Does sugar weaken your immune system?

We found no evidence that it does in healthy people, and also no trial that tested the question directly. A meta-analysis of 64 controlled feeding trials in 4,094 mostly healthy adults found that sugars containing fructose did not raise the inflammatory markers CRP, TNF-alpha or IL-6. No randomized trial we could find counted colds or infections in people eating more or less sugar. The clearest measured infection risk is in people with diabetes, and those data compare people with and without the disease, not people eating more or less sugar.

Unsettled. The popular claim that a sugary snack switches off white blood cells for hours traces to a 1973 study. We could not obtain its abstract or full text, so we cannot check what it measured, and it is not used as evidence on this page.

What the trials found

Inflammatory markers in ordinary diets

In the 64-trial meta-analysis, total fructose-containing sugars lowered CRP when they were added on top of a diet and had no effect when they replaced other foods or were cut out. Inside the replacement trials the source mattered. Mixed diets that included sugary drinks raised CRP by 0.64 mg/L (95% CI 0.12 to 1.17) across 6 energy-matched trials, with high heterogeneity, and that result lost significance when single trials were removed. Two trials of 100% fruit juice lowered CRP by 1.09 mg/L (95% CI for the drop 0.17 to 2.01). CRP is a marker of inflammation. It does not tell you whether someone fights off an infection.

Unsettled. A crossover trial in 29 healthy young men pointed the other way. Drinking 20 fl oz (600 mL) a day of sweetened drinks with 40 to 80 g of fructose, glucose or sucrose for 3 weeks raised hs-CRP by 60 to 109 percent. That trial was small and short, and the larger meta-analysis above found no overall rise. Neither measured infections.

Sugar drinks during exercise

Hard exercise briefly shifts immune markers, and sugar drinks soften part of that shift. A meta-analysis of 20 studies in 286 exercising people found carbohydrate drinks lowered IL-6 and the anti-inflammatory IL-10 and IL-1ra, while TNF-alpha rose. Those are short-term cytokine changes from small trials. An older systematic review of 45 trials in 1,603 athletes, where its authors rated most trials as poor quality, found carbohydrate softened the rise in cortisol and neutrophils after heavy exercise, and no other effect on the outcomes it studied was significant or consistent. In a crossover trial of 20 cyclists riding 75 km, a 6% sugar drink blunted the surge in white cells after the ride. It did not prevent the fall in natural killer cell numbers and activity 1.5 hours later.

Sugar drinks before surgery

A meta-analysis of 13 randomized trials in 996 people having bowel surgery tested a carbohydrate drink before the operation, mostly maltodextrin made for hospitals. It did not reduce overall complications or deaths. It went with lower odds of respiratory infection after surgery (OR 0.35, p = 0.04), a secondary outcome across varied trials. That is a clinical drink in fasting surgical patients and says nothing about sugar in an everyday diet.

Unsettled. No trial has shown that sugar in an ordinary diet causes colds or flu in healthy people, and none of the trials above counted infections in everyday life. The claim is untested, not disproven.

Where sugar and immune-related disease meet, on a risk scale

Observational data

Above 1 means more cases. Every row is observational and none can show cause. The diabetes rows compare people with diabetes against matched people without it and say nothing about sugar in the diet. The bowel disease studies were mostly retrospective. Sources: cards ev-sgim-08, ev-sgim-09 and ev-sgim-11 below.

Who should be careful

People with diabetes carry the clearest measured risk of infection. Among English adults with diabetes in primary care, infection-related hospital admissions ran 3.71 times the rate of matched people without diabetes in type 1 (95% CI 3.27 to 4.21) and 1.88 times in type 2 (95% CI 1.83 to 1.92). The abstract gives the ratios without the underlying rates, so we cannot turn them into cases per thousand.

In pregnancy, a meta-analysis of 4 cohorts linked more free sugar with 7 percent higher odds of asthma in the child (OR 1.07, 95% CI 1.00 to 1.14), with sugar intake self-reported and confounding possible. In children, a 2026 review of 17 studies tied sugary drinks and fructose to asthma, with two- to five-fold odds in cross-sectional US surveys, in a review that was mostly observational and mixed in design. A meta-analysis of 45 studies, most of them retrospective, linked higher added sugar with 66 percent higher odds of Crohn's disease (OR 1.66, 95% CI 1.21 to 2.29) and 59 percent higher odds of ulcerative colitis (OR 1.59, 95% CI 1.25 to 2.02). All of these are odds from observational data.

Not for everyone. If you have diabetes, your risk of serious infection is higher, and how to manage it is a matter for you and the person treating you. If you are pregnant, the asthma signal is weak, with an interval that touches no effect, and it is one more reason to keep free sugars low rather than a reason for alarm.

What expert bodies say

The WHO recommends that adults and children keep free sugars below 10% of their daily energy. The WHO release does not mention immune function or infection. We found no expert body that gives sugar limits for the sake of immunity.

How we searched

Searched: a local copy of PubMed for sugar, sucrose, fructose, added sugar and sugary drinks with immune, inflammation, CRP, infection and neutrophil terms (779 records, top 60 screened). Narrower searches covered sugars and inflammatory markers in trials (30 screened), sugar or high blood glucose and infection or immune function (30), carbohydrate and immunity in athletes (20), and sugary drinks with asthma, allergy and atopy (20). ClinicalTrials.gov returned nothing for sugar and immunity. We also used the open web and Europe PMC for studies outside our PubMed copy, read the WHO sugars release and checked all cited studies against the Retraction Watch database, which listed none of them. Search run on 7 October 2026.

Included: four meta-analyses or systematic reviews of trials (feeding trials and CRP, exercise and cytokines, athletes and immune markers, carbohydrate before surgery), two randomized trials, four observational studies or reviews on diabetes and infection, sugar in pregnancy and child asthma, sugar and asthma in children, and sugar and bowel disease, and the WHO guideline.

Excluded: CRP-only trials and a network meta-analysis already covered on our sugar and inflammation page, a small runners' trial with no numbers in its abstract, a 13-person training study covered by the athletes review, a Cochrane review of mixed nutrition before surgery, a 2026 umbrella review with no immune outcome, and a soy milk review with only an indirect sugar angle.

What we read: abstracts, plus the results section of the 64-trial meta-analysis in full text. The abstracts of the 3-week drinks trial and the English diabetes cohort came from Europe PMC.

What we could not get: the 1973 white blood cell study behind the popular claim, which has no abstract online and a paywalled full text. Any randomized trial of more against less dietary sugar that counted colds, flu or other infections in healthy people. The full texts of the drinks trial and the diabetes cohort.

What would change this answer

A randomized trial that assigns healthy adults to more or less added sugar for a winter and counts respiratory infections.
A modern repeat of the white blood cell experiment, with realistic sugar doses, a published method and a control drink.
Cohorts of sugar intake and infection rates in people without diabetes, adjusted for body weight.

More on sugar itself, including the WHO limit on free sugars: Sugar.

The food behind this question

Sugar: the two WHO numbers are not one rule — numbers, evidence and safety

Sources

ev-sgim-01 · Meta-analysis or systematic review · systematic review and meta-analysis of controlled feeding trials with GRADE · n = 4,094
Total fructose-containing sugars decreased CRP in addition trials and had no effect in substitution, subtraction or ad libitum trials. No effect was observed on other outcomes at any level of energy control.
Who: predominantly healthy adults of mixed weight or with overweight or obesity; trials of at least 7 days
Effect: CRP substitution MD 0.07 mg/L (95% CI -0.08 to 0.22); addition MD -0.18 mg/L (-0.33 to -0.03); no effect on TNF-alpha or IL-6; GRADE mostly moderate to low
Certainty: Inflammatory markers, not infection outcomes; same study as ev-sgi-01 on the inflammation page, used here for the immune question.
Nutrients, 2022 · checked 2026-10-07 · we read the abstract
ev-sgim-02 · Meta-analysis or systematic review · subgroup of a meta-analysis of controlled feeding trials · n = 6 trials
An interaction by food source was detected in the substitution trials (p = 0.010), where sweetened dairy alternative from soy (1 trial; MD: −0.96mg/L; 95% CI: −1.67, −0.25mg/L; PMD = 0.008) and 100% fruit juice (2 trials; MD: −1.09mg/L; 95% CI: −2.01, −0.17mg/L; PMD = 0.021; no heterogeneity, I2 = 0.0%, PQ = 0.590) resulted in decreased CRP, while mixed sources (with SSBs) (6 trials; MD: 0.64mg/L; 95% CI: 0.12, 1.17mg/L; PMD = 0.016; substantial heterogeneity, I2 = 82.9%, PQ < 0.001) increased CRP.
Who: substitution trials, energy-matched replacement of sugars
Effect: mixed sources with SSBs: CRP MD +0.64 mg/L (95% CI 0.12 to 1.17), I2 82.9%, moderate certainty; 100% fruit juice MD -1.09 mg/L (-2.01 to -0.17)
Certainty: Subgroup, high heterogeneity; lost significance when single trials were removed. Food matrix, not sugar alone, seems to drive it.
Nutrients, 2022 · checked 2026-10-07 · we read the fulltext
ev-sgim-03 · Meta-analysis or systematic review · systematic review and meta-analysis · n = 286
Carbohydrate supplementation significantly reduced levels of IL-6, IL-1ra and IL-10, while TNF-α levels increased significantl, and changes in IL-1β were not significan.
Who: people performing exercise at different intensities, acute carbohydrate supplementation vs placebo
Effect: IL-6, IL-1ra, IL-10 reduced; TNF-alpha increased; IL-1beta no change; glucose plus sucrose strongest IL-6 reduction in high-intensity exercise
Certainty: Short-term cytokine responses to exercise, not infection risk; small trials; pooled effect sizes not given in the abstract.
Crit Rev Food Sci Nutr, 2026 · checked 2026-10-07 · we read the abstract
ev-sgim-04 · Meta-analysis or systematic review · systematic review and meta-analysis of randomized and controlled trials · n = 1,603
Carbohydrate supplementation attenuated the increase in cortisol and neutrophils after exercise; vitamin C attenuated the decrease in lymphocytes after exercise. No other interventions had significant or consistent effect on any of the studied outcomes.
Who: athletes in randomized and/or controlled trials of supplements after heavy exercise; 20 trials on carbohydrate
Effect: carbohydrate attenuated post-exercise cortisol and neutrophil increases; no other significant or consistent effects
Certainty: 2007; trial quality mostly poor per authors.
Eur J Clin Nutr, 2007 · checked 2026-10-07 · we read the abstract
ev-sgim-05 · Meta-analysis or systematic review · systematic review and meta-analysis of RCTs · n = 996
Carbohydrate loading did not significantly affect overall complications or mortality. Secondary outcomes suggested reduced respiratory infections (OR 0.35; p = 0.04), shorter time to first flatus (MD - 0.60; p < 0.01), earlier defecation (MD - 0.58; p = 0.01), shorter LOS (MD - 0.98; p < 0.01) and earlier mobilisation (MD - 0.49; p < 0.01).
Who: adults undergoing elective colorectal surgery; carbohydrate loading vs water or overnight fasting
Effect: overall complications and mortality no difference; respiratory infections OR 0.35 (p = 0.04); length of stay MD -0.98 days
Certainty: Secondary outcome; heterogeneous trials; clinical carbohydrate drinks (mostly maltodextrin), not dietary sugar.
Int J Colorectal Dis, 2026 · checked 2026-10-07 · we read the abstract
ev-sgim-06 · Randomized controlled trial(s) · randomized crossover trial · n = 20
Carbohydrate ingestion from BAN or SUG attenuated immediate post-exercise increases in leukocyte, neutrophil, and NK cell counts, but did not counter the 1.5 h decreases in NK cell counts and NKCA.
Who: cyclists, overnight fasted, three 75 km time trials with water, bananas or 6% sugar beverage
Effect: carbohydrate attenuated immediate post-exercise rises in leukocyte, neutrophil and NK counts; did not counter the 1.5 h fall in NK counts and NK cytotoxicity
Certainty: Small, single bout of exercise; immune cell counts, not infections.
Nutrients, 2018 · checked 2026-10-07 · we read the abstract
ev-sgim-07 · Randomized controlled trial(s) · randomized controlled crossover trial · n = 29
Fasting glucose and high-sensitivity C-reactive protein (hs-CRP) increased significantly after all interventions (by 4-9% and 60-109%, respectively; P < 0.05); leptin increased during interventions with SSBs containing glucose only (MG and HG: P < 0.05).
Who: healthy normal-weight young men, six 3-week interventions in random order
Effect: hs-CRP +60 to 109% and fasting glucose +4 to 9% after all SSB interventions (P < 0.05)
Certainty: Small, short; CRP is a marker, not immune failure. Larger meta-analysis (ev-sgim-01) finds no overall CRP rise. Outside our PubMed snapshot, so the passport is unchecked by machine and the quote is checked by eye only. NCT01021969.
Am J Clin Nutr, 2011 · checked 2026-10-07 · we read the abstract
ev-sgim-08 · Observational data · retrospective matched cohort study · n = —
IRRs for infection-related hospitalizations were 3.71 (95% CI 3.27-4.21) for T1DM and 1.88 (95% CI 1.83-1.92) for T2DM.
Who: primary care patients with diabetes vs matched people without diabetes, England
Effect: infection hospitalization IRR 3.71 (95% CI 3.27 to 4.21) T1DM, 1.88 (1.83 to 1.92) T2DM
Certainty: About chronically high blood glucose in diabetes, not dietary sugar in healthy people. Outside our PubMed snapshot, so the passport is unchecked by machine and the quote is checked by eye only.
Diabetes Care, 2018 · checked 2026-10-07 · we read the abstract
ev-sgim-09 · Observational data · systematic review and meta-analysis of cohort studies · n = 4 cohorts
The limited meta-analysis showed that a mother's increased free sugar intake during pregnancy was associated with an increased risk of developing asthma in offspring (odds ratio 1.07 [95% CI, 1.00 to 1.14; I2 = 0%]).
Who: mother-child pairs in 4 unique cohorts, children followed from infancy to 5 years
Effect: offspring asthma OR 1.07 (95% CI 1.00 to 1.14), I2 0%
Certainty: Only 4 cohorts, self-reported sugar; OR, not risk; confounding possible.
Nutr Rev, 2022 · checked 2026-10-07 · we read the abstract
ev-sgim-10 · Observational data · systematic review of epidemiologic studies, narrative · n = 17 studies
Cross-sectional analyses from NHANES and the National Children's Study showed stronger associations, with greater fructose exposures linked to two- to five-fold higher odds of asthma.
Who: children in cohort, case-control, cross-sectional and feeding studies
Effect: NHANES and National Children's Study: 2 to 5 times higher odds of asthma with greater fructose exposure
Certainty: Mostly observational and heterogeneous; mechanism (gut barrier, type 2 immunity) proposed, not proven.
Nutrients, 2026 · checked 2026-10-07 · we read the abstract
ev-sgim-11 · Observational data · systematic review and meta-analysis of observational studies · n = 523,730 (Crohn's, 14 studies)
Pooled analysis showed that added sugar intake was associated with increased risk of Crohn's disease (OR 1.66; 95% Cl 1.21-2.29; n = 523,730; 14 studies) and ulcerative colitis (OR 1.59; 95% CI 1.25-2.02; n = 787,228; 18 studies).
Who: 11 prospective and 34 retrospective studies of sugar intake and IBD
Effect: Crohn's disease OR 1.66 (95% CI 1.21 to 2.29); ulcerative colitis OR 1.59 (1.25 to 2.02; n = 787,228; 18 studies)
Certainty: Mostly retrospective (recall bias); OR, not risk. Same study as ev-sgi-11 on the inflammation page.
United European Gastroenterol J, 2025 · checked 2026-10-07 · we read the abstract
ev-sgim-12 · Position of an expert body · agency guideline (news release) · n = —
A new WHO guideline recommends adults and children reduce their daily intake of free sugars to less than 10% of their total energy intake.
Who: adults and children
Effect: free sugars <10% of total energy
Certainty: T2, level E. The guideline does not mention immune function or infection.
WHO, news release 4 March 2015: WHO calls on countries to reduce sugars intake among adults and children · checked 2026-10-07 · we read the section

How this page was made. Evidence was extracted from primary sources into cards, every number was re-checked against the source, and the text was written from those cards. Bioma Learn has no human medical reviewer at this time, and we say so rather than invent one. Methodology. This is information, not medical advice.