Vitamin D: Why New Zealanders Are Deficient — And What It Means for Your Nervous System
- Dr Anupa Dharamsi
- Jun 11
- 12 min read
By Dr. Anupa Dharamsi, Chiropractor | Handcrafted Chiropractic, Māngere, Auckland Nervous system-focused chiropractic care | ACC & WINZ welcome
That Winter Feeling
Every year, somewhere around May, I start hearing the same things in clinic.
A kind of general flatness. Aching that's hard to pinpoint — not a specific injury, just a body that feels heavier and less resilient than usual. Fatigue that doesn't fully clear with sleep. Mood that dips without an obvious reason. Pain that was settling well through summer starting to return.
Most people chalk it up to the cold, or being busy, or getting older. But in many cases, there's a specific and measurable biological factor contributing to all of it.
Vitamin D.
New Zealand has a sunshine reputation that masks a quiet and widespread deficiency problem — particularly in South Auckland, and particularly among the communities I work with every day.
Understanding why deficiency is so common here, and what vitamin D actually does in the body, changes how you approach winter health in a meaningful way.
This isn't a post about taking a tablet and feeling better. It's a post about understanding what vitamin D really is, what it does at a cellular and neurological level, and how to address it intelligently.
First — A Reframe That Changes Everything
Most people think of vitamin D as a nutrient. Something you get from sunlight and fish, take in a capsule when you don't get enough sun, and check off the list alongside vitamin C and omega-3.
But vitamin D is not technically a vitamin at all. It's a steroid hormone — one that communicates with virtually every cell in your body through a structure called the vitamin D receptor (VDR). These receptors are found in immune cells, nerve cells, muscle fibres, bone cells, cardiovascular tissue, and the gut lining.
This is why vitamin D deficiency doesn't just affect one thing. It's not like iron deficiency, which primarily affects red blood cells. Low vitamin D is a body-wide communication problem — affecting the function of multiple systems simultaneously, often in subtle and overlapping ways that are easy to miss or misattribute.
Once you understand this, the range of symptoms associated with deficiency makes complete sense. And once you understand how common deficiency is in New Zealand, particularly in winter, the pattern many people experience seasonally starts to have a clear explanation.
What Vitamin D Actually Does in Your Body
Through its receptor system, vitamin D influences:
Immune regulation — vitamin D modulates both the innate (immediate) and adaptive (learned) immune response. It supports the body's ability to mount a rapid, well-regulated defence against infection and to resolve that response without excessive collateral inflammation. Research consistently shows a protective association between adequate vitamin D levels and lower rates of respiratory tract infections — particularly in people with deficient baseline levels.
Musculoskeletal health — vitamin D is essential for calcium absorption from the gut and for the mineralisation of bone and cartilage. Without adequate vitamin D, calcium cannot be properly absorbed regardless of how much you consume. Muscle fibres also require vitamin D signalling for optimal contraction and recovery. Low vitamin D is directly associated with muscle weakness, non-specific musculoskeletal aching, and slower recovery from physical load.
Nervous system function — this is the connection most vitamin D content doesn't cover, and the one most relevant to the patients I see. Vitamin D plays multiple roles in the central nervous system: it supports neural proliferation and differentiation during development, regulates the synthesis of neural growth factors, supports neuroprotection and neuroplasticity, and influences neurotransmitter systems including the dopaminergic pathway. In plain language — your nervous system requires adequate vitamin D to maintain its capacity to adapt, regulate, and recover.
Mood and cognitive function — emerging evidence consistently links vitamin D status to mood regulation, depression risk, and cognitive performance. A 2025 cross-sectional analysis found that higher dietary vitamin D intake positively influenced brain function and mental performance. A dose-response meta-analysis found a 1.2% reduction in dementia risk for every 10 nmol/L increase in serum vitamin D — a modest but consistent signal that vitamin D status matters for long-term neurological health.
Inflammation regulation — vitamin D modulates inflammatory signalling pathways, supporting the resolution of inflammation and helping prevent the chronic low-grade inflammatory state that underlies so many of the health patterns I discuss in When Stress Drives Inflammation.
The New Zealand Problem
Here is the uncomfortable reality for a country with as much sunshine as ours.
Most adults in New Zealand and Australia are unlikely to obtain more than 5–10% of their vitamin D requirement from dietary sources. The main source of vitamin D is exposure to sunlight.
This means we are almost entirely dependent on skin exposure to UV-B radiation for our vitamin D status. And UV-B radiation in New Zealand is only available at sufficient levels for vitamin D synthesis for part of the year — typically October through April in most regions. Outside of these times, the body produces very little or no vitamin D, leading to the risk of deficiency — with the prevalence of vitamin D deficiency in New Zealand adults varying substantially by month.
By the time Auckland winter arrives in June and July, many people's vitamin D reserves — built up over summer — are running low. By August and September, those who didn't store adequately over summer are often clinically deficient.
And certain groups are at significantly higher risk.
The groups at greatest risk of vitamin D deficiency include dark-skinned people — particularly those who are modestly dressed — and people who regularly avoid sun exposure or work indoors. For adults in disadvantaged communities at high risk of vitamin D deficiency, such as dark-skinned migrants from low socioeconomic backgrounds, a strong case could be made to routinely provide vitamin D supplements.
I want to be direct about what this means for the community I work with in South Auckland.
Pacific and Māori communities have higher melanin concentrations in skin — which requires significantly more UV exposure to produce the same amount of vitamin D as lighter skin. Many members of these communities work indoors, cover skin for cultural or practical reasons, or live in lower-sunlight environments. The combination of these factors creates a disproportionate deficiency risk that, in my clinical experience, is not widely enough understood or addressed.
If you're Pacific, Māori, or from another community with darker skin tones and you've been experiencing the symptoms below — vitamin D status is worth investigating with your GP.
Signs Your Body May Be Low in Vitamin D
Because vitamin D's receptors are distributed throughout the body, deficiency signals can be diffuse and easy to dismiss. Common patterns include:
Persistent fatigue that doesn't fully resolve with adequate sleep
Generalised aching — bones, muscles, and joints without a specific injury
Muscle weakness, particularly in the legs and lower back
Frequent illness or slow recovery from infections
Low mood, particularly worsening through autumn and winter
Brain fog — reduced concentration, slower mental processing
Bone pain, particularly in the back and lower limbs
Slow recovery from physical training or injury
Headaches with no clear structural explanation
None of these alone confirms deficiency — and several overlap with other patterns (nervous system dysregulation, magnesium depletion, chronic stress) that I cover in the other posts in this series. But if you're experiencing multiple symptoms from this list, particularly in the winter months, vitamin D status is one of the first things worth checking.
Vitamin D and the Nervous System — The Handcrafted Connection
This is the piece I most want to bring into focus.
Chiropractic care works at the intersection of structure and neurology. Adjustments restore movement to restricted spinal joints — and in doing so, they improve the quality of sensory signalling between the spine and the brain. Over a course of care, the nervous system receives accumulating evidence that movement is possible, the body is safe, and the protective posture of chronic guarding can ease.
But that neurological reorganisation — neuroplasticity — requires a nervous system that is biologically capable of adapting.
Evidence from animal and in vitro studies provides compelling evidence that vitamin D has a crucial role in proliferation, differentiation, neurotrophism, neuroprotection, neurotransmission, and neuroplasticity.
In other words: the nervous system's capacity to change, adapt, and reorganise is partly dependent on vitamin D status. A person who is significantly vitamin D deficient is walking into their adjustments with a nervous system that is neurobiologically less equipped to do the work of adaptation.
I see this clinically — though indirectly. Patients who are generally well-nourished, who sleep reasonably, who are consistent with care — and whose progress still feels slower or less stable than expected. Often these are patients I'll ask about vitamin D status, sun exposure, and whether their GP has ever tested their levels. The answer is often "never tested" and "very little sun."
It's not that vitamin D replaces chiropractic care. It's that it supports the cellular and neurological environment in which care happens. The adjustment opens the door. The nutritional foundation is what lets the body walk through it.
The Magnesium Connection — Why Supplementing Vitamin D Alone Often Doesn't Work
This is the most important clinical insight in this post — and the one most people don't know.
Magnesium assists in the activation of vitamin D. All of the enzymes that metabolise vitamin D seem to require magnesium, which acts as a cofactor in the enzymatic reactions in the liver and kidneys.
To state this plainly: your body cannot properly convert vitamin D supplements into their active, usable form without adequate magnesium. The inactive form (25-OHD, what's measured on a blood test) must be converted in the liver and kidneys into the biologically active form (1,25(OH)₂D). Every enzyme in that conversion process requires magnesium.
Low magnesium levels impede the conversion of 25(OH)D to 1,25(OH)₂D — meaning a person can have apparent vitamin D resistance despite supplementing.
For patients who are chronically stressed — which describes a significant proportion of the people
I see — magnesium is also being rapidly depleted by the stress response itself. The same patient who is low in vitamin D is often simultaneously low in magnesium, and supplementing vitamin D without addressing magnesium status means the supplementation may be far less effective than expected.
This is why in clinic I often discuss these two nutrients together. They are not independent variables. They are partners in a metabolic system, and deficiency in one undermines the other.
If you haven't read the post on Magnesium: The Mineral Your Nervous System Is Quietly Asking For — it's worth reading alongside this one.
"You can be supplementing vitamin D every day and still not activating it properly — if your magnesium is depleted. These two nutrients are metabolic partners, not independent supplements."
Vitamin D3 and K2 — Why They Work Better Together
There's a second pairing worth understanding: vitamin D3 and vitamin K2.
Vitamin D enhances the absorption of calcium from the intestinal tract into circulation, while vitamin
K2 aids calcium's transport into bone tissue. Without adequate K2, the calcium that vitamin D helps absorb can be directed toward soft tissue and blood vessel walls rather than bone — which is counterproductive and potentially problematic over time.
Vitamin K2 (specifically the menaquinone form, MK-7) activates proteins responsible for directing calcium to bone and preventing its deposition in arteries. The D3+K2 combination is now the standard clinical recommendation for supplementation, rather than D3 alone.
When choosing a vitamin D supplement, look for:
D3 (cholecalciferol) rather than D2 (ergocalciferol) — D3 is the form the body produces naturally and is more bioavailable
Combined D3+K2 — most quality formulations now include this pairing
Third-party tested, clear IU dosing, minimal fillers
Getting Vitamin D Through Food
It's worth being honest about the dietary picture here: unlike most nutrients, vitamin D is genuinely difficult to obtain adequately through food alone.
The best dietary sources include:
Oily fish — salmon, sardines, mackerel, tuna — the most concentrated food sources
Eggs (particularly yolks) — modest contribution but consistently available
Mushrooms exposed to sunlight — a useful source for those on plant-based diets; UV exposure during growth significantly increases D2 content
Fortified foods — some NZ dairy products and cereals are vitamin D fortified; check labels
Even a diet consistently rich in these sources will typically meet only 5-10% of vitamin D requirements. This is why the sun remains the primary lever — and why, for many people in South Auckland, supplementation from April through September is a genuinely sensible strategy.
Smart Supplementation — Test First
This is where I want to be clear and responsible.
Unlike magnesium and vitamin C — which are water-soluble and readily excreted — vitamin D is fat-soluble and accumulates in tissue. High-dose vitamin D supplementation without knowing your baseline status is inadvisable. Toxicity from excess vitamin D, while rare, is real.
My recommendation: test before you supplement at therapeutic doses.
A simple blood test (25-OHD serum level) through your GP gives you a clear baseline. A serum 25-OHD level of ≥50 nmol/L at the end of winter is required for optimal musculoskeletal health — though many practitioners consider 75-100 nmol/L optimal for broader neurological and immune function.
If testing isn't immediately accessible, a maintenance-level supplement (1,000-2,000 IU of D3 daily) is generally considered safe for adults without known contraindications. For people with confirmed deficiency or those in high-risk groups, therapeutic doses (2,000-4,000 IU daily, or a short course of higher doses under GP guidance) may be more appropriate.
Key practical points:
Take vitamin D with a meal containing fat — it's fat-soluble and absorption improves significantly
Take it in the morning or early afternoon — some people find evening vitamin D supplementation affects sleep, possibly through interaction with melatonin pathways
Ensure your magnesium is adequate — the activation problem described above is real and clinically significant
Combine with K2 for optimal calcium direction
Re-test after 3-4 months of supplementation to confirm levels have reached an adequate range
Frequently Asked Questions
How do I know if I'm vitamin D deficient in NZ? The only reliable way is a blood test measuring your serum 25-hydroxyvitamin D (25-OHD) level. This can be requested through your GP. Routine testing isn't always funded, but it's worth requesting — particularly if you're experiencing the symptom patterns described above, if you have darker skin, work indoors, or are heading into or out of winter. A result below 50 nmol/L suggests deficiency by NZ musculoskeletal health standards.
Is 10 minutes of NZ sun enough to make vitamin D? It depends on season, time of day, latitude, skin type, and how much skin is exposed. In Auckland, UV-B radiation sufficient for vitamin D synthesis is generally available from approximately October to April, during the middle of the day. Darker skin requires significantly more exposure than lighter skin to produce equivalent vitamin D. Outside of summer months, sun exposure alone is often insufficient — which is why supplementation from autumn through early spring makes sense for many people.
Can vitamin D help with chronic back and muscle pain? The evidence is supportive. Studies of patients with chronic musculoskeletal pain consistently find high rates of vitamin D inadequacy — in some studies across 48 to 100% of patients. Restoring adequate vitamin D levels supports muscle function, reduces musculoskeletal pain sensitivity, and improves the inflammatory environment around joints and soft tissue. It is not a treatment for specific structural problems, but it is a meaningful part of the nutritional environment that supports recovery.
How does vitamin D interact with the other supplements I might be taking? Vitamin D works closely with magnesium (which is required for its activation), vitamin K2 (which directs the calcium vitamin D helps absorb), and vitamin C (which supports connective tissue repair in the same structural systems vitamin D supports). If you're supplementing all of these, sequencing and dosing matters — taking D3+K2 together with a meal, and magnesium separately (often in the evening), tends to work well for most people. If you're on medications, particularly those affecting calcium metabolism, check with your GP before adding high-dose vitamin D.
Why do I feel worse in winter even though I eat well? Dietary sources contribute only 5-10% of vitamin D requirements at best. Regardless of diet quality, if you're in Auckland during the winter months and not getting significant midday sun exposure on adequate skin area, your vitamin D production is likely minimal. The fatigue, low mood, generalised aching, and increased susceptibility to illness that many people experience from May through September has a real biological basis — and vitamin D is a significant part of that picture.
Part of a Bigger Nutritional Picture
Vitamin D doesn't stand alone. It is one member of an interconnected group of nutrients that collectively support the biological environment your nervous system, immune system, and musculoskeletal system operate within.
Alongside magnesium — which activates it. Alongside vitamin C — which supports the connective tissue that chiropractic care works with. Alongside omega-3s — which modulate the inflammatory signalling that determines how well repair proceeds. Alongside quality sleep and nervous system regulation — which determine the mode the whole system operates in.
None of these nutrients is a quick fix. Together, they create a body that has the resources it needs
to heal, regulate, and adapt.
That's the conversation I have at Handcrafted Chiropractic — not just where the pain is, but what the body is carrying, and what it needs to genuinely recover and build resilience.
Ready to Look at the Whole Picture?
If you've been managing fatigue, generalised aching, slow recovery, or mood that dips through winter — it may be time to look at what your body is working with at a fundamental level.
At Handcrafted Chiropractic in Māngere, we work with people across South Auckland — Māngere, Māngere Bridge, Ōtāhuhu, Māngere East — to understand and address the whole-system picture, not just the symptom. ACC and WINZ welcome.
Not sure how your nervous system is currently functioning? Take our free 2-minute Nervous System Quiz — it's a useful starting point.
Dr. Anupa Dharamsi is the founder and lead chiropractor at Handcrafted Chiropractic, a nervous-system-focused, trauma-informed practice in Māngere, Auckland. She works with patients to understand the whole nutritional and neurological picture behind their health — supporting genuine resilience rather than symptom management. ACC & WINZ welcome.
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