Vitamin C, Vitamin D and Zinc for Immunity: Deficiency, Prevention and Colds

Why these three

Vitamin C, vitamin D and zinc are the nutrients most often linked with "immune support" and colds. The question to ask is not "is it good for immunity?" but "for which outcome, in whom, and compared with what?" The table separates four different questions. Evidence labels: established recommendation, promising but limited, mixed, insufficient evidence.

Four questions, kept apart

Vitamin C

• Prevention. A Cochrane review of 29 trial comparisons (11,306 participants) found regular vitamin C did not reduce the chance of catching a cold in the general community (pooled risk ratio 0.97). In 598 marathon runners, skiers and soldiers on subarctic exercises, colds were fewer (risk ratio 0.48). [2]
• Duration. Regular use shortened colds by 8% in adults and 14% in children. Taking vitamin C only once symptoms began showed no consistent effect. [2]
• Label: mixed. The NHS says adults aged 19 to 64 need 40 mg a day and should get all they need from a varied diet. [1]
• Safety. More than 1,000 mg a day can cause stomach pain, diarrhea and gas, which fade after stopping. [1]

Vitamin D

• Deficiency. The NHS (UK government advice) says everyone should consider a daily supplement during autumn and winter, and some people at higher risk of low levels should take one all year, such as people rarely outdoors, those who cover most of their skin outdoors, and people with dark skin. [3] US advice differs; ask your clinician.
• Infections. A 2017 individual-participant meta-analysis of 25 trials (10,933 participants) found a reduced risk of acute respiratory infection (odds ratio 0.88), with stronger effects in people with very low starting levels (below 25 nmol/L) and with daily or weekly dosing. [4] A 2025 update of 40 studies (61,589 participants) found the overall effect no longer statistically significant (odds ratio 0.94, 95% CI 0.88 to 1.00), and no evidence that age, baseline level or dose changed the effect. [5]
• Label: mixed. It is more plausible as correcting a shortage than as a general infection shield.
• COVID-19. The NHS says there is not enough evidence to support taking vitamin D solely to prevent or treat it. [3]
• Safety. The NHS says not to take more than 100 micrograms (4,000 IU) a day, and that long-term excess can cause calcium build-up that can weaken bones and damage kidneys and heart. [3] Note that this NHS page was last reviewed in 2020.

Zinc

• Prevention. The 2024 Cochrane review included 34 studies (8,526 participants); zinc may have little or no effect on preventing colds (risk ratio 0.93). [7]
• Treatment. When used for treating colds, zinc may reduce the duration (about 2.4 days shorter on average), but certainty is low and results varied widely. Most studies used lozenges at 45 to 276 mg of zinc gluconate a day, which are study amounts, not recommendations. [7]
• Side effects. Non-serious adverse events were probably more common when zinc was used to treat colds (risk ratio 1.34). The review counted events such as unpleasant taste, loss of smell, vomiting, stomach cramps and diarrhea. [7]
• Label: mixed, low certainty.
• Upper limit. The NHS says do not take more than 25 mg of zinc supplements a day unless advised by a doctor; high doses reduce copper absorption and can lead to anemia and weakened bones. [6]

The bigger picture

A systematic review of 39 randomized studies in healthy people (including vitamins A, C, D, E and zinc) said gaps prevented firm conclusions about immune-health benefits. [8] The NHS states there is little evidence that vitamin C, echinacea or garlic prevent colds or speed recovery. [9] Nutrient profiles will be on the Vitamins and Supplements section.

Safety and when to get help

Check labels for duplicate ingredients across products: amounts and duplicate ingredients, reading supplement labels and medication interactions. Ask a clinician before using supplements if pregnant, taking medicines, or with kidney problems: talking with a health professional. Frequent infections are a reason for medical review, not just a supplement: getting sick often. For symptoms of colds, see colds, flu and respiratory infections.