The gastrointestinal tract digests and ferments a large variety of carbohydrates, mainly different types of starches, sugars, polyols, and dietary fiber. The tract has evolved and is regulated to maximize carbohydrates as a source of fuel; however, the rate and extent of digestion and fermentation depend more on food and carbohydrate structures, with health implications. The absence of evolutionary adaptation and mutations in adapted genes has left some individuals without adequate gastrointestinal function and manifests in the maldigestion of starches and sugars and the malabsorption of glucose and galactose, which markedly reduce energy availability from foods to the point of failure to thrive.