Vitamins in winter: what changes and what doesn't

In winter, the thing the sources single out is vitamin D. Between October and early March the body does not make enough of it through the skin, and official UK advice is that everyone should consider taking a daily vitamin D supplement in autumn and winter. As for vitamin C and zinc, the NHS says you should be able to get what you need from your daily diet.

If you take one idea from this page, take this one: sort out the vitamin D, keep eating normally, and don't spend money trying to boost your immune system with products that promise more than they can deliver.

Why vitamin D is the one thing that genuinely changes in winter

Vitamin D is made in the skin when sunlight falls on it. In the low-sun months that process more or less stops. The NHS states that from about late March or early April to the end of September, most people can make all the vitamin D they need — but that between October and early March the body does not make enough. It adds a detail that tends to surprise people: vitamin D is found in very few foods. Oily fish, red meat, egg yolks and fortified foods. This is not a matter of eating better. It is a matter of the main source being shut for months.

There is an honest geographical nuance here. The figures that set these dates come from a UK government page, and the Harvard T.H. Chan School of Public Health explains that the sun's UVB rays are weaker at higher latitudes, so blood vitamin D levels tend to be lower in those places. Neither source gives specific dates for Portugal or southern Europe. You probably get more winter sun in Faro than in Manchester — but nobody can tell you, from the data available, the exact day when that stops being enough. The advice to consider a daily supplement in autumn and winter is the safest thing on offer.

Vitamin C and zinc: why the answer is different

Here the story flips. The NHS is clear: vitamin C cannot be stored in the body, so you need it every day, and good sources include citrus fruit, peppers, strawberries, blackcurrants, broccoli, Brussels sprouts and potatoes. The same source concludes that you should be able to get all the vitamin C you need from your daily diet. If you eat an orange or a pepper salad at lunch, it is already there.

Zinc follows the same logic. Meat, shellfish, dairy, bread and cereal products are good sources, and the NHS says you should be able to get all the zinc you need from your daily diet. Notice what that means: if your diet already supplies the zinc you need, raising the dose in December adds nothing that was not already there.

What European law allows you to say about these nutrients is modest, and it is worth knowing it exactly as it stands. Regulation (EU) No 432/2012 authorises the claims that vitamin C contributes to the normal function of the immune system and that zinc contributes to the normal function of the immune system — but only for foods that are at least a source of that nutrient. The same regulation permits, for vitamin D, claims about the maintenance of normal bones, muscles and teeth, normal absorption of calcium and phosphorus, and normal immune function. "Contributes to the normal function of" is not the same as "boosts", "protects" or "prevents". It is the difference between keeping what already exists and promising a barrier.

How to fit this into a real day

You don't need a protocol. You need two or three decisions that survive a bad week.

• Vitamin D at the same time every day. With breakfast, or after dinner — it doesn't matter. What matters is hitching it to something you already do. Leave it for "when I remember" and you will have forgotten by January.
• One source of vitamin C with each main meal. This is not a complicated nutritional target: put peppers in the sandwich, eat a clementine, add broccoli to the plate.
• Zinc across the week, not every day. Meat, shellfish, eggs, bread. If your diet is varied, this sorts itself out.

On the days that go badly — travel, illness, a week of frozen food — don't try to make up for it with a double dose. Vitamin D accumulates in the body, and too much over time has consequences described further down. One missed day does not undo a consistent month.

The mistakes that wreck the plan

The first is treating a vitamin D supplement as automatically safe because it is "just a vitamin". The NHS advises not taking more than 100 micrograms (4,000 IU) of vitamin D a day, because it can be harmful: too much vitamin D over a long period can lead to too much calcium building up in the body, known as hypercalcaemia. That is not a decorative warning. It is the reason you do not "strengthen" a low dose by doubling it.

The second mistake is too much zinc. The same source advises no more than 25 mg a day from supplements, unless a doctor tells you otherwise. And there is a third: high doses of vitamin C. Above 1,000 mg a day it can cause stomach pain, diarrhoea and wind. Gram-strength effervescent tablets, taken out of habit, are the easiest way to get there without noticing.

The mistake that matters less than people fear: not taking the supplement at the "right" time. The sources we looked at do not name a correct hour for taking vitamin D, so regularity is the part that is actually in your hands.

Where a supplement fits, and where it doesn't

A vitamin D supplement in autumn and winter has solid backing: it is the recommendation of a public health authority, not a trend. That is where the evidence is firmest. For vitamin C and zinc in people who eat normally, the official starting point is that food is enough — a supplement does not add what is already there.

If you are looking for a product in this category, there are the Immunity & Defense Gummies with Echinacea, with 5 µg of vitamin D3 per 2-gummy serving (100% of the reference intake), 80 mg of vitamin C (100% of the NRV) and 5 mg of zinc (50% of the NRV), among other ingredients. The Multivitamin Gummies include 50 µg of vitamin D per 2-gummy serving, alongside various other vitamins and minerals. Neither of these gummies replaces a varied diet and a healthy lifestyle — and neither treats or prevents disease. Read the label, check the amounts, and don't combine two products that overlap without doing the arithmetic.

A warning that comes with the product itself: do not exceed the recommended daily dose, keep out of reach of young children, and consult a healthcare professional before use if you are pregnant, breastfeeding, suffering from a medical condition or taking medication. The gummies contain polyols and excessive consumption may produce laxative effects.

When this stops being a question about vitamins

Honesty requires saying this: if after two or three weeks of taking vitamin D consistently and eating reasonably well you still feel tired all the time, keep getting infections, have bone or muscle pain, or simply feel "off" without being able to explain why — this is not the moment to increase doses. It is the moment to book an appointment. Vitamin D has a role in maintaining normal bones and muscles, but persistent tiredness has many causes, and none of them is solved by guessing.

The same applies before you start. If you take medication regularly, if you have a diagnosed health condition, if you are pregnant or breastfeeding, talk to your doctor or pharmacist before adding any supplement. A pharmacist can, in a five-minute conversation, check for interactions and overlaps that no article can anticipate.

The honest limit of what is known

A lot of what you hear about "boosting your defences in winter" goes beyond what is established. What we know reliably is limited and useful: vitamin D production in the skin falls in the winter months at mid and high latitudes, vitamin D exists in very few foods, and health authorities recommend considering a daily supplement during that period. We also know that vitamin C and zinc come from a normal diet for most people, and that high doses carry costs — stomach, gut, and in the case of vitamin D, serious risks with prolonged use above the limit.

What we do not know, and shouldn't pretend to: there are no specific dates for Portugal, there is no guarantee that a supplement will stop you catching a cold, and there is no single nutrient that replaces sleep, movement and reasonable food. Start with vitamin D. Leave the rest alone until you have a concrete reason to change something.