Chokeberry capsules for national-team rowers: the gut and stress markers came out ahead, mostly because the placebo group's went the wrong way

Hard endurance exercise is rough on the gut. Blood is pulled away from the intestine to the working muscles, and the gut lining can come under strain. Sports scientists call it exercise-induced gastrointestinal syndrome, and they have been looking for foods that might soften it. Berries rich in anthocyanins, the pigments that make them purple-black, are one candidate. Black chokeberry, Aronia melanocarpa, is a rich source. A team from the Poznan University of Physical Education tested it on members of Poland's national rowing squad, and the results appeared in Nutrients on 29 October.

The design was a proper one in miniature. Eighteen young men, about 20 years old and 190 cm tall on average, were randomly given either chokeberry extract or a placebo in identical opaque capsules: three a day with meals, 600 mg of extract standardised to 18% anthocyanins, for eight weeks of a training camp in spring 2023. Neither the rowers nor the researchers knew who had what. Three dropped out because they were away at championships, which left 7 on chokeberry and 8 on placebo. Before and after the eight weeks each man rowed 2,000 m flat out on an ergometer, a test that also counted towards selection for the championship crew, and gave blood before, straight after and an hour later.

The blood was tested for three markers tied to the gut lining: claudin-3, a protein from the seals between gut cells; I-FABP, released when those cells are damaged; and LBP, which rises with exposure to bacterial toxins. Cortisol was measured as the stress hormone. After eight weeks the chokeberry group had lower resting claudin-3 (p = 0.037) and resting LBP (p = 0.013) than the placebo group, and lower cortisol an hour after the test (p = 0.008). I-FABP showed no effect of the supplement. Rowing performance did not change: both groups covered 2,000 m in about 6 minutes 13 seconds before and after.

Read the details and the picture shifts. The changes the paper reports within a group are almost all in the placebo group, whose resting LBP and resting I-FABP rose over the eight weeks, as did its cortisol an hour after the test. The chokeberry group did not show a significant improvement of its own. So the gap looks more like the berry group being spared a rise than getting better, and the authors admit they cannot rule out that it was something else. The placebo group already had higher LBP straight after the first test, before any capsules. The two groups also ate differently: the chokeberry group ate more protein in the day before testing (199 g vs 167 g after the eight weeks), and more fat but much less fibre (29 g vs 48 g). In an exploratory analysis, more protein went with lower claudin-3. And the week of training before the second test differed from the week before the first, which can move these markers by itself.

The authors spell out the rest. With 15 men the study could only detect large effects, so big effect sizes from a handful of people are fragile. Many markers at many time points mean some significant results will turn up by chance. None of the three gut markers measures leakiness directly, and no one recorded gut symptoms. The trial was registered after it ran, and it says nothing about women, other sports or recreational exercisers. Their own conclusion is that the findings "should not be interpreted as evidence of improved intestinal barrier function". In our reading this is a reason to run a bigger trial, not to buy chokeberry capsules for a stomach that misbehaves in races. Chokeberries themselves, fresh, frozen or as juice, are a perfectly good purple fruit to eat.