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By Riaana Bajaj
Introduction
From veganism to intermittent fasting, there are hundreds of methods that claim to link health, diet, and immunity. However, such claims do not have strong scientific evidence backing them up, due to the reason that conducting studies to track the impact of diet can pose a challenge.
Modern diets, specifically of the Western world, have been said to change our immune responses in ways that have weakened our ability to resist diseases. On the other hand, it is stated that diet could be used to treat various health problems, from cancer to chronic immune disorders.
In order to settle this debate, researchers have developed ingenious approaches to nutrition immunology, such as tools that allow them to look closely at both the beneficial and the damaging short-term effects of smaller food groups and fixed dietary components, and to study how food affects the immune system at a molecular level.
Obesity
Prior to this study, the exploration of links between diet and health dates back to the era of Hippocrates, Greek physician. In 1912, Polish biochemist Casimir Funk suggested that a deficiency of essential nutrients, later named vitamins, caused diseases like scurvy and rickets, with research confirming their role in immunity.
In recent years, ‘omics’ techniques, which can identify and analyze all types of biomolecules, like genes and proteins, in cells or tissues, have allowed scientists to explore how diets affect the immune system and health. Immunologist Steven Van Dyken's research focuses on chitin, a fiber found in mushrooms and crustaceans, how it sets off type 2 immunity, and its effects on metabolism.
He conducted a study and observed that mice on a high-chitin diet showed increased stomach stretching, activating type 2 immunity and triggering a chitin-digesting enzyme. Mice who were modified and unable to produce this enzyme gained less weight and had better insulin sensitivity. Chitin also raised levels of glucagon-like peptide-1 [GLP-1], a hormone that suppresses appetite, offering potential for obesity treatments. Obesity is also linked to autoimmune conditions like psoriasis, in which skin cells collect in dry, scaly patches. It is two to three times more frequent in people with obesity compared to those without, and weight loss has been proven to improve symptoms of psoriasis.
Immunologist Chaoran Li's research revealed that obesity disrupts immune cells that prevent psoriasis-related inflammation. Additionally, previous research indicated that high-fat diets worsen psoriasis by activating immune cells that cause inflammation. Using RNA sequencing, Li and his team studied skin immune cells in lean mice and identified a type of T cell that helps control psoriasis-related inflammation. In obese mice on a high-fat diet, they found lower levels of these T cells, which led to increased inflammation. By conducting studies on people with psoriasis, the results showed similar disruptions in these cells.
Fasting
Since overeating and obesity have a negative impact on health and the immune system, one might think that fasting has the opposite effect. While fasting can lower the risk of health issues like hypertension, diabetes, and asthma, excessive calorie restriction can weaken immune responses.
Cheng Zhan, a neuroscientist at the University of Science and Technology of China, studied a group of neurons in the brainstem that regulate the immune system to see if they could be manipulated for beneficial effects. In a recent study, Zhan's team found that fasting activates these neurons in mice, prompting T cells to move from the blood, spleen, and lymph nodes to the bone marrow. Using a mouse model of multiple sclerosis, Zhan demonstrated that continuous activation of these neurons significantly reduced paralysis, prevented weight loss, and increased survival rates.
Filip Swirski found that fasting led to a significant drop in monocytes, increasing infection susceptibility upon re-feeding.
Diet Changes
Diet changes also quickly affect immunity. Siracusa's studies showed that a short-term high-fat diet suppressed immune function in mice, and similar effects were seen in human volunteers who switched to a low-fiber diet.
Human TrialsResearch into the effects of different diets on the human immune system is challenging due to difficulties in controlling the diets of participants.
Kevin Hall and his team at the US National Institutes of Health have combatted this by conducting trials in which volunteers stayed at a hospital where their diets were strictly controlled.
Immunologist Yasmine Belkaid’s study in 2024 explored how a ketogenic diet and a vegan low-fat diet influenced the immune system. The study proved that the ketogenic diet enhanced the adaptive immune system, whereas the vegan diet boosted the innate immune response.
While these results are promising, more research is needed to apply dietary interventions in clinical settings. Future trials could focus on specific conditions like lupus or cancer, where diet may enhance immune-based therapies. Researchers hope that within a decade, personalized, science-based dietary advice could be used in medical treatments.
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