How We Digest Our Food

Food not only tastes good, but provides our bodies with an energy source to function. In order to be converted to energy, the meals we eat must first be broken down into smaller particles – macronutrient molecules consisting of monosaccharides from carbohydrates, amino acids from proteins, and fatty acids from fats. This happens through the digestion system. These broken down molecules are then taken into the cells; this is absorption.

The interior of the entire gastrointestinal tract is lined with mucosa cells, which ultimately absorb the digested nutrients [1]. The mucosal lining additionally secretes mucus, which keeps the mucosal cells from damage while adding lubrication to help the food travel down [1].

Mouth

Food first enters the body through the mouth, where the teeth grind up the food and mix it with saliva to liquefy it [2]. The teeth are critical in expanding the surface area of chewed food in order to increase the ability of enzymes to act upon the food and digest it more efficiently, as well as breaking up fibrous particles to release nutrients so they can be absorbed [1, 2]. Tasting food through the tongue’s taste buds causes the salivary glands to release saliva, which is comprised of both water that helps the ground-up food to be swallowed and move along the digestive tract, and salivary amylase and lipase, enzymes that  begin breaking down carbohydrates and fats respectively into smaller units [1, 2]. The tongue additionally blends food with saliva [1]. Once formed, the food is swallowed and passes through the pharynx, a funnel that directs food and liquid into the esophagus and away from the airway [1].

Once in the esophagus, the food is moved deeper into the digestive tract through muscular contractions [1]. The esophageal sphincter, at the end of the esophagus, controls the flow of food into the stomach while preventing the stomach acid and food from leaving the stomach and returning to the mouth [2]. If you have acid reflux, you might have heard your doctor talking about this sphincter not closing properly.

Stomach

Once the food travels into the stomach through the esophageal sphincter, the real work begins: The stomach muscles mix and mash the food with hydrochloric acid and protein-digesting enzymes (pepsinogen) to break down the food into smaller pieces [1]. Some water, alcohol, certain pharmaceuticals, and some mineral salts are absorbed by the stomach into the body [1, 2].

Small intestine

While the stomach is responsible for most of the body’s protein digestion, the small intestine has a larger job. It is responsible for fat and carbohydrate digestion plus absorption of all protein, fat, and carbohydrates. Mucosal cells line the small intestine to release enzymes that aid in digestion and absorb digested nutrients and release mucus to block toxins from leaving the digestive tract and absorbing into the cells [1, 2].

When food starts entering into the small intestine, the pancreas is alerted to release bicarbonate. Yes, like baking soda! This neutralizes the stomach acids; fat and carbohydrate enzymes (lipases, proteases) can’t work in acidic environments [1, 2]. The liver is also signaled to tell the gallbladder to release bile into the small intestine [1]. Bile emulsifies fats to aid in their digestion and absorption [2]. Bile additionally provides bicarbonate to further neutralize the environment [2]. Once the foods are broken down into tiny molecules, digestion is accomplished, and the simple sugars (monosaccharides), proteins (amino acids), and fats (fatty acids) can now be absorbed by the cells, as can be cholesterol and unused bile salts [1, 2].

Large intestine

Now that the carbohydrates, fats, and proteins plus cholesterol have been dealt with, what’s left? Water, vitamins, minerals including electrolytes, and fiber. The large intestine reabsorbs large quantities of water; absorbs electrolytes and other minerals and vitamins; hosts vitamin-producing gut bacteria; and, from the digestion of indigestible fiber by these bacteria, also absorb resultant short-chain fatty acids and volatile fatty acids [2]. The large intestine is additionally tasked with concentrating and forming stool, and then helps push it out of the body [1, 2].

Amazing what happens when we put food into our mouths, huh?

References
[1] Smolin, L.A. & Grosvenor, M.B. (2012). Nutrition: Science and applications (3rd ed). Hoboken, NJ: Wiley.

[2] Lipski, E. (2012) Digestive wellness: Strengthen the immune system and prevent disease through healthy digestion (4th ed.). New York: McGraw Hill Education.