Winter Is Not a Vitamin D Emergency

Every winter, the vitamin D conversation starts again.

The days get short. The UVB disappears. Everyone gets their bloodwork done, watches the number fall, and starts slamming vitamin D pills like our bodies have somehow been caught completely unprepared by December.

I don't.

I live in Calgary and spend part of the year in Montana. Winter here is not subtle. For months, the sun sits too low in the sky for us to make meaningful vitamin D through our skin.

And yet I don't look at that and think: nature screwed this up. Better replace the missing sunlight with a pill.

I think winter is supposed to be different.

My ancestors did not evolve in July 

This is the part I keep coming back to.

Humans have lived through seasonal darkness for a very long time. Populations living at northern latitudes adapted to radically different light environments than populations living near the equator.

Lighter skin itself appears to be one of those adaptations: less melanin allows UVB to penetrate more efficiently when UVB is scarce. Human pigmentation changed as people moved into environments with lower and much more seasonal ultraviolet light.

That doesn't mean our ancestors never experienced low vitamin D. It doesn't mean food wasn't important — traditional northern diets containing fish, meat and animal fat likely mattered enormously.

It means seasonality is not an accident.

The northern human body did not evolve expecting July-level UVB twelve months of the year.

So I don't automatically interpret a winter decline in vitamin D as proof that something has gone wrong.

Vitamin D levels normally rise and fall seasonally. They tend to peak after summer and fall through winter. The body can also store vitamin D in fat tissue rather than requiring every molecule to arrive the day it is used.

Winter physiology is different physiology.

And I'm becoming increasingly reluctant to override every seasonal change simply because we have a supplement capable of moving a lab value.

I care more about the signal of winter

Vitamin D is one signal from sunlight.

It isn't sunlight.

This distinction matters to me.

You can swallow vitamin D in January. That does not recreate the spectral environment of July. It doesn't create a long day. It doesn't give your eyes sunrise. It doesn't give your skin UVA, red light or infrared. It doesn't tell your circadian clock what time it is.

So rather than trying to chemically turn January into July, I lean harder into what January actually gives me.

Light when there is light. Darkness when there is darkness. Cold when it is cold.

That's the winter program.

In winter, I think about melatonin

This might be the biggest shift in how I think about winter health. Everyone is obsessed with what disappears in winter.

I'm interested in what increases.

Darkness.

Melatonin is fundamentally a darkness signal. The brain begins releasing it at night when the light signal falls away. Under natural light conditions, longer winter nights can extend that nighttime melatonin signal.

That is fascinating to me.

We tend to talk about a long dark winter as something unfortunate that must be endured with SAD lamps, bright houses, screens and supplements until summer returns.

But what if some of that darkness is doing something?

What if winter is partly a recovery season?

Honestly it's started to feel like that for me. The long summer days where the kids go to bed so late is pretty exhausting tbh. I'm ready for less solar intensity, longer nights, more darkness, more opportunity for sleep and repair.

(Obviously this falls apart if we spend those extra dark hours under bright LEDs staring into a phone until midnight.)

You don't get the biology of darkness simply because the sun went down outside: you actually have to let it get dark.

**My winter obsession isn't melatonin pills either. It's protecting my own melatonin signal.

Bright natural light during the day, dim evenings, dark nights.

And yes, I still go outside

Lack of UVB does not make winter sunlight useless.

This is one of those ideas that seems obvious once you see it, but somehow we reduce sunlight to vitamin D constantly.

If I can't make vitamin D from the January sun, why bother going outside?

A grey winter morning outside is still a completely different light environment than standing underneath LEDs in my kitchen.

I want the actual sunrise hitting my eyes. I want outdoor brightness telling my brain this is morning. I want the changing colour and intensity of daylight throughout the day. I want sunset telling the system that the day is ending.

The shorter the light window gets, the more interested I am in catching the light that actually exists.

I'm not trying to squeeze vitamin D out of a December sunrise.

I'm trying to give my body December.

Then there is cold

It's no secret that I'm very into Jack Kruse.

One of his ideas that has completely changed the way I think about northern winters is that when environmental light falls, cold becomes part of the energy story.

He talks about cold thermogenesis as a way of increasing endogenous — internal — light.

That's obviously a very different framework from conventional physiology.

The conventional description is simpler and well established: cold activates thermogenesis. Brown adipose tissue becomes more active, mitochondrial metabolism increases and the body burns energy to produce heat.

Kruse takes that several steps further into quantum biology and biophoton production.

I also don't need to pretend every piece of it has been settled by mainstream physiology to notice the larger pattern:

Low light arrives with cold.

Nature paired them.

We invented centrally heated houses.

The body didn't.

So while everyone else is trying to recreate tropical conditions indoors all winter — 23°C house, tropical fruit, bright lights until 11 p.m., vitamin D pills to recreate a summer blood level — I'm increasingly interested in doing almost the opposite.

Experience some cold.

Eat differently.

Sleep more.

Use the darkness.

Get outside for whatever light exists.

Stop demanding that February behave like August.

This doesn't mean vitamin D deficiency isn't real

I want to make this distinction because the internet is terrible at nuance.

I'm not saying nobody should ever take vitamin D.

Vitamin D is biologically important. True deficiency exists. Certain medical conditions, ages, diets and circumstances change the equation.

I'm saying I don't assume that winter itself represents a disease state that needs to be corrected with large doses of supplemental vitamin D.

My winter protocol is boring

I don't try to win winter.

I try to participate in it.

I get outside.

I expose myself to some cold instead of living permanently at thermostat temperature.

I eat seasonal, local nutrient-dense food.

I let evenings get dark.

I sleep.

And perhaps most importantly, I stop interpreting every seasonal change in my body as a problem that needs to be fixed. (Lay off the blood tests, friends).

I think summer is for building.

Light. UV. Long days. Heat. Melanin. Energy.

And winter is the counterweight.

Cold. Darkness. Long nights. Stillness. Repair.

Maybe the goal isn't to carry summer physiology through all twelve months.

Maybe the oscillation is the point.

I don't want twelve Julys.

I want to get very good at winter.

xo Caitlin

 

Further reading

• Human adaptation to latitude / lighter skin / vitamin D: Nina Jablonski & George Chaplin’s Human skin pigmentation as an adaptation to UV radiation. This is probably your best citation for the idea that depigmentation evolved in populations living under lower and more seasonal UVB. It specifically discusses northern environments where useful UVB can disappear for months. Read the paper
  There’s also a newer review that adds an important nuance you may like: northern adaptation wasn't just lighter skin; diet, culture and vitamin-D metabolism were part of the package too. Human skin pigmentation, genetics, environment and culture
• Vitamin D genuinely is seasonal: this systematic review shows circulating 25(OH)D follows the annual UV cycle, with summer exposure affecting the winter nadir. This is useful for your argument that a winter drop is, at least in itself, an expected seasonal phenomenon rather than proof the body has suddenly malfunctioned. UV exposure and seasonal vitamin D levels
  NIH also has a very straightforward vitamin D overview confirming that season, latitude, day length, skin pigmentation and time of day all affect cutaneous vitamin-D production. NIH Vitamin D Fact Sheet
• The “we don't necessarily need to chase huge vitamin-D numbers” piece: the 2024 Endocrine Society guideline is surprisingly useful here. For generally healthy adults under 50, it recommends against routine vitamin-D supplementation above the established dietary intake, and it also recommends against routine vitamin-D testing in healthy adults. Ages 50–74 get essentially the same recommendation regarding extra routine supplementation. It explicitly says outcome-specific optimal blood thresholds haven't been established in clinical trials. Endocrine Society 2024 Vitamin D Guideline
  This is probably the strongest conventional source for the “I don't slam high-dose vitamin D all winter” line. It isn't saying vitamin D doesn't matter; it just doesn't support treating every healthy adult as needing extra high-dose supplementation.
• Longer darkness → longer melatonin signal: this old but very clean human experiment by Thomas Wehr exposed people to a summer-like 16h light/8h dark cycle and then a winter-like 10h light/14h dark cycle. Under the short winter photoperiod, nocturnal melatonin secretion lasted significantly longer, as did sleep duration. The durations of human melatonin secretion and sleep respond to changes in daylength
  Wehr's review on human photoperiodism is good further reading too—the machinery that encodes seasonal day length through the duration of melatonin secretion appears to be conserved in humans, even though how strongly humans use that seasonal signal remains an open question. Photoperiodism in humans and other primates
• Why actually letting winter evenings be dark matters: a controlled study of 116 people found ordinary room light before bed delayed melatonin onset in almost everyone studied and shortened its duration by about 90 minutes compared with dim light. This is perfect for your line that you don't automatically get the biology of a long dark winter if you artificially illuminate the whole thing. Room light before bedtime suppresses melatonin
• Why winter daylight still matters even without useful UVB: outdoor light is doing far more than making vitamin D. A large UK Biobank study found more daytime outdoor-light exposure was associated with earlier circadian timing, easier waking, fewer insomnia symptoms and other sleep/mood differences. It's observational, so I wouldn't make causal promises from it, but it's excellent support for “I'm not going outside in January to squeeze out vitamin D; I'm going outside because light is information.” Outdoor light, sleep and circadian rhythms
• Cold thermogenesis—the conventional part: mainstream physiology is quite comfortable saying that cold activates brown adipose tissue and non-shivering thermogenesis. Brown fat dissipates stored chemical energy as heat, and cold stress activates it through the sympathetic nervous system. Brown adipose tissue activation and metabolism in humans
  A 2024 systematic review/meta-analysis also looks specifically at metabolic effects of cold-induced brown-fat activation in humans. Cold exposure and brown fat: systematic review
• Cold as “internal light”—this is where I would explicitly cite Kruse, not present it as established conventional physiology. Cold Thermogenesis 6: The Ancient Pathway is probably the best foundational Kruse piece for the winter/cold evolutionary framework. Jack Kruse — Cold Thermogenesis 6: The Ancient Pathway
  And this one gets closer to the mitochondrial heat/light language you're invoking: Ubiquitination 1. Kruse argues that cold changes mitochondrial heat release, water chemistry and energetic efficiency. Those broader quantum-biological interpretations go beyond what the brown-fat literature above establishes, so I'd keep the attribution to him very obvious. Jack Kruse — Ubiquitination 1