We all know that we need to eat protein to support muscle function, strength and mass. After all, protein is the building blocks of the body.  When we eat a source of protein like fish or eggs, it is broken down by the digestive system into its component amino acids.  The liver then rearranges them and sends them off to become new proteins at various sites throughout the body. This includes skin proteins, immune cell proteins and of course, muscle proteins. 

In this respect, it seems obvious that our gut can affect muscle mass. For example, we eat protein, it is then digested and absorbed through our digestive system (AKA gut).  If none of this is working as it should, then our ability to use protein for muscle function and strength would be compromised.  But what if we were to tell you that there is an ecosystem in your gut known as the microbiome, and this has been found to play a role in both hypertrophy (muscle gain) and sarcopenia (muscle loss). In addition, that we find this relationship is particularly relevant as we age. 

Let’s get to grips with how our gut affects muscle mass, for better and for worse.

The Gut

When we talk about the gut, we are referencing the gastrointestinal tract. This is the long tube from your mouth to your back passage.  When food is ingested, it enters through the mouth, gets to the stomach, then into the small intestine and finally the large intestine, before any waste products are excreted.   For this topic we’re going to zoom in on the large intestine. 

The large intestine has four main purposes:

Hydration – the large intestine reabsorbs fluid and electrolytes.  Here the contents from the gut turn from liquid into solid (when everything is working as it should).

Nutrient absorption – whilst most of the nutrient absorption occurs in the small intestine, some does occur in the large intestine.

Waste compactor – the end of the large intestine, the rectum, stores and compacts the waste produced by the body, like the parts of dead blood cells which make faeces brown.

Its final function is to house the microbiome, and this is what we’re particularly interested in.  There are microbes found throughout the gastrointestinal tract, but the majority of them set up home here.  These guys can make vitamins, amino acids, hormones, and chemical messengers.  They train the immune system, strengthen the gut barrier, prevent invasion from bad bugs and influence gut movement and function.  Not only that but they directly communicate with many systems in the body, like the brain and muscles.  You may have heard of the gut-brain axis which is how the gut influences how we feel and behave. But we also have a gut-muscle axis, and this describes how those microbes found in our gut can influence muscle function, strength, and size.

The Gut and Muscle Mass

The study of any gut axis usually starts by studying germ free mice. We then rightly or wrongly apply these theories to humans.    

In these studies, mice are raised in sterile environments, so they have no bacterial colonisation. They are like a blank bacterial sheet.  Generally, we then reintroduce bacteria through faecal samples from age-matched mice who have been conventionally raised, to see if any changes occur.

When exploring the gut-muscle axis, germ-free mice regularly demonstrated increased levels of sarcopenia. Which as we know is the age-related decline in muscle mass and function.  To further develop the idea, researchers have moved onto piglets and found that germ-free piglets suffered a 40% reduction in body weight and lean mass compared to conventionally raised piglets. 

We also have the human data to back it up – researchers have found an overall reduced diversity in the microbiome of sarcopenic patients compared to non-sarcopenic individuals. 

One school of thought is that certain bacteria found in the gut support protein digestion and use in the body. If we are lacking these bacteria due to poor diversity in our gut, then our ability to use protein is compromised. That said, there is also other data indicating that there is poor expression of certain genes associated with skeletal muscle growth in germ-free animals. This suggests a possible role of certain bacteria in gene function too.

On the other side of the scale, we have noticed hypertrophy (muscle gain) in cases of probiotic supplementation. Probiotics are live microorganisms that we introduce to the body to positively influence the microbial diversity of the microbiome. This is not to suggest that probiotics are the answer to our muscle wasting worries, but it shows us what is possible with a diverse microbial community in our gut.

How Do We Optimise Gut Health For Microbial Diversity?

An unhealthy balance of microbes in our gut is known as gut dysbiosis and there are a number of contributing factors. 

The colonisation (introduction and settlement) of microbes starts when we are babies.  We are exposed to our mother’s bacterial community through pregnancy, and then through birth.  Caesarean section births are associated with poorer diversity as are formula fed babies.  Natural birth with breastfeeding is associated with improved diversity. 

As we develop, we are exposed to microbes through our environment – the sand and soil we play in, the pets we cuddle and the water we swim in.  The food we eat also influences the community found in our gut.  Those microbes eat fibre so if we’re not feeding them, they can’t survive!  Poor diversity is often noted in low fibre diets.

In order to optimise our gut health to support all the systems it communicates with, including our muscles, we can:

  • Get outside – expose ourselves to microbes in the soil, the sand and water.
  • Eat a fibre rich diet – try to get 30 plant points per week.  1 plant = 1 point.  You can only count it once, even if you have more than one portion through the week.  Plants include fruits, veggies, nuts, seeds, herbs, spices, and legumes.
  • Manage stress – high levels of perceived stress have been associated with poor gut diversity.
  • Prioritise sleep/rest/recovery – poor sleep/wake cycles are linked with poor gut diversity.
  • Limit use of antibacterial products where possible – broad spectrum products are exactly that, they wipe out all bacterial populations, including the good guys we need to influence all these body systems.  Whilst there is a place to eradicate harmful bacteria, we need to preserve the beneficial ones.

It’s clear that our gut is important to so many aspects of our health and this includes our muscle function, strength, and mass.  As we age, there is a natural decline in the microbial community found in our gut and it is this decline that researchers are suggesting plays a role in the development of sarcopenia (muscle loss).  Armed with this information, it gives us the opportunity to optimise our gut health as we age so it can keep doing what it does best.