“Does eating sugar suppress your immune system?” is a common question, particularly around cold and flu season. The evidence is more nuanced than the yes/no framing suggests, some effects are real and mechanistically established, others are overstated.

The Linus Pauling-Era Claim and What’s Behind It

A 1973 study by Sanchez et al., published in the American Journal of Clinical Nutrition, is the primary source for the oft-repeated claim that “sugar suppresses immune function for hours.” The study had 10 subjects consume 100g of glucose, fructose, sucrose, honey, or orange juice, then measured the ability of their neutrophils to engulf bacteria (phagocytic activity) over 5 hours.

All sugar forms reduced neutrophil phagocytic activity compared to water. The nadir occurred at about 2 hours and returned to baseline by 5 hours. The magnitude was substantial: phagocytic activity approximately halved.

This study is frequently cited but has significant limitations: only 10 subjects, 100g of sugar (more than a can of soda), and no blinding. It’s never been definitively replicated at scale. The mechanism proposed. That glucose and vitamin C compete for the same GLUT transporters on neutrophils, so high blood glucose reduces vitamin C uptake and therefore neutrophil activity, is plausible but hasn’t been confirmed causally in larger studies.

Glycation and Antibody Function

A better-established mechanism involves glycation. Antibodies are proteins; glucose reacts with proteins to form advanced glycation end products (AGEs). Glycated antibodies show reduced binding affinity for their antigens, they’re structurally impaired.

In diabetics with chronically elevated blood glucose, glycated immunoglobulin G (IgG) concentrations are measurably higher. The clinical correlate: diabetics have significantly higher rates of bacterial infections, impaired wound healing, and poorer vaccine responses. A systematic review by Critchley et al. in Cochrane found that diabetics on immunosuppression (transplant recipients) had substantially lower vaccine immunogenicity.

Whether this effect operates at non-diabetic blood glucose levels after a single sugary meal is much less established.

Sugar, Inflammation, and the Innate Immune System

Chronic high sugar intake promotes systemic inflammation through several pathways (AGEs, gut permeability, uric acid, reactive oxygen species). This low-grade inflammation isn’t the same as an acute immune response. It actually impairs immune defense.

When innate immune cells are chronically activated by metabolic inflammation, they can become less responsive to acute infectious threats (a phenomenon called “nutritional training” or, in its negative form, “metabolic exhaustion”). The macrophages and dendritic cells that should mount vigorous responses to bacteria and viruses are partially occupied with adipose tissue inflammation and AGE-RAGE signaling.

This is plausible and consistent with the observation that obese individuals, who typically have high-sugar diets and chronic metabolic inflammation, have worse outcomes with respiratory infections, including COVID-19.

What the COVID-19 Data Showed

A 2021 analysis by Yates et al. in Diabetes Care found that among UK Biobank participants infected with SARS-CoV-2, those with metabolic syndrome (which includes insulin resistance and elevated blood glucose) had 2-4 times higher risk of severe disease and death. Obesity and type 2 diabetes were the strongest individual predictors.

This isn’t a direct study of sugar and immune function, but it establishes that the metabolic state driven by excess sugar consumption, insulin resistance, chronic inflammation, elevated blood glucose, meaningfully worsens infectious disease outcomes.

What’s Overstated

The direct “eating sugar before you get a cold gives you a cold” claim doesn’t have rigorous support. Infection requires exposure to a pathogen; immune suppression from a single high-sugar meal is modest and transient. The long-term effects of chronically high sugar intake on immune function are more defensible than acute meal-to-meal effects.

Sugar also does not cause cancer directly through immune suppression — this is a different mechanism (Warburg effect) covered in the cancer article.

Primary Sources

  • Sanchez A et al. “Role of sugars in human neutrophilic phagocytosis.” AJCN 1973. The foundational (but small) study
  • Hotamisligil GS. “Inflammation and metabolic disorders.” Nature 2006
  • Yates T et al. “Obesity and risk of COVID-19: analysis of UK Biobank.” Diabetes Care 2021