Aging Blunts a Protective Gut Lipid Defense That Keeps COVID-19 Mild

Why COVID-19 hits older people so much harder has been one of the pandemic’s most stubborn questions, and much of the search for answers has focused on the lung, the immune system, and well-known comorbidities. A new study in GeroScience shifts the spotlight to an unexpected organ: the intestine. Working with young and aged Syrian hamsters, researchers led by AhmedElmontaser Mergani and Katrin Wirz at the University of Veterinary Medicine Hannover found that age dramatically reshapes how the gut handles oxysterols, oxidized derivatives of cholesterol with potent antiviral and immune-regulating activities. In young animals, infection triggered a robust surge of protective side-chain oxysterols throughout the intestine and bloodstream, while aged animals failed to mount this response and instead accumulated ring oxysterols that tracked with worse disease. The findings sketch a previously underappreciated intestinal–systemic–pulmonary axis that may help determine who weathers SARS-CoV-2 infection and who deteriorates.

The experimental design was deliberately rigorous. The team infected 7-to-10-week-old and 12-month-old female and male Syrian golden hamsters intranasally with the BavPat1/2020 SARS-CoV-2 isolate, then followed them for five days under biosafety level 3 conditions. Crucially, the researchers paired classic disease readouts with a comprehensive lipidomic analysis: they quantified viral RNA in oropharyngeal swabs, bronchoalveolar lavage fluid, lung, duodenum, and colon by real-time reverse transcription PCR, scored pulmonary lesions histopathologically, and measured cholesterol and six distinct oxysterol species by high-performance liquid chromatography in intestinal tissues, lung, and serum. The lipid measurements were performed after methanol-mediated viral inactivation, validated with spiking experiments, so that samples could safely leave the high-containment laboratory for analysis.

Clinically, the model reproduced what is now well established for aged hamsters. From day three post infection onward, the older animals deteriorated markedly: by day five their mean clinical score reached 6.5 points, compared with 3.7 points in young animals whose condition had stabilized, and aged hamsters lost the most weight relative to their starting body mass. Hypersalivation, particularly pronounced in aged females, was an additional distinctive sign. Yet when the pathologists examined the lungs, the picture was surprisingly uniform. Total lesion scores, comprising alveolar, airway, and vascular changes, rose significantly in all infected groups but did not differ between age groups, and mild rhinitis and tracheitis appeared across the board. The aged animals were sicker without having visibly worse lungs.

The intestine told a different story. Although microscopic examination of the duodenum and colon revealed no overt tissue damage or SARS-CoV-2 nucleoprotein immunoreactivity in either age group, molecular testing showed that aged hamsters carried significantly higher SARS-CoV-2 RNA loads in both intestinal segments than their young counterparts. Respiratory viral RNA in swabs and lavage fluid was also slightly higher in the aged groups. This combination, a morphologically silent but virologically active gut infection concentrated in older hosts, positioned the intestine as a key site where age-dependent differences in viral burden and metabolic response could be dissected without the confounding noise of severe tissue pathology.

The lipidomic data revealed a stark divergence. Upon infection, young hamsters showed a striking upregulation of the side-chain oxysterols 25-hydroxycholesterol and 27-hydroxycholesterol in duodenum and colon, and this pattern was mirrored in serum, where both molecules rose significantly. These compounds are not passive metabolic byproducts. 25-hydroxycholesterol in particular is recognized as a central component of innate antiviral defense, capable of blocking viral fusion and replication, suppressing inflammasome activation through liver X receptor–dependent inhibition of sterol regulatory element-binding protein signaling, and participating in interferon-mediated immune regulation. In aged hamsters, this protective response was essentially absent: intestinal 27-hydroxycholesterol was mostly undetectable even after infection, and the modest rise in 25-hydroxycholesterol never reached statistical significance in any tissue.

Instead, the aged animals showed a selective accumulation of ring oxysterols, oxidized at the sterol ring rather than the side chain. Colonic 4β-hydroxycholesterol surged in infected aged hamsters and correlated strongly and positively with clinical score, age, and viral RNA loads in both bronchoalveolar lavage fluid and oral swabs, while showing negative correlations with serum cholesterol and the protective side-chain oxysterols. This finding echoes earlier reports that 4β-hydroxycholesterol rises in colitis in humans and mouse models, hinting at a role in colonic inflammation. Meanwhile 7β-hydroxycholesterol, another ring oxysterol, was significantly downregulated in the intestines of infected aged animals, whereas 7-ketocholesterol and 5β,6β-epoxycholesterol remained unchanged, indicating a selective remodeling of specific oxysterol pathways rather than a blanket increase in oxidative cholesterol products.

A plausible mechanism for this age-dependent shift lies in the biology of aging itself. Advancing age is associated with mitochondrial dysfunction and elevated production of reactive oxygen species, which drive nonenzymatic autooxidation of cholesterol into ring oxysterols, a hallmark of age-related oxidative stress frequently observed in tissues of elderly individuals with inflammatory comorbidities. Side-chain oxysterols, by contrast, are generated enzymatically, and their coordinated induction in young animals reflects a targeted metabolic defense. Notably, oxysterols were undetectable in lung tissue regardless of age or infection status, yet young hamsters showed a significant depletion of lung cholesterol after infection while aged animals showed only a nonsignificant trend, suggesting that the pulmonary response to SARS-CoV-2 operates through cholesterol depletion rather than local oxysterol production.

The correlation analyses tied these threads into a coherent, if still correlative, framework. Intestinal and serum side-chain oxysterols were positively correlated with each other, supporting the intestine as a major source of circulating oxysterols, and both showed significant negative correlations with viral RNA in bronchoalveolar lavage fluid and with clinical severity. Serum 25-hydroxycholesterol inversely correlated with age. Perhaps most strikingly, side-chain oxysterols from the duodenum and colon, and serum 25-hydroxycholesterol, all correlated negatively with total lung cholesterol, which itself was inversely associated with viral load and clinical score. Because SARS-CoV-2 entry depends on cholesterol-rich lipid rafts that anchor the ACE2 receptor, reduced cholesterol availability in pulmonary cells could plausibly restrict viral entry, offering a mechanistic rationale for how gut-derived oxysterols might protect the lung from a distance.

The study also uncovered systemic signatures with practical potential. Because oxysterol measurement is not routine in clinical settings, the team searched for surrogates among standard blood chemistry parameters. Serum 25-hydroxycholesterol correlated positively with total bilirubin and negatively with total protein and globulin, markers that were themselves linked to advanced age and more severe disease, while declining potassium, inorganic phosphate, and creatine phosphokinase tracked with better outcomes and younger age. The drop in potassium and phosphate in aged hamsters alongside higher intestinal viral loads hints at an infection-induced malabsorptive phenotype, possibly involving a shift of the epithelium toward goblet cell hyperplasia at the expense of absorptive enterocytes, a remodeling described in other coronaviruses through disruption of Notch signaling. The authors are appropriately cautious: elevated 25-hydroxycholesterol may reflect an effective host response rather than cause it, and distinguishing these possibilities will require targeted mechanistic studies. They also propose that antioxidant nutritherapeutics, such as alpha-tocopherol and polyphenols, which counteract ring oxysterol toxicity, could merit exploration as a strategy to improve outcomes in elderly patients, pending validation in larger cohorts and clinical trials.

Subject of Research: Age-dependent intestinal oxysterol remodeling during SARS-CoV-2 infection in Syrian hamsters

Article Title: Intestinal oxysterol remodeling underlies severe COVID-19 in an aged hamster model

Article References: Mergani, A., Wirz, K., Reineking, W., Richter, A., Lechler, V., Pilchová, V., Störk, T., Michaely, L. M., Zdora, I., Buettner, M., Volz, A., Bleich, A., Baumgärtner, W., Schulz, C., & von Köckritz-Blickwede, M. (2026). Intestinal oxysterol remodeling underlies severe COVID-19 in an aged hamster model. GeroScience. https://doi.org/10.1007/s11357-026-02531-1

Image Credits: AI Generated

DOI: 10.1007/s11357-026-02531-1

Keywords: SARS-CoV-2, COVID-19, oxysterols, 25-hydroxycholesterol, aging, cholesterol metabolism, intestine, lipidomics, Syrian hamster, gut-lung axis, innate immunity, GeroScience