A no-carb, no-sugar diet plan is one that prohibits the consumption of all carbohydrates, including sugar. A more common diet allows some carbohydrates each day, but the amount is drastically less than what the average person consumes.
The human body turns carbohydrates into glucose in the bloodstream, which is the human body's main source of energy. When the body is deprived of sufficient carbohydrates to fuel the body, it goes into a state of ketosis where fat reserves are broken down and converted into the needed energy. A diet that prohibits the consumption of carbohydrates is trying to force the system into that state of ketosis to quickly burn off fat.
A more popular variation of a no-carb diet is a low-carb diet, which allows the consumption of some complex carbohydrates, such as whole grains, especially those that are high in fiber. The 2010 Dietary Guidelines for Americans recommend an optimal carbohydrate intake of 130 grams per day, but many low-carb diets limit the amount to 50 grams or less. Many of these diets also recommend a focus on fat intake along with a moderate intake of protein to ensure that the body has what it needs to continue making energy.