To extract nutrients from the foods you eat, they must be small enough for the cells of your small intestine to absorb. The process of digestion reduces your food particles to a size sufficient for absorption to occur. However, some foods you eat contain macronutrients, such as sugars and amino acids, already small enough without requiring digestion. Other nutrients, including vitamins and minerals, exist in foods as small molecules able to be absorbed as is.
The carbohydrates in your diet consist of starches and sugars. During digestion, your body breaks down starch into single glucose molecules as well as maltose, a two-sugar unit made of two glucose sugars. Other food carbohydrates you consume include the disaccharides sucrose, comprising a glucose and a fructose joined together, and lactose, composed of a glucose bound to a galactose. All of these carbohydrates require digestion to single sugar units prior to absorption. However, some of the sugars you eat are already in the form of monosaccharides, or single sugar molecules. These single sugars include glucose, galactose and fructose, and your small intestine absorbs them without needing to digest them first.
Amino acids are also known as the building blocks of proteins. The proteins you consume undergo digestion to release individual amino acids, which your small intestine then absorbs for transport to the cells of your body. Your cells combine these amino acids in a specific order to create new proteins as you need them. Because amino acids are so tightly bound within the food protein molecule, you consume few free amino acids. You can, however, supplement with free amino acids, and these molecules are ready for absorption by the cells of your small intestine without the process of prior digestion.
Vitamins are organic, or carbon-containing, molecules your body requires in small amounts for optimal health. Although they naturally exist in association with various foods and may be carried through your gastrointestinal system as your foods undergo digestion, they are small enough for your small intestine to absorb without digestion. Vitamins are classified as either water-soluble or fat-soluble, depending on their ability to mix with water. Special proteins in the cells of your small intestine bind to water-soluble vitamins for absorption, while fat-soluble vitamins absorb into your intestinal cells along with digested fat molecules.
Minerals are essential elements you need in your diet to fulfill roles including structure, cell signaling, fluid balance, nutrient metabolism, nerve function and muscle contraction. As with vitamins, minerals are bound with foods and pass through your intestinal tract during digestion. However, they do not experience digestion but rather undergo absorption directly into the cells lining your intestines.