http://www.atkins-diet.cc/Article46.phtml
"The MIT research looked at serotonin levels in the brains of 100 volunteers who ate different diets, either with a lot of meat and other high-protein foods, or with more carbohydrates, found in breads and cereals.
They found that the brain only made serotonin after a person ate sweet or starchy carbohydrates.
But they said the ?kicker? was that the carbohydrates needed to be eaten in combination with very little or no protein."