Sugar is a pure substance because its composition is homogeneous throughout. Pure substance can be either elements or compounds. Sugar or sucrose is a compound with the chemical formula C12H22O11.
Table Salt (Sodium chloride) is a pure substance. In chemistry, the term "salt" is a reference to a variety of compounds formed in the reaction of a base and an acid. Compounds are considered to be pure substances.
When dieting and omitting sugar and salts from your diet you have to remember that sugar and salt are substances and your body crave them just like an addict body will crave it's substance of choice like cocaine or heroin. You'll experience side effects from eliminating the substance.