What Training Does to a Fighter's Gut and Minerals

Fight nutrition talk usually starts and ends with the scale. The research on combat-sport nutrition is thin and the studies are small, but two of them point at the same overlooked idea: hard training quietly changes your internal chemistry, and the gut and the minerals in your blood are two places it shows up.

Neither study below is a blockbuster. Both are worth knowing anyway, as long as you hold them at the right confidence level.

The gut study

A 2023 trial in Frontiers in Nutrition took 23 male MMA athletes and split them in two for four weeks. Both groups took vitamin D3. One group also took a multi-strain probiotic.

The probiotic group improved their time to exhaustion on a graded cycling test, from about 496 to 559 seconds, a statistically significant jump (p = 0.023). The vitamin-D-only group didn't budge. The probiotic group also cut a marker of gut inflammation (calprotectin) roughly in half and shifted their gut bacteria toward more beneficial strains. Maximal oxygen uptake didn't change in either group, so this wasn't a bigger engine. The authors suggest it was a better-functioning gut supporting endurance.

The same team reported the anaerobic half of the trial, on the same 23 athletes, in a companion paper, and it pointed the same way: the probiotic plus vitamin D group had lower lactate 60 minutes after an all-out sprint (4.73 versus 5.88 mmol/L in the vitamin-D-only group) and improved mean power and total work. Endurance and anaerobic markers both moved in the supplemented group, which is at least internally consistent for one small trial split across two papers.

One thing the headline tends to garble: because both groups took vitamin D, the improvement tracks the probiotic, not the vitamin. And the caveats are real. It's 23 people, men only, four weeks, the product is a named commercial mix, and an author had a tie to a probiotic company. None of that makes it wrong, but it is a lead to watch, not a supplement to rush out and buy.

The mineral study

The second study, from 2017, followed 21 elite Muay Thai athletes (15 men, 6 women) through a 10-day high-intensity camp and a competition, measuring trace minerals in their blood at each stage.

Things moved. Zinc inside the red blood cells dropped in both sexes. Iron dropped in the men after competition. Manganese in the plasma fell after the bout. Other markers moved in the other direction, so this is not a simple "training depletes everything" story. Possible contributors are the usual suspects of hard training in a weight-class sport: heavy sweating, the tissue stress of repeated impact, and the calorie restriction that comes with making weight.

This is a small, exploratory study in an obscure journal, so don't over-read any single number. But it lines up with something well established outside combat sports: athletes who sweat hard and restrict food, which is to say most fighters cutting weight, have reason to watch iron and zinc status. And low iron is one of the most common, most fixable causes of "my gas tank is empty and I don't know why."

What to actually do

Put the two together and the message is narrower than "buy this probiotic" or "megadose that mineral":

• Don't build your nutrition around one study. Both of these are small. Neither earns a hard rule.
• If you cut weight or train at high volume, take iron and zinc seriously. The scale is not the only number worth checking. Get tested if your endurance has cratered for no clear reason, rather than guessing.
• Treat gut health as part of recovery, not a wellness fad. The probiotic signals here, whatever their limits, fit a growing picture that what happens in your gut can affect what happens in the last round.

The research

• Przewłócka K, Folwarski M, Kaczmarczyk M, et al. Combined probiotics with vitamin D3 supplementation improved aerobic performance and gut microbiome composition in mixed martial arts athletes. Frontiers in Nutrition, 2023;10:1256226. doi:10.3389/fnut.2023.1256226
• Przewłócka K, Kujach S, Sawicki P, et al. Effects of probiotics and vitamin D3 supplementation on sports performance markers in male mixed martial arts athletes: a randomized trial. Sports Medicine - Open, 2023;9:31. doi:10.1186/s40798-023-00576-6
• Ugras A. Effect of high intensity interval training on Muay Thai athletes' mineral levels. The Anthropologist, 2017;27(1–3):125–133. doi:10.1080/09720073.2017.1311678