This article was first published in Rouleur Issue 141

In over a century of bike racing, there has been a never-ending search for things to measure and tweak. From the first flag drop, teams have weighed equipment, food and riders’ bodies. Watts, carbohydrates and blood lactates have been tracked. Wind tunnels have refined aerodynamics, research has demystified training programmes, and supplements (legal and illegal) have changed human chemistry. So, what else can be measured and altered in the pursuit of speed?

“Our riders do two stool samples each year,” said Dr Julien Louis, who alongside his research role at Liverpool John Moores University is also a nutritionist at Decathlon CMA CGM (formerly Decathlon AG2R La Mondiale).

“We do the stool analysis to get an idea of the microbiome and how it works.”

The microbiome is the collection of trillions of bacteria, viruses, and other microorganisms that live in and on the human body. These microbes help digest food, support the immune system, and influence overall health. In the past decade or so, interest in the microbiome has grown as it has proven to be integral to everything from metabolism and immunity, to mental health and athletic performance.

In a sport where gains are measured in fractions of a percent, professional cycling teams have begun to explore the microbiome as another variable that can be monitored and influenced.

Dr Louis is a busy man. He balances academia with elite sports nutrition, what he admits are effectively two full-time jobs. He has taken some time out from Decathlon’s December camp – a key period for building riders’ fitness and healthy microbiomes – to talk to me about everything from periodising fibre, high-carbohydrate diets, the influence of ultra-processed foods (UPFs) and what comes out at the end of it all. Literally.

An inflammatory topic

The Decathlon riders do their stool samples twice a year – once before their major goals, like a Grand Tour, and once at the end of the season. Dr Louis and the team of nutritionists analyse the findings looking for inflammatory markers.

“We have some data regarding the type of marker we need to look at. For example, a big one for us is one of the short chain fatty acids called butyrate, and this is a very good marker of inflammation. If the level of butyrate is too low, it means the body is probably struggling to fight against this inflammation,” Dr Louis explained.

The team’s nutritionists also look for signs of ‘gut permeability’, which is when the intestinal lining that usually controls what passes from the gut into the bloodstream becomes compromised, letting unwanted molecules like toxins, microbes, and undigested food particles leak through, potentially causing further inflammation.

“We would look at indicators of gut permeability – there is a specific protein called Lipopolysaccharide-binding protein (LBP) and we look at this one especially because it shouldn’t be too present in the stools,” Dr Louis continued.

An inflamed or permeable gut can have a wide range of effects on a riders’ health. Symptoms range from gastrointestinal discomfort and diarrhoea, to a compromised ability to ingest carbohydrates, and even developing more resistance to insulin secretion, which could lead to issues with weight management.

Decathlon even use their end of season stool samples to influence decisions about the riders’ off seasons: “If the stool sample presents some inflammation, the rider would perhaps need more time off the bike during the off season in November, like six weeks, perhaps, instead of four weeks or three weeks, to make sure they can go back to a level that is acceptable. We don’t want them to start training hard and being exposed to big loads, creating potentially more inflammation if the body has not fully recovered,” Dr Louis told me.

A powerful performance indicator indeed, but studying the microbiome isn’t without its difficulties. Price (upwards of £500 per stool sample), accuracy, lack of rigorous research and the question of what is a normal microbiome, all present challenges to fully optimising a rider’s microbial potential.

“At this stage, for me, it’s more like an exploratory approach, because there is no straight guideline on the type of profile of microbiota to perform maximally, or vice versa,” he said.

Other teams take different approaches to Decathlon’s. James Moran, a clinical dietitian with extensive experience in sports nutrition, now working at Uno-X Mobility, talked me through the way the Norwegian team has looked at the microbiome.

“I think there’s so much more that we don’t know about the microbiome than what we actually do. We are constantly monitoring the literature, and speaking with experts in different companies, but always through a lens of: ‘So what? What are these diagnostic tests or monitoring? Does it actually affect performance? Do we trust the data?’ At the moment, I would say it’s probably not robust enough for us to be wasting a lot of time or using a lot of time on going into this level of depth,” said Moran.

That isn’t to say Uno-X doesn’t take the topic seriously, but the way they go about monitoring it is at the completely opposite end – no pun intended – to Decathlon’s.

“We do some oral microbiome testing, which is part of a bigger project we’re doing with Maurten, looking at oral health and how it changes and how it correlates with gut health, wider health and performance.”

Moran explained that the project is a long term approach, which they are now starting to get the results on, which will help put into practice corresponding measures.

“The thing we’re interested in is specifically oral hygiene, and how that translates to the health of the oral microbiome, specifically around the consumption of carbohydrate drinks and carbohydrate gels. The volume of carbohydrates the riders consume has a big impact on their oral microbiome.”

Periodisation and preservatives

Professional cycling lives at the extremes, with excessive training loads and extraordinary energy demands; inevitably the nutrition follows suit. The modern peloton runs on carbohydrates, often delivered in forms that sit firmly within the ultra-processed food (UPF) category – gels, drinks, bars engineered for speed, shelf life and palatability. But even the stuff that isn’t designed to be eaten on the bike – white rice, pasta and the like – is far from the wholefood approach and recommended fibre intakes the rest of us should adhere to.

“I must say, we are quite far from what would be considered a healthy diet, because they are elite athletes, and they have a diet that is adapted to their needs. It’s an extreme sport with extreme energy expenditure. In order to reach energy balance, it’s an extreme behaviour in the way they eat,” admitted Dr Louis.

But living at the extremes is only possible for short periods of time: “As soon as the load goes down, we try to go back to a more healthy diet to make sure we keep the riders in good health, because this is the number one priority. If they are not healthy, they won’t be able to compete.”

Moran and Uno-X take the same approach. At elite level, performance dictates digestion. When training load and racing intensity rise, tolerance to fibre drops sharply. Simple carbohydrates dominate because they are reliable under stress.

“You actually need some of these ultra-processed foods in order to get the amount of carbohydrates and calories into the rider,” he says. “It’s a double-edged sword.”

At Decathlon and Uno-X, off the bike, riders eat conventionally. The UPFs are largely confined to the moments where digestion must not interfere with performance.

“If they train for four hours, there’s still 20 hours in the day where they’re not on the bike,” said Moran. “We try to encourage eating normal foods as much as possible. It’s not like there’s lots of ultra-processed food off the bike. It’s more the volume of energy bars and gels that they need to consume on the bike.”

The problem is that UPFs don’t just deliver carbohydrates. They also introduce additives, something which Dr Louis is concerned about. He told me he would like nutrition brands to avoid adding UPFs. The increased consumption of UPFs in the western world has changed eating behaviour; the additives have made food hyper-palatable with sugar, salt, and fat, hijacking our brains’ dopamine reward system, while their engineered texture and low fibre lead to faster eating and lower satiety levels, overriding natural hunger signals and promoting overconsumption.

For riders burning thousands of calories per day, overeating may seem irrelevant. But Dr Louis says the gut still bears the cost: “All these additives that are added to preserve them, to make them nicer, better texture, better flavour – these could have some impacts on the microbiome.”

Sorbitol is one example of a UPF that can be found in sports products, and Dr Louis explained it can alter a rider’s gut balance: “Sorbitol is used because it helps retain moisture, but if the dose is too high, it could accelerate transit time. So we don’t want the riders to go to the toilet too often. It’s actually quite common that riders would struggle to find the brand that works for them – not because they’re lactose intolerant, but very likely because of additives.”

The gut instinct

Despite different approaches, both practitioners see seasonal patterns. Dr Louis noted that gut health markers are typically worst at the end of the season: “When we do it in October, it’s the worst values we get. There must be an impact of the high amount of carbohydrates and not enough fibre.”

Grand Tours, with their extreme demands on the body and gut, exacerbate the issue. Riders can suffer from constipation after a number of days avoiding fibre and after the three-week races, its reintroduction isn’t always smooth.

“They go back home, go back to whatever they want to eat,” Dr Louis said. “There is a transition. It must be a bit difficult to readjust.”

Moran said at Uno-X, they deliberately avoid over-structuring the post-Grand Tour diets.

“They’ve been in a very pressurised, prescriptive environment. For that week after the Tour, we don’t give them too much structure.”

But where does this leave the rest of us? Those who aren’t planning on pushing our bodies to the absolute extremes, but would still like to see improvements in both health and performance. Both nutritionists agree that fibre is essential — just not always if you’re still looking to perform your best.

“For general health, at least 30g per day,” Dr Louis advises. “That would be a good target.”

For the pros on their race days and for us when we have a target event, that figure can drop dramatically: “When we want a low-residue diet, we go below 10 grams. It’s not sustainable, but if it’s well periodised, it’s recommended.”

Moran is wary of prioritising microbiome optimisation over practical fuelling: “If you focus on the microbiome rather than what the athlete is actually consuming, you’re more likely to get negative symptoms.”

Professional cycling will never be a template for perfect health. It is too extreme for that. But it does offer a lesson. Performance can tolerate a health compromise, but biology cannot ignore it forever.

From what we can see from the current data, the microbiome remains stubbornly human. It resists neat targets and simple optimisation. You can’t train it like VO2 max, or bolt it on like a faster wheel-set. It reacts, adapts and sometimes rebels.

In a sport that has always believed there must be something left to measure, the gut may be the final frontier, not because it offers easy gains, but because it exposes the limits of control. The microbes that help a rider absorb fuel today may inflame their body tomorrow.

The same diet that wins a race may quietly undermine a season.

Cycling has spent more than a century trying to turn bodies into machines. Watts replaced feel. Blood lactates replaced instinct. Now, teams peer into riders’ mouths and what they flush away, searching for answers in what was once discarded or ignored.

But the deeper they look, the clearer one truth becomes: performance isn’t just engineered – it’s negotiated, between stress and recovery, between fuel and damage, and between speed and health. The stopwatch still decides races. But increasingly, the battle begins somewhere far less glamorous – deep in the gut, where trillions of unseen organisms remind one of the most measured sports in the world that it is still, after all, human.