Who is most at risk of weak immunity from poor nutrition

Key takeaways

• Malnutrition is the world’s leading cause of immune deficiency, according to a 2024 Boston University release and the authors of a 2025 study, and it accounts for about 1 in 5 new tuberculosis cases, more than twice the share tied to HIV¹²⁹.
• In a study of 1,022 vaccinated adults, 9.8% of those with obesity caught flu or a flu-like illness, compared with 5.1% of healthy-weight adults, even though their antibody levels looked the same³.
• Most older adults don’t get enough protein, vitamin B12 or fluids, a federal aging agency says, and undernourished older patients get more infections⁴⁵.
• In pooled human studies, every 10 to 20 grams of alcohol a day went with about 8% higher relative risk of pneumonia, and a 2025 federal review describes that risk climbing in a straight line with intake⁶.
• Fixing a true deficiency helps, but the NIH Office of Dietary Supplements says extra vitamins and minerals don’t prevent infections in people who aren’t deficient⁷.

In January 2024, a team led by Boston University researchers made a pointed comparison in the journal BMC Global and Public Health. Tuberculosis, they noted, sickened 10.6 million people in 2021 and killed 1.6 million².

About 1 in 5 of those cases could be traced to malnutrition, more than double the share tied to HIV². By our math, that’s roughly 2 million cases in a single year.

Like HIV, the authors wrote, malnutrition weakens the immune system, a condition some call N-AIDS, for nutritionally acquired immunodeficiency syndrome². Malnutrition, the university’s announcement said, is the world’s leading cause of immune deficiency¹. Researchers writing in eLife in 2025, citing a 2010 review, make the same point about immune weakness that isn’t inherited⁹.

The remedy they pointed to was low-tech. In one study the team reviewed, an inexpensive food basket given to household members of people with TB cut their TB rates by 40%, though the announcement didn’t give the underlying rates¹. “TB is not simply a medical disease, it’s a social one,” said Pranay Sinha, the paper’s corresponding author¹.

Most Americans will never face that kind of hunger. But food and immunity collide here in quieter ways: older adults who eat too little protein, heavy drinkers, people on narrow diets, and people with obesity, whose flu shots may protect them less. This guide covers who’s at risk, and why a varied plate beats any “immune” product.

Hunger shrinks the places where immune cells train

A 2016 review in Trends in Immunology, led by researchers at Queen Mary University of London, found that malnourished children are most likely to die of common infections rather than starvation alone⁸. The damage runs both ways: poor nutrition weakens immunity, and repeated infections deepen malnutrition⁸.

Malnourished children have fewer white blood cells, and germs slip more easily through their skin and gut linings⁸. Even stunting, being too short for one’s age, may come with weaker immune development in children who otherwise look healthy⁸. The Wellcome Trust, the Gates Foundation and British government agencies funded the work⁸.

A 2025 mouse study shows how deep the damage can go. Researchers cut the animals’ food by 40%, enough for them to lose about a tenth of their weight in roughly four weeks⁹. Their thymus, spleen and lymph nodes shrank, and more of them died from a bacterial infection⁹.

After refeeding, weight and T cells came back, but the bone marrow’s output of some infection-fighting cells, and the mice’s resistance to infection, didn’t fully recover⁹. The journal eLife’s editors rated the work important but said its explanation of how this happens is incomplete, and mice aren’t people⁹.

Vitamin A and measles. One nutrient shows the link clearly. WHO estimates that measles killed about 95,000 people worldwide in 2024¹¹. Vitamin A deficiency is a risk factor for severe measles, according to the NIH Office of Dietary Supplements (ODS), and a WHO analysis tied 11,200 child measles deaths in 2013 to it¹⁰.

WHO’s measles fact sheet, updated July 2026, recommends two doses of vitamin A, a day apart, for every suspected case in children under 5¹¹. WHO says the vitamin restores levels that drop during measles even in well-fed children, helps prevent eye damage and blindness, and may reduce deaths¹¹. CDC stresses that vitamin A doesn’t prevent measles and isn’t a substitute for the vaccine, as the fact-checking charity Full Fact reported in 2025¹¹. Our article on answering health misinformation covers the 2025 vitamin A claims, and our zinc article covers WHO’s zinc advice for children with diarrhea.

In the U.S., the shortfalls are quieter

Older adults. Most older Americans don’t get enough protein, vitamin B12 or fluids, according to Administration for Community Living guidance that cites the 2020–2025 Dietary Guidelines⁴. Those shortfalls can feed muscle loss, anemia and dehydration⁴.

They can feed infections, too. Calder’s 2021 review describes undernutrition as a driver of immune decline in older people, and notes that less well-nourished older hospital patients had more infections⁵. In one study he cites, 50% of frail older adults got a flu-like illness after vaccination, compared with 10% of those who weren’t frail⁵.

Zinc matters here as well. Older adults are among the groups most likely to fall short, and poor zinc status is a risk factor for pneumonia in older people, ODS says¹⁰.

Your body needs amino acids, the building blocks of protein, to make antibodies¹². Our protein article explains what the 2025–2030 Dietary Guidelines advise and who benefits most.

Smokers and narrow diets. About 7% of Americans 6 and older are deficient in vitamin C, ODS reports¹⁰. Smokers need more of it than nonsmokers¹⁰. People who eat a limited range of foods, including some older adults and people with alcohol or drug use disorders, are also more likely to fall short¹⁰.

Restricting food on purpose carries the same risk. If cutting back has started to feel hard to stop, the National Alliance for Eating Disorders runs a weekday helpline staffed by licensed therapists at 1-866-662-1235. In a crisis, call or text 988.

Obesity: plenty of calories, weaker flu protection

Over the 2013–14 and 2014–15 flu seasons, University of North Carolina researchers followed 1,022 adults who had all received a standard flu shot³. About 44% had obesity, 28% had overweight and 27% were at a healthy weight¹³.

Among those with obesity, 9.8% caught lab-confirmed flu or a flu-like illness, compared with 5.1% of healthy-weight adults³. That’s about twice the risk, or by our math about 47 more sick people per 1,000³¹³.

The surprise was in the blood. Antibody levels after the shot looked the same in both groups³. The team suggested that in people with obesity, antibody tests may overstate how well a vaccine protects³. Calder’s review offers a possible reason: obesity is linked to weaker activity of T cells, B cells and natural killer cells⁵.

One caution: everyone in the study was vaccinated, so it can’t show that the shot fails people with obesity¹³. The authors reported no relevant financial ties¹³.

Obesity also tracks with more severe infections. In a pooled analysis of 22 observational studies cited by Calder, people with obesity had about three times the odds of severe COVID-19 symptoms, an estimate drawn from 4 of those studies⁵. Those are relative odds; the analysis doesn’t say how many extra people got very sick.

CDC lists people with a body mass index (BMI) of 40 or higher among those at increased risk of flu complications¹⁴. In 2017–2018, about 9.2% of U.S. adults had a BMI that high, and 42.4% had obesity of any degree, the editors of the journal Obesity noted in 2020¹⁴.

None of this is about blame. Calder lists obesity alongside older age, frailty and poor nutrition as conditions that can impair immunity⁵. If you live in a larger body, ask a clinician which vaccines and precautions fit you; our vaccine schedules article explains who recommends what.

Alcohol weakens the lungs’ first responders

Heavy drinking is linked to pneumonia, sepsis, slow wound healing and acute respiratory distress syndrome (ARDS), a dangerous lung injury, according to a 2015 overview in the federal alcohol institute’s journal¹⁵. Alcohol damages the gut’s barrier and weakens both fast and slow immune defenses¹⁵.

A 2025 review in the same journal put numbers on the lung risk⁶. In a pooled analysis of human studies, every 10 to 20 grams of alcohol a day raised the relative risk of community-acquired pneumonia by about 8%⁶. That’s a relative figure; how much it adds for you depends on your starting risk.

Alcohol use disorder roughly doubles the risk of ARDS, which kills 30% to 50% of people who develop it⁶. At a blood alcohol level of 0.10%, people recruited fewer neutrophils, the front-line germ fighters, to where they were needed⁶.

In mice, long-term alcohol exposure raised flu deaths to about 50%, compared with about 1% in mice not given alcohol⁶. In people, the review describes pneumonia and ARDS risk climbing steadily with the amount consumed⁶. Our alcohol article covers the wider debate over moderate drinking and ways to cut back.

Deficiency hurts, but extra doesn’t help

ODS lists more than a dozen nutrients whose deficiency may weaken immune function: vitamins A, B6, B12, C, D, E and K, plus folate, copper, iodine, iron, magnesium, selenium and zinc¹⁰. Each shortfall hits a different part of the system¹⁰:

• Vitamin A: the linings of your airways and gut, your first barrier, grow weaker.
• Folate: the thymus and spleen suffer, and T cells drop.
• Vitamin B12: neutrophils get worse at swallowing germs.
• Vitamin e: natural killer cells, which destroy infected cells, become less active.
• Copper: the risk of infection rises.
• Selenium: low levels are tied to some viruses becoming more harmful.

The good news, Calder writes, is that these problems can be reversed by restoring the missing nutrient⁵. In a small trial of 28 men, about half of whom started with low vitamin C, 1,000 mg a day for eight weeks cut the relative risk of a cold by 45% compared with a placebo¹⁰. In a national survey, 24% of people with the lowest vitamin D levels reported a recent cold or similar infection, compared with 17% of those with the highest levels, a link rather than proof¹⁰.

Topping up a full tank is another story. If you aren’t deficient, extra vitamins and minerals don’t help prevent infections or speed recovery, ODS says⁷.

Vitamin E shows that more can even backfire. In a year-long trial of 617 nursing home residents 65 and older, 200 IU a day didn’t change how often or how long they had respiratory infections¹⁰. In another trial of 652 adults 60 and older, about 15 months of 200 mg a day didn’t prevent infections, and those who got sick were ill longer, with worse symptoms¹⁰.

Our immune-boosting article covers the big vitamin C, vitamin D and zinc trials.

How strong is the evidence?

Claim	Evidence level	Why
Severe undernutrition weakens immunity and raises deaths from infection	Strong evidence	Consistent findings in malnourished children; animal studies show how89
Fixing a true deficiency restores immune function	Strong evidence	Reviews agree; WHO treats measles with vitamin A511
Better food cuts TB among exposed family members	Promising but early	One trial in a 2024 review: 40% fewer cases; base rates not given1
Obesity weakens flu-shot protection	Promising but early	One study of 1,022 adults; antibody levels looked normal3
Obesity goes with more severe COVID-19	Strong evidence	A pooled analysis of 22 observational studies5
Heavy drinking raises pneumonia and ARDS risk	Strong evidence	Pooled human studies plus lab work615
Moderate drinking protects against pneumonia	Evidence says no, or unsafe	Pooled data show risk rising steadily with intake6
Extra vitamins prevent infections in well-nourished people	Evidence says no, or unsafe	No benefit without a deficiency, per ODS7
Vitamin E pills prevent infections in older adults	Evidence says no, or unsafe	Two trials: no fewer infections, and one found worse illness10
A single “immune food” prevents infection	Not enough evidence	No specific food shown to prevent or treat viral infections12

The immune nutrition check

Find the row that sounds like you. For food amounts, see our vitamins and minerals guide.

If you are…	Nutrients to watch	Food first	Ask a clinician or pharmacist when…
65 or older and eating less	Protein, vitamin B12 and fluids4; zinc10	Eggs, fish, beans, yogurt or meat at each meal; a drink with every meal	You’re losing weight or muscle, or just left the hospital; ask whether B12 needs checking45
A smoker	Vitamin C; smokers need more10	Citrus, peppers, kiwi, broccoli	Your diet is also limited10
Drinking heavily	Vitamin C10; zinc, per our zinc article	Regular meals with fruit, vegetables and protein	You want help cutting back; ask about a nutrition check too
On a narrow or restrictive diet	Vitamin C10; vitamin B12 if fully vegan, per our vegan article	Add back one food group at a time	Restriction feels hard to stop: 1-866-662-1235
Thinking of vitamin D for colds	Most Americans eat less than recommended yet have adequate blood levels10	Fatty fish, fortified milk	You want to know whether a blood test makes sense for you
Taking orlistat, acitretin or bexarotene	Vitamin A pills interact with these drugs7	Food isn’t the concern	Before any vitamin A supplement7
Taking a statin or a thiazide water pill	Vitamin D pills interact with these drugs7	Food isn’t the concern	Before any vitamin D supplement7
Taking antibiotics or a blood thinner	Zinc interacts with antibiotics; vitamin E with blood thinners7	Food isn’t the concern	Before zinc or vitamin E pills7

A varied plate beats any “immune” food

If you’ve long treated food as your first medicine, the immune research mostly agrees. A varied, nourishing diet can supply everything a healthy immune system uses, ODS says, and people who come up short may not fight off or bounce back from infections as well⁷.

Calder made the flip side clear in a 2020 talk, as summarized by the Nutrition Society. No single supplement stops infection, no specific food or commercial probiotic has been shown to prevent or treat viral infections, and supplements can’t replace a healthy diet¹².

Variety also feeds your gut bacteria, which help train immune cells, as our companion article on microbes and immunity explains.

Bring the same skepticism you’d bring to a drug company. Calder, whose review is cited here, reports research funding from Bayer Consumer Care and consulting work for companies including BASF, DSM, Cargill and GSK Consumer Healthcare⁵. That doesn’t make his work wrong, but it’s worth knowing.

Questions readers ask

The bottom line

Around the world, hunger is among the biggest drags on immunity, and it drives more tuberculosis cases than HIV does. In the U.S., the risks are quieter: older adults short on protein and zinc, smokers and narrow eaters short on vitamin C, heavy drinkers, and people with obesity whose flu shots may protect them less.

The fix is mostly food. Eat a varied diet, fill real gaps with a clinician’s help, and skip pills meant to push a well-fed immune system past normal. They haven’t been shown to work, and daily vitamin E pills may even backfire.

Related reading

• Your immune system works more like a team than a volume knob, and some habits help it
• Vitamin pills mostly failed big prevention trials, though a few still earn their place
• Who needs the new protein goal of up to twice the RDA
• Two federal alcohol reviews disagreed, yet both found cancer risk in moderate drinking
• Amish house dust protected mice from asthma, and the lesson isn’t to keep a dirtier home

Sources

1. Boston University Medical Campus. “Nutritional Acquired Immunodeficiency (N-AIDS) Is Leading Driver of TB Pandemic.” BU News, January 12, 2024. bumc.bu.edu
2. Dauphinais MR, Koura KG, Narasimhan PB, Mehta S, Finkelstein JL, Heysell SK, Sinha P. “Nutritionally Acquired Immunodeficiency Must Be Addressed With the Same Urgency as HIV to End Tuberculosis.” BMC Global and Public Health, 2024;2 (DOAJ record, doi:10.1186/s44263-023-00035-0). doi.org
3. UNC Gillings School of Global Public Health. News release on Neidich SD, Beck MA, et al., International Journal of Obesity, published online June 6, 2017. June 7, 2017. sph.unc.edu
4. Administration for Community Living, U.S. Department of Health and Human Services. “Nutrition Requirements of the Older Americans Act.” ACL, undated (based on the 2020–2025 Dietary Guidelines). ACL.gov
5. Calder PC. “Nutrition and Immunity: Lessons for COVID-19.” Nutrition & Diabetes, June 23, 2021, co-published in European Journal of Clinical Nutrition (University of Southampton eprint, with conflict-of-interest statement). pubmed.ncbi.nlm.nih.gov
6. Samuelson DR. “Alcohol’s Effects on the Lung and Lung Disease.” Alcohol Research: Current Reviews, 2025;45(1):11. National Institute on Alcohol Abuse and Alcoholism. arcr.niaaa.nih.gov
7. NIH Office of Dietary Supplements. “Dietary Supplements for Immune Function and Infectious Diseases: Fact Sheet for Consumers.” NIH, updated March 10, 2025. ods.od.nih.gov
8. Queen Mary University of London. “Malnutrition Results From More Than Just Poor Diet” (on Bourke CD, Berkley JA, Prendergast AJ, Trends in Immunology, 2016;37(6):386–398). QMUL News, 2016.
9. Sukhina A, et al. “Malnutrition Drives Infection Susceptibility and Dysregulated Myelopoiesis That Persists After Refeeding Intervention.” eLife, 2025 (reviewed preprint version 2, June 30, 2025; eLife lists a Version of Record dated July 15, 2025). pubmed.ncbi.nlm.nih.gov
10. 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
11. World Health Organization. “Measles.” Fact sheet, July 15, 2026. who.int and Kalake L. “Can Vitamin A ‘Prevent and Treat’ Measles?” (quoting CDC). Full Fact, May 29, 2025. fullfact.org
12. Gouaille J. “Nutrition, Immunity and COVID-19” (summary of a webinar by Philip Calder). The Nutrition Society blog, May 1, 2020. nutritionsociety.org
13. Healio. “Influenza Vaccine Not as Effective in Adults Who Are Obese.” Healio Primary Care, June 20, 2017. healio.com
14. Centers for Disease Control and Prevention. “People at Increased Risk for Flu Complications.” CDC, updated September 11, 2024. cdc.gov and Ryan DH, Ravussin E, Heymsfield S. “COVID 19 and the Patient With Obesity: The Editors Speak Out.” Obesity, 2020;28(5):847. covid-19.conacyt.mx
15. Sarkar D, Jung MK, Wang HJ. “Alcohol and the Immune System” (editors’ note). Alcohol Research: Current Reviews, 2015;37(2):153–155. National Institute on Alcohol Abuse and Alcoholism. arcr.niaaa.nih.gov