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

Gut health and muscle health are often thought of as separate concerns. The first brings to mind eating fibre and probiotics, while the other centres 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 could also be beneficial for muscle health.

My colleagues and I recently conducted a review that investigated 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 – influence both 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 the processes relevant to muscle function and metabolism.

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

The research also demonstrated that the gut acts as a barrier between the contents of the intestine and the rest of the body. A healthy gut barrier helps keep potentially harmful substances from entering circulation. Changes in the gut microbiome and this barrier can influence inflammation, which may in turn affect muscle and wider health.

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

Our review found that the communication goes both ways, as well. Muscles produce signalling molecules and metabolic by-products that can influence other tissues, including the gut.

Given the many ways that the gut and the muscles may be connected, we wondered whether eating specific foods could benefit muscles and the gut. So we decided to investigate what the research shows about protein-rich fermented foods – such as yoghurt, kefir, cottage 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 body to use. These microbes also produce new, bioactive compounds – some of which may have antioxidant or anti-inflammatory effects, which can prevent cell damage.

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

The fermentation of high protein foods might therefore allow them to more readily influence different parts of the gut-muscle axis. 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 cut.

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

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

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

Research will need to test the idea directly, by measuring both what happens in the gut and what happens to muscle. This would also help to establish whether fermentation provides benefits beyond simply eating a similar amount of protein in a non-fermented food.

So while it’s too early to say whether foods such as kefir, yoghurt, 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 could benefit both gut and muscle health.