So they say but not necessarily for all people. They discovered almost 30 years ago that low tryptophan diets duplicated the effects of caloric restriction (in lab rats). It's not so difficult to lower tryptophan in a vegetarian diet, for instance, if your primary protein source is yogurt.. This works because tryptophan and phenylalanine compete to cross the blood-brain barrier and you can easily identify which foods have the highest ratio of phe to try. I made it part of my life extension program decades ago, and heh, it works for me. The group of Bulgarian centenarians who have eaten large amounts of yogurt their whole lives supports it too.