Nutrition Basics: Digestion

Today's topic is on the digestion, absorption, transport, and excretion of nutrients! I will incorporate both science, textbook-like information, and fun facts. For health professionals, this may serve as a brush-up on your past knowledge.

The GI Tract
The primary roles of GI tract are to: extract macronutrients from ingested food and drink, absorb micronutrients, and serve as a protective barrier to microorganisms and foreign material. Depending on the diet consumed, about 92-97% of food/drink is digested and absorbed. Since humans lack the enzymes to hydrolyze the chemical bonds of plant fibers, most of unabsorbed material is of plant origin.  The health of the body depends on the health and function of your GI tract. The GI tract is more susceptible to micronutrient deficiencies, drugs, and protein malnutrition. The nutrients passing through the lumen supply energy to both the small intestine and the colon. With a few days of starvation, the GI tract will begin to break down (decrease in absorption and function).

Digestion Overview
Mouth: limited starch and lipid digestion have taken place.
Stomach: protein digestion has just begun.
Small Intestine: macronutrients, vitamins, minerals, trace elements, and most water is absorbed. Most fat digestion takes place.
Colon/Rectum: remaining fluid and electrolytes are absorbed.
Enzymes: HCL, bile, sodium bicarbonate, lipases, etc.
Hormones: peptides, gastrin, secretin, CCK, GIP, GLP-1, GLP-2, motilin, somatostatin

Interesting Facts About Digestion
-Most of a liquid meal will empty the stomach in 1-2 hours.
-Most of a solid meal will empty the stomach in 2-3 hours.
-When eaten alone, carbohydrates leave the stomach the most rapidly, followed by protein, fat, and fibrous food.
-The small intestine is the primary site for digestion of food and nutrients.

Nutrient Absorption
-The small intestine is the primary organ of absorption.
-Fun fact: Each day, the small intestine absorbs 200-300g of monosaccharides, 60-100g of fatty acids, 60-120g amino acids and peptides!
-The large intestine absorbs the remaining of the water and salts.
-Transit time from your mouth to your anus can vary from 18-72 hours (fruits and vegetable rich diet decreases this time).

Fiber
-Valuable for maintaining healthy cells in the colon.
-Prevents excessive intracolonic pressure.
-Prevents constipation.

Carbohydrate Digestion/Absorption
-Glucose ("blood sugar") is transported from the liver to the tissues (some is stored in liver and muscles as glycogen).
-Consuming large amounts of lactose (milk sugar), fructose (fruit sugar), and sugar alcohols can result in them passing unabsorbed in the colon. This can lead to gas and diarrhea.
-Cellulose, pectin, gums, and other fiber forms cannot be digested by humans; thus, they are passed unchanged into the colon.

Just a final note about the processing of food:
-Cooking food at high temperatures for prolonged periods can destroy nutrients (ascorbic acid and folate), but in general, cooked food is more digestible than raw food.
-Cooking food helps to soften dietary fiber. This helps to make the nutrients attached to the fiber more available for digestion.
-During grain refinement for bread and cereal, nutrients, phytochemicals, and fiber are lost. The "enrichment" of the product will replace some nutrients but not the fiber or phytochemicals.





Mahan L, Escott-Stump, S. "Food, Nutrition, and Diet Therapy." Saunders. 2004.