WHY PEOPLE ARE NOT MICE: How Popular Diets Affect the Health of Humans and Animals
Illustration by David Plunkert / nytimes.com
Science is accumulating more and more data on the link between diet and immunity. Personalized diets are increasingly being used as an additional tool in the treatment of various diseases. However, physiologists also warn about potential health risks posed by trendy fasting systems and popular diets. Let’s explore the real issues, limitations, and prospects of this field of knowledge.
ADVERTISING DOES NOT EQUAL BENEFIT
What could be more ordinary than food? We eat every day, and to give us a sense of purposeful consumption, supermarket shelves and news feeds bombard us with messages about the connection between diet, health, and immunity. Headlines and labels often promise new insights.
For instance, we’re told that fasting can «reset» our immune system, that replacing coffee with mushroom tea can protect us from cancer, or that feeding the «good» bacteria inside us can transform our health. The problem with such claims is that they rely on limited evidence.
The scientific validity of these dietary theories is often questionable because conducting rigorous research in this field is nearly impossible. Moreover, the significance of many findings is frequently exaggerated for commercial gain.
DIETS DISTORT IMMUNE RESPONSES
Over the past five years, scientists have developed innovative approaches to nutritional immunology. Previously, researchers focused on studying the long-term effects of vaguely defined diets. Today, new tools allow them to concentrate on short-term effects and the molecular mechanisms underlying how food influences immunity.
This topic was highlighted at the 2024 summit «Food as Medicine». Among the key points raised was the claim that popular Western diets distort immune responses and weaken immune resilience. However, other researchers remain more optimistic, emphasizing the notable success of personalized nutrition in treating a range of diseases.
NOT USED TO EATING INSECTS? THINK AGAIN!
Over the past decade, scientists have cataloged and analyzed entire sets of biomolecules, helping uncover mechanisms through which different dietary components affect the immune system. For example, researchers at Washington University School of Medicine in St. Louis studied type 2 immune responses.
Typically triggered by allergens and parasites, this response can also be stimulated by chitin — dietary fibers found in mushrooms, shellfish, and insects. Mice fed a chitin-rich diet exhibited stomach expansion, which activated type 2 immunity. This, in turn, triggered an enzyme responsible for digesting chitin.
Interestingly, genetically modified mice that could not produce this enzyme gained less weight, accumulated less fat, and showed better insulin sensitivity compared to their counterparts. Understanding the links between chitin, immunity, and health opens new possibilities for treating obesity and developing appetite-suppressing drugs.
It is also well-known that obesity disrupts skin immunity. Consequently, psoriasis is 2–3 times more common in people with obesity than in those without it. Weight loss and a low-fat diet have been shown to reduce psoriasis symptoms.
CAN WE FAST WITHOUT STARVING?
Scientists are accumulating growing evidence that fasting reduces the risks of a wide range of conditions, including hypertension, atherosclerosis, diabetes, and asthma. In some cases, these benefits are linked to the immune system. For instance, fasting decreases the number of monocytes — cells that protect the body from foreign invaders but can also indicate autoimmune diseases.
In the brainstem, there is a group of neurons that help regulate the immune system. Experiments on mice conducted by Chinese researchers revealed that these neurons are activated in response to fasting, causing monocytes to retreat from the blood, spleen, and lymph nodes to their central reservoir — the bone marrow.
As a result, survival rates significantly improved in mice suffering from atherosclerosis and paralysis. These neurons can also be activated in humans through electrical stimulation, potentially enhancing the health benefits of fasting without requiring prolonged food deprivation.
FASTING CAN BACKFIRE
Over the past decade, evidence supporting fasting as a therapeutic method has grown. However, scientists caution that fasting generally dampens immune responses, and in certain circumstances, reducing calorie intake may have harmful effects.
Immunologists at the Icahn School of Medicine at Mount Sinai (New York) confirmed observations made by their Chinese colleagues: fasting mice showed a 90% reduction in monocytes in their blood, while these cells increased in the bone marrow, where they are produced.
However, when the mice resumed normal feeding after 24 hours of fasting, their blood was flooded with an abnormally high number of monocytes, leading to monocytosis — a condition typically associated with infectious and autoimmune diseases.
When researchers exposed these monocytotic animals to bacterial pneumonia, they died earlier and in greater numbers than control animals that had not fasted. This suggests that while the body conserves monocytes as a protective mechanism during periods of low energy reserves, prolonged fasting may tip the balance toward increased risks rather than benefits.
FEASTS AND CELEBRATIONS — A BLOW TO IMMUNITY
Not just fasting but any dietary changes can redistribute immune cells within hours, affecting immunity. When researchers switched mice from a «gentle» diet to a high-fat regimen, it suppressed their immune systems and impaired the function of cells responsible for detecting and remembering pathogens.
Additionally, a lack of fiber disrupted gut microbiomes, which support these immune cells. It took only three days on such a diet for the immune system to suffer serious damage! A similar effect was observed in human volunteers who agreed to simulate «holiday feasting» for the study.
EASIER WITH MICE THAN WITH HUMANS
Still, scientists warn that humans are not mice. Studies on animals can only offer clues about what happens in the human body. Validating these clues poses a significant challenge, as getting people to accurately recall and record their daily diets over long periods is extremely difficult. Unlike mice, humans cannot be kept in cages, making fully controlled experiments nearly impossible.
Even when physiologists at the U.S. National Institute of Diabetes, Digestive, and Kidney Diseases instructed volunteers to stick strictly to provided meals, participants still consumed an average of 400 extra calories outside the study’s guidelines.
The only solution is to recruit volunteers willing to remain under strict supervision in a hospital research unit for extended periods.
KETO DIET AND VEGANISM: THERE IS A DIFFERENCE
The Pasteur Institute in Paris successfully conducted a study involving 20 adults who stayed in a hospital for four weeks, adhering to either a keto or vegan diet. Throughout the study, researchers regularly collected blood samples.
The results showed that the keto diet boosted levels and activity of T and B cells — components of the adaptive immune system that precisely recognize specific threats. In contrast, vegans exhibited enhanced immune responses that were faster but less specific than adaptive reactions.
While physiologists see great potential in these findings, they are not yet ready to make dietary recommendations based on them.
OUTLOOK: PERSONALIZED DIETS
The research team plans to expand the study to include individuals with lupus, psoriasis, diabetes, and cancer to test the effects of different diets. Some predictions about how the bodies of these patients might respond already exist.
For example, evidence suggests that the keto diet serves as an effective complementary therapy for psoriasis and type 1 diabetes. Similarly, high-fiber diets have shown positive effects by engaging the immune system in the fight against cancer.
Scientists predict that the vast clinical potential of data-driven nutrition will spark a surge of interest in personalized diets in the coming years.
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