A few immune-boosting foods were tested against real infections, and most weren’t

Key takeaways

• In a Belfast trial of 83 older adults, those who ate at least five portions of fruit and vegetables a day for 16 weeks made a stronger antibody response to a pneumonia vaccine than those who ate two or fewer¹².
• In a 90-day trial of 120 adults, a daily aged garlic extract didn’t prevent colds, though users logged 61% fewer days with symptoms; the extract’s maker supplied it and helped pay for the study⁵⁶.
• In a Japanese trial, 4 of 98 health care workers taking green tea catechin capsules were diagnosed with flu, compared with 13 of 99 on placebo, but lab-confirmed cases didn’t differ significantly¹¹.
• In a 2022 Cochrane review, probiotics, often given to children and older people in milk-based foods, cut the share of people with at least one cold-type infection from 447 to 340 per 1,000, with low certainty⁷⁸.
• In 2010, the FTC barred Dannon from claiming that any yogurt or probiotic drink lowers the chance of a cold or flu unless the FDA approves the claim⁹.

In 2009, researchers at Queen’s University Belfast reported an unusual food experiment¹. They enrolled 83 healthy people aged 65 to 85 who ate two or fewer portions of fruit and vegetables a day¹².

Half kept eating as usual. The other half added produce until they reached at least five portions a day, and kept it up for 16 weeks¹. A portion was modest: about 80 grams of fruit, three heaping tablespoons of vegetables or three-quarters of a cup of juice².

At week 12, everyone got two vaccines, one against pneumonia bacteria and one against tetanus¹. The five-a-day group made a stronger antibody response to the pneumonia vaccine¹². Tetanus responses didn’t differ¹.

The trial was small, and its first report showed wide margins of error¹. Still, it did something most “immune-boosting” food claims never do. It measured a response tied to real protection, in real people eating real food.

Popular immune lists usually feature citrus, garlic, ginger, turmeric, yogurt, green tea, bone broth and honey. A few have been tested against actual infections. Others have only lab or blood-marker studies, and some have no infection research at all. We walk through what each one has behind it, and why your whole plate matters more than any single item.

An immune marker isn’t the same as a cold you didn’t catch

There’s no simple blood test for how well your immune system protects you, as our companion article on immune tests explains. The surest sign that a food helps is fewer or milder infections in the people who eat it.

That gives food research a ladder of proof, from weakest to strongest:

• Lab dishes. A food extract changes immune cells in a test tube. That’s a reason to keep studying, nothing more.
• Blood markers. Some immune cells or proteins shift after people eat the food. That shows it reaches the body, not that it prevents illness.
• Symptom diaries. People record their colds. Better, but people may guess which group they’re in.
• Counted infections. In a randomized controlled trial, chance decides who gets the food or a look-alike, and a clinician or lab confirms infections.

Most immune-food claims stop on the first two rungs. Many trials that climb higher tested a capsule or extract, not the food in your kitchen.

Garlic: an extract cut sick days, not colds

Garlic’s reputation against colds is old, but its test record is thin. A 2014 Cochrane review, which pools the best available trials, found too little evidence to say whether garlic prevents colds, according to the National Center for Complementary and Integrative Health (NCCIH)⁴. Our garlic article explains why that review rested on a single small trial.

One newer test used aged garlic extract. Aging converts garlic’s smelly sulfur compounds into ones that dissolve in water⁶.

In a University of Florida trial published in 2012, 120 healthy adults took 2.56 grams of the extract or a placebo every day for 90 days⁵⁶. Their average age was 26⁶.

The result was mixed in a telling way. The garlic group didn’t catch fewer colds or flu-like illnesses⁵. But in their diaries, they reported 21% fewer symptoms, 61% fewer days with symptoms and 58% fewer missed days of work or school⁵. In lab tests of their blood, two kinds of immune cells also multiplied better⁵.

The abstract reports those results only as percentages, so we can’t translate them into sick days per person. According to a trade report, the extract’s maker, Wakunaga of America, supplied it and helped pay for the study along with the university’s agricultural experiment station⁶. Independent trials would make the finding more convincing.

Garlic is safe as food for most people. NCCIH notes that garlic supplements can cause bad breath and body odor and may interact with blood thinners⁴.

Yogurt and probiotic drinks: many cold trials used dairy

A 2022 Cochrane review, which our article on immune “boosting” summarizes briefly, pooled trials of probiotics against colds and other upper respiratory infections⁷⁸. The detail that matters here is how people took them. Children usually got their probiotics in a milk-based food, older people in milk-based food or capsules, and adults mostly as powders⁷.

What the review found for every 1,000 people⁸:

• At least one infection: 447 in the comparison groups and 340 with probiotics. By our math, that’s about 107 fewer. Certainty was low.
• Antibiotics for those infections: 100 versus 58, about 42 fewer. Certainty was moderate.
• Side effects: about the same in both groups, mostly mild stomach complaints.

Most trials ran through the winter and spring⁷. Some participants may have known which group they were in, which can tilt self-reported results⁷. The reviewers also found that industry-funded trials were more likely to report a benefit, and that 10 trials were paid for by the company that made the probiotic⁸.

So a carton of yogurt isn’t automatically a tested probiotic. The trials used named strains at set doses, and your cup may carry different microbes. Our probiotic articles explain how to check a strain code.

Marketing has outrun this evidence before. Dannon promoted its DanActive drink with a bacterium it trademarked as “L. casei Immunitas”¹⁰. Connecticut’s attorney general said the company claimed, without proof, that the drink supported immunity and prevented colds and flu¹⁰.

In December 2010, Dannon agreed to pay the states $21 million⁹. The FTC’s order barred it from claiming that any yogurt, dairy drink or probiotic food lowers the chance of a cold or flu unless the FDA approves that claim⁹. Dannon didn’t admit breaking the law⁹.

Probiotic foods are safe for most healthy people. NCCIH notes that serious infections from probiotics have been tied to people with serious underlying illnesses⁴.

Green tea: the flu trial used capsules, not cups

One green tea trial ran through the 2009–2010 flu season at three elder-care facilities in Higashimurayama, Japan¹¹. Researchers led by Keiji Matsumoto of the University of Shizuoka gave 197 health care workers capsules or a placebo for five months¹¹¹².

The capsules held 378 milligrams of green tea catechins, the plant compounds tea is known for, plus 210 milligrams of theanine, an amino acid found in tea¹¹.

Four of the 98 workers on capsules (4.1%) were diagnosed with flu based on their symptoms, compared with 13 of the 99 on placebo (13.1%)¹¹. By our math, that’s about 9 fewer cases for every 100 workers. But a lab confirmed only six cases, 1 versus 5, and that gap could have been chance¹¹.

NCCIH’s cold and flu guide lists green tea as having too little evidence, and it also cites a separate trial of green tea gargling in high school students⁴. The sources we could open didn’t say who paid for the capsule trial.

A cup of tea and a pill are different products. NCCIH says liver injury, though uncommon, has been reported mainly with green tea extracts in tablets or capsules¹³. High doses of green tea can also lower blood levels of nadolol, a beta-blocker medicine¹³. Tea contains caffeine, which pregnant people should keep to moderate amounts¹³.

Kiwifruit and citrus: vitamin C from the plate

Citrus gets top billing on immune lists because of vitamin C. Most research on vitamin C and colds tested pills, and a later article in this series covers those trials.

Whole fruit has a smaller record. In a New Zealand pilot study, 32 adults aged 65 and older ate the equivalent of four gold kiwifruit a day for four weeks and two bananas a day for another four weeks, with a break in between¹⁴. Kiwifruit didn’t lower the number of colds and other respiratory infections¹⁴.

Among people who did get sick, though, symptoms passed faster during kiwifruit weeks. A sore throat lasted about 2 days instead of about 5.4, and head congestion lasted under a day instead of 4.7¹⁴. Blood vitamin C rose, but natural killer cell activity didn’t change¹⁴. Zespri International, whose branded gold kiwifruit the volunteers ate, funded the study¹⁴.

That’s an early clue from a very small group. The plainer point is that produce covers vitamin C needs easily. The NIH Office of Dietary Supplements (ODS) lists 95 mg in half a cup of raw red pepper, 70 mg in a medium orange and 64 mg in a medium kiwifruit¹⁵.

Adult women need 75 mg a day and men 90 mg¹⁵. By our math, one serving of any of those foods covers most or all of a day’s need. A later article in this series covers vitamin C doses and limits.

Ginger, turmeric, broth and soup: comfort with little testing

Some favorites on immune lists have almost no infection research behind them. NCCIH’s guide to cold and flu remedies, last updated in 2016, weighs garlic, honey, probiotics, green tea and several supplements⁴. It doesn’t cover ginger, turmeric, bone broth or chicken soup⁴.

• Ginger. Our ginger article found its best support is for nausea, and rated it not enough evidence for colds.
• Turmeric. Our turmeric article explains why curcumin pills have a shaky record and a rare risk of liver injury.
• Bone broth. It’s warm and filling, but we know of no trial that tested it against colds or flu.
• Chicken soup. Our sick-day article describes a 2000 lab study in which soup slowed white blood cells in a dish, and why its authors called any benefit in sick people untested.
• Honey. It may ease a cough, but never give it to a baby under 1⁴. Our honey articles weigh the cough evidence.
• Mushrooms and elderberry. Medicinal mushrooms get their own article later in this series. Elderberry’s small, maker-funded trials are covered in our elderberry article.

Warm drinks and a familiar meal can make a sick day easier. They just haven’t earned the word “boosting.”

The immune food scorecard

This is our own summary, using the four-level evidence scale: Strong evidence, Promising but early, Not enough evidence, and Evidence says no, or unsafe.

Food	Best human evidence on infections	Form and amount tested	Rating	Caution
Fruit and vegetables, mixed	Stronger pneumonia-vaccine response in 83 older adults; fewer weeks of infection in a second trial12	At least five portions a day for 16 weeks1	Promising but early	None for most people
Aged garlic extract	Same number of colds; fewer days with symptoms by diary5	2.56 grams a day for 90 days5	Promising but early	Maker supplied it6; blood thinners4
Garlic as food	Too little evidence in a 2014 Cochrane review4	Varies	Not enough evidence	Breath and body odor4
Yogurt and probiotic drinks	Fewer infections and antibiotic courses, low to moderate certainty8	Specific strains, often in milk-based foods, through a winter and spring7	Promising but early	Your carton’s strains may differ
Green tea capsules	Fewer flu cases diagnosed by symptoms; lab-confirmed gap not significant11	378 mg catechins plus 210 mg theanine daily for five months11	Promising but early	Rare liver injury from extracts; nadolol13
Green tea as a drink	Listed by NCCIH as not enough evidence4	Not what the capsule trial tested	Not enough evidence	Caffeine13
Kiwifruit	No fewer infections; shorter sore throat and congestion in 32 older adults14	The equivalent of four gold kiwifruit a day for four weeks14	Not enough evidence	Funded by Zespri, whose fruit was tested14
Citrus and other vitamin C foods	Studied mostly as vitamin C pills	Ordinary servings; see the ODS amounts above15	Not enough evidence as foods	Adult upper limit is 2,000 mg a day from food and pills combined15
Ginger, turmeric, bone broth	Not covered in NCCIH’s cold guide4	Not tested against infections	Not enough evidence	Turmeric pills: see our turmeric article
Honey	May ease children’s nighttime cough4	Not tested for prevention	Promising but early, for cough only	Never under age 14

Why the whole plate beats any one food

Belfast wasn’t the only test of a better overall diet. A University of Sheffield team led by Elizabeth Williams assigned 217 people aged 65 to 85 to one of three plans for three months². One group ate at least five daily portions of produce, fish twice a week, nuts weekly and whole-grain bread. Others took a low-dose vitamin and mineral pill or a placebo².

The number of infections didn’t differ. But the diet group reported fewer weeks with an infection than either other group².

These trials fit what ODS says: eating a variety of nutritious foods helps keep the immune system working, and shortfalls in several vitamins and minerals may weaken it³. The foods that supply those nutrients are ordinary³:

• Vitamin c: citrus, peppers, kiwifruit, broccoli, strawberries, tomatoes and potatoes.
• Vitamin e: nuts, seeds, vegetable oils and leafy greens.
• Zinc: oysters, crab, beef, pork, poultry, beans, nuts, whole grains and dairy.
• Selenium: Brazil nuts, seafood, meat, poultry, eggs and dairy.

What pills add when you aren’t short on anything is a separate question, covered in our article on immune formulas. Our nutrition and immunity article explains who’s most likely to fall short.

If you’ve long treated food as your first medicine, the research mostly backs your instinct, with one twist. The strongest food evidence came from eating more ordinary produce, not from one “super” ingredient. Many single-food trials tested concentrated capsules, which is the supplement model, not the kitchen one.

Tradition still matters. Garlic, tea, soup and honey carry generations of family care, and comfort is real. You can honor that and still ask what a product was tested to do.

Questions readers ask

The bottom line

A handful of so-called immune-boosting foods have been tested against real infections, and the results are modest. Aged garlic extract, green tea catechin capsules and kiwifruit each have small trials, often run with products from the companies that sell them, and probiotic dairy has a larger but low-certainty body of research. Ginger, turmeric, bone broth and chicken soup rest mostly on tradition and comfort.

The most encouraging results came from eating more ordinary fruits and vegetables, which strengthened a vaccine response in one trial and cut weeks of infection in another. Fill your plate with variety, treat any single “immune” food as a pleasure rather than a shield, and ask what a product was tested to do before you pay extra for it.

Related reading

• Your immune system works more like a team than a volume knob, and some habits help it
• Who is most at risk of weak immunity from poor nutrition
• Garlic pills nudge blood pressure down, but a Stanford trial found no cholesterol benefit
• Beyond the Gut, Most Probiotic Claims Outrun the Evidence. A Few Hold Up
• What immune labels and tests mean, given that no single test measures your immunity

Sources

1. Gibson A, Neville CE, Gilchrist SECM, et al. “Increased Fruit and Vegetable Consumption and Immune Function: The Ageing and Diet Intervention Trial (ADIT).” Proceedings of the Nutrition Society, 2009;68(OCE3):E102 (conference abstract; full results later published in the American Journal of Clinical Nutrition, 2012). cambridge.org
2. Tufts University. “Healthy Diet Fights Infection by Boosting Immune System.” Tufts Health & Nutrition Letter, November 15, 2013, updated September 17, 2019. nutritionletter.tufts.edu
3. NIH Office of Dietary Supplements. “Dietary Supplements for Immune Function and Infectious Diseases: Fact Sheet for Health Professionals.” NIH, last modified September 8, 2026. ods.od.nih.gov
4. National Center for Complementary and Integrative Health. “Colds, Flu, and Complementary Health Approaches: Usefulness and Safety.” NIH, last updated November 2016. nccih.nih.gov
5. Nantz MP, Rowe CA, Muller CE, Creasy RA, Stanilka JM, Percival SS. “Supplementation With Aged Garlic Extract Improves Both NK and γδ-T Cell Function and Reduces the Severity of Cold and Flu Symptoms: A Randomized, Double-Blind, Placebo-Controlled Nutrition Intervention.” Clinical Nutrition, 2012;31(3):337-344 (abstract copy). chiro.org
6. NutraIngredients. “Aged Garlic Reduces Cold and Flu Severity: RCT Data.” January 26, 2012 (trade press; used for funding and product details). nutraingredients.com
7. Cochrane. “Can Probiotics (Live Micro-Organisms) Prevent Upper Respiratory Tract Infections, Such as the Common Cold?” Plain language summary of Zhao Y, Dong BR, Hao Q, CD006895, published August 25, 2022. cochrane.org
8. Zhao Y, Dong BR, Hao Q. “Probiotics for Preventing Acute Upper Respiratory Tract Infections.” Cochrane Database of Systematic Reviews, 2022, Issue 8, CD006895.pub4. pubmed.ncbi.nlm.nih.gov
9. Federal Trade Commission. “Dannon Agrees to Drop Exaggerated Health Claims for Activia Yogurt and DanActive Dairy Drink.” Press release, December 15, 2010. ftc.gov
10. Connecticut Office of the Attorney General. “Attorney General Announces Agreement With Dannon Over Activia and DanActive Claims About Digestive Health.” Press release, December 15, 2010. portal.ct.gov
11. Matsumoto K, Yamada H, Takuma N, Niino H, Sagesaka YM. “Effects of Green Tea Catechins and Theanine on Preventing Influenza Infection Among Healthcare Workers: A Randomized Controlled Trial.” BMC Complementary and Alternative Medicine, 2011;11:15 (DOAJ record). pubmed.ncbi.nlm.nih.gov
12. Infection Control Today. “Green Tea Catechins and Theanine May Ward Off Influenza Infection.” 2011. infectioncontroltoday.com
13. National Center for Complementary and Integrative Health. “Green Tea: Usefulness and Safety.” NIH, last updated February 2025. nccih.nih.gov
14. Hunter DC, Skinner MA, Wolber FM, et al. “Consumption of Gold Kiwifruit Reduces Severity and Duration of Selected Upper Respiratory Tract Infection Symptoms and Increases Plasma Vitamin C Concentration in Healthy Older Adults.” British Journal of Nutrition, 2012;108(7):1235-1245. cambridge.org
15. NIH Office of Dietary Supplements. “Vitamin C: Fact Sheet for Health Professionals.” NIH, updated July 31, 2025. ods.od.nih.gov