A new study from the University of British Columbia Okanagan (UBCO) suggests that fasting does not affect all body types equally, with people living with obesity showing different metabolic and immune responses compared to lean individuals.
Fasting, particularly when paired with very low-carbohydrate ketogenic diets, has grown in popularity as a strategy to burn stored fat for energy. While many people report benefits, researchers at UBCO’s School of Health and Exercise Sciences caution that its impact may vary depending on body composition.
“Fasting and low-carbohydrate meals can provide health benefits, but our findings show that people with obesity may respond differently, especially when it comes to immune system activity,” said Dr. Hashim Islam, assistant professor at UBCO and co-lead of the study.
The research, published in iScience, involved participants with obesity and lean counterparts who fasted for 48 hours. Blood samples collected before, during, and after the fast were analyzed for hormone levels, metabolites, inflammation markers, and the activity of T cells—white blood cells that help fight infections but can also drive chronic inflammation.
The results revealed that people with obesity had a higher number of pro-inflammatory T cells and continued producing inflammatory signals despite fasting. They also showed a smaller rise in ketones and reduced chemical interactions linked to immune regulation. In contrast, lean participants’ immune cells adapted to fasting by burning more fat, shifting toward a more anti-inflammatory state.
“Fasting is often promoted for its ability to switch the body from burning sugar to burning fat, which produces ketones that may strengthen immunity and reduce inflammation,” said lead author Dr. Helena Neudorf. “However, our findings suggest these benefits may be blunted in individuals with obesity.”
Islam emphasized that while fasting can improve health, its effectiveness as a therapeutic strategy for obesity remains uncertain. “We don’t yet know whether these differences are ultimately harmful or protective,” he said. “What is clear is that nutrition, metabolism, and immune function are deeply interconnected, and more research is needed to understand how fasting can be tailored to different body types.”
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