Scientists have known that sugar is bad for the brain since 1927, when a biochemist named Herbert Crabtree discovered that elevated glucose levels lower mitochondrial function... sugar has been shown to decrease the number of dopamine receptors in our brains... All forms of sugar are bad for your brain, but fructose — found in fruit, high-fructose corn syrup, and agave nectar — is the worst. Fructose creates oxidative stress and feeds the bad bacteria in your gut, leading to even more inflammation. Fructose is implicated in damaging mitochondria in skeletal muscle cells, harming the mitochondrial membrane, and impairing cellular respiration and energy metabolism. While your brain won’t suffer too much in you eat moderate amounts of whole, seasonal fruits, you should avoid consuming excessive amounts of fructose and completely stay away from fruit juice and foods that contain high-fructose corn syrup and agave nectar. Try to limit your intake to about 20 grams of fructose a day.