How do humans meet the brain’s glucose demand of 420kcal (105g sugar) per day on a healthy diet? Wouldn't the protein demand in case of gluconeogenesis be tremendous?
Most of your diet is carbohydrates, which are or can be directly converted into sugars, Vegetables, fruits, and grains are all primarily carbohydrates. Unless you are eating a exclusively animal based diet getting glucose for the brain is not difficult.
Contrary to what fad diets claim a healthy diet contains a lot of carbohydrates. What is unhealthy is when your diet contains only refined pure carbohydrates and not the vitamins fiber and other nutrients.