Brain development is connected to proteins and fats, not sugars.
Eat enough sugar, more than your body uses, and you will get fat, because any unused calories get stored as fat.
Indians also have very high rates of Type II Diabetes.
The Chinese do eat protein. They're not vegetarians, as are many Indians.
There's an excellent movie about the effect on an infant of parents feeding it a strictly vegan diet. The cranium doesn't grow, and therefore neither does the brain. It's called "Hungry Hearts". It's heartbreaking and beautiful, and I really recommend it.