Obesity worsens outcomes of infectious diseases

Researchers from a large international study report that obesity significantly increases the risk of hospitalization and death from many infectious diseases, including flu, COVID-19, pneumonia, gastroenteritis, urinary tract infections and respiratory tract infections.

The findings were published Feb. 9, 2026 in The Lancet.

The study’s lead author, Professor Mika Kivimaki (University College London – Faculty of Brain Sciences), said, “Obesity is well known as a risk factor for metabolic syndrome, diabetes, cardiovascular disease, and many other chronic conditions. Here we have found robust evidence that obesity is also linked to worse outcomes from infectious diseases, as becoming very ill from an infection is markedly more common among people with obesity.”

The new analysis included pooled data on 67,766 adults from two Finnish studies and 479,498 from the UK Biobank. The average age at baseline was 42 years old for the Finnish cohorts and 57 years old for the UK Biobank cohort.

Participants had their body mass index (BMI) assessed when they entered the studies (1998–2002 in the Finnish studies; 2006–10 in UK Biobank), and they were subsequently tracked for an average of 13-14 years.

They were followed through national hospitalization and mortality registries for hospital admission and death due to infectious disease. At baseline, the subjects had no recent history of infection related hospitalization.

For BMI, they were classed as having healthy weight (18.5–24.9 kg/m²), overweight (25.0–29.9 kg/m²) or obesity — class I obesity (30.0–34.9 kg/m²) class II (35.0–39.9 kg/m²) or class III (≥40.0 kg/m²).

During follow-up, there were 8,230 infection cases in the Finnish cohorts and 81, 945 in UK Biobank cohort.

The investigators reported that subjects with obesity, defined as BMI ≥30 kg/m², had a 70% higher risk of hospitalization or death from any infectious disease compared to those with a BMI between 18.5 to 24.9.

Notably, subjects with the most severe obesity (BMI ≥40 kg/m²) had three times the risk of death from infection compared to those with a healthy weight.

Author Dr Solja Nyberg, University of Helsinki (Finland), says “Our findings suggests that people living with obesity are significantly more likely to become severely ill or to die from a wide range of infectious diseases. As obesity rates are expected to rise globally, so will the number of deaths and hospitalizations from infectious diseases linked to obesity.”

Kivimäki added, “Our finding that obesity is a risk factor for a wide range of infectious diseases suggests that broad biological mechanisms may be involved. It is plausible that obesity weakens the immune system’s ability to defend against the infectious bacteria, viruses, parasites or fungi, therefore resulting in more serious diseases. Evidence from trials of GLP-1 weight-loss drugs fits with this, as reducing obesity also appears to lower the risk of severe infections, alongside many other health benefits. That said, additional research is required to confirm the mechanisms underlying these associations.”