Here’s where mechanisms meet the real world results, and I love to see it because my most recent Substack article explains much of the why behind this

Most think it’s impossible for Vitamin D levels to rise through a cold, “sun and UV poor” Canadian winter despite no supplementation

1. Mobilization of fat stores during cold exposure. 25(OH)D is fat-soluble and stored in body fat. Frequent cold exposure increases sympathetic tone, brown adipose tissue activation, and lipolysis, which mobilizes stored lipophilic compounds, including Vitamin D metabolites, into circulation

In winter, you’re not necessarily producing new Vitamin D through skin UV-B exposure, but your body is liberating what you stored during summer, raising serum levels

2. Enhanced sulfation and circulation dynamics in cold

Cold exposure upregulates hepatic sulfotransferase activity and improves peripheral circulation

Sulfated Vitamin D metabolites like 25(OH)D₃-3-sulfate) are more water-dispersible and can act as a circulating reserve pool, re-entering active circulation when lipolysis and transport proteins shift

Likely making more Vitamin D bioavailable or detectable in standard blood tests depending on the assay

3. Seasonal hormonal and binding protein fluctuations

Vitamin D–binding protein and albumin levels are not static, they fluctuate seasonally and under stress. Cold exposure, changes in liver metabolism, and altered circadian photoperiods can shift DBP levels. A lower DBP concentration can increase the proportion of free and bioavailable 25(OH)D, which can raise measured total levels depending on the test

4. Reduced conversion to active metabolites

In colder months, metabolic rates and sun exposure patterns shift. If less 25(OH)D is being converted to 1,25(OH)₂D (the active form), the circulating pool of 25(OH)D might actually RISE because less is being drawn downstream

Think of it like reduced spending during hibernation, the checking account balance creeps up because withdrawals slow down

5. Circadian and photoperiod adaptations

Even without external UV-B, proper light entrainment through getting AM light daily keeps the hepatic-renal axis and hormonal cascades in sync, further supporting efficient recycling and conservation of Vitamin D metabolites

Circadian alignment creates more efficient Vitamin D homeostasis

•••

All this points to some fascinating overlap that most people do not consider

A winter rise in Vitamin D likely reflects a synergy of three processes:

1. Mobilization of stored lipophilic D metabolites through cold-induced lipolysis

2. Enzymatic reconversion of conjugated metabolites into measurable 25(OH)D

3. Endogenous biophoton UV-B photochemistry acting on cholesterol/sulfated precursors, sustaining low level Vitamin D production even in the absence of external UV-B

This is why I advocate for sunbathing when the time is right, so you can build that reserve of Vitamin D metabolites

In winter, especially with cold exposure and good circadian hygiene, your body liberates and conserves those stores more efficiently, oftentimes enough to increase serum Vitamin D, even without new synthesis

The human animal is built for this