Why You Get Sick Every Winter, and the Honest Truth About Immune-Boosting Foods

By Dr. Raj Dhadwal

It happens on schedule. The mornings turn cold, the office windows stay shut, the children bring something home from school, and by the second week of it you are on the sofa with a box of tissues and a sore throat, wondering what you did wrong. So you do what most people do. You open a search bar and you type some version of foods to boost immunity, and you get back a wall of confident lists telling you that this berry or that root or this powder will armour you for the winter. I want to walk you through the honest version of that question, because the biology is more interesting than the marketing, and because the research on one of those very berries, sea buckthorn, turned out to say something you would never guess from the headlines.

This is education about normal physiology and food composition, not medical advice, and nothing here describes treating, curing or preventing any condition. As a clinician, the single most useful instinct I can give you is to be sceptical of anything, in a shop or on a screen, that promises a food will make you sick less often. That promise is almost always bigger than the evidence behind it.

What does it even mean to boost your immune system?

Start with the word itself, because the word is where the trouble begins. Boost implies a dial you can turn up, as if the immune system were a single engine that runs faster when you feed it the right fuel. That is not what the immune system is. It is a layered network of barriers, patrolling cells, signalling molecules and memory, and most of the time you do not want it turned up. An immune system running hotter than it needs to is not health, it is inflammation, allergy and autoimmunity. What you actually want is an immune system that is well supplied, well rested and responds appropriately, which is a very different thing from one that is cranked to maximum.

There is also a simple arithmetic problem with the boosting story. A common cold is caused by a virus you caught from another person, most often a rhinovirus, and how often you catch one is driven far more by exposure than by breakfast: how many people you are near, how much they are shedding virus, how well ventilated the room is, how much you touch your face. A food eaten in the morning does not stand between you and the person coughing on the train. This is why the honest researchers who study this are careful to separate two questions that marketing blurs together: does a food change some measurement in the blood, and does it change how often a real person actually gets sick. Those are not the same question, and a food can move the first without touching the second.

So do nutrients matter for immunity at all?

They do, but the shape of the effect is not the shape the adverts imply. The clearest place nutrition matters is deficiency. If someone is genuinely short of a nutrient the immune system needs, such as vitamin C, vitamin D, zinc, protein or several others, correcting that shortfall lets the system work as it is meant to. That is repletion, filling a gap, and it is real. What that is not is a bonus effect in someone who is already well fed. Pouring more of a nutrient into a person who already has enough does not keep pushing the immune response higher and higher. The curve flattens. Enough is enough, and past that point extra is just expensive.

This is the distinction that gets lost. Vitamin C is a genuine example: it is a nutrient the body cannot make and must get from food, which is why a real deficiency causes real problems, and we go into that in why your body cannot make vitamin C. But needing it to avoid deficiency is a world away from the idea that loading up on it turns an ordinary winter into an invincible one. Most of the careful reviews land in the same unglamorous place: a varied diet that keeps you out of deficiency supports normal immune function, and no single food layered on top of an already adequate diet has been shown to reliably cut how often healthy adults catch colds. We look at the vitamin C part of the story specifically in the truth about berries and vitamin C, and at the broader cold-season picture in nutrition through cold season.

What happened when they actually tested a berry?

Here is where I want to be precise, because this is the part that earns your trust. Sea buckthorn is one of the foods most often waved around as an immune food, largely because it is dense in vitamin C and other compounds. So it is worth reporting exactly what happened when researchers put it to a proper test, including, and especially, the part that did not go the way the marketing would want.

Study	Design	What it used	What it reported
Larmo and colleagues, European Journal of Clinical Nutrition, 2008 (PMID 17593932)	Double-blind, randomized, placebo-controlled trial, 254 volunteers	Frozen sea buckthorn berry preparation taken over the cold-and-flu season, against a placebo	Reported NO reduction in the number of colds and no meaningful reduction in their duration. Participants eating the berries got sick about as often as those who did not. The inflammation marker C-reactive protein (CRP) was lower in the sea buckthorn group, a blood-measurement difference, not a change in how often anyone actually caught a cold.

Read that as it stands, because it is a rare and valuable thing. This was not a small, casual study. It was randomized, placebo-controlled, double-blind, and it followed 254 people through the season, which is precisely the design built to answer the question that matters: do people who eat this food get fewer colds. The answer it reported was no. The berry did not reduce the number of colds. That is the headline the marketing never prints, and it is the honest one.

The CRP finding is worth handling carefully, because it is exactly the kind of result that gets spun. CRP is a general marker of inflammation measured in the blood. It moved in the sea buckthorn group. But a change in a blood marker is a change in a blood marker; it is not the same as getting sick less often, and in this very trial the two came apart, the marker shifted while the colds did not. That is the whole lesson of this article in one dataset. A food can nudge a measurement without changing the outcome you actually care about. Reporting the CRP number as if it were an immunity benefit would be exactly the bridge I am telling you to distrust, so I am not going to build it.

What actually helps you catch fewer colds?

The measures with the best evidence are the boring ones, which is probably why they do not sell as well as a jar. Wash your hands and keep them off your face, because most colds enter through hands to eyes, nose and mouth. Ventilate shared indoor spaces. Guard your sleep, because short sleep is one of the more consistently studied things that leaves people more susceptible. Stay up to date with the vaccines your clinician recommends for the viruses that have them. And eat a varied diet that keeps you out of any nutrient deficiency, which supports the immune system doing its ordinary job, a job it does perfectly well without being boosted. If you catch colds and also feel run down and slow to bounce back, that recovery question is its own topic, covered in why you recover slowly.

Notice what is not on that list: a single hero food. That absence is the point. Nutrition matters for immunity mainly by keeping you out of deficiency, and beyond that the reliable levers are behaviour and exposure, not a single food in a shot glass.

Where does a berry honestly fit, then?

Human Renaissance is the only sea buckthorn puree in the world sold in a single-serve pouch. 190+ naturally occurring compounds, omega 3, 6, 7 and 9, 201 mg of vitamin C per pouch, 0 g sugar, 5,640 hand-picked berries per box.

That is a statement about what is in a whole food, and it is where the honest statement ends. Sea buckthorn is a nutrient-dense berry, and vitamin C is a nutrient you have to eat because your body cannot make it. But density on a label is not a promise about your winter. The best test done on this berry reported that it did not reduce the number of colds, and I would rather you heard that from me than heard the opposite from someone selling you a winter shield.

The short version

Boost is a marketing word, not a physiological one; the immune system is a layered network you generally do not want turned up, and how often you catch a cold is driven mostly by exposure, ventilation, hand hygiene and sleep rather than by breakfast. Nutrition matters chiefly through deficiency: filling a genuine gap in a nutrient like vitamin C lets the system work normally, but piling more onto an already adequate diet does not keep raising immunity. When sea buckthorn, a berry often sold as an immune food, was actually tested in a double-blind randomized placebo-controlled trial of 254 people (PMID 17593932), it reported no reduction in the number of colds, even though the inflammation marker CRP fell in that group, a blood-measurement change that is not the same as getting sick less. The reliable moves are the unglamorous ones: hands, air, sleep, recommended vaccines and a varied diet. This is education about physiology and food, not medical advice, and nothing here treats, cures or prevents any condition.