Molecules, absorption and how we found out

Each day the digestive tract handles up to 10 litres of food, drink and juices, yet less than one litre reaches the large intestine.

Big molecules into small ones

A slice of bread, an egg and a pat of butter look nothing alike, but digestion treats them the same way: it cuts large molecules into small building blocks that can cross the wall of the gut. The NIDDK lists three main conversions. Carbohydrates are broken into simple sugars. Proteins are broken into amino acids. Fats are broken into fatty acids and glycerol.

Each food group starts its breakdown in a different place but finishes in the same one. Starch digestion begins in the mouth with salivary amylase and continues in the small intestine with pancreatic amylase. Protein digestion begins in the stomach with pepsin and continues in the small intestine with trypsin and other pancreatic enzymes. Fat digestion barely starts in the stomach with lingual lipase; most of it happens in the small intestine, where bile emulsifies fat and pancreatic lipase splits it. The final steps for carbohydrates and proteins happen right at the brush border, where enzymes are fixed to the membranes of the microvilli.

Two roads out of the gut

Absorbed nutrients take two routes. Simple sugars and amino acids, along with glycerol, some vitamins and salts, pass into the blood capillaries of the villi and travel to the liver through the hepatic portal system. The liver stores, processes and sends them on when needed. Fatty acids and fat-soluble vitamins go into the lacteals instead and travel in the lymph before they join the blood. OpenStax gives some numbers for how complete this is. Almost all protein, 95 to 98 percent, is digested and absorbed in the small intestine, along with about 95 percent of lipids, and the small intestine can absorb simple sugars at an estimated 120 grams per hour.

How we found out

Much of this was worked out through careful experiments over two centuries. You met William Beaumont in the lesson on the stomach. His book of 1833 described 238 experiments on Alexis St. Martin's stomach and settled the question of whether digestion in the stomach is chemical. Later in the century the Russian physiologist Ivan Pavlov studied the nerves that control digestive glands, working with dogs. He received the Nobel Prize in Physiology or Medicine in 1904 for his work on the physiology of digestion.

Even late in the 20th century there were surprises. In 1982, the Nobel committee later wrote, stress and lifestyle were considered the major causes of peptic ulcers. The pathologist Robin Warren in Perth, Australia, had noticed small curved bacteria in the lower part of the stomach in about half of the patients whose biopsies he examined, always close to signs of inflammation. With the young doctor Barry Marshall he studied biopsies from 100 patients; Marshall managed to grow the unknown bacterium, later named Helicobacter pylori, and the two showed that it was present in almost all patients with gastric inflammation or ulcers. The two received the 2005 Nobel Prize in Physiology or Medicine. Their story is a reminder that well-established ideas about the body can still change when someone looks closely.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.