The effects can be profound – headaches, dizziness, cravings, anxiety, sadness, general crankiness, and changes in bodily functions – but they usually disappear within a week or two.
Processed food is packed with multiple forms of sodium, which causes the body to retain fluids. Go cold-turkey on all that salt, and you’ll urinate profusely and feel (literally) drained. When you start eating fruits, vegetables, and whole-grain carbohydrates, all that fiber will affect your digestive system in unpleasant ways. Also, I don’t know if there’s any science behind this, but I’ve read that fat and sugar cause chemical effects in the brain that suddenly stop when such foods are eliminated.
As soon as a person adjusts, however, he/she will likely feel wonderful and energized.