To keep your running performance and recovery on track, you might be diligent about getting enough protein, reaching that recommended 0.4 grams of the macronutrient per pound of body weight. Or maybe you consume an even higher amount, like 0.5 to 0.9 grams of protein per pound body weight, as suggested by the American College of Sports Medicine for maintaining muscle mass during training.
But, according to a study in the journal mSystems, consuming too much protein could come with added risks.
In the study, researchers assessed the stability of gut microbes in trained endurance athletes, and how microbial composition may change with protein and carbohydrate consumption.
Albeit small (the study looked at just 16 runners), the results were informative. Researchers analyzed the participants' athletic performance and gut bacteria diversity (via stool samples) for seven days while on a high-protein diet, which was made up of 40% protein, 30% carbs, and 30% fat. This was compared to a group on high-carb diet of 10% protein, 60% carbohydrate, and 30% fat.
Researchers found that when protein was the biggest component, it resulted in a disturbance in the stability of the gut microbiome-with less bacterial diversity-and a notable reduction of about 23% in running performance, as measured by a time trial to exhaustion. With the high-carb diet, however, there was an improvement in performance of about 6%, as well as more stability in the gut microbiome.
These results suggest that there's a strong connection between your gut bacteria and athletic performance, study co-author Justin Roberts, Ph.D., associate professor in health and exercise nutrition at Anglia Ruskin University in the U.K. told Runner's World.
"We can't be certain that the high amount of protein in the body was entirely responsible for the significant drop in performance," he said. "But it was found that there were certainly changes to the microbiome following a short-term, high-protein diet, which appeared to be associated with performance."
One drawback to the recent study is its very small participant size, but previous, larger-scale research has also indicated that high amounts of protein could be a gut changer, according to Amanda Kostro Miller, R.D., registered dietitian and medical reviewer at Botanical Institute.
For example, a research review published in Frontiers in Physiology in 2017 points out that different protein sources affect gut microbiota. Another meta-analysis published in February 2022 says protein can negatively and positively affect the microbiome, depending on sources, content, and diet as a whole, among other factors. (Keep in mind the 29 studies involved in the meta-analysis were not conducted on humans, but rather on animals and in vitro.)
The negative affects of high amounts of protein are particularly potent when accompanied by low carbohydrate consumption, Miller told Runner's World.
"High-protein, low-carb diets can have a detrimental effect on gut health by negatively changing the microbiota of the gut," she said. "Following that kind of diet can reduce the production of short-chain fatty acids (SCFAs) within the gut." Research published in Frontiers in Microbiology notes that SCFAs produced by gut microbiota are recognized as potential regulators of skeletal muscle metabolism, which plays a key role in exercise.
That's a big deal, and not just for athletic performance, she added. Produced when you eat foods rich in fiber and resistant starch-such as bananas, cooked and cooled rice, beans, avocados, apples, and oats-SCFAs may influence the gut-brain connection, and also play an important role in maintaining the integrity of the intestinal barrier of the gut.
These SCFAs may also reduce inflammation, regulate the immune response, and can lower risk of developing some cancers, particularly colon cancer. In fact, one particular type of SCFA called butyrate has been shown in studies to potentially inhibit tumor growth in a variety of colon cancer cell types. Butyrate may also help with inflammatory conditions, like inflammatory bowel disease, as other research suggests.
"Because of this, following a diet with adequate carb intake is essential for optimal health within the gut, and for your overall health and athletic performance," said Miller.
The bottom line here: While there is more research needed on the effects of gut bacteria on your health and athletic performance, studies do point to the importance of carbohydrates in creating a healthy microbiome which can play out in your well-being in various ways.
In the recent study, researchers noted that it wasn't just choosing a high-protein or high-carb diet that could have an effect on endurance-it was also switching between the two, or making a sudden shift between a high-carb and low-carb diet, in a strategy known as periodization. This is frequently employed during athletic training, said Roberts.
"Athletes undertaking dietary periodization should be aware of potential negative impacts of drastic changes to dietary composition on gut microbial status and, in turn, endurance performance," he added. Those with rock-solid gut stability are less likely to see these effects, but those with reduced gut health could see unexpected outcomes, like slower running performance and even some digestive upset.
Keep in mind that endurance exercise on its own has been shown to improve gut function, Roberts said, so periodization may be a cinch for you. But if you're seeing unwelcome changes-especially fatigue, lack of motivation, mood changes, and bloating, which are all indicative of less-than-ideal gut health-it may be better to shift eating habits much more gradually.
Rather than send your protein consumption zooming upward in a quest to build muscle mass faster, he advises the approach you'd take with a new training program: Start adding in more protein gradually, progress over time, and pay attention to what your body is telling you.