As winter looms and the layers pile on, I'm starting to hear familiar phrases from my patients: “I'm catching every bug going.” “My mood’s off.” “I’m feeling more sluggish.” Most people assume they’re just tired. But here’s what I’ve found from the testing I do: more often than not, their vitamin D is tanking. When we think about Vitamin D we think about bones and brittle nails - but vitamin D is doing far more behind the scenes.
As winter looms and the layers pile on, I'm starting to hear familiar phrases from my patients: “I'm catching every bug going.” “My mood’s off.” “I’m feeling more sluggish.” Most people assume they’re just tired. But here’s what I’ve found from the testing I do: more often than not, their vitamin D is tanking. When we think about Vitamin D we think about bones and brittle nails - but vitamin D is doing far more behind the scenes. It’s not simply about skeletons, it’s about systems. Vitamin D receptors are found across almost every major system - immune, brain, gut, muscle and metabolic. It directly regulates hundreds of genes and indirectly influences thousands more. That’s why low vitamin D can influence so many processes at once: immunity, metabolism, insulin sensitivity, reproductive hormones, mood regulation, mitochondrial function, inflammation, calcium balance, gut integrity, and even circadian biology. So when levels fall, so does your resilience. And the smartest moment to act? Before winter truly begins. Let’s dive in.
Where insight becomes impact
What I’ve been looking at this week…
The science has shifted far beyond vitamin D for bone density. It’s now understood as a multi-system signal.
A growing body of studies show that vitamin D₃ not only strengthens skeletal health but also reshapes the gut microbiome, increasing beneficial phyla like Firmicutes and Bacteroidetes that produce butyrate, the gut’s anti-inflammatory fuel. PMID: 30159755
And here’s why that matters: vitamin D isn’t really a vitamin at all. It’s a pro-hormone that relies on this whole-body ecosystem to work.
Here’s how it actually works:
Vitamin D₂ (ergocalciferol) comes from plants and some fortified foods. It raises vitamin D levels, but less effectively and less reliably than D₃.
Vitamin D₃ (cholecalciferol) is made in your skin when UVB hits cholesterol or is taken as a supplement. It’s the form your body recognises best. Both D₂ and D₃ travel to the liver, where they’re converted into 25-hydroxyvitamin D [25(OH)D], also called calcidiol - this is the storage form we measure in blood tests. This tells us how much vitamin D you have available.
From there, the kidneys, immune cells, and even the brain convert 25(OH)D into 1,25- dihydroxyvitamin D [1,25(OH)₂D], also called calcitriol - this is the active hormone that switches on genes for calcium balance, immune regulation, mitochondrial activity, and neurotransmitter production.
This final step depends on magnesium, healthy mitochondria, hormones, and a well-fed microbiome. Low butyrate, low magnesium, or chronic inflammation, and your “on” switch becomes a dimmer.
And then there’s the genetic layer. Variants in genes such as VDR determine how you absorb, activate, and transport D. I’ve had patients who've been told their Vitamin D blood levels are “normal” but they're still functionally deficient because their receptors are sluggish or their conversion enzymes slow. This then shows up as lower immunity, coughs and colds you can't seem to shake, and inflammation that ages you. This is exactly why, when I test vitamin D, I also extend the analysis to include genetics as well - to see not just how much you have, but how well your system can use it - that's the most important part most people miss.
Add in stress, and the plot thickens. Cortisol, your dominant stress hormone, suppresses the same enzyme that activates vitamin D. That’s why the HPA axis (your hypothalamic–pituitary–adrenal stress circuit) and vitamin D are deeply intertwined. So yes, you can eat well, supplement regularly, and still feel flat if your stress hormones are interfering with activation. That's why alongside testing Vitamin D levels I also couple this with measuring your cortisol curve.
Track it to hack it.
What I’ve been monitoring this week…
This week, I’ve been pairing data from my Oura Ring with cortisol mapping, because low vitamin D often mirrors low HRV and shallow sleep. What'I've seen with my patients is that when D levels rise, parasympathetic tone usually follows and resilience increases.
If you're not sure why your Oura data is not mirroring your efforts, Vitamin D may lie at the core.
This is my testing suite to see the full picture:
25(OH)D serum: the gold-standard baseline
PTH and calcium: to check whether D is working or simply accumulating.
Magnesium: essential for D-activation enzymes.
Cortisol curve: a four-point saliva test to reveal whether stress is blunting conversion.
Gut microbiome + butyrate levels: to assess how well your gut supports D receptor activity. Genetic panel (VDR): to personalise dosing and predict responsiveness.
Most doctors will tell you your vitamin D level is “normal” if it sits between 50–75 nmol/L. That range simply prevents rickets and severe deficiency. It is not where optimal function happens. From a longevity perspective, where we’re looking at immune resilience, metabolic stability, inflammation control, mood, sleep, mitochondrial efficiency, and winter robustness, the sweet spot is far higher. Aim for 100-175 nmol/l
And remember, if your D looks normal or even optimal, but your PTH is high, your cortisol curve is flat, and your gut diversity is low, you don’t have a deficiency of intake; you have a deficiency of activation.
From knowing to doing.
What this means for YOU…
Here’s what this means for you. 1️⃣ Test before you top-up. Know your 25(OH)D, PTH, magnesium, cortisol, gut and gene status before winter. 2️⃣ Choose D₃, not D₂. It’s the biologically active form your body recognises. 3️⃣ Pair it smartly. D₃ brings calcium in; K₂ (100–200 µg MK-7) sends it to your bones; magnesium (~400 mg/day) activates the enzymes. 4️⃣ Mind your meal. Take D with your fattiest meal for up to 50 % better absorption. 5️⃣ Manage stress. Your HPA axis is part of your D pathway. Modulate cortisol with 6-60 Breathflow, morning sunlight, and consistent sleep. 6️⃣ Feed your flora. Diversity is key: 30+ plant foods a week, fermented fibres, and resistant starch boost butyrate, your microbiome’s gift to your VDR. 7️⃣ Retest every 12 weeks. Adjust dose with data, not guesswork.
💼 Case in point:
When I first met James, 52, a tech CEO, he showed me his recent bloodwork with a smile. “My GP says my vitamin D is normal,” he said. It was sitting at 65 nmol/l, right in the range most GPs consider fine (55–75 nmol/L). Yet he felt anything but fine. He was constantly tired, waking unrefreshed, and fighting off the same lingering cold for weeks. When we dug deeper, the picture changed completely. His genetics revealed a VDR polymorphism, meaning his vitamin D receptors were less efficient at using the vitamin D he did have. His cortisol curve showed a stress-driven flattening, suppressing activation further, and his gut results showed low butyrate producers, reducing VDR expression even more. So instead of telling him to take more vitamin D, we fixed the system that activates vitamin D:
supported receptor sensitivity with K₂ + magnesium,
rebuilt his gut butyrate,
and rebalanced his cortisol rhythm with simple daily stress-modulation work.
Twelve weeks later his vitamin D had moved into the optimal range at 115 nmol/L, his activation markers normalised, his sleep deepened, and no more distracting colds. Which meant he could lead his team at full capacity again.
That’s the power of precision — from guessing to knowing.
🎯 If you'd like to make decisions with precision based on your personal data and using devices that are specific for you, then book a private consultation and let's start to decode your health.
Where you’ll find me…
Last week: in Palm Beach, building a longevity network. I'll be back!
This week: won an award for Female Speaker of The Year... again!!! So if you're looking for a speaker for your events, get in touch. And very excited to be speaking at Oura's Harrods launch this week!
Next week: It's "Grab a Seat" week in The Million Hour Club Grow Pods I host every 2 weeks - laser- focused action-taking to live your longevity lifestyle.
A thought to pause on…
“Vitamin D doesn’t just build bones, it builds resilience. It’s the light your body listens to when the sun goes silent.” So before the winter really begins, test your D, track your stress, and tune into your genes. That’s how you keep your internal sunshine shining, no matter what the season does outside.
Wishing you a wonderful week ahead- that's 168 hours of your million hour life! Dr Alka Be sure to send this email to anyone who you also want to live their longest, healthiest life!
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Take the free LQThis article is general education, not medical advice, and reading it does not create a doctor-patient relationship. If something has been going on for a while, please get it properly looked at by your own doctor.