Vitamin D is one of the most frequently tested and most frequently misunderstood micronutrients. GPs order it. Pharmacies sell it. But the interpretation of results (what counts as sufficient, when supplementation helps, and what dose is appropriate) is more contested than most patient information leaflets suggest.

In perimenopause, vitamin D matters for reasons that go beyond the standard bone health conversation. Its roles in muscle function, immune regulation, insulin sensitivity, and possibly mood become relevant at the exact time estrogen loss is altering all of those same systems.

How Vitamin D Works

Vitamin D is not strictly a vitamin. It functions as a steroid hormone precursor. The form that enters the body, either through ultraviolet B radiation on skin or through food, is biologically inactive. It undergoes two conversion steps: first in the liver (to 25-hydroxyvitamin D, the form measured in blood tests) and then in the kidneys and other tissues (to 1,25-dihydroxyvitamin D, the active hormonal form, also called calcitriol).

Vitamin D receptors are expressed in almost every tissue in the body, including bone, muscle, the gut, the brain, immune cells, and the pancreas. This broad receptor distribution explains why insufficiency produces such varied and non-specific symptoms, and why its effects extend far beyond calcium absorption.

Reading Your Blood Test

Blood tests measure 25-hydroxyvitamin D (25(OH)D), the storage form. Interpretation is complicated by the fact that different organisations use different thresholds.

The UK's NHS and the Scientific Advisory Committee on Nutrition (SACN) define deficiency as below 25 nmol/L (10 ng/mL) and insufficiency as 25–50 nmol/L.[1] The US Endocrine Society uses a higher threshold, defining sufficiency as above 75 nmol/L (30 ng/mL).[2] The Institute of Medicine (now the National Academy of Medicine) considers above 50 nmol/L sufficient for bone health.[3]

For practical purposes: if your level is below 50 nmol/L, supplementation is warranted. Between 50 and 75 nmol/L is a borderline zone where supplementation is reasonable, particularly in winter or with limited sun exposure. Above 75 nmol/L is unlikely to produce further benefit for most outcomes, and toxicity risk, while low, becomes relevant above 250 nmol/L.

Most adults in the UK are insufficient during winter. A 2020 analysis of National Diet and Nutrition Survey data found that 20% of UK adults had levels below 25 nmol/L in winter and spring, rising to 40% among people of South Asian or Black African heritage.[4]

Bone: The Core Mechanism

Calcium is absorbed in the gut via a vitamin D-dependent transport mechanism. Without adequate vitamin D, only 10–15% of dietary calcium is absorbed; with adequate vitamin D, absorption rises to 30–40%.[5] Vitamin D does not build bone directly; it determines how much dietary calcium actually makes it into circulation.

Combined with calcium, vitamin D supplementation reduces fracture risk in postmenopausal women. A Cochrane meta-analysis found that vitamin D plus calcium reduced hip fracture risk by 16% and total fracture risk by 13% in older adults; vitamin D alone without additional calcium did not produce the same benefit.[6]

The practical implication: taking vitamin D while low in dietary calcium is less effective than taking both. The reference nutrient intake for calcium is 700 mg per day (UK) or 1000 mg (US, rising to 1200 mg for women over 50). Dairy, fortified plant milks, sardines with bones, and calcium-set tofu are the main dietary sources.

Muscle Strength and Fall Risk

Vitamin D receptors in skeletal muscle are required for normal muscle protein synthesis and force generation. Low vitamin D status is associated with reduced muscle strength, slower reaction time, and higher fall risk in older adults.[7]

For perimenopausal women already experiencing estrogen-driven changes in muscle mass and function, concurrent vitamin D deficiency removes an additional support. A meta-analysis by Bischoff-Ferrari et al. (2009) found that vitamin D supplementation in older adults reduced fall risk by 19% compared to placebo, primarily through improvements in muscle function rather than bone density.[8]

Muscle weakness and a general sense of physical fragility are symptoms women commonly attribute to ageing. Checking vitamin D status and correcting deficiency addresses one potentially reversible contributor.

Mood

The association between low vitamin D and depression has been observed across multiple epidemiological studies, but causal direction has been difficult to establish. People who are depressed spend less time outdoors, which could lower vitamin D through reduced sun exposure, rather than low vitamin D causing depression.

A systematic review and meta-analysis of 61 studies found an association between low vitamin D and depression after controlling for multiple confounders, though the association was stronger in observational studies than in intervention trials.[9]

Randomised trials of supplementation for depression have been mixed. The VITAL-DEP trial found no significant effect of vitamin D3 supplementation (2000 IU daily) on depression or mood over five years in a general adult population.[10] Sub-group analyses suggest people who were deficient at baseline may see more benefit than those who were already sufficient.

Vitamin D is unlikely to function as an antidepressant in the clinical sense. But correcting deficiency removes one contributor to fatigue and low mood, symptoms that can compound what hormonal changes are already producing.

Immune Function and Inflammation

Vitamin D has immunomodulatory effects. It promotes T regulatory cell function (which suppresses excessive immune responses) and reduces pro-inflammatory cytokine production.[11] Low vitamin D has been associated with higher circulating inflammatory markers, including CRP and IL-6.

Low-grade inflammation is one pathway that worsens vasomotor symptoms (inflammatory cytokines sensitise the hypothalamic thermostat) and contributes to mood symptoms and cognitive difficulties. Vitamin D's anti-inflammatory role is relevant in the perimenopausal context beyond its direct effects on any single system.

Dosing and Supplementation

The SACN (UK) recommends 10 micrograms (400 IU) per day for all adults as a baseline supplement from October to March, and year-round for people at risk of deficiency.[1] This is a conservative minimum to prevent deficiency, not a therapeutic dose for someone who is already insufficient.

For correcting deficiency (below 50 nmol/L), 1000–2000 IU (25–50 micrograms) daily is widely used in clinical practice and supported by multiple guideline bodies. The UK government-approved tolerable upper limit is 100 micrograms (4000 IU) per day for adults, though most people supplementing at 1000–2000 IU remain well within a safe range.[12]

Vitamin D3 (cholecalciferol) raises and maintains 25(OH)D levels more effectively than vitamin D2 (ergocalciferol) and is the form used in most quality supplements.[13] Most formulations derive it from lanolin (sheep's wool); vegan D3 sourced from lichen is available and equally effective.

Taking vitamin D with a fat-containing meal improves absorption, as it is a fat-soluble vitamin.[14]

Know your baseline before choosing a dose. If you have access to a blood test (through your GP, or self-pay via private laboratories), it is worth knowing your level before supplementing. Taking 1000 IU when you are already at 90 nmol/L achieves nothing; taking 1000 IU when you are at 25 nmol/L may be insufficient to restore adequate levels quickly. Retesting after 3 months confirms the level has moved into the adequate range.

References

[1] Scientific Advisory Committee on Nutrition. (2016). Vitamin D and Health. Public Health England. https://www.gov.uk/government/publications/sacn-vitamin-d-and-health-report

[2] Holick, M. F., Binkley, N. C., Bischoff-Ferrari, H. A., et al. (2011). Evaluation, treatment, and prevention of vitamin D deficiency: An Endocrine Society clinical practice guideline. Journal of Clinical Endocrinology and Metabolism, 96(7), 1911–1930. https://doi.org/10.1210/jc.2011-0385

[3] Institute of Medicine. (2011). Dietary Reference Intakes for Calcium and Vitamin D. National Academies Press.

[4] Bates, B., Cox, L., Nicholson, S., et al. (2020). National Diet and Nutrition Survey: Years 9 to 11 (2016/2017 to 2018/2019). Public Health England.

[5] Heaney, R. P., Dowell, M. S., Hale, C. A., Bendich, A. (2003). Calcium absorption varies within the reference range for serum 25-hydroxyvitamin D. Journal of the American College of Nutrition, 22(2), 142–146. https://doi.org/10.1080/07315724.2003.10719287

[6] Avenell, A., Mak, J. C., O'Connell, D. (2014). Vitamin D and vitamin D analogues for preventing fractures in post-menopausal women and older men. Cochrane Database of Systematic Reviews, 4, CD000227. https://doi.org/10.1002/14651858.CD000227.pub4

[7] Halfon, M., Phan, O., Teta, D. (2015). Vitamin D: A review on its effects on muscle strength, the risk of fall, and frailty. BioMed Research International, 2015, 953241. https://doi.org/10.1155/2015/953241

[8] Bischoff-Ferrari, H. A., Dawson-Hughes, B., Staehelin, H. B., et al. (2009). Fall prevention with supplemental and active forms of vitamin D: A meta-analysis of randomised controlled trials. BMJ, 339, b3692. https://doi.org/10.1136/bmj.b3692

[9] Shaffer, J. A., Edmondson, D., Wasson, L. T., et al. (2014). Vitamin D deficiency and depressive symptoms: A systematic review and meta-analysis. Psychosomatic Medicine, 76(3), 190–196. https://doi.org/10.1097/PSY.0000000000000044

[10] Okereke, O. I., Reynolds, C. F., Mischoulon, D., et al. (2020). Effect of long-term vitamin D3 supplementation vs placebo on risk of depression or clinically relevant depressive symptoms and on change in mood scores. JAMA, 324(5), 471–480. https://doi.org/10.1001/jama.2020.10224

[11] Aranow, C. (2011). Vitamin D and the immune system. Journal of Investigative Medicine, 59(6), 881–886. https://doi.org/10.2310/JIM.0b013e31821b8755

[12] European Food Safety Authority. (2012). Scientific opinion on the tolerable upper intake level of vitamin D. EFSA Journal, 10(7), 2813. https://doi.org/10.2903/j.efsa.2012.2813

[13] Tripkovic, L., Lambert, H., Hart, K., et al. (2012). Comparison of vitamin D2 and vitamin D3 supplementation in raising serum 25-hydroxyvitamin D status: A systematic review and meta-analysis. American Journal of Clinical Nutrition, 95(6), 1357–1364. https://doi.org/10.3945/ajcn.111.031070

[14] Dawson-Hughes, B., Harris, S. S., Lichtenstein, A. H., Dolnikowski, G., Palermo, N. J., Rasmussen, H. (2015). Dietary fat increases vitamin D-3 absorption. Journal of the Academy of Nutrition and Dietetics, 115(2), 225–230. https://doi.org/10.1016/j.jand.2014.09.014