They'll certainly help bulk you out, but they could cause health problems in middle-age and could even reduce your lifespan, researchers are warning.
The shakes are rich in one type of amino acid—the branch-chain amino acids (BCAAs)—and we need more, and different, amino acids if we're going to have a healthy diet, researchers from the University of Sydney have found.
The BCAAs block another amino acid, tryptophan, which helps in the production of serotonin, the 'happiness chemical' that also helps give us a good night's sleep. But serotonin has another role; it also regulates our appetite, and without it, we tend to keep on eating.
The researchers discovered the knock-on effects of the BCAAs when they tested them on mice. Their serotonin levels dropped, and they ate more, and quickly put on weight.
BCAAs are essential amino acids—and they're found in high-protein foods such as red meat, dairy, chicken, fish and eggs—but they need to be balanced with tryptophan, which is in seeds and nuts, soy beans and chicken, the researchers say.
The BCAAs in protein shakes usually contain whey protein, which is sourced from dairy by-products.