target="_blank" rel="nofollow" href="#fb3_img_img_a5e026c0-032f-5432-a8a3-85696b8e6aa8.png" alt="Technical stuff"/> The lining of the small intestine is a series of folds covered with projections that have been described as “fingerlike” or “small nipples.” The scientific name for these small structures is villus (singular) and villi (plural). Each villus is covered with smaller projections called microvilli, and every villus and microvillus is programmed to accept a specific nutrient — and no other.
Nutrients are absorbed not in their order of arrival in the intestine but according to how fast they’re broken down into their basic parts, as follows:
Carbohydrates — which separate quickly into single sugar units — are absorbed first.
Proteins (as amino acids) go next.
Fats — which take the longest to break apart into their constituent fatty acids — are last. That’s why a high-fat meal keeps you feeling fuller longer than a meal such as chow mein or plain tossed salad, which are mostly low-fat carbohydrates.
Vitamins that dissolve in water are absorbed earlier than vitamins that dissolve in fat.
After you’ve digested your food and absorbed its nutrients through your small intestine, this is what happens:
Amino acids, sugars, vitamin C, the B vitamins, iron, calcium, and magnesium are carried through the bloodstream to your liver, where they’re processed and sent out to the rest of the body.
Fatty acids, cholesterol, and vitamins A, D, E, and K go into the lymphatic system and then into the blood. They, too, end up in the liver, are processed, and are shipped out to other body cells.
Inside the cells, nutrients are metabolized — burned for heat and energy or used to build new tissues. The metabolic process that gives you energy is called catabolism (from katabole, the Greek word for casting down). The metabolic process that uses nutrients to build new tissues is called anabolism (from anabole, the Greek word for raising up).
How the body uses nutrients for energy and new tissues is, alas, a subject for another chapter. In fact, this subject is enough to fill seven different chapters, each devoted to a specific kind of nutrient. For information about metabolizing proteins, turn to Chapter 6; for fats, Chapter 7; for carbohydrates, Chapter 8; for alcohol, Chapter 9; for vitamins, Chapter 10; for minerals, Chapter 11; and for water, Chapter 12.
Your large intestine
When every useful, digestible ingredient other than water has been wrung out of your food, the rest — indigestible waste such as fiber — moves into the top of your large intestine, the area known as your colon. The colon’s primary job is to absorb water from this mixture and then to squeeze the remaining matter into the compact bundle known as feces.
Feces (whose brown color comes from leftover bile pigments) are made of indigestible material from food, plus cells that have sloughed off the intestinal lining and the bodies of bacteria, members of the microbiome (see the nearby sidebar “Prebiotics and probiotics: The good gut twins”). In fact, about 30 percent of the entire weight of the feces comprises the bodies of these microorganisms, which live in permanent colonies in your colon, where they
Manufacture vitamin B12, which is absorbed through the colon wall
Produce vitamin K, also absorbed through the colon wall
Break down amino acids and produce nitrogen (which gives feces a characteristic odor)
Feast on indigestible complex carbohydrates (fiber), excreting the gas that sometimes makes you physically uncomfortable — or a social pariah
When the bacteria have finished and their bodies have been incorporated into the waste, the feces — small remains of yesterday’s copious feast — pass down through your rectum and out through your anus. But not necessarily right away: Digestion of any one meal may take longer than a day to complete.
PREBIOTICS AND PROBIOTICS: THE GOOD GUT TWINS
A little more than 100 years ago, Ilya Ilyich (Elie) Metchnikoff, a Russian scientist, decided that humans who died young had succumbed to the effects of “putrefying bacteria” in their intestines. Searching for a remedy, Metchnikoff ended up with Bulgarian peasants, a significant number of whom lived well into their late 80s. Historians may argue that the only way to have lived long in Bulgaria was to avoid Bulgarian politics, but Metchnikoff credited the national longevity to yogurt, the first time someone recognized the benefits of probiotics, a discovery that won him the Nobel Prize in physiology and medicine in 1908.
The National Institutes of Health (NIH) defines probiotics as “live microorganisms that are similar to the beneficial microorganisms found in the human gut.” The most common probiotics in our food are members of the Lactobacilli family, such as the ones in Metchnikoff’s magic Bulgarian yogurt. Probiotic microbes are also active in other fermented food, including kefir, tempeh, some pickles, sauerkraut, and kimchi. (Note: Heat kills these microbes, so to be useful they must be live as noted on the labels for many yogurts.) Recently, a new term has entered the conversation: prebiotics. As you can assume, the “pre” means something that comes before probiotics — in this case, dietary fiber your body cannot absorb but which serves as food for those protective probiotic microbes. (See Chapter 8 for more on dietary fiber.)
While food is your best natural source of prebiotics and probiotics, the hot market right now is in supplements: probiotic tablets, capsules, powders, lozenges, and gums whose worldwide sales are expected to rise from $2.5 billion in 2018 to an astronomical $74.69 billion by 2025. And no wonder. Healthwise, proponents claim a slew of benefits for probiotics, including the prosaic but highly welcome ability to prevent or ease diarrhea due to an infection or treatment with antibiotics that wipes out normal bacterial colonies in the intestines. In 2014, a metanalysis for 24 different trials showed that probiotics also helped prevent a life-threatening intestinal inflammation in newborn premature infants.
Some studies also suggest that probiotics may alleviate symptoms of digestive disorders such as irritable bowel syndrome (IBS) and improve and perhaps relieve depression, but no evidence supports claims that they strengthen your immune system or make it easier for you to lose weight. And there’s this: Probiotic products are supplements, which means the Food and Drug Administration (FDA) regulates them as food, not drugs, so they don’t have to prove that they’re safe or effective. (See Chapter 13 for more on supplement safety.) That matters because with all the hoopla surrounding a “natural” product, there is a clear lack of data regarding the possible risks of long-term use of these supplements.
The bottom line: Probiotics are a promising field of research, and one day they may be used to treat or help prevent many disorders. But right now, not enough evidence exists to recommend their widespread use.
