You: On a Diet plus Collins GEM Calorie Counter Set. Michael Roizen F.

your brain tells you to eat, that’s exactly what you do. You eat. Maybe you gorge. Maybe you nibble. And then maybe you forget about that hefty portion of mac ’n’ cheese until it winds up on the back of your thighs. But in between mouth and thighs, there’s an amazing system of digestion that takes place—a system that determines whether that food gets burned, stored, or expelled faster than a delinquent high schooler.

      Now that you know the biochemical reasons why you shuttle food to your mouth, it’s time to start exploring the biology of what happens to food once it’s in there. In this chapter, we’ll discuss what happens in the early part of your digestive system, and in the next chapter, we’ll discuss the effects of food as it interacts with the rest of your digestive organs.

      Your Digestive Highway: The On-ramp

      On your gastrointestinal interstate, everything enters via your physiological toll booth: your mouth. The nutritious powerhouses slide through the express toll to give you the power, energy, stamina, and strength to live your life. Toxic (though sometimes tasty) foods can enter too, but you’ll pay a heavier toll later for the damage they do along the way and after. Throughout its journey, your food and all of its nutrients (and toxins) will pull over at various organs, slow down on winding roads, speed up, merge with other nutrients, and even get pulled over by the bowel brigade for nutritional violations. (See Figure 3.1.)

      During every trip, your food hits a symbolic three-pronged fork in the road:

       Either it will be broken down and picked up by your bloodstream and liver to be used as energy.

       Or it will be broken down and stored as fat.

       Or it will be processed as waste and directed to nature’s recycling pot: the porcelain junkyard.

      Figure 3.1 Gutting It Out Food pulls over at various spots in the intestinal track so disease of these areas can cause nutritional deficiencies even if two people are eating the identical foods. Not all of the nutrients that come from food and supplements get absorbed in the same place; they’re absorbed throughout your GI tract. Here are the rest stations where nutrients are absorbed:

       Stomach: alcohol

       Duodenum (first part of the small intestine; takes off from the stomach): calcium, magnesium, iron, fat-soluble vitamins A and D, glucose

       Jejunum (middle part of the small intestine): fat sucrose, lactose, glucose, proteins, amino acids, fat-soluble vitamins A and D, water-soluble vitamins like folic acid

       Ileum (last part of the small intestine; leads to large bowel): proteins, amino acids, water-soluble vitamins like folic acid, vitamin B12

       Colon (also known as the large bowel): water, potassium, sodium chloride

      FACTOID

      The average person has 10,000 taste buds, which are onion-shaped structures. People regenerate new taste buds every three to ten days, but these regenerate at a slower rate as people get older. Elderly people may have only 5,000 taste buds.

      Here’s how the system starts: Before a morsel even reaches the tollbooth, your body has a radar gun to let you know that food is coming—powered by such physiological cues as sight, smell, and the fact that you’ve been drooling like an overheated Saint Bernard at the thought of a fried-cheese appetizer special. In response to that sensory information, glands in your mouth start to secrete enzymes to help break down your food; then your stomach quickly constructs its version of a roadside welcome center by pumping out stomach acid to help prepare your body for the digestion process.

      Now, don’t underestimate your stamp licker as a player in this digestion process. Back in the day of buffalo-hide cocktail dresses, people relied on their tongues (and their noses) for survival; if it tasted good, then it was safe, and if it tasted like dinosaur dung, then it could be poisonous or toxic.

      FACTOID

      Maybe the old days were right: It used to be that young docs would criticize older docs for giving B12 shots, calling them nothing more than placebos. But nearly 40 percent of Americans may suffer from a vitamin B12 deficiency.

      We do the same things, but in slightly different ways. Since our bodies use our senses to process information, we rely on our tongue for information about food. The information we acquire sends messages to the brain, and then the brain sends messages to our forks: keep eating or stop eating. That message largely comes from our five tastes (sweet, sour, salty, bitter, and unami, which recognizes the inherent deliciousness in foods like juicy filet mignon), but it also comes from what we smell. Some researchers say that three-quarters of how we “taste” certain foods actually comes from how we smell it. What’s this have to do with your waist growing? For one, there’s the obvious: the more you like a bad-for-you food, the more likely you are to keep eating it. But the genetics of taste and taste buds may play an even more subtle and fascinating role. As you’ll see in the box on page 70 (“Are You a Supertaster?”), the physiological makeup of your tongue could make you more or less disposed to eating good or bad foods.

      Figure 3.2 Taste Tester The most powerful muscle in the body, the tongue, tastes food with papillae that sense the chemicals in foods and tell you whether they’re worth your continued attention.

      Figure 3.3 Chewing the Fat One of the reasons we can gain weight so readily is the efficiency of our teeth, which fit perfectly with one another to ensure that every morsel of food is crushed completely. Salivary glands near the lower teeth and at the back of mouth secrete enzymes to hasten digestion before swallowing. The sight and smell of food warn these systems of what’s to come.

      FACTOID

      Eating nuts does not create the calorie intake that you might expect because 5 percent to 15 percent of the calories are not absorbed by the intestinal system. That’s because the nuts’ skin and how well we chew nuts influence digestion. An added bonus: The slow release of calories throughout the intestinal system leads to prolonged satiety.

      Unlike other animals, we waste very little energy eating because of our highly efficient perfectly opposing molars (see Figure 3.3). The powerful crushing motion helps us extract every possible calorie from the prime rib deluxe. Other animals waste or burn a lot of calories while they eat because their teeth do not efficiently mush the food when they move prey to belly. In humans, once that food actually does breeze past the toll booth, it accelerates onto the on-ramp of the esophagus—that’s the tube that links your mouth to the interstate that is your GI system.

      After your Double Whopper slides down the on-ramp, it has to make a tricky merge in the form of a sharp turn to enter the stomach. That angle—the gastroesophageal junction—is what keeps stomach acid from spilling back into your esophagus and making your chest feel like an arson victim. (When you have extra fat in your belly, that angle is pried open, allowing acid to spill upward and cause heartburn. See “The Word on Gerd,” page 64.) Once your Whopper chunks have entered your stomach, serious digestion begins. The food is held in your stomach until your body directs it to the small intestine, where most of the nutrients are absorbed and passed along to the rest of your body through your bloodstream (to the liver, which is the next stop for absorbed nutrients), or to the large intestine on the way to evacuation.

      Food Processor:

      How Your Body Breaks Down Nutrients

      In terms of weight gain, a calorie is a calorie is a calorie. Calories not used immediately by your body for energy are either eliminated as waste or stored as fat. YOU-reka! But that doesn’t mean that all calories are treated equally by your body. For example, protein and fiber