From Caffeine to Polyphenols: Scientists Trace What Happens to Coffee in the Gut

Coffee interacts with the gut in a much more complex way than it may seem: some of its components are rapidly absorbed by the body, while others reach the colon and become substrates for gut bacteria. This is the conclusion reached by the authors of a new review study published in the journal Metabolites.

Coffee is one of the most popular beverages in the world and an important source of polyphenols and caffeine. Observational studies have linked regular coffee consumption to a lower risk of type 2 diabetes and some favorable markers of liver health. However, such studies cannot determine what role the gut microbiota plays in these effects.

The researchers examined what happens to the main components of coffee after consumption and how they interact with microorganisms in the gut.

Caffeine is rapidly absorbed in the upper digestive tract and is then converted in the liver into paraxanthine, theobromine, and theophylline. In contrast, a significant portion of other coffee compounds is not absorbed in the small intestine.

This applies, in particular, to chlorogenic acids, one of the main groups of coffee polyphenols. They can reach the colon, where they are exposed to bacteria. Some microorganisms break them down into simpler phenolic acids, some of which are then absorbed and enter the bloodstream.

Some polysaccharides in coffee beans are also not fully digested. Gut bacteria can ferment them, producing short-chain fatty acids, including acetate, propionate, and butyrate. These compounds are involved in the regulation of metabolic and immune processes.

Roasting and the method used to prepare coffee affect the composition of the beverage. Water temperature, contact time, pressure, grind size, and filtration determine which compounds end up in the cup and which of them can reach the gut. As a result, espresso, drip coffee, French press coffee, instant coffee, and cold coffee can differ in the amounts of compounds potentially available to the microbiota.

At the same time, the researchers found no basis for claiming that any particular brewing method consistently provides a better effect on the microbiota or metabolism. However, there is a separate and more firmly established observation concerning filtration: paper filters trap cafestol and kahweol, compounds found in unfiltered coffee that can raise LDL cholesterol levels when consumed regularly.

The researchers are also examining a possible link between coffee and the so-called gut-liver axis. Products of bacterial activity can affect bile acid metabolism, while bile acids, in turn, play a role in regulating metabolic processes and immune responses.

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