Obesity Accelerates MS Disability Only in People Lacking a Key Immune Gene, Swedish Study Finds

Obesity has long been flagged as a bad companion for people living with multiple sclerosis, but a sweeping Swedish study now suggests that its damaging effects are far from universal. According to research published in the Journal of Neurology, excess body weight at the time of diagnosis sharply increases the risk of long-term disability progression only in patients who lack a specific immune gene variant, HLA-A*02:01. In those who carry the variant, obesity appears to leave the disease course largely untouched. The finding, drawn from more than two thousand patients followed for a median of fourteen years, adds a striking genetic dimension to one of the most modifiable risk factors in MS care and hints that the immune pathways governed by this gene may determine whether adipose tissue becomes a driver of neurological decline or a bystander.

Multiple sclerosis is a chronic inflammatory and neurodegenerative disease of the central nervous system and a leading cause of non-traumatic neurological disability in young adults. Its origins lie in a tangled interplay between genetic susceptibility and environmental exposures, ranging from Epstein-Barr virus infection to smoking, low sun exposure and adolescent obesity. Yet while much has been learned about what triggers the disease, far less is understood about what shapes its long-term trajectory after diagnosis. Genome-wide studies have identified only a handful of genetic loci linked to severity, and polygenic risk scores built from susceptibility markers predict disability trajectories poorly. This has left clinicians with limited tools for forecasting who will deteriorate and who will remain stable.

Obesity stands out among lifestyle factors because its influence is potentially reversible. Adipose tissue is no longer viewed as an inert energy depot but as an active immunometabolic organ. It secretes adipokines such as leptin, sustains chronic low-grade systemic inflammation, and promotes immune dysregulation, mechanisms that have been implicated in inflammatory conditions from rheumatoid arthritis to psoriasis. In MS, these obesity-related pathways may sustain peripheral immune activation and foster a pro-inflammatory environment that facilitates tissue injury in the central nervous system. Previous studies have indeed linked obesity to faster disability accumulation, worse cognitive functioning and reduced quality of life, but whether those harms apply equally to every patient has remained an open question.

The new study, led by Jie Guo, Anna Karin Hedström and colleagues at the Karolinska Institute, drew on two Swedish population-based case-control studies, the Epidemiologic Investigation of Multiple Sclerosis (EIMS) and Genes and Environment in Multiple Sclerosis (GEMS). The researchers focused on 2,117 individuals with MS who had a disease duration of five years or less at study entry and for whom HLA genotyping was available. Restricting the sample to recently diagnosed patients was a deliberate design choice: it ensured that body mass index reflected body weight close to the time of diagnosis and minimised the chance that years of living with the disease had already altered participants’ weight. Obesity was defined as a BMI above 30 kilograms per square metre, calculated from self-reported height and weight.

To track disability, the team linked participants to the Swedish MS registry, a nationwide database integrated into routine clinical documentation in which neurologists prospectively record disease activity, neurological function and treatment at every visit. The primary outcome was confirmed disability worsening, defined as a sustained increase of at least one point on the Expanded Disability Status Scale, the standard neurological rating instrument, between two visits at least six months apart. The researchers also examined the time it took patients to reach confirmed EDSS scores of 3 and 4, milestones that mark the transition from relapsing-remitting disease to progressive accumulation of impairment. Cox proportional hazards models estimated hazard ratios, adjusted for age, sex, ancestry, disease phenotype, disease duration, baseline EDSS, smoking status and category of first disease-modifying therapy.

The results were unambiguous. Among individuals lacking HLA-A02:01, obesity was associated with a 40 percent higher risk of confirmed disability worsening, with a hazard ratio of 1.40. When BMI was modelled as a continuous variable, each additional kilogram per square metre raised the risk by roughly two percent per year in this group. By contrast, among HLA-A02:01 carriers, the association was weak and statistically indistinguishable from no effect. The pattern grew starker when the researchers compared each group against non-obese carriers of the protective allele: obese individuals lacking HLA-A*02:01 had a 49 percent higher risk of confirmed disability worsening, a 90 percent higher risk of reaching EDSS 3, and a 130 percent higher risk of reaching EDSS 4. Obese carriers of the allele showed no convincing elevation in risk on any outcome.

Additive interaction analyses reinforced the picture. The attributable proportion due to interaction, a measure of whether the combined effect of two exposures exceeds the sum of their individual contributions, was significantly greater than zero for the combination of obesity and absence of HLA-A02:01. In other words, the two factors do not merely coexist; they amplify one another in a clinically meaningful way. Longitudinal mixed-effects models, which tracked predicted EDSS trajectories over time, told the same story from a different angle. Obese HLA-A02:01-negative patients consistently showed the highest predicted disability scores throughout follow-up, with the divergence from other groups becoming statistically significant around year seven and persisting thereafter. Obese carriers of the allele, meanwhile, tracked alongside non-obese participants.

Why would a single class I HLA allele so profoundly alter the consequences of obesity? HLA-A02:01 encodes a major histocompatibility complex class I molecule that presents antigens to CD8-positive T cells and orchestrates antiviral immune responses, particularly those directed against Epstein-Barr virus, the herpesvirus now considered a leading cause of MS. Recent protein-quantitative trait locus work suggests that protective HLA class I alleles are also tied to broader differences in immune regulation, including type I interferon signalling. The absence of HLA-A02:01 may therefore define an immunogenetic context in which inflammatory pathways are regulated differently, leaving carriers of that context more vulnerable to the chronic immune activation that adipose tissue promotes. Notably, the same research group has previously reported that the adverse effect of smoking on MS disability progression is also largely confined to individuals lacking HLA-A*02:01, suggesting a shared mechanism through which this gene shapes responses to environmental exposures after disease onset.

The study’s strengths lie in its size, its population-based design, its long follow-up and its use of prospectively collected registry data rather than retrospective recall. The authors also tested the robustness of their findings by treating disease-modifying therapy as a time-varying covariate and by additionally adjusting for past infectious mononucleosis, alcohol consumption, sun exposure and physical activity; neither adjustment changed the pattern. Still, some caveats apply. BMI was self-reported and measured only once, at diagnosis, so the analysis could not capture how weight changes over the disease course might influence progression. Some participants lacked HLA data, although obesity prevalence was similar in those with and without genotyping, making major selection bias unlikely. Residual confounding cannot be excluded, and the mechanistic interpretation remains speculative pending experimental work.

The clinical implications are nonetheless compelling. Weight management is already advisable for people with MS for general health reasons, but these findings suggest it may be especially critical for the subset of patients who lack HLA-A*02:01, a group in whom obesity appears to act as a genuine accelerant of neurological decline. They also point toward a future in which risk stratification in MS integrates both lifestyle and immunogenetic information, identifying early in the disease course those patients who stand to benefit most from aggressive weight interventions. As the authors conclude, obesity may represent an important, potentially modifiable determinant of long-term disability accumulation in genetically susceptible individuals, and HLA class I-related immune pathways may hold the key to understanding why the same extra kilograms mean so much more for some patients than for others.

Subject of Research: Gene-dependent effects of obesity on disability progression in multiple sclerosis

Article Title: Genetic modification of obesity-associated disability progression in multiple sclerosis

Article References: Guo, J., Wu, J., Olsson, T., Alfredsson, L., & Hedström, A. K. (2026). Genetic modification of obesity-associated disability progression in multiple sclerosis. Journal of Neurology, 273(10), Article 578. https://doi.org/10.1007/s00415-026-14091-7

Image Credits: AI Generated

DOI: 10.1007/s00415-026-14091-7

Keywords: multiple sclerosis, obesity, HLA-A*02:01, disability progression, EDSS, immunogenetics, body mass index, Epstein-Barr virus, neuroinflammation, Swedish MS registry, Cox regression, adipokines