Vitamin D: Why “Normal” Isn’t Always Optimal

“Fine” Depends Entirely On Who’s Measuring

In conventional medicine, vitamin D is generally treated as adequate once it climbs above roughly 50 nmol/L. Below 25 nmol/L is almost universally agreed to be insufficient, that part isn’t in dispute. That threshold exists to rule out rickets and severe bone disease, and for that narrow purpose, it works. A growing number of researchers are now arguing that this cutoff is set too low even for that purpose, let alone for optimal health.¹

But that’s the key difference: conventional ranges are built around the question “has this person a disease or not?” In naturopathy, we ask a different question entirely. We’re not aiming for “no disease,” we’re aiming for optimal health.

Through that lens, the range we look for to support meaningful immune regulation and optimal health is 75 to 125 nmol/L, more than double the conventional cutoff.² Someone can sit at 60 nmol/L, be told their vitamin D is fine, and still be running an immune system that has nowhere near what it needs to keep itself in check.

That gap between “not deficient” and “optimal” is where I see a lot of autoimmune symptoms quietly living.

Why It Matters So Much When Your Immune System Is Already Confused

In a healthy immune system, vitamin D helps regulate T-helper cells, the cells that decide whether your immune response ramps up or stands down.³ It also lowers inflammatory markers such as IL-6 and TNF-alpha, the same messengers that show up again and again in bloodwork for people with RA, Graves’, lupus, and other autoimmune conditions.⁴

Vitamin D also activates the insulin gene and improves insulin sensitivity, which is part of why deficiency is so consistently linked to insulin resistance and type 2 diabetes.⁵ For anyone managing an autoimmune condition alongside blood sugar swings, that connection is worth knowing about.

One of vitamin D’s most fundamental jobs is maintaining serum calcium and phosphorus homeostasis, keeping the levels of these two minerals in your blood balanced and stable.² That balance affects far more than bone health, it feeds into heart function and nervous system functioning too.

Here’s the wider picture of what it’s doing throughout your body:

• Bone health: supports bone density, alongside vitamin K2, by increasing intestinal calcium absorption.²
• Immune function and regulation: beyond regulating T-helper cells and lowering inflammatory cytokines, vitamin D also enhances the innate immune system, stimulates the production of antibacterial peptides, and inhibits eosinophils.⁶
• Gut health: the vitamin D receptor (VDR) helps regulate mucosal inflammation, supports healthy commensal bacteria colonisation, and stabilises the tight junctions between your intestinal cells.⁷
• Cancer prevention and support: enhances the anti-tumour activity of innate immune cells and inhibits angiogenesis, the process tumours rely on to build their own blood supply.⁸
• Blood sugar regulation: activates transcription of the insulin gene, increasing insulin secretion, and increases cellular sensitivity to insulin.⁵
• Works hand in hand with vitamin A: the vitamin D receptor pairs with vitamin A’s retinoid X receptor (RXR) to bind DNA together, so healthy levels of both vitamins are needed for correct gene transcription.

Vitamin D receptors sit in your gut lining too, where they help regulate the tight junctions between your intestinal cells.⁷ When those junctions are compromised (what most people call “leaky gut”), your immune system gets exposed to particles it was never meant to see, and for a lot of us, that’s where the autoimmune spiral tends to start.

So when someone comes to me with joint pain, fatigue, or a new autoimmune diagnosis, vitamin D level is also one of the things I want to understand, not because it’s a magic fix, but because it has an important influence on our physiology.

What’s Actually Driving The Deficiency

A low result is often just not enough sun in this era where we spend a lot of time indoors and on devices, but it’s not always that. These are all the factors to consider:

• Not enough UVB, or too much sunscreen. Sunscreen blocks the specific UV wavelengths your skin needs to make vitamin D, sometimes almost entirely.⁹
• Excess animal protein or calcium intake, which affects how vitamin D is formed and cleared
• Low dietary fat, since vitamin D is fat-soluble and needs dietary fat present to be absorbed
• Breastfeeding without adequate sun or supplementation. If your baby is exclusively breastfed (which is a very good thing) and not getting adequate sun exposure, it’s very important to consider vitamin D supplementation.
• Impaired liver function, from alcohol, medications, caffeine or pesticide load
• Age, or kidney disease affecting activation
• Poor intestinal absorption, from coeliac disease, dysbiosis, or a lack of bile

Signs You Might Be Running Low

None of these confirm a deficiency on their own, but together they’re worth paying attention to and getting checked further:

• Bone or non-specific musculoskeletal pain (including fibromyalgia)
• Fatigue (including chronic fatigue)
• Frequent infections or poor immune regulation (including autoimmunity)
• Severe asthma in children
• Insomnia
• Low mood or depression
• Menstrual irregularities (including PMOS, formerly known as PCOS, and infertility)
• Bone demineralisation, osteoporosis, or fractures over the longer term

Three Things You Can Do, Without A Lab Test

1. Do the shadow test. Stand outside around midday. If your shadow is the same height as you or shorter, your skin can make vitamin D from that sunlight. If your shadow is longer than you are (early morning, late afternoon, most of winter in a lot of climates), it can’t. This alone tells you a lot about whether your body has had a real chance to make its own vitamin D lately. As a benchmark, 10 minutes of summer sun exposure produces around 400 IU of vitamin D in fairer skin types, and it can take 3 to 6 times longer for darker skin to produce the same amount.¹⁰

2. Know what blocks it. Sunscreen and glass windows both block the UVB rays your skin needs, so a sunny drive or a sunscreen-covered afternoon outside won’t move the needle.⁹ If you want to top up naturally, that means some unprotected skin time, in moderation, ideally around midday. If you’re heading out for longer, wait 10 to 15 minutes before applying sunscreen so your skin has a chance to start making vitamin D first. Skin tone matters too, the more melanin you have, the longer you need in the sun to make the same amount of vitamin D, up to 3 to 6 times longer for darker skin compared to fairer skin.¹⁰

3. Eat vitamin D rich foods. Oily fish (herring, mackerel, sardines, wild salmon), cod liver oil, egg yolks and mushrooms all contribute dietary vitamin D. But here’s the part almost nobody mentions: magnesium is required to activate the vitamin D you already have.¹¹ If your diet or your labs suggest you’re low in magnesium (leafy greens, nuts, seeds, cacao), supplementing D alone may not do much.

What This Doesn’t Cover

I’m deliberately not getting into supplement dosing in this article, and that’s on purpose. The right amount of vitamin D depends entirely on your current blood levels, your skin tone, the climate you live in, your liver and kidney function, and what else is going on with your autoimmune picture. Too little does nothing. Too much, sustained over months, can cause its own problems. And if you want to support your bone health, you’ll want vitamin K2 alongside it.

This is exactly the kind of thing I map out for you, where I look at your actual history, your labs, understand your health goals, and build a plan that’s specific to your body and compatible with the current medications you might be on, rather than a generic recommendation.

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References

¹ Christie, S., Uday, S., Hill, T.R., Rhodes, J.M., Sempos, C.T., Ruxton, C.H.S. and Hewison, M. (2026) ‘Vitamin D in the UK: an urgent call to redefine the threshold for deficiency’, Frontiers in Endocrinology, 17, 1870408.

² Holick, M.F., Binkley, N.C., Bischoff-Ferrari, H.A., Gordon, C.M., Hanley, D.A., Heaney, R.P., Murad, M.H. and Weaver, C.M. (2011) ‘Evaluation, treatment, and prevention of vitamin D deficiency: an Endocrine Society clinical practice guideline’, Journal of Clinical Endocrinology and Metabolism, 96(7), pp. 1911-1930.

³ Sîrbe, C., Rednic, S., Grama, A. and Pop, T.L. (2022) ‘An update on the effects of vitamin D on the immune system and autoimmune diseases’, International Journal of Molecular Sciences, 23(17), 9784.

⁴ Dashti, F., Mousavi, S.M., Larijani, B. and Esmaillzadeh, A. (2021) ‘The effects of vitamin D supplementation on inflammatory biomarkers in patients with abnormal glucose homeostasis: a systematic review and meta-analysis of randomized controlled trials’, Pharmacological Research, 170, p. 105727.

⁵ Mitri, J. and Pittas, A.G. (2014) ‘Vitamin D and diabetes’, Endocrinology and Metabolism Clinics of North America, 43(1), pp. 205-232.

⁶ Liu, P.T., Stenger, S., Li, H., Wenzel, L., Tan, B.H., Krutzik, S.R., Ochoa, M.T., Schauber, J., Wu, K., Meinken, C., Kamen, D.L., Wagner, M., Bals, R., Steinmeyer, A., Zugel, U., Gallo, R.L., Eisenberg, D., Hewison, M., Hollis, B.W., Adams, J.S., Bloom, B.R. and Modlin, R.L. (2006) ‘Toll-like receptor triggering of a vitamin D-mediated human antimicrobial response’, Science, 311(5768), pp. 1770-1773.

⁷ Kong, J., Zhang, Z., Musch, M.W., Ning, G., Sun, J., Hart, J., Bissonnette, M. and Li, Y.C. (2008) ‘Novel role of the vitamin D receptor in maintaining the integrity of the intestinal mucosal barrier’, American Journal of Physiology-Gastrointestinal and Liver Physiology, 294(1), pp. G208-G216.

⁸ Feldman, D., Krishnan, A.V., Swami, S., Giovannucci, E. and Feldman, B.J. (2014) ‘The role of vitamin D in reducing cancer risk and progression’, Nature Reviews Cancer, 14(5), pp. 342-357.

⁹ Matsuoka, L.Y., Ide, L., Wortsman, J., MacLaughlin, J.A. and Holick, M.F. (1987) ‘Sunscreens suppress cutaneous vitamin D3 synthesis’, Journal of Clinical Endocrinology and Metabolism, 64(6), pp. 1165-1168.

¹⁰ Clemens, T.L., Henderson, S.L., Adams, J.S. and Holick, M.F. (1982) ‘Increased skin pigment reduces the capacity of skin to synthesise vitamin D3’, The Lancet, 1(8263), pp. 74-76.

¹¹ Uwitonze, A.M. and Razzaque, M.S. (2018) ‘Role of magnesium in vitamin D activation and function’, Journal of the American Osteopathic Association, 118(3), pp. 181-189.