How to strengthen your immune system: the 4 science-backed nutrients
"Boosting your immunity" has become an omnipresent marketing claim. Behind the phrase, however, lies precise biology: four nutrients whose effect on immune function is rigorously documented. Here is which ones, in what forms, and at what doses.
1. Zinc: cofactor of cellular immunity
Zinc is a critical trace element for more than 300 enzymes and is particularly concentrated in immune cells (neutrophils, T lymphocytes, NK cells). Its direct action against RNA viruses, notably by inhibiting intracellular viral replication, is documented in vitro and in vivo.
The Cochrane review on zinc supplementation shows that 75 mg/day lozenges reduce the duration of a cold by 33% when started within the first 24 hours of symptoms. For ongoing prevention, intakes of 5 to 15 mg/day maintain optimal status.
The preferred forms are zinc citrate, bisglycinate and picolinate; zinc oxide has very low bioavailability and is not a relevant choice in micronutrition.
2. Vitamin C: front-line antioxidant
Vitamin C accumulates at high concentrations in leukocytes, particularly neutrophils and macrophages. It facilitates their migration to sites of infection, supports cytokine production and regenerates membrane antioxidants such as vitamin E.
Its preventive effect on cold frequency becomes clinically significant in people under intense physical stress, with a documented 50% reduction in incidence. During active infection, intakes of 5 to 20 g/day, split across the day, are recommended.
3. Selenium: antioxidant and immune cofactor
Selenium is the indispensable cofactor of selenoproteins, including glutathione peroxidase (GPx), the central enzyme of oxidative detoxification. A selenium deficiency impairs the adaptive immune response and increases susceptibility to viral infections, with some viruses mutating faster in a selenium-deficient organism.
Swiss soils are naturally poor in selenium, which is reflected in the micronutrient density of locally produced foods. Average intakes in Switzerland often remain below WHO recommendations (55 µg/day). L-selenomethionine (an organic, yeast-derived form) has a bioavailability above 90%, versus less than 25% for inorganic sodium selenite or selenide.
A narrow therapeutic window: selenium is one of the micronutrients with the smallest margin between efficacy and toxicity. The maximum tolerated level set by the Swiss FSVO is 165 µg/day. Selenium supplementation should stay within the preventive range (100 to 200 µg/day) and not be stacked without first assessing total dietary intake.
4. Vitamin D3: regulator of innate and adaptive immunity
Vitamin D3 (cholecalciferol) acts as a pro-hormone that modulates the expression of more than 200 genes, a significant share of which belong to the immune system. It strengthens innate immunity by stimulating antimicrobial peptides (cathelicidin, defensins), while regulating adaptive immunity to prevent excessive autoimmune responses.
Several meta-analyses agree: a serum 25(OH)D level below 30 ng/mL is associated with 40 to 60% more respiratory infections. In the Swiss winter (latitude 47°N, no skin synthesis from October to March), intakes of 10,000 to 50,000 IU/day are generally needed to reach the target.
Vitamin D3 and fat solubility: an essential technical point. Because vitamin D3 is fat-soluble, its intestinal absorption depends on the presence of fats in the meal. The practical rule is to take it with the fattiest meal of the day (olive oil, avocado, nuts, oily fish).
NUHO formulation note: vitamin D3 is not included in base One. Its fat solubility makes it incompatible with a water-soluble powder formula, and its optimal dose varies considerably with individual blood level and season. The coherent strategy is to measure serum 25(OH)D, then take a separate vitamin D3 in drops or lipid capsules, ideally alongside magnesium.
What the science actually validates
The scientific literature clearly distinguishes high-impact interventions from marketing formulas. Zinc and vitamin D3 are the two interventions with the most robust documented effect in the Swiss context, where deficits in these two micronutrients are common. Vitamin C is a relevant support, particularly in people exposed to high physical or oxidative stress.
Most plant-based "immune boosters" (echinacea, elderberry, multi-component herbal blends) have an insufficient or methodologically fragile evidence base to justify replacing documented micronutrients. Probiotics have real but strictly strain-specific effects; most mass-market products are underdosed and formulated without therapeutic targeting.
Immunity is built over weeks, not a one-off response to trigger in an emergency. Continuous micronutrient coverage, started ahead of the at-risk season, is physiologically more effective than reactive supplementation begun at the time of infection.