Resistance Training Can Change Your Gut Microbiome in Eight Weeks

Lifting weights only two or three times weekly could markedly alter the trillions of bacteria in your gut - potentially within as little as eight weeks.

That is the finding of a recent study, which has not yet been peer-reviewed. It showed that people who had previously been inactive experienced meaningful shifts in their gut microbiome, the collection of microbes that lives in the digestive system, after starting resistance training.

Your gut contains bacteria, fungi, viruses and other microscopic organisms, with most residing in the large intestine. These microbes break down foods your body cannot digest alone, helping you obtain additional nutrients and vitamins.

Some bacteria are regarded as beneficial because they are commonly present in people with good physical and mental health. They also create compounds that seem to promote wellbeing.

The composition of the gut microbiome is not permanent. It can be altered by factors including diet, age and sleep quality - and, as this research suggests, exercise too.

Resistance training and the gut microbiome

Researchers from the University of Tübingen in Germany recruited 150 people who did not usually exercise. They were asked to complete resistance training two to three times per week for eight weeks. Participants either used lighter weights for higher repetitions (15 to 20), or heavier weights for fewer repetitions (eight to ten).

Each method led to comparable gains in strength and body composition. The programme involved chest presses, abdominal exercises, leg curls, leg presses and back exercises, with two sets of each movement.

To monitor changes in gut bacteria, the team collected stool samples at the beginning of the programme, then again after four weeks and after eight weeks.

Strength gains and changes in gut bacteria

Strength improved far more quickly in some participants than in others. The researchers grouped people as "high responders" - the top 20%, whose strength rose by more than 33% on average - and "low responders" - the bottom 20%, who improved by less than 12.2%.

A person's starting level of strength seemed to be the main factor in determining whether they were a high or low responder.

However, the researchers identified another notable pattern. Those who made the greatest strength gains had subtle yet significant changes in their gut bacteria that were not seen in the other participants.

In high responders, 16 types of bacteria increased and 11 declined. Two bacteria were particularly notable: Faecalibacterium and Roseburia hominis.

Both bacteria produce butyrate, a compound known as a short-chain fatty acid. Gut bacteria generate these compounds while breaking down fibre. They have several functions, providing the body with energy and helping to preserve a healthy gut lining that stops harmful bacteria entering the bloodstream.

Other studies examining exercise and the gut have reported similar rises in these bacteria. In this study, though, the researchers did not detect an increase in short-chain fatty acids in the stool samples - only higher levels of the bacteria that produce them.

The gut microbiome is not that simple

It can be appealing to describe bacteria as "good" or "bad", but the reality is more complicated. During the study, some bacteria generally associated with good health fell in number, whereas others that have previously been connected with poor health increased.

This underlines an important point: every person's microbiome is distinct. The same bacteria may have different functions in different individuals, depending on the person and their broader health.

Nor can we be sure whether shifts in gut bacteria produced the strength improvements, or whether becoming stronger led to the bacterial changes. Research of this kind can reveal associations, but cannot establish cause and effect - too many influences affect the microbiome for all of them to be controlled.

Diet, for example, has a substantial impact on gut bacteria. Although participants were asked not to alter their diets during the study, accurately monitoring what people eat is extremely difficult.

Some high responders may have adjusted their diet as they became more focused on fitness. This might have contributed to both their changes in bacteria and their improvements in strength.

What can be stated with greater certainty is that exercise seems to support overall physical and mental health. It should form part of a healthy lifestyle, regardless of its effect on gut microbes.

This was a small study that must still undergo peer review, the process in which other scientists formally examine the work. Even so, it could add to the growing evidence that lifestyle choices, including how much we move, can shape the microscopic world within us.

Rosie Young, PhD Candidate, Gut Microbes in Health and Disease, Quadram Institute

This article is republished from The Conversation under a Creative Commons license. Read the original article.