Recently appointed personal professor Clara Belzer investigates how the gut microbiome supports human health

The microorganisms in your gut can reveal whether you are a coffee drinker or whether you eat sushi often. That is because gut bacteria adapt to compounds in coffee and seaweed. “This illustrates how closely our gut microbiome and diet are intertwined, and that we can influence our microbiome,” says Clara Belzer. 

Although everyone’s microbiome has a unique composition of bacteria, viruses and fungi, these communities perform similar functions. They help digest food, produce compounds that strengthen the immune system, and extract additional energy from our food. According to Belzer, their contribution is still often underestimated. “We usually assume that food is processed directly by our bodies, but part of what we eat is handled by our microbes. That means the same meal can have different effects in different people.”

Belzer’s research focuses on what she calls the gut microbiome–glycan nexus: the relationship between gut bacteria and complex sugars produced by the human body. These include sugars in breast milk and in the mucus layer that protects the intestinal wall. Over evolution, some bacteria have specialised in feeding on precisely these compounds.

One well-known example is Akkermansia muciniphila, a bacterium that uses the mucus lining of the gut as its food source. At first glance, that might seem harmful, but it is not, Belzer explains. “They trim away part of a molecular protective layer, making it accessible for other microorganisms. By breaking down the mucus layer, they stimulate the gut to produce fresh mucus.” This creates a dynamic balance in which humans and microbes maintain each other.

This cooperation may have consequences far beyond the gut. Belzer and colleagues have previously shown that people with obesity and related metabolic disorders tend to have lower levels of Akkermansia in their intestines. In animal studies, adding this bacterium to a high-calorie diet reduced weight gain in mice. Early human studies are now emerging. This also means that the nutritional value of a meal is not fixed, and that food labels do not necessarily apply equally to everyone. What you extract from a product depends on your gut microbes, meaning one person may absorb more nutrients or calories (and thus more energy) from the same food than another.

According to Belzer, this is clearly illustrated in germ-free animals, animals free from microorganisms. “Those animals need to eat far more to maintain their weight and obtain enough nutrients to stay healthy.” A similar effect is seen in premature infants. Studies show that some nutrients from breast milk pass through their bodies unused, because their microbiome is not yet fully developed enough to process the food. For Belzer, such findings are only a starting point. She is not only interested in whether a bacterium has an effect, but specifically in how the effect occurs. Which molecules transmit signals? Which receptors respond? And why do some bacteria disappear from a microbial ecosystem and fail to return?

Belzer is also interested in how diet can subtly modulate existing microbial communities. Can we encourage certain bacteria to produce more beneficial compounds? Can we design nutrients that mimic the complex sugars found in breast milk or intestinal mucus? And how can such ideas be tested without immediately experimenting in humans? That is why her research group now develops so-called synthetic microbial communities: carefully composed mixtures of gut bacteria that function in bioreactors as simplified models of the human gut microbiome. These systems allow researchers to disentangle more precisely which bacteria do what, and ultimately how diet and the microbiome work together to support health.

Clara Belzer studied Biology at Utrecht University and obtained her PhD in 2007 at Erasmus Medical Centre, where she researched interactions between humans and gut microbes. She then worked as a postdoctoral researcher at Harvard Medical School, studying gut microbiome symbioses using germ-free mouse models. Since 2010, she has been based at Wageningen University & Research, where she established her own research group within the Laboratory of Microbiology. Alongside her research, she developed the course The Human Microbiome, which has reached tens of thousands of participants worldwide, and she actively contributes to education and public communication.