I believe they do. There are many books out there that tell the medical reasons that they do. I believe they do because I have experienced it. Overeating sugar makes me feel tired, drowsy and down. Salmon and tuna makes me feel good. I believe our individual bodies make some of us prone or sensitive to side effects of foods and medicines, both pro and con. I am one of those sensitive people. The Yeast Connection books are very supportive of this theory and it's makes total sense. Check it out and good luck!