Vitamin D3

Vitamin D is the only vitamin your body can make on its own. Your skin produces it when UVB light from the sun hits cholesterol in the lower epidermis. From October to March in northern Europe the sun never sits high enough to drive that reaction. Your body cannot synthesise vitamin D from sunlight at the latitude of Berlin for roughly half the year.^[1]

Deficiency is one of the most common nutrient gaps in European women. It sits behind a wide range of issues from low mood to weakened immunity. This page covers which form absorbs best, how much you need and what the research shows.

Forms of vitamin D

Vitamin D in supplements comes in two chemical forms. They differ in how efficiently your body raises and holds blood levels.

Vitamin D3 (cholecalciferol) is the form your skin makes from sunlight. It is also found in oily fish and egg yolks. Supplemental D3 is typically extracted from sheep wool lanolin. It raises serum 25-hydroxyvitamin D more and holds the level for longer than the same dose of D2.^[1] This is the form used in nōuxx Cycle Routine.

Vitamin D2 (ergocalciferol) is plant-derived. It is made by exposing yeast or mushrooms to UV light. It is cheaper but less bioavailable per unit dose.^[1]

Vitamin D is fat-soluble. Oil-based softgels or drops absorb better than dry tablets, especially with a meal that contains some fat.^[1]

How much you need

The European Food Safety Authority sets an adequate intake of 15 µg (600 IU) of vitamin D per day for adults. This assumes minimal endogenous synthesis from sun exposure. The 25 µg dose in nōuxx sits well within the EFSA safe upper limit of 100 µg per day.^[1]

Despite the modest target, deficiency is widespread. Across European populations, around 40% of adults have serum 25-hydroxyvitamin D below 50 nmol/l, the threshold most often used to define insufficiency. Rates climb in winter and in women with darker skin or lower sun exposure.^[3] The only way to know your status is a blood test for 25(OH)D. Many GPs and labs offer it. Levels above 75 nmol/l are generally considered sufficient. Levels below 30 nmol/l indicate deficiency.^[1]

The science on women's health

Mood and seasonal depression

A 2024 dose-response meta-analysis of 31 randomised controlled trials in more than 24,000 adults found that each additional 1000 IU of vitamin D per day reduced depressive symptom scores. The effect was stronger in people who already had depressive symptoms at baseline.^[4]

A 2024 meta-analysis of 13 studies on perinatal depression found that pregnant and postpartum women with depression had significantly lower serum vitamin D than women without depression.^[5] A separate 2024 review of 25 studies reached the same conclusion across both pregnancy and the postpartum period.^[6]

The biology fits. Vitamin D receptors sit in brain regions involved in mood. The seasonal drop in vitamin D from October to March overlaps with the seasonal rise in low mood many women report through winter.

Immune function

Vitamin D modulates both arms of the immune system. It supports the antimicrobial response and helps keep inflammatory signalling in check. The evidence on whether supplementation prevents respiratory infections has shifted as larger trials reported in.

A 2025 meta-analysis of 40 randomised trials in more than 61,000 participants found a small protective effect of vitamin D against acute respiratory infections. The pooled odds ratio sat just at the edge of statistical significance.^[7] The strongest signal in earlier reviews was in people who started deficient. For women heading into a northern European winter with low baseline levels, correcting deficiency is the rational target.

EU authorised health claims

Under EU law, vitamin D is officially authorised to carry the following health claims:

• Vitamin D contributes to the normal function of the immune system
• Vitamin D contributes to the maintenance of normal bones
• Vitamin D contributes to the maintenance of normal teeth
• Vitamin D contributes to normal absorption/utilisation of calcium and phosphorus
• Vitamin D contributes to normal blood calcium levels
• Vitamin D contributes to the maintenance of normal muscle function
• Vitamin D has a role in the process of cell division^[2]

References

[1] National Institutes of Health, Office of Dietary Supplements. Vitamin D: Fact Sheet for Health Professionals. ods.od.nih.gov/factsheets/VitaminD-HealthProfessional

[2] Commission Regulation (EU) No 432/2012 establishing a list of permitted health claims made on foods. Official Journal of the European Union, 25 May 2012. eur-lex.europa.eu

[3] Cui A, Zhang T, Xiao P, Fan Z, Wang H, Zhuang Y. Global and regional prevalence of vitamin D deficiency in population-based studies from 2000 to 2022: a pooled analysis of 7.9 million participants. Frontiers in Nutrition 2023;10:1070808. doi.org/10.3389/fnut.2023.1070808

[4] Ghaemi S, Zeraattalab-Motlagh S, Jayedi A, Shab-Bidar S. The effect of vitamin D supplementation on depression: a systematic review and dose-response meta-analysis of randomized controlled trials. Psychological Medicine 2024;54(15):3999-4008. doi.org/10.1017/S0033291724001697

[5] Yuan Y, Qu L, Sun Q, He P, Zhou X. The association between vitamin D deficiency and perinatal depression: a systematic review and meta-analysis. Alpha Psychiatry 2024;25(6):669-675. doi.org/10.5152/alphapsychiatry.2024.241553

[6] Centeno LOL, Fernandez MDS, Muniz FWMG, Longoni A, de Assis AM. Is serum vitamin D associated with depression or anxiety in ante- and postnatal adult women? A systematic review with meta-analysis. Nutrients 2024;16(21):3648. doi.org/10.3390/nu16213648

[7] Jolliffe DA, Camargo CA, Sluyter JD, et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. The Lancet Diabetes & Endocrinology 2025;13(4):307-320. doi.org/10.1016/S2213-8587(24)00348-6