Just because the human body is capable of digesting/assimilating Sugar doesn't mean it has to, the body is very versatile. The body had to be in order to endure what it has through out history. The body can live in a state known as ketosis, burning mainly fats and some protein for fuel. The evidence of this are the Arctic inuit who eat only raw animal protein for most of the year and thrive on it. There is no better scientific method than having actual evidence such as the inuit to prove this point. While taste buds imply what people should eat, people actually thriving on certain diets proves otherwise.
There is no specific diet that people are meant to eat. As is shown by the wide varieties of diets of people from around the world. What a person can easily digest and assimilate is the best diet for that person, and that is closely linked to their ancestry.