative review in Nutrients looked specifically at breast cancer and vitamin D, exploring the biological mechanisms through which vitamin D may influence tumour cell behaviour, including its ability to regulate cell growth and reduce proliferation. This is an active area of research and the full picture is not yet established. What is clear is that vitamin D's role in cancer biology is real, significant and continuing to be studied at the highest levels.

Mood, Mental Health and Perimenopause

Vitamin D receptors are present in areas of the brain involved in emotional regulation, and vitamin D plays a role in the production of both serotonin and dopamine. Low levels are consistently associated with increased rates of depression, and research in perimenopausal and postmenopausal women specifically has found that those with higher vitamin D intake were less likely to develop depressive symptoms, with one study reporting a 20% reduction in risk over three years.

For women navigating perimenopause, this is worth noting. Declining oestrogen reduces the body's ability to synthesise and absorb vitamin D efficiently, meaning that just at the point when mood stability, sleep quality and energy levels are under pressure from hormonal change, vitamin D status is also more likely to be suboptimal. A review published in Frontiers in Physiology noted that vitamin D insufficiency is common in perimenopausal and postmenopausal women and is associated with a range of menopausal symptoms beyond bone loss, including mood disturbance and fatigue.

How Deficient Are Women Actually?

Quite often. Approximately 40% of adults in Europe have low vitamin D status, and women going through perimenopause and menopause are at particular risk. In the UK, the NHS recommends that everyone considers a daily supplement of 10 micrograms (400 IU) between October and March. The Endocrine Society's updated 2024 clinical guidelines specifically recommend supplementation for certain groups, acknowledging benefits that extend well beyond bone health.

Getting vitamin D levels tested via a blood test is the most reliable way to know whether supplementation is warranted, and at what dose. Self-supplementing at standard doses is generally considered safe, but higher-dose supplements should be taken under medical guidance, as vitamin D is fat-soluble and excess levels can accumulate.

What Women Should Take Away From This

The research on vitamin D has changed considerably over the past decade. It is no longer a one-note conversation about calcium and bone density. The evidence places it at the intersection of immune regulation, inflammation, hormonal health, mental wellbeing and cancer biology. That does not make it a cure-all, and no responsible reading of the science suggests it is. What it does suggest is that for many women, especially those living with chronic inflammatory conditions, approaching perimenopause or experiencing persistent low mood and fatigue, knowing their vitamin D level is a reasonable first step.

References

1. Lappe JM, et al. Vitamin D and calcium supplementation reduces cancer risk. American Journal of Clinical Nutrition. 2007.

2. Ogata S, et al. Vitamin D and cancer mortality: umbrella review. Biochimica et Biophysica Acta. 2025.

3. Torres A, et al. The impact of vitamin D in breast cancer prevention. Nutrients. 2024;16(5):573.

4. Manson JE, et al. VITAL trial: vitamin D supplementation and cancer incidence. New England Journal of Medicine.2019.

5. Zhou IW, et al. Vitamin D for primary dysmenorrhea and endometriosis-related pain. PLOS ONE.2025;20(4):e0321393.

6. Ghasemi M, et al. Vitamin D supplementation and fibromyalgia-related pain: systematic review and meta-analysis. Nutrients. 2025;17(20):3232.

7. Xie B, et al. Association between vitamin D and endometriosis among American women. PLOS ONE.2024;19(1):e0296190.

8. Li X, et al. The role of vitamin D in menopausal women's health. Frontiers in Physiology. 2023.

9. Holick MF. 2024 Endocrine Society vitamin D clinical practice guidelines: a critical review. Conexiant. 2024.

Editorial Note

This article is for informational purposes only and is not a substitute for professional medical advice. If you have concerns about your health or are considering making any changes to your health regime, speak to a qualified healthcare professional first.