The gut-muscle link: why fermented foods boost muscle health

Good health and muscle health are often thought of as separate concerns. The first recalls eating fiber and probiotics, while the other centers on exercise and protein.

But researchers are increasingly finding that the two may be connected through what is described as the “gut-muscle axis” – a two-way communication network between the gut and our muscles. This connection could be key when investigating whether foods that are good for gut health are also good for muscle health.

My colleagues and I recently conducted a review examining what is known about the gut muscle axis. We also explored whether fermented foods such as kefir, yoghurt and cottage cheese – which are also rich in protein – affect gut and muscle health.

The gut and its community of microbes (collectively known as the gut microbiome) can influence many processes throughout the body. Our review looked at processes relevant to muscle function and metabolism.

The research we explored shows that the gut microbiome produces molecules that can influence how the body uses nutrients, regulates blood sugar and controls inflammation.

Research has also proven that the gut acts as a barrier between the contents of the gut and the rest of the body. A healthy intestinal barrier helps potentially harmful substances to enter the circulation. Changes in the gut microbiome and this barrier can affect inflammation, which can affect muscle and wider health.

The research we looked at showed that the signals produced in the gut can also affect our hormones and metabolism. The gut microbiome can change how nutrients are broken down and absorbed. Both factors can affect the availability of amino acids – the building blocks of protein – that muscles need to grow and function.

Our review found that communication also goes two ways. Muscles produce signaling molecules and metabolic byproducts that can affect other tissues, including the gut.

Because of the many ways that the gut and muscles can be connected, we wondered if eating specific foods could benefit the muscles and the gut. So we decided to investigate what the research shows about protein-rich fermented foods – such as yogurt, kefir, hard cheese and tempeh. These are foods that have been transformed by microbes through fermentation.

Fermented proteins

Fermentation can change food in several ways. Microbes can break down proteins and other components in food, potentially making the protein easier to digest and the amino acids more readily available for our bodies to use. These microbes also produce new, bioactive compounds – some of which may have antioxidant or anti-inflammatory effects that prevent cell damage.

Depending on the food and how it is made, fermented products may contain live microbes or components of microbes. These microbes interact with the gut microbiome and the immune system, which could affect health beyond the gut, including muscle health.

The fermentation of high-protein foods could therefore allow them to affect different parts of the intestinal muscle axis more easily. But when we looked at the evidence, the effects that these protein-rich fermented foods exerted on the gut-muscle axis weren’t quite so clear.

We found evidence that some protein-rich fermented foods can affect aspects of gut health, metabolism and inflammation. But there is little evidence showing these changes translate into improvements in muscle mass or strength.

In fact, there has been very little research that has explored what effects protein-rich fermented foods have had on muscle protein synthesis (the process of building more muscle). This means that we cannot say whether these foods improve muscle growth through the intestinal muscle axis or not.

Instead, much of the evidence suggests that any benefits of fermented foods for muscle health come through less direct pathways—such as changes in the gut microbiome, our metabolism, or less inflammation in the body.

Research needs to test the idea directly by measuring both what happens in the gut and what happens to the muscle. This would also help determine if fermentation offers benefits beyond simply eating a similar amount of protein in a non-fermented food.

So while it’s too early to say whether foods like kefir, yogurt, cottage cheese and tempeh are a proven, superior way to improve muscle health through the gut, that doesn’t mean you should stop including these foods in your diet. Protein-rich fermented foods are still useful sources of protein, nutrients and microbes that can benefit both gut and muscle health.