How to Prepare Your Immune System for Autumn – Sage Green

There is a moment, usually in the last week of August, when the light changes before the weather does. The sun is still warm at noon, but it comes in at a lower angle, and the shadows on the kitchen floor are longer than they were a month ago. Nothing has happened yet. The tomatoes are still ripening. Nobody is ill.

This is the most useful two weeks of the year for your immune system, and almost nobody uses them.

We tend to think about immunity reactively — in November, with a sore throat, standing in a pharmacy queue. But the immune system is not a switch you flip when you need it. It is a slow, distributed, resource-hungry network that depends on nutrient stores built up over weeks, on a gut microbiome shaped by months of eating, and on a circadian rhythm that takes time to shift. By the time you feel the first scratch in your throat, the decisions that mattered were made in September.

So let's look at what actually changes between now and the end of October — mechanically, not mystically — and what can be done about it while it's still easy.

What genuinely changes in September

Woman walking outdoors in early morning autumn light, the daily light exposure that anchors circadian rhythm

Four things shift at once, which is why the season feels like it arrives all at once.

The light collapses. At 57° north — the latitude of Rīga, Tallinn and Vilnius — daylight falls from nearly eighteen hours in late June to under seven by mid-December. More importantly, the angle of the sun drops below the threshold at which UVB radiation can penetrate the atmosphere enough for the skin to synthesise vitamin D. Somewhere between late September and early October, at these latitudes, your skin effectively stops making vitamin D altogether, and does not resume until March [1].

People move indoors and mix. Schools reopen, offices refill, windows close. Respiratory viruses do not become more virulent in autumn; they become more transmissible, because we spend more hours sharing the same volume of air with more people.

The air dries out. Heated indoor air in the Baltic autumn drops to 20–30% relative humidity. The mucous membranes lining the nose and throat are your first physical barrier, and they only work when they are moist. Dry mucosa clears pathogens measurably more slowly.

Sleep gets confused. The circadian system takes its main cue from morning light. When you wake in darkness, the signal is late and weak, and the timing of the whole system drifts. Sleep is not a luxury item for immunity — it is when much of the adaptive immune work actually happens.

None of this is a catastrophe. All of it is predictable, which means all of it can be prepared for.

The vitamin D cliff — and what the evidence really says

Vitamin D levels in northern populations follow a sine wave: peak in late August, trough in February–March. Right now, at the end of August, most people in the Baltics are at their highest level of the year. From here it falls, by a few nmol/L a month for most people, until spring.

This matters because vitamin D is not really a vitamin at all. It functions as a hormone, and immune cells — monocytes, macrophages, T cells — carry receptors for it [2]. Under EU nutrition rules we can state plainly what is authorised: vitamin D contributes to the normal function of the immune system.

Beyond that sentence, it is worth being precise, because this is a field with a great deal of overreach. Early meta-analyses of supplementation trials were encouraging [3]. The most recent update, which added further trials to the pool, is more sober: it no longer finds a statistically significant overall effect, and finds no evidence that baseline status or dosing regimen changes the picture [4]. A capsule is not a shield.

What remains uncontroversial is the simpler thing — that being frankly deficient is worth avoiding, and that at this latitude, in February, a great many people are. So the point of the autumn handover is not to megadose. It is to know your number. That belongs to your doctor and a blood test, and late August is an unusually good time to take one: a reading now tells you what your best level of the year looks like.

Vitamin C is a daily account, not a savings account

Vitamin C behaves in the opposite way to vitamin D, and this is where food does its real work.

It is water-soluble. It is not stored in any meaningful quantity. Plasma levels respond to intake within hours and decline within days. Immune cells — neutrophils in particular — actively concentrate ascorbate to levels far above plasma, and consume it during an immune response, which is one reason plasma vitamin C falls during infection [5].

Ripe red rosehips on a thorny branch in autumn, one of the most vitamin C–dense fruits grown at northern latitudes

The practical consequence is that a heroic dose on the day you feel ill is close to pointless, while a modest, boring, daily intake is not. This is where the Nordic berries earn their reputation:

Rosehip is one of the most vitamin C–dense fruits that will grow at this latitude, along with a distinctive set of carotenoids and galactolipids. Ours is pressed into Nordic Petal.
Sea buckthorn carries several times more vitamin C by weight than an orange, alongside a fatty-acid profile that almost nothing else in the plant world has. It's worth its own article — we've written one — and it lives in our sea buckthorn range.
Blackcurrant and acerola are the reliable everyday workhorses.

The single most useful habit here is not a supplement. It is frequency. A small amount of a vitamin C–dense food, twice a day, keeps plasma levels closer to saturation than one large hit.

Your gut does most of the negotiating

A large share of the body's lymphoid tissue — commonly estimated at around 70% — sits in and around the gut wall [6]. The relationship runs in both directions: the microbiome trains immune cells, and immune signalling shapes which microbes thrive.

Fresh aronia berries and berry juice in a glass

The lever most people can actually pull is polyphenols — the dark pigments in aronia, blackcurrant, haskap, pomegranate. Only a small fraction of dietary polyphenols is absorbed in the small intestine. The rest travels onward to the colon, where bacteria metabolise them into smaller compounds that are absorbed and biologically active, and where they appear to shift the microbial community itself [7]. Human trials with aronia have reported changes in microbial composition over eight to twelve weeks [8].

This is a slow lever. It works on the timescale of weeks, which is exactly why now is the right moment rather than November. And it wants company: polyphenols and fermentable fibre act on the same microbial population, and the combination does more than either alone.

The five-week handover

Here is the plan, laid out honestly. None of it is dramatic. That is the point.

Week 1 — audit. Book a vitamin D blood test if you have never had one. Note how much sugar and how little fibre the summer actually contained. No changes yet.

Week 2 — anchor the morning. Get outside within an hour of waking, even under cloud. Ten minutes of outdoor light in the morning is worth more to your circadian system than an hour indoors by a window. Attach one wellness habit to something you already do without thinking — coffee, brushing your teeth. A shot taken beside the kettle survives. A shot that requires you to remember it does not.

Week 3 — darken the plate. Add one deeply coloured plant food to every meal. Aronia, beetroot, red cabbage, blackcurrant, black radish. Colour in the plant world is almost always chemistry doing something. If you want a shortcut to the darkest end of that shelf, it is our pressings that go nearly black in the glass — Bitter Sweet, Go Berry, Blue North.

Dark berry juice being poured from a bottle into a glass as part of a morning routine

Week 4 — fix the air and the sleep. Get a humidifier or put a bowl of water near the radiator before the heating comes on. Set a wake time and hold it on weekends within an hour. This is the least glamorous item on the list and probably the most powerful.

Week 5 — set the winter shelf. Decide now what the winter pantry looks like, and buy it before you need it. The failure mode is not lack of knowledge; it is standing in a cold kitchen in November with nothing in the fridge. This is precisely what a set like Nordic Shield is for.

What this will not do

An honest article owes you this section.

No juice, berry, root or shot prevents infection. Immunity is not a quantity that can be "boosted" upwards indefinitely — an immune system running hot is called inflammation, and it is not a goal. What nutrition realistically does is remove the constraints: a system that is not short of vitamin C, vitamin D, zinc, protein or sleep responds better than one that is.

Megadoses are not a shortcut. Water-soluble vitamins above saturation are simply excreted; fat-soluble ones accumulate, and vitamin D in particular has a real upper limit. More is not the strategy. Sooner is.

And if you already have a specific condition, are pregnant, or take regular medication — grapefruit and statins, or vitamin K–rich foods and warfarin, being the classic examples — the right conversation is with a pharmacist or doctor, not a blog.

Back to the light

The shadows on the kitchen floor are a countdown, and the useful thing about a countdown is that it is visible before the deadline.

Nothing about late August feels urgent. That is exactly what makes it valuable. Everything described here — the light, the plate, the sleep, the pantry — takes effect over four to six weeks. Start in November and you are treating. Start now and you are simply arriving in winter already prepared, which is a much quieter and much more pleasant thing.

The handover happens whether you take part in it or not. You may as well be there.

References

Webb AR, Kline L, Holick MF. Influence of season and latitude on the cutaneous synthesis of vitamin D3. J Clin Endocrinol Metab. 1988;67(2):373–378.
Liu PT et al. Toll-like receptor triggering of a vitamin D–mediated human antimicrobial response. Science. 2006;311(5768):1770–1773.
Jolliffe DA et al. Vitamin D supplementation to prevent acute respiratory infections: a systematic review and meta-analysis of aggregate data from randomised controlled trials. Lancet Diabetes Endocrinol. 2021;9(5):276–292.
Jolliffe DA et al. Vitamin D supplementation to prevent acute respiratory infections: systematic review and meta-analysis of stratified aggregate data. Lancet Diabetes Endocrinol. 2025;13(4):307–320.
Carr AC, Maggini S. Vitamin C and immune function. Nutrients. 2017;9(11):1211.
Vighi G et al. Allergy and the gastrointestinal system. Clin Exp Immunol. 2008;153(Suppl 1):3–6.
Cardona F et al. Benefits of polyphenols on gut microbiota and implications in human health. J Nutr Biochem. 2013;24(8):1415–1422.
Istas G et al. Effects of aronia berry (poly)phenols on vascular function and gut microbiota: a double-blind randomized controlled trial in adult men. Am J Clin Nutr. 2019;110(2):316–329.