Vitamin D: Beyond Bones – How D Builds Resilience
Originally sent to subscribers on 23 November 2025
Winter Blues or Vitamin D?
As winter looms and layers pile on, patients tell me: “I’m catching every bug,” “My mood’s off,” “I’m feeling sluggish”. Often, their vitamin D is tanking. We think vitamin D is about bones, but it’s doing far more.
Vitamin D receptors are found in immune, brain, gut, muscle and metabolic systems. Low D affects immunity, metabolism, insulin sensitivity, reproductive hormones, mood, mitochondrial function, inflammation, calcium balance, gut integrity, and circadian biology. When levels fall, so does your resilience. The smartest moment to act? Before winter truly begins.
DATA
Where insight becomes impact
The science has moved beyond bone density. Vitamin D₃ reshapes the gut microbiome, increasing beneficial phyla like Firmicutes and Bacteroidetes that produce butyrate. Vitamin D is not just a vitamin; it’s a pro‑hormone that relies on the whole body to work.
How D Works
D₂ (ergocalciferol) comes from plants and fortified foods; it raises D levels less reliably than D₃.
D₃ (cholecalciferol) is made in your skin when UVB hits cholesterol or taken as a supplement.
Both travel to the liver and convert to 25(OH)D (calcidiol)—the storage form measured in blood tests.
From there, kidneys, immune cells, and the brain convert 25(OH)D into 1,25(OH)₂D (calcitriol)—the active hormone that switches on genes for calcium balance, immune regulation, mitochondrial activity, and neurotransmitters.
This final step depends on magnesium, healthy mitochondria, hormones, and a well‑fed microbiome.
Genetics & Stress
Variants in VDR genes determine how you absorb, activate, and transport D. Patients told their levels are “normal” can still be functionally deficient. Cortisol suppresses the enzyme that activates vitamin D; stress and the HPA axis are intertwined with D. Low butyrate, low magnesium, or chronic inflammation dim your D “on” switch.
DEVICES
Track it to hack it
This week I paired Oura Ring data with cortisol mapping; low D often mirrors low HRV and shallow sleep. Here’s my testing suite:
25(OH)D serum: baseline measure.
PTH & calcium: to see if D is working or accumulating.
Magnesium: essential for activation.
Cortisol curve: a four‑point saliva test to reveal stress blunting.
Gut microbiome & butyrate: to assess support for VDR activity.
VDR genetic panel: to personalise dosing.
Most doctors call 50–75 nmol/L “normal”; that range prevents rickets. Optimal function sits higher—aim for 100–175 nmol/L. If D looks normal but PTH is high, cortisol is flat, or gut diversity is low, you don’t have a deficiency of intake; you have a deficiency of activation.
DECISIONS
From knowing to doing
Test before you top up: Know your 25(OH)D, PTH, magnesium, cortisol, gut and gene status.
Choose D₃, not D₂.
Pair smartly: D₃ brings calcium in; K₂ (100–200 µg MK‑7) sends it to your bones; magnesium (~400 mg/day) activates the enzymes.
Mind your meal: Take D with your fattiest meal for up to 50 % better absorption.
Manage stress: Modulate cortisol with 6‑60 Breathflow, morning sunlight and consistent sleep.
Feed your flora: Diversity is key—30+ plant foods a week, fermented fibres, resistant starch; butyrate boosts VDR.
Retest every 12 weeks. Adjust with data, not guesswork.
Case in point
James, 52, had a vitamin D level of 65 nmol/L—“normal”. Yet he was tired, unrefreshed, and fighting a lingering cold. We found a VDR polymorphism, a flattened cortisol curve and low butyrate producers. Instead of increasing D, we supported receptor sensitivity with K₂ and magnesium, rebuilt gut butyrate, and rebalanced cortisol. Twelve weeks later his vitamin D reached 115 nmol/L, activation markers normalised, sleep deepened, and no more colds.
DIARY
Last week: Building a longevity network in Palm Beach.
This week: Won Female Speaker of The Year and speaking at Oura’s Harrods launch.
Next week: Hosting “Grab a Seat” week in the Million Hour Club Grow Pods.
DISTINCTION
“Vitamin D doesn’t just build bones, it builds resilience”. Before winter begins, test your D, track your stress, and tune your genes.
Wishing you a wonderful week ahead—that’s 168 hours of your million hour life.
Dr Alka