Ectomorph: Naturally thin with a fast metabolism, long limbs, and difficulty gaining weight or muscle. Mesomorph: Naturally muscular and athletic with a medium frame, gains and loses weight relatively easily. Endomorph: Naturally higher body fat, wider frame, slower metabolism, tends to gain weight more easily.
Your somatotype is largely determined by genetics and cannot be fundamentally changed. However, with consistent training and proper nutrition, you can significantly alter your body composition — building more muscle or reducing fat — regardless of your natural body type. Many people are a blend of two types (e.g., Ecto-Meso).
The somatotype theory, originally developed by William Sheldon in the 1940s, is debated in modern scientific literature. Most people fall on a spectrum between types rather than fitting neatly into one category. Nonetheless, it remains a useful practical framework for personalizing fitness and nutrition approaches.