Glutamine is the most common loose amino acid in your body, and you make plenty on your own. Two mechanisms look real on paper: the cells lining your gut burn it for fuel ¹, and your white blood cells use it while they work. After a marathon, blood glutamine drops about 20% ², and the industry built a category on that dip.

In healthy lifters, the story falls apart. Six weeks of resistance training with glutamine or placebo gave the same strength and lean mass ³; in a post-workout shake, it adds nothing above the protein alone ⁴. The round-up of every athlete trial found nothing on performance, body composition, or immunity ⁵.

The immune claim didn't hold up. Topping the blood back up after that marathon dip changes nothing, and the most-cited review calls the hypothesis unsupported by trials ⁶. Salivary antibodies after hard exercise don't budge either ⁷.

Where it works, the populations are narrow. A 230-person trial cut sickle cell pain crises by about a quarter, earning FDA approval as Endari ⁸. And in 106 people whose diarrhoea started after a stomach infection and who tested positive for a leaky gut barrier, eight weeks dropped symptoms in four of five, against one in twenty on placebo ⁹.

Screen yourself before you buy. Work out which bucket you're in.

What you gain by stopping. Week one: about $80 a year stays in your account and the morning loses a step that did nothing. Month three: the next podcast guest who pairs "leaky gut" with "tight junctions" gets caught, because you know that trial was 106 people with a specific diagnosis. And the readers in the two indicated groups stop walking past their own medicine.

For recovery, spend on protein near 1.6 g/kg and creatine. For general IBS, the low-FODMAP diet. For immunity around hard training, sleep is the dominant lever.