Short answers, pulled from the story.
A carbohydrate is a sugar or sugar derivative built only from carbon, hydrogen, and oxygen. In the simplest carbohydrates these atoms appear in a 1:2:1 ratio, and the molecules count among the major families of biomolecules alongside amino acids, fats, and nucleic acids.
The German chemist Carl Schmidt first proposed the term carbohydrate in 1844. Earlier, William Prout had given the same group of substances the name saccharine after analyses by Joseph Louis Gay-Lussac and Thenard.
Carbohydrates are classified by degree of polymerization into sugars, oligosaccharides, and polysaccharides. Sugars include monosaccharides such as glucose, galactose, fructose, and xylose, and disaccharides such as sucrose, lactose, and maltose, while polysaccharides exceed nine units and include starch, glycogen, and cellulose.
Carbohydrates store energy as starch and glycogen and form structures such as cellulose in plants and chitin in arthropods and fungi. The 5-carbon sugar ribose builds the backbone of RNA and coenzymes like ATP, while deoxyribose forms part of DNA, and saccharides also function in immunity, fertilization, blood clotting, and development.
Simple sugars yield 3.87 kilocalories of energy per gram, and complex carbohydrates in most foods yield 3.57 to 4.12 kilocalories per gram. The oxidation of one gram of carbohydrate yields roughly 16 kilojoules, about 4 kilocalories.
The glycemic index measures how much a carbohydrate raises blood glucose compared with pure glucose, which is set at 100. Foods are grouped as high-GI above 70, moderate-GI from 56 to 69, and low-GI below 55.
The ketogenic diet is established as a medical diet for treating epilepsy, but as a weight-loss fad it carries risks such as low energy, insomnia, nausea, and gastrointestinal discomfort. Low-carbohydrate diets show no long-term metabolic advantage, and the British Dietetic Association named the ketogenic diet one of the top 5 worst celeb diets to avoid in 2018.