Scientists discover the one nutrient beneficial parasites can't live without

The findings, published in Nature Communications, come from parasitologists at the Biology Centre of the Czech Academy of Sciences.

Why Scientists Are Studying Intestinal Worms

For most of human history, intestinal parasites were a normal part of the digestive system. In industrialized countries, however, they have become far less common because of improved hygiene and modern medicine. During that same period, autoimmune disorders and inflammatory bowel diseases have become increasingly common.

This trend prompted researchers about two decades ago to investigate whether certain intestinal worms could be used to treat inflammatory conditions. The approach, known as helminth therapy, has shown promise, but its results have been inconsistent.

"However, the results of helminth therapy have been inconsistent -- sometimes the worms suppress inflammation, sometimes they do not. That's why we focused on factors that may influence their effects in the gut," says Kateřina Jirků from the Institute of Parasitology, Biology Centre CAS.

The team's research pointed to one major factor: the amount of dietary fiber in the host's diet.

How Fiber Changes Worm Behavior

To better understand the connection, the researchers examined how an intestinal parasite responds to different levels of dietary fiber. They used the rat tapeworm Hymenolepis diminuta, a non-pathogenic species commonly used to study interactions among parasites, gut microorganisms, and the immune system because of its known anti-inflammatory properties.

Their experiments revealed a striking difference between fiber-rich and fiber-poor diets.

"We found that when the diet contains a high proportion of structural fiber, the tapeworm is not only in excellent condition but is also able to induce an anti-inflammatory response in the host. When fiber is lacking, the worm enters an energy-saving state resembling hibernation in mammals, and its anti-inflammatory effect disappears," explains Jirků.

When the animals consumed a low-fiber diet, the tapeworms were several times smaller, never reached sexual maturity, and failed to produce eggs. Genetic testing also showed widespread shifts in gene expression affecting the worms' development, metabolism, and reproduction.

Fiber Reshapes the Gut Microbiome

The researchers also found that dietary fiber transformed the host's gut microbiome.

A fiber-rich diet encouraged the growth of bacteria associated with a healthy intestinal environment. In contrast, a Western-style diet reduced microbial diversity and allowed bacteria linked to dysbiosis to become more abundant. These microbial changes were mirrored by differences in the host's immune response.

According to the researchers, the study highlights how strongly diet influences the entire gut ecosystem, from parasites and microbes to the immune system itself.

Health organizations generally recommend that adults consume about 25 to 30 grams of fiber each day, yet average intake across many Western countries falls below that level. Traditional populations, by comparison, are estimated to consume between 80 and 120 grams of fiber daily.

Previous research has shown that too little dietary fiber can weaken the gut microbiome, which plays an important role not only in digestive health but also in immune function, brain health, and mental well-being. An unhealthy microbial balance has been associated with a higher risk of allergies, depression, anxiety, and neurodegenerative diseases, including Alzheimer's disease.