A few days of unhealthy eating may have a bigger impact on your immune system than you think. A new study by researchers at the University of California, San Francisco (UCSF), has found that even a brief switch to a high-fat diet can trigger stronger inflammatory responses in the body, with effects that may persist even after returning to a healthier eating pattern.

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Published in the journal Nature on October 28, the study examined how short-term dietary changes affected inflammation in mice. Researchers found that mice fed a high-fat diet for just four days developed more severe skin inflammation when exposed to an irritating cream than those on a low-fat diet.

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The findings also revealed something more surprising. Even after the mice returned to a low-fat diet and the initial inflammation subsided, their immune systems remained primed to react more strongly when exposed to the same irritant again.

How does a high-fat diet affect the immune system?

To understand whether severe inflammation was linked to obesity or the food itself, the researchers conducted experiments involving mice fed different diets. They initially observed that mice with obesity, which had been regularly consuming high-fat food, experienced more severe inflammation after researchers applied imiquimod, a cream used to trigger skin irritation.

However, when normal-weight mice were switched to a high-fat diet for just four days, they developed similarly severe inflammatory reactions. In contrast, obese mice that were moved to a low-fat diet for the same period experienced milder inflammation.

This suggested that the type of food consumed could influence the immune response independently of body weight.

Why could the effects last even after changing your diet?

The researchers found that a high-fat diet rapidly attracted immune cells to the skin, intensifying the inflammatory response. Although the initial inflammation eventually subsided, another type of immune cell, known as memory T cells, remained in the affected area.

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These cells appeared to leave the tissue prepared for a stronger reaction when it encountered the same irritant again. In other words, the effects of a short-term dietary change did not necessarily disappear when the mice returned to their regular diet.

The team also observed a similar increase in inflammation in the lungs of mice following just a few days of consuming high-fat food.

What do the findings mean for human health?

The study highlights how quickly dietary choices can influence immune activity. While researchers have long established a connection between obesity, diet and inflammation, this research points to the possibility that even short-term eating habits could shape how the immune system responds to irritation.

Sagar Bapat, professor of pathology at UCSF and the study's corresponding author, said the research revealed a more immediate influence of diet on the immune system than previously understood.

However, the findings were based on experiments in mice and do not establish that a few days of high-fat eating will trigger the same response in humans. More research is needed to understand how these mechanisms work in people.

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The researchers also pointed out that diet is not the only factor that could influence immune health. Exercise and sleep may also play important roles in shaping the body's immune response.