Resistance Training Can Change Your Gut Microbiome in Eight Weeks

Strength training two or three times weekly may substantially alter the trillions of bacteria in the gut, with changes potentially emerging within just eight weeks.

That is the finding of a recent study, which has not yet undergone peer review. It found that people who had previously been inactive experienced measurable shifts in their gut microbiome – the population of microbes that inhabits the digestive tract – after taking up resistance training.

The gut contains bacteria, fungi, viruses and other microscopic life forms, with the majority residing in the large intestine. These microbes assist in digesting foods the body cannot process by itself, helping us obtain additional nutrients and vitamins.

Certain bacteria are regarded as helpful because they are commonly identified in people with good physical and mental health. They also make compounds that seem to promote wellbeing.

The composition of the gut microbiome is not permanent. It can be affected by what you eat, your age and sleep quality – as well as, this research suggests, your exercise habits.

Resistance training and the gut microbiome

Scientists at the University of Tübingen in Germany enrolled 150 people who did not usually exercise. They completed resistance-training sessions two or three times each week over eight weeks. Some participants lifted lighter weights for higher repetitions (15 to 20), while others used heavier weights for fewer repetitions (eight to ten).

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

To monitor changes in gut bacteria, the researchers took stool samples before the programme began, at four weeks and again after eight weeks.

Strength gains varied considerably between participants. The team categorised people as "high responders" – the top 20%, who increased their strength by more than 33% on average – and "low responders" – the bottom 20%, who gained less than 12.2%.

Starting strength seemed to be the strongest predictor of whether a person became a high or low responder.

However, the researchers identified another noteworthy pattern. Those with the greatest strength improvements had small yet meaningful alterations in their gut bacteria that were not seen in the other participants.

Among high responders, 16 bacterial types became more abundant and 11 declined. Two types were particularly notable: Faecalibacterium and Roseburia hominis.

Both bacteria produce butyrate, a compound known as a short-chain fatty acid. Gut microbes generate these compounds when they break down fibre. Short-chain fatty acids have several roles: they supply the body with energy and support a healthy gut lining, helping to stop harmful bacteria from passing into the bloodstream.

Other research examining exercise and the gut has also reported similar rises in these bacteria. Yet this study did not detect a corresponding increase in short-chain fatty acids in stool samples; it found only a higher number of the bacteria capable of producing them.

Why gut bacteria changes are difficult to interpret

It may be appealing to describe particular bacteria as "good" or "bad", but the reality is more complicated. During the study, some bacteria generally connected with good health fell in number, while others formerly associated with poor health became more common.

This underlines a key issue: every person's microbiome is distinct. Depending on the individual and their wider state of health, the same bacteria may have different functions.

Nor can we know with certainty whether gut bacterial changes led to increased strength or whether becoming stronger produced the bacterial shifts. Research of this kind can reveal links, but it cannot establish cause and effect, because the microbiome is shaped by far too many influences to control fully.

Diet and lifestyle factors

Diet, for example, has a substantial influence on gut bacteria. Although participants were instructed to maintain their usual eating patterns throughout the study, accurately recording what people consume is exceptionally challenging.

Some high responders may have adjusted their diets as they became more engaged with fitness. That could have played a part in both the changes to their bacteria and their strength improvements.

What can be stated more confidently is that exercise seems to support general physical and mental health. It should form part of a healthy lifestyle, whatever effect it may have on gut microbes.

This was a small study and it must still complete peer review, the formal process in which other scientists assess the work. Even so, it could contribute to increasing evidence that lifestyle decisions, including how active we are, may affect the microscopic ecosystem 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 licence. Read the original article.