Question & Context

Strengthening the immune system – what really makes a difference?

In brief

A healthy immune system cannot be “boosted”, but you can give it good working conditions. Enough sleep, regular exercise and the recommended vaccinations are best supported. Among supplements there are some bright spots: in studies, probiotics lowered the number of respiratory infections, zinc may possibly shorten a cold, and vitamin C helps mainly people under extreme physical strain. According to the latest meta-analysis, vitamin D does not measurably protect against infections on average; correcting a deficiency still makes sense.

What matters first

Sleep at least seven hours

Sleep is immune work. In one study, 164 healthy adults were measured with a motion sensor for a week and then deliberately exposed to a cold virus. Those who slept less than five hours had an approximately 4.5-fold higher risk of a cold than people with more than seven hours; at five to six hours it was approximately 4.2-fold. Between six and seven hours, no increased risk could be detected any longer.

Exercise regularly

In a Cochrane review of 14 studies with 1,377 adults, moderate endurance training such as brisk walking or cycling did not lower the number of infections, but it did lower the number of sick days during the observation period by a good two days, as well as the severity of symptoms. The data are still uncertain, but the direction fits everything that is known about exercise and health.

Check your vaccination status

Vaccinations are the most precise tool there is for the immune system: they train it specifically against individual pathogens. Germany's Standing Committee on Vaccination (STIKO) recommends the annual flu vaccination for, among others, all people aged 60 and over, pregnant women and people with chronic illnesses. A look at your vaccination record at your next family doctor appointment is worthwhile.

Correct deficiencies instead of supplementing blindly

Zinc, vitamin D, iron or vitamin B12 are indispensable for the body's defenses. But the people who really benefit are mainly those who have too little of them. A targeted blood test when a deficiency is suspected makes more sense than an all-round product whose benefit for healthy adults could not be demonstrated in large studies.

When you should have this checked by a doctor

Two to three colds a year are normal in adults and not a sign of a weak immune system. You should have the following checked by a doctor: repeated pneumonia or sinus infections, infections that do not heal despite treatment, unusual pathogens or persistent fungal infections, as well as unintended weight loss, night sweats and persistently swollen lymph nodes. See a doctor immediately in case of shortness of breath, high fever lasting several days, chest pain or confusion. Frequent infections can also have treatable causes such as iron deficiency, undetected diabetes or medications that dampen the immune system.

Why you cannot simply turn up an immune system

The immune system is not a muscle that gets stronger the more you stimulate it. A good defense responds quickly and specifically to pathogens and then calms down again. A permanently ramped-up immune system would rather be a problem, as allergies and autoimmune diseases show. That is why experts prefer to speak of supporting the defenses or closing gaps.

This is exactly how the study data behave as well. The large effects are found where something is missing: with too little sleep, with a nutrient deficiency or with extreme physical strain. In people who are well supplied, additional products usually bring little or nothing.

What the supplement studies show

For vitamin C, the data are almost uniquely good. A Cochrane review evaluated 29 trial comparisons with 11,306 participants: in the general population, the risk of catching a cold practically did not fall, but in five studies with marathon runners, skiers and soldiers it roughly halved. With regular intake, colds in adults were about 8 percent shorter.

For zinc, the current Cochrane review of 34 studies shows a possible shortening of a cold by a good two days, but with extremely variable results and low certainty. Mild side effects occurred more often with zinc. For vitamin D, the verdict in 2021 was still: a small protective effect. With the new large trials, the 2025 result lay just outside statistical certainty, with no difference between people with low or good baseline levels.

In the Cochrane analysis, probiotics lowered the proportion of people with at least one respiratory infection by about a quarter, as well as the number of antibiotics prescribed. The certainty of these findings is low to moderate, and which strains carry the effect is still open.

What the database says

Sorted by strength of evidence. The BK-Score rates the state of knowledge, not the substance.

Vitamin D3 Evidence 8/10 · Well supported, heavily overhypedVitamin D is important for immune cells, and a deficiency should be corrected. In the latest meta-analysis of 40 studies with 61,589 participants, however, protection against respiratory infections was no longer statistically established.
Vitamin C Evidence 8/10 · Well supported, heavily overhypedVitamin C practically does not lower the risk of catching a cold in the general population, but slightly shortens colds with regular intake. Marathon runners, skiers and soldiers under extreme strain benefited clearly.
6,000 to 10,000 steps a day Evidence 8/10 · Well supported, heavily overhypedEveryday movement is the basis: in large cohorts it is clearly linked to lower mortality, and in studies people who exercised were ill for shorter periods.
Zinc Evidence 7/10 · Supported, with caveatsZinc lozenges may possibly shorten a cold, but the studies vary widely. No reliable preventive effect could be demonstrated, and high amounts of zinc over the long term disturb copper balance.
Probiotics Evidence 7/10 · Supported, with caveatsIn a Cochrane review of 23 studies with 6,950 participants, people taking probiotics less often had at least one respiratory infection. The effect depends on the strain, not on the product class.
Sleep & sleep hygiene Evidence 7/10 · Supported, with caveatsThe foundation from the defenses' point of view: short sleep demonstrably makes you more susceptible to cold viruses. The page shows what sleep hygiene and regularity achieve.
Cold showers (Wim Hof) Evidence 4/10 · Thin human evidenceIn a large study, a cold shower at the end of the normal shower reduced sickness absence from work, but not the number of days of illness. A feel-good tool with an interesting side effect.
Colostrum (bovine) Evidence 5/10 · Thin human evidenceCow's first milk is promoted for the immune defenses and the gut barrier. A meta-analysis found no or only a small effect on immune markers.
Micronutrient concentrates (LaVita & co.) Evidence 9/10 · Well studied – effect not confirmedLiquid vitamin concentrates and multivitamins are very well studied. For healthy adults, large randomized trials show no measurable benefit.

Frequently asked questions

How many infections a year are normal?

Adults have on average two to three colds per year, children often more. That is not a sign of a weak immune system, but everyday life. You should pay attention to repeated pneumonia, infections that do not heal, or unusual pathogens.

Is vitamin D worth it against colds?

If a deficiency has been proven, correcting it makes sense, for bones and general well-being. As pure protection against infection, vitamin D showed no statistically established benefit in the latest meta-analysis of 40 studies. If you are unsure, have your level measured.

Does zinc help once the cold has already started?

Possibly, especially as a lozenge right at the start. On average the studies show a shorter duration of illness, but they vary widely. If you try it, you should do so only briefly, because permanently high amounts of zinc disturb copper balance.

Does exercise make you more prone to infections?

Regular moderate exercise is associated with shorter and milder infections. Under extreme strain such as marathon runs or winter military exercises, vitamin C helped most clearly in studies. If you are ill, it is better to take a break, especially with a fever.

Do cold showers strengthen the immune system?

In a large Dutch study, cold showerers called in sick less often, but were not ill less often. The shower seems to improve well-being and resilience rather than the defenses themselves. As a morning ritual, it is nevertheless a good idea for healthy people.

Do I need a combination product for my defenses?

For healthy adults with a varied diet, large studies show no benefit. It makes more sense to look specifically for a concrete deficiency and correct it. With a vegan diet, in pregnancy or at an older age, advice can help.