The building blocks of carbs are simple sugars, or monosaccharides. Your body only absorbs carbs once they have been broken down into monosaccharides. Carbs can be chains of one, two, a few or many sugars. These are called mono-, di-, oligo- and polysaccharides respectively. Polysaccharides can be either straight or branched chains of sugars. You might be familiar with glucose and fructose, which are monosaccharides, and sucrose, a disaccharide of fructose and glucose. Specialized enzymes break polysaccharides apart.
Protein consists of building blocks called amino acids. Unlike carbs, proteins are only made of straight, not branched, chains of amino acids. Enzymes called proteases break protein down into amino acids. Only nine amino acids are considered essential for human health and must be derived from food; your body can synthesize all other amino acids.
Fats are normally found in foods as triglycerines, a short backbone with three fatty acid tails. Hormones called lipases cleave the fatty acid tails off the triglyceride, allowing your body to absorb the fatty acids and the glycerol backbone. Because fats are not soluble in water, large amounts of fats in the small intestine will slow the digestion of other nutrients.
Vitamins and Minerals
Vitamins and minerals are not broken down by enzymes in your body. However, vitamins and minerals can be trapped in the structure of food, and cannot be absorbed until the food is broken down. Once food is broken down, and most of the other nutrients have been absorbed, your small intestine draws vitamins and minerals into the blood.
Fiber consists of materials, mainly carbs, that your body cannot break into individual pieces, as your body lacks the specific enzymes. Fiber helps carry undigested nutrients and waste through the small intestine and into the large intestine, to be disposed of as solid waste.
- "Molecular Biology of the Cell (fourth edition)"; Alberts et al; 2002
- "Pharmacological Reviews"; "Is the Role of the Small Intestine in First-Pass Metabolism Overemphasized?"; Jiunn Lin et al; June 1, 1999
By Daniel Smith
A research scientist since 2003, Daniel Smith has been writing health and fitness articles since 2010. Smith specializes in neuroscience and behavior with a broad expertise in biology and medicine. He holds a Master of Science in neuroscience from Harvard University and a Bachelor of Science in biological sciences from Carnegie Mellon.