The body can handle, as in metabolize, six teaspoons of added sugar per day. The problem is that people consume a lot more than that. Most Americans devour over three times what they should be, with teens and men munching on the largest amounts. On average, people in the U.S. eat about 20 teaspoons of sugar a day, data show. The result is chaos, stress and overload for the body that can lead to both physical and mental illnesses. Cognitive impairment, especially dementia, is a possible result, according to research.