"Supplementation of BCAAs resulted in high levels of BCAAs in the blood, which competed with tryptophan for transport into the brain," observes co-lead author Prof. Stephen Simpson. "Tryptophan," he explains, "is the sole precursor for the hormone serotonin, which is often called the 'happiness chemical' for its mood-enhancing effects and its role in promoting sleep. But, serotonin does more than this, and therein lay the problem." <br. The researchers found that the competition between BCAAs and tryptophan in the blood led to lower-than-normal serotonin levels in the brain, which had unwanted consequences. "This then lowered serotonin levels in the brain, which in turn was a potent signal to increase appetite," says Prof. Simpson, adding, "The serotonin decrease caused by excess BCAA intake led to massive overeating in our mice, which became hugely obese and lived shorter lives."