Scientists reviewed 120 studies involving over 100,000 adults to see which natural supplements work best for preventing respiratory infections like colds and flu. They found that certain supplements - including catechin (found in green tea), specific probiotics, and high-dose vitamin D - showed promising results in reducing infection rates and shortening illness duration. The research suggests these supplements may offer a safe, natural way to boost your body’s defenses against common respiratory illnesses, though more direct comparison studies are still needed.

The Quick Take

  • What they studied: How well different natural supplements prevent respiratory infections like colds, flu, and COVID-19 in adults
  • Who participated: Over 100,000 healthy adults from 120 different research studies conducted worldwide
  • Key finding: Catechin, certain probiotics, and high-dose vitamin D reduced infection rates by 10-34% compared to fake pills
  • What it means for you: Taking specific natural supplements may help you get sick less often and recover faster, but talk to your doctor first

The Research Details

This was a network meta-analysis, which means researchers combined data from 120 separate studies to get a bigger picture of what works. They only included high-quality randomized controlled trials - the gold standard of medical research - where some people got real supplements and others got fake pills, and neither group knew which they were taking. The researchers looked at various natural supplements including vitamins, plant compounds like catechin from green tea, and beneficial bacteria called probiotics.

By combining many studies together, researchers could compare different supplements more reliably than any single study could. This approach helps identify which supplements truly work and which don’t, giving us more confidence in the results.

The studies included were well-designed with proper control groups and random assignment. The researchers used strict quality standards and rated their confidence in the findings from low to high certainty, helping readers understand how trustworthy each result is.

What the Results Show

Three supplements stood out as most effective for preventing respiratory infections. Catechin, a compound found in green tea, reduced infection risk by 21% with high confidence in this result. A specific probiotic strain called Bifidobacterium animalis also reduced infections by 21%, while multi-strain probiotics (containing several beneficial bacteria types) reduced infections by 10%. For COVID-19 and flu specifically, high-dose vitamin D was particularly effective, reducing infection risk by 34%.

The supplements didn’t just prevent infections - they also helped people recover faster when they did get sick. Catechin shortened illness duration by about 2.6 days per infection, while multi-strain probiotics reduced symptoms by nearly one day. Multi-strain probiotics also made symptoms less severe overall. Importantly, none of the supplements caused more side effects than fake pills, suggesting they’re generally safe.

These findings support and strengthen previous research suggesting that certain natural supplements can boost immune function. However, this study provides the most comprehensive comparison to date, helping identify which specific supplements work best rather than just whether supplements in general might help.

The main limitation is that most studies compared supplements to fake pills rather than directly comparing different supplements to each other. The supplements were also given in different forms - some as pills, others as drinks or even mouthwashes - making it harder to determine the best way to take them. More research is needed to find the optimal doses and delivery methods.

The Bottom Line

Based on this research, adults looking to prevent respiratory infections might consider catechin supplements, Bifidobacterium animalis probiotics, or multi-strain probiotics with moderate to high confidence. For COVID-19 and flu prevention specifically, high-dose vitamin D appears promising. However, consult with a healthcare provider before starting any supplement regimen, especially for vitamin D dosing.

Healthy adults who frequently get colds or respiratory infections may benefit most. People with compromised immune systems, those taking medications, or anyone with chronic health conditions should definitely consult their doctor first before trying these supplements.

The studies typically ran for several months to see effects, so don’t expect immediate results. Most benefits appeared after consistent use for at least 8-12 weeks, and effects were measured over entire cold and flu seasons.

Want to Apply This Research?

  • Track frequency and duration of respiratory infections (colds, flu, etc.) before and after starting supplements
  • Consider adding evidence-based supplements like catechin or probiotics to your daily routine while monitoring infection patterns
  • Log daily supplement intake, symptom onset and severity, and recovery time to identify personal patterns and effectiveness

This information is for educational purposes only and should not replace professional medical advice. Consult with your healthcare provider before starting any supplement regimen, especially if you have existing health conditions or take medications.