“Immune-boosting” is a marketing word. Here’s what the immune system actually needs

Every September, the same word appears on the same shelves. Boost. Immune-boosting this, immunity-boosting that, usually in green, usually next to a picture of a lemon.

Here's a thing worth knowing about that word: no supplement sold in the UK is permitted to make an immune-boosting claim. Not because of an oversight, and not because the regulator is slow. Because the evidence doesn't support it, so the claim was never authorised.

Which raises the reasonable question, how is it on all those boxes? And the slightly more interesting one: if "boost" is off the table, what is actually true about nutrition and immunity?

That second question has a genuinely good answer. It's just quieter than the packaging.

In the UK, health claims on food and supplements aren't a free-for-all. There's a register. A nutrient gets a claim only after the evidence has been assessed and accepted, and the approved wording is then fixed, you use it as written, or you paraphrase it faithfully, and you don't embellish.

For immunity, the authorised claim reads like this:

"Vitamin C contributes to the normal function of the immune system."

That's it. That's the strongest thing anyone is allowed to say. And the same construction covers a surprisingly long list of nutrients:

Vitamin D · Vitamin C · Zinc · Vitamin A · Selenium · Copper · Folate · Iron · Vitamin B6 · Vitamin B12

Ten nutrients, all carrying the same claim: contributes to the normal function of the immune system.

Now read that wording again, slowly, because every word in it is doing work.

"Contributes to" — it plays a part. It isn't the whole system, and it isn't a switch.

"Normal function" — this is the important bit. The claim is that the nutrient is required for your immune system to work the way it's supposed to. It is emphatically not a claim that more of it makes your immune system work better than normal. There is no authorised claim anywhere on that register for exceeding normal function, because that's not a thing the evidence shows happens.

So the honest version of the whole category is this: these nutrients don't boost your immune system. They're part of what it's built from. Run short, and it doesn't work properly. Top back up, and it works properly again. Keep going past that point, and you're not buying extra immunity, you're buying expensive urine, and in a few cases (we'll come to selenium) something less benign than that.

Set the regulation aside for a moment, because there's a deeper problem with the word.

"Boost" implies your immune system has a volume dial, and that turning it up is good. But your immune system isn't a volume dial. It's a coordinated system of physical barriers, patrolling cells, chemical signals and antibody memory, and the thing it prizes above all else is appropriate response, going hard at genuine threats, and standing down for everything else.

An immune system running permanently louder isn't a healthier one. Conditions where the immune system attacks the wrong target are, by definition, immune systems doing more, not less. Nobody selling a lemon-flavoured sachet is genuinely offering to turn the whole thing up, and it's just as well.

What you actually want is an immune system that's adequately resourced, one that has the raw materials to build cells, mount responses, and resolve them properly when the job's done. That's a supply question, not a volume question. Supply is boring. Supply is also the entire truthful story.

So let's do the genuinely geeky bit, what those nutrients are doing in there, because "contributes to normal function" is accurate but tells you nothing satisfying.

Vitamin C turns up in immune cells at concentrations far higher than in blood plasma, your neutrophils, the first responders of the immune system, actively concentrate it. It's also required for the resolution phase, the clean-up after a response, which is a part of immunity nobody puts on packaging because "helps tidy up afterwards" doesn't sell. Its other authorised claims are worth knowing too: contributes to the protection of cells from oxidative stress, and contributes to the reduction of tiredness and fatigue. Immune responses are metabolically expensive and generate a lot of oxidative mess; vitamin C is involved in both sides of that.

Vitamin D is the one with the most interesting story here, because immune cells carry vitamin D receptors, they're not just passive recipients, they're actively listening for it. This is a genuinely active area of research and we're not going to overstate where it's got to. What's settled enough to be authorised is the claim itself: vitamin D contributes to the normal function of the immune system. What's also settled is that in a British winter, a large number of people don't have much of it (see our blog on that, where the real numbers live).

Zinc is required for the production and function of immune cells, and it's a cofactor for hundreds of enzymes across the body. It's also one of the nutrients where "am I getting enough" is a genuinely reasonable question for plant-based eaters, since the zinc in beans, grains and nuts comes bound up with phytates that reduce how much you absorb.

Vitamin A maintains the epithelial barriers, your skin, and the linings of your gut and airways. Which is a nice reminder that a large chunk of immune defence isn't glamorous cellular warfare, it's just having intact walls.

Selenium, copper, folate, iron, B6 and B12 each hold the same authorised claim, all doing quieter jobs: antioxidant enzymes, cell division, the manufacture of new immune cells. Immune responses require making an awful lot of cells very quickly, and cell-making is where the B vitamins earn their keep.

Notice what's missing from that list. There's no exotic mushroom extract, no proprietary blend, no berry flown in from somewhere with good photography. Ten fairly ordinary nutrients, most of which you've heard of since primary school.

Because we'd rather say this than not: on this list, overshooting isn't harmless.

Selenium has an unusually narrow comfortable range, the gap between "enough" and "too much" is smaller than for almost any other nutrient. Long-term high intakes are associated with real problems. This is a nutrient to have at roughly 100% of requirement and then leave alone.

Iron shouldn't be supplemented on a hunch. Too little causes problems; so does too much, and iron overload is not a rare curiosity. If you suspect you're low, that's a blood-test conversation, not a shelf-browsing one.

Zinc at sustained high doses interferes with copper absorption which is a nice illustration of why single-nutrient megadosing is a blunt instrument. Two nutrients on the same immune list, and hammering one depletes the other.

This is the practical case for a formulated multivitamin over a shelf of individual pots: not that it's more powerful, but that someone has already worked out the proportions.

Our MultiVitis built on exactly that, and it's worth being specific about rather than waving at the word "balanced". Every nutrient on the immune list where overshooting genuinely matters sits at 100% of NRV and no higher — zinc, selenium, copper, vitamin A, vitamin C and iron. That's deliberate, and it's this whole section in practice: selenium's comfortable range is narrow, sustained high-dose zinc competes with copper, and iron isn't something to nudge upwards on spec.

A few nutrients do sit above 100%, and they're all B vitamins a long way below their upper limits, B1, B2 and B6 at 150%, folate at 200%. And then there's B12, at 50µg, which reads as an eye-watering 2000% of NRV until you know what that number is actually measuring.

It isn't a bigger-is-better dose. It's an absorption dose.

B12 gets into you by two routes. The first uses a carrier protein called intrinsic factor: efficient, but with a hard ceiling. The second is simple passive diffusion: no ceiling, and hopelessly inefficient. A 2024 study in Nutrients by Kashyap and colleagues measured exactly where that ceiling sits, using stable-isotope-labelled B12 in healthy adults. Swallow 2.5µg and you absorb about half of it. Swallow 10µg and you absorb about 15%. The percentage collapses because the amount absorbed barely moves, it plateaus at roughly 1.2 micrograms per dose, more or less whatever you take. Everything past that point is left to passive diffusion, which picks up around 1%.

It's a small study (nine adults) so hold the precise figures loosely. But it's measuring something long established, and it explains why every B12 supplement on the market looks faintly absurd on paper. To reliably land a useful amount from one daily tablet, you have to start with a great deal more than the amount you're trying to land.

Which is the point we made about the gut back in July: what you swallow and what you absorb are two different quantities. Where that gap is real, the dose reflects it. Where it isn't, the minerals above, the dose stays at 100%.

One — eat like someone who reads the list. Most of those ten nutrients are widely available in a varied plant-based diet. Supplements fill gaps; they're not a substitute for the food.

Two — cover the two the winter genuinely takes away. Vitamin D is the standout, because from October the sunlight simply stops supplying it — D3 2500iu is our autumn dose. Vitamin C is the other sensible one, and ours comes with citrus bioflavonoids, the compounds that occur alongside vitamin C in whole fruit.

Three — sleep, move, and manage the stress you can. These affect immune function more than any supplement will, they're free, and they're irritatingly hard to sell in a bottle. That's precisely why nobody advertises them at you in September.

Four — ignore anything promising a boost. Now you know it isn't allowed to say that, you can read the shelf properly. A brand willing to bend the rules on the front of the box has told you something useful about the rest of it.

Your immune system doesn't need boosting. It needs building materials, sleep, and not to be lied to. We can help with two of those, and we're honest about the third.