Protein powders, especially the whey variety, have long been glorified in fitness circles for their muscle-building benefits. “Pump some iron and chug back a protein shake” is a preferred muscle-making formula. But research published in the journal Nature Metabolism, led by scientists from the University of Sydney, suggests that while the essential branched-chain amino acids (BCAAs) leucine, isoleucine and valine—found in high amounts in certain protein powders—can indeed help muscles recover and grow in response to training, excessive consumption of BCAAs may have unwanted side effects if taken at the expense of other amino acids from additional protein sources.

For instance, supplementing with high doses of BCAAs leads to high levels in the blood, potentially triggering competition with another amino acid, tryptophan, for transport to the brain. This can negatively affect mood, as tryptophan is a precursor for the feel-good hormone serotonin. Lower brain levels of serotonin can also stimulate appetite, leading to possible excess consumption of calories and, in the end, weight gain.

The take-home message is that it’s fine to include a scoop of whey protein in your daily diet to support training needs, but it’s best to eat a wide range of protein foods—including those from plant sources like legumes—to get a better balance of amino acids.