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Immunity · Athletes and travel

Training hard or travelling: the one place vitamin C earns its keep

For most people, vitamin C does not change the odds of catching a cold. Marathon runners, skiers and soldiers training in the cold are the exception: in trials, regular vitamin C roughly halved their colds. If your winter includes a race block or a long-haul trip, here is what the evidence supports.

By Kymata Health Editorial Sources cited inline Prices checked Oct 29, 2026

Quick answer

The quick answer

  • In five trials of people under short periods of extreme physical stress (598 people), regular vitamin C roughly halved the risk of a cold (risk ratio 0.48).
  • The effect needs regular use before and during the stress. Starting vitamin C after symptoms begin did not help consistently.
  • On long-haul flights, elderberry did not cut the number of colds in an industry-funded trial, though cold days were fewer.
  • Sleep loss, crowded cabins and dry air are the bigger levers. Protect sleep in race week and wash or sanitize hands when travelling.

We may earn a commission if you buy through these links. That never changes a rank. See how partner links work.

Story

A story you might recognize

Composite story built from common reader accounts; details changed. Not a testimonial.

She is 38 and training for her first marathon. Every big race she has run ended the same way: a cold in the week after, sometimes the week before. Her running club swears by mega-dose vitamin C started when the throat gets scratchy.

The trials say that timing is backwards. Vitamin C helped people under heavy physical stress when they took it regularly before and through the hard period. She started a modest daily dose two weeks before race week, protected her sleep, and kept zinc lozenges in her race bag in case.

In depth

Why heavy exertion and travel are different

Very long or intense exercise, especially in the cold, is followed by a window of more upper-respiratory symptoms in many studies. Race events add crowds, travel and poor sleep. That is the setting where the vitamin C trials found an effect.

Long flights add hours in a shared cabin, dry air, time-zone shifts and short sleep. The elderberry trial ran in exactly this setting.

The plan

The race-block or trip plan

  1. Two weeks out: start daily vitamin C. The stress trials gave vitamin C regularly before and during the hard period, not as a rescue at the first sniffle.
  2. Protect sleep. Short sleep raises the chance of catching a cold after virus exposure. Bank sleep before race week and plan around jet lag.
  3. Pack zinc lozenges. If a cold starts anyway, the zinc lozenge trials started within about 24 hours of symptoms.
  4. Travel hygiene. Hand sanitizer before eating, a mask in crowded spaces during peak season if you are close to a race, and fluids on the flight.
  5. After the event. Stop the vitamin C when the period of heavy stress ends; there is no evidence for carrying it on all year in this context.

The stack

The race-block and travel stack

Daily vitamin C through the hard period, lozenges on hand, and elderberry only as an optional extra around a flight. Evidence grade: A strong · B moderate · C limited · D preliminary.

  1. 01

    Vitamin C (ascorbic acid)

    The one group where regular vitamin C roughly halved the chance of catching a cold.

    What the research shows: In the Cochrane review, five trials in 598 people under short periods of extreme physical stress found regular vitamin C roughly halved the risk of a cold (RR 0.48). In the general population it made no real difference to the risk (RR 0.97 across about 11,000 people), and starting it after a cold began had no consistent effect. (PMID 23440782)

    • Adult upper limit: 2,000 mg a day (NIH Office of Dietary Supplements). Large doses cause loose stools.
    • People with a history of kidney stones or with hereditary hemochromatosis should ask before taking regular high doses.

    In the studies

    • Trials in people under heavy physical stress (marathon runners, skiers, soldiers in subarctic exercises) gave at least 200 mg of vitamin C a day, taken regularly before and during the period of stress.

    A record of what researchers gave, in whom; not a dose for you.

    Evidencemoderate (Grade B)

    Quick pick · HerbsPro

    Brands that pay us are shown first in this list.

    NOW Foods Vitamin C-500, 100 Tabs

    1 tablet: 500 mg vitamin C (ascorbic acid) with 40 mg rose hips powder

    $5.62price checked Oct 14, 2026 on HerbsPro’s site

    Shop at HerbsPro (paid link, opens in new tab)

    We may earn a commission if you buy through these links. That never changes a rank. See how partner links work.

    Also from our partners

    Only brands that pay us are listed here.

  2. 02

    Zinc acetate or gluconate lozenges

    The one supplement with consistent trial data for a shorter cold, when it is started early. It does little to stop you catching one.

    What the research shows: In three pooled zinc acetate lozenge trials (199 adults), colds were about 2.7–2.9 days shorter, against an average cold of 7 days. A 2017 meta-analysis of seven trials (575 people; PMID 28515951) found colds 33% shorter with lozenges giving more than 75 mg a day, with no extra benefit above 100 mg. The 2024 Cochrane review rated the evidence for shorter colds low certainty because trials disagreed, which is why this is a B and not an A. (PMID 27378206, PMID 28515951, PMID 38719213)

    • The lozenge trials went above the US upper limit for zinc (40 mg/day from all sources, NIH Office of Dietary Supplements) for a few days only. Weeks of zinc at 50 mg/day or more can lower copper status; do not carry cold-dose zinc into a daily habit.
    • Nausea and a bad taste are the common side effects. Zinc nasal sprays and gels have been linked to lasting loss of smell; lozenges dissolve in the mouth and are a different product.
    • Separate zinc from quinolone or tetracycline antibiotics (take the antibiotic at least 2 hours before or 4–6 hours after) and from penicillamine.

    In the studies

    • Trials in the 2017 meta-analysis gave 80–92 mg of elemental zinc a day from lozenges dissolved slowly in the mouth, for up to about a week, in adults with a fresh cold.

    A record of what researchers gave, in whom; not a dose for you.

    Evidencemoderate (Grade B)

    What to look for on the label

    Lozenges (not capsules or tablets you swallow) that name zinc acetate or zinc gluconate and the elemental zinc per lozenge, with no citric acid or tartaric acid, which bind zinc in the mouth. Not a nasal spray or gel.

    We only name brands we have vetted and work with; a specific pick will appear here once one is approved.

  3. 03Optional

    Standardized elderberry extract

    Optional. A small, industry-funded trial hints at fewer cold days after flying, but it did not cut the number of colds.

    What the research shows: In 312 economy-class travellers, colds were not significantly less common with elderberry (12 vs 17), though total cold days were lower (57 vs 117) from only 29 colds. The extract maker funded the trial and supplied the capsules. A later independent trial in 87 people with lab-confirmed flu found no benefit at all. (PMID 27023596, PMID 32929634)

    • Raw or unripe berries, leaves, stems and bark can cause nausea, vomiting and severe diarrhea; use commercially prepared products only.
    • Little is known about safety in pregnancy and breastfeeding. Ask first if you take diabetes medicines or diuretics.

    In the studies

    • The air-travel trial gave 600 mg/day of elderberry extract for the days before travel and 900 mg/day while travelling and for a few days after.

    A record of what researchers gave, in whom; not a dose for you.

    Evidencelimited (Grade C)

    Quick pick · HerbsPro

    Brands that pay us are shown first in this list.

    NOW Foods Elderberry, 500 mg, 60 Veg Caps

    1 veg capsule: 500 mg elderberry fruit 10:1 concentrate (Sambucus nigra)

    $8.92price checked Oct 14, 2026 on HerbsPro’s site

    Shop at HerbsPro (paid link, opens in new tab)

    We may earn a commission if you buy through these links. That never changes a rank. See how partner links work.

Listed picks above total $14.54 at one-time prices on the brands' own sites; most hold about a month of servings at the label's serving size.

Left out

What we left out, and why

Gram-plus vitamin C started at the first sniffle
Therapeutic vitamin C started after symptoms began had no consistent effect on cold length in the Cochrane review. Doses above 2,000 mg a day exceed the upper limit and cause loose stools.
Airborne-style effervescent "immune" tablets
These combine vitamin C, zinc and herbs in amounts not matched to the trials, and the zinc is swallowed rather than released in the throat.

Safety

Safety and interactions

  • Vitamin C upper limit: 2,000 mg a day (NIH Office of Dietary Supplements). People with kidney stones or hereditary hemochromatosis should ask before regular high doses.
  • Zinc lozenge trials exceeded the 40 mg/day upper limit for a few days only; do not continue cold-dose zinc for weeks.
  • See a clinician promptly for trouble breathing, chest pain, confusion, a fever lasting more than 4 days, symptoms that improve and then get worse, or symptoms lasting more than 10 days (CDC guidance).
  • Babies, adults over 65, pregnant women and anyone with asthma, COPD, heart disease, diabetes or a weakened immune system should call early: flu and COVID-19 have prescription options that work best in the first 48 hours.
  • Not medical advice. Supplements are not evaluated by the FDA to diagnose, treat, cure or prevent any disease.

Questions

Frequently asked questions

Does vitamin C help athletes avoid colds?

In five trials of marathon runners, skiers and soldiers under short periods of extreme physical stress, regular vitamin C roughly halved the risk of a cold. This is the one group where it clearly helped.

How much vitamin C did those trials use?

Those trials (PMID 23440782) gave at least 200 mg a day, taken regularly before and during the period of stress, and the risk of a cold dropped by about half. Doses above the 2,000 mg/day upper limit add side effects without proven extra benefit.

What helps avoid getting sick after a flight?

Sleep, hand hygiene and fluids matter most. In one industry-funded trial, elderberry around a long-haul flight did not reduce the number of colds, though cold days were fewer.

Sources

Sources

Studies were checked on PubMed on Oct 31, 2026. Product prices come from each brand's own site, read on Oct 29, 2026; each product shows the date its price was read.

  1. 1Hemilä H, Chalker E. Vitamin C and the common cold (Cochrane review). Cochrane Database Syst Rev 2013 (opens in new tab)PMID 23440782
  2. 2Tiralongo E et al. Elderberry supplementation and cold duration and symptoms in air-travellers: randomized, double-blind placebo-controlled trial. Nutrients 2016 (opens in new tab)PMID 27023596
  3. 3Hemilä H et al. Zinc acetate lozenges for the common cold: individual patient data meta-analysis. Br J Clin Pharmacol 2016 (opens in new tab)PMID 27378206
  4. 4NIH Office of Dietary Supplements. Vitamin C fact sheet for health professionals (opens in new tab)

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