Which supplements to strengthen winter immunity?

Why does immunity need specific support in winter?

What does Swiss winter change for the immune system?

Swiss winter reduces the natural production of vitamin D in the body and exposes you to a denser viral promiscuity, two factors that weigh on the body’s defences against pathogens. Between December and March, about 60% of the Swiss population is not adequately supplied according to the FSVO^[1].

This winter drop is due to the lack of natural light at Swiss latitudes: under reduced daylight, the skin barely synthesises any vitamin D between October and March. The Federal Nutrition Commission sets the recommended daily intake at 600 IU for adults and 800 IU from age 60^[2]. This molecule contributes to the normal functioning of the immune system^[3] — this is the claim officially validated and reproduced by the FSVO in annex 14 of the Ordinance on Food Information. Add to this indoor confinement, heating that dries out respiratory mucous membranes, and dry air in enclosed spaces that favours the circulation of viruses during the winter months.

Who should pay particular attention to winter deficiencies?

Older people, young children, pregnant or breastfeeding women, and people with dark skin or little exposure to natural light are the most at risk according to the FSVO^[1]. For these profiles, a sufficient intake is recommended all year round, not only in winter.

Skin synthesis decreases with age: in seniors, the FSVO recommends 800 IU per day on a continuous basis^[1]. People who cover their skin for cultural or medical reasons, and those with obesity, are also among the groups to monitor. In an obese person, vitamin D is stored in fatty tissue and becomes less available in the bloodstream. Some treatments such as corticosteroids or antiepileptics also accelerate its degradation. Endurance athletes who train indoors in winter and night workers also accumulate deficiency factors — their lifestyle, sleep quality and lack of daylight weigh on the body’s balance.

Which supplements are really backed by recent studies?

Vitamin D: for whom and at what dose in Switzerland?

Vitamin D remains the supplement best justified in winter in Switzerland to strengthen immunity, but its impact on respiratory infections is more modest than the earlier literature suggested. The 2025 Jolliffe meta-analysis published in the Lancet Diabetes & Endocrinology, which gathers 46 randomised trials and more than 64,000 participants^[4], no longer shows a statistically significant protective effect on the risk of acute respiratory infection.

This update incorporates a large recent trial of 15,804 people^[4] that tipped the overall result. The subgroup analysis also reveals no different benefit by age, dose or starting status. The Swiss justification for supplementation therefore remains first and foremost the correction of the deficiency itself, which plays a key role for bone health and the immune function of cells. The recommended doses are 600 IU per day for adults and 800 IU from age 60^[2]. The product is best taken with a meal rich in good fats, in the form of drops, capsules or tablets, to improve absorption; some preparations such as cod liver oil contain it naturally. For the specific prevention of flu or COVID, some work suggests that supplementation may reduce the risk^[5]. That falls within a specific medical framework that goes beyond self-supplementation and free use of the product.

Zinc, probiotics and catechins: what do meta-analyses retain?

Zinc lozenges to be dissolved may shorten the duration of the cold by about two days according to the Cochrane Nault meta-analysis published in 2024^[6], but do not prevent it — the authors stress that the evidence remains of low certainty. Probiotics, flavonoids (including catechins, antioxidants from green tea) and synbiotics are among the oral supplements most useful in reducing the risk of respiratory infections in adults, according to the network meta-analysis by Zhu and colleagues published in 2025^[5].

The mineral acts locally, in direct contact with the mucous membrane of the throat, when the lozenge slowly dissolves in the mouth: intake in swallowed capsule form does not have this benefit. The Cochrane review also notes an increase in digestive adverse effects (nausea, bad taste) at high doses^[6]. These minerals play a role in the functioning of the immune system but must remain on a reasoned intake basis. For probiotics, the documented effectiveness concerns multi-strain formulas of live bacteria taken for several weeks to rebalance the intestinal flora and support digestion, not commercial yoghurts. A balanced gut microbiota participates in the body’s immune response. Vitamin C at the usual dose (1 g per day or less) has not shown a robust preventive effect on the cold in the general population^[10]; a marked benefit exists, on the other hand, in people very exposed to physical stress, such as marathon runners or soldiers in cold climates — the Cochrane Hemilä meta-analysis reports a relative risk of 0.48 in this subgroup^[10]. Substances such as acerola (rich in vitamin C), royal jelly or some plants in essential oils are often used by preference or habit, but echinacea and elderberry do not have a validated European health claim for the immune system^[3].

How to choose and use an immunity supplement well?

Which quality criteria should you look at first?

The form of the nutrient, the dosage in pure mineral, compliance with FSVO claims and the nature of the excipients are the four priority checkpoints before any purchase. In Switzerland, only the claims listed in annex 14 of the Ordinance on Food Information may appear on the packaging^[3]; at the time of choice, the advice of a healthcare professional at a pharmacy remains the best counsel to validate the use of a targeted product.

The displayed quantity must correspond to the quantity of pure mineral (elemental zinc, for example), not to the total salt. Check the absence of claims prohibited by the FSVO (“cures”, “prevents flu”): their presence signals a manufacturer who does not respect the Swiss framework^[7]. On a website, also check the secure payment terms, the privacy policy, the displayed sale price and the delivery terms before ordering a product. These preparations are regulated by the Federal Act on Foodstuffs and Utility Articles and the FDHA Ordinance on Food Supplements^[8].

• ①Form of the nutrient: D3 rather than D2 for vitamin D; bisglycinate or citrate for zinc.
• ②Dosage in pure mineral displayed clearly, not in total salt.
• ③Limited excipients: avoid unnecessary colourings and flavourings, especially in gummies.
• ④Claims compliant with annex 14 of the Ordinance on Food Information, with no promise of cure.
• ⑤Consistency with the diet: no overlap with a multivitamin already dosed in zinc and vitamin D.

Which common mistakes should you avoid during a winter course?

Stacking several immunity formulas, exceeding the recommended doses, or waiting for symptoms to appear before starting vitamin D are the three most frequent mistakes when facing winter. The FSVO reminds people that these products neither prevent nor treat diseases^[9]: they correct a deficiency or provide a targeted micronutrient.

Stacking a multivitamin, an immunity complex and an added syrup often leads to exceeding the tolerable upper intake levels, in particular for vitamin A and zinc. For vitamin D, starting intake from October allows you to reach a sufficient status before the winter trough — full effectiveness requires several weeks of regular intake to durably strengthen the functioning of the immune system. A balanced diet rich in fresh fruit and vegetables, regular exercise, good sleep and the reduction of stress and alcohol remain the basics to strengthen the immune system over time. Lastly, the self-diagnosis of a “drop in immunity” often leads to mistargeting the need: chronic fatigue, repeated infections or digestive disorders justify medical advice, not a stack of products.

• ⚠Never exceed the doses indicated on the packaging, in particular for vitamin A, zinc and iron.
• ⚠A course does not replace a varied diet, sufficient sleep and regular physical activity.
• ⚠Ask the pharmacist or doctor for advice in case of chronic treatment, pregnancy or for a child.

Frequently asked questions about winter immunity supplements