A voyage of discovery through the gut

In 2021, 50 subjects swallowed a capsule the size of the outer joint of a thumb while eating their breakfast. The capsule then began its journey through the stomach, small intestine and large intestine to collect information about pH, temperature and pressure. The capsule came out in the faeces between 12 and 72 hours later, and the researchers already then noticed that both gut environment and travel time through the gut varies from person to person.

“We could see, for example, that it took 2 hours for the capsule to pass through the small intestine in some people and 10 hours in others. Since we already know that we absorb most of our nutrients in the small intestine, differences in the travel time in the small intestine probably have an impact on how much of the nutrients we absorb and how much passes on to the large intestine, where the gut bacteria kick in,” says Associate Professor Henrik Roager, Department of Nutrition, Exercise and Sports at the University of Copenhagen, who led the study.

Previously, the activity in the gut has most often been examined via stool samples, which have been compared to what the person had eaten. The capsule gives more precise insight into how the environment changes throughout the gut.

“The capsule means that we can collect information that may help explain individual differences in digestion, nutrient uptake and bowel movement patterns. This provides us with far greater knowledge than we have previously been able to obtain via dietary patterns and stool samples.” explains Associate Professor Henrik Roager.

The environment in the gut: from the acidic stomach to the alkaline small intestine

In their journey through the digestive system the capsule and the food came first of all to the stomach. Here, the capsule registered a very low pH value, because in the stomach acid is released that breaks down the food. Then the food and the capsule moved into the small intestine. Here, gut cells release the alkaline bicarbonate that neutralizes the stomach acid, and it is here that nutrients are absorbed.

The indigestible remainder of the food and the capsule then passed on to the large intestine, where the food was fermented by the gut bacteria. The gut bacteria produce fatty acids, which cause the pH value to fall again in the first part of the colon. However, the pH value increases incrementally along the length of the large intestine as the fatty acids are gradually absorbed through the wall of the gut and the activity of the gut bacteria changes.

“The capsule registered all these changes in pH values, and we can estimate how long the food was in the different parts of the gut on the basis of the changes in pH. We know that pH is a crucial factor in bacterial growth and activity, so it made perfect sense that we could see that gut environment and pH are linked to differences in the composition and activity of the gut bacteria. This means that the environmental conditions we each have in our gut can help explain why we have different bacteria in the gut.” says Henrik Roager.

Personal nutrition

According to Associate Professor Henrik Roager, the new knowledge could be very useful for future nutritional guidelines.

“Our results show that we are all unique — also in our gut.” says Henrik Roager and continues: “We are used to assuming that we all digest and absorb food in the same way and to the same extent, but we can also see that this is not always the case. Our study provides further evidence that individuals react differently to food — and here differences in our gut environment could very well play an important role.”

The results indicate that the physiology and environment of the gut play an important role in the individual differences in the human gut microbiome and metabolism.

Facts about the study

The capsules swallowed by the 50 subjects measured 26 x 13 mm. The test subjects consumed the capsule at the same time as a standardised breakfast, which consisted of rye bread with butter and jam, a boiled egg, a portion of plain yoghurt with nuts and blueberries, and a glass of water.

The study was led by Nicola Procházková, who was a PhD student and postdoc at Department of Nutrition, Exercise and Sports, at the University of Copenhagen from 2020-2024. The study is published as the scientific article Gut physiology and environment explain variations in human gut microbiome composition and metabolism in the scientific journal Nature Microbiology. It was carried out in collaboration with researchers from DTU Food and KU Leuven, Belgium, and it is part of the Challenge project PRIMA.

The study was supported by the Novo Nordisk Foundation.