not all the fat in your food gets stored on your hips. Everything has the potential to turn into fat if it’s not used by your body for energy at the exact time it is absorbed through your intestines. And energy is energy is energy (see Figures 3.4a and 3.4b). Here’s how the different nutrients are processed:
Simple sugars (as in a cola): When sugar, which is quickly absorbed and sent to the liver, meets the liver in the digestion process, the liver tells your body to turn that sugar into a fat if it can’t be used immediately for energy.
Complex carbohydrates (as in whole-grain foods). They take longer to digest, so there’s a slower release of the carbohydrates that have been converted in your bowel to sugar to become sugar in your bloodstream. That means your digestive system is not stressed as much. Still, if your body can’t use this slower sugar when it’s released, it gets converted to fat.
Figure 3.4a Department of Energy The three major types of energy are contained in carbohydrates, proteins, and fats, which can come from healthy or waist-busting forms. Complex carbs enter the blood slowly, so we do not tax our hormones. Amino acids are converted inefficiently to sugars, and fats cannot be converted at all. Fats come in forms our bodies recognize (like nuts) and naturally less common forms that poison us (like trans fats). Most foods, like meat, are a combination of energy sources; as the food digests (or sometimes rots) in your intestines, nutrients are absorbed in different places. By the way, even though the liver is the symbolic center of the metabolic universe, the intestines, as evidenced by your bathroom time, aren’t really a closed loop.
Figure 3.4b Food Use Simple sugars from carbohydrates are the most versatile energy source, so they’re preferentially used by our organs, especially the finicky brain, which refuses to tolerate any other source. Fats are a backup system to supply muscle with energy; this is why actually using muscles is needed to selectively lose fat and why exercise works so well. Amino acids from proteins are crucial for building the body, but are used only as a last resource for exercise energy.
Protein (as in meat): It gets broken down into small amino acids, which then go to the liver. If the liver can’t send them to your muscles (say, if you’re not exercising and don’t need them for muscle growth or maintenance), then, yep, they get converted to glucose, which then gets converted to fat if you can’t use it for energy.
Fat(as in funnel cake): It gets broken into smaller particles of fat and gets absorbed as fat. Good fats (like those found in nuts and fish) decrease your body’s inflammatory response, and bad fats increase it. That inflammatory response, which we’ll explain in the next chapter, is a contributing factor to obesity and its complications. If you’re exercising and have used up all readily available carbohydrates (sugar), your muscles can use fat for energy, which is a great way to erode your love handles.
Your Digestive Highway: The Main Drag
At the bottom of your stomach and top of your intestines, your food hits an important traffic signal: It’s the red light that tells your brain you’re full and don’t need another large order of onion rings (or the cheese sauce for dipping or the beer to wash it down). That red light is delivered by the vagus nerve, which is a large nerve that comes from the brain and stimulates the contraction of the stomach (see Figure 3.5). The vagus nerve is also the main cable controlling the parasympathetic system, which is the relaxation section of your nervous system. YOU-reka! The key messenger switching the vagus on is a peptide produced in your gastrointestinal track called CCK, which is released when your bowel senses fat. Technically, it stands for cholecystokinin, but for our purposes, let’s think of it as the Crucial Craving Killer because its main purpose is to tell your brain via the vagus nerve that your stomach feels fuller than a Baywatch bathing suit.
Figure 3.5 Stop Sign Food entering the small bowel stimulates release of the substance CCK into the wall of the stomach. That’s where the vagus nerve senses that we’re full and informs the satiety center in our brains to tell our hand to put down the buttered popcorn.
FACTOID
While the majority of taste buds are located on your tongue, you also have taste buds on the roof of your mouth.
Without having to go through the chemical pathways of your body (your bloodstream), CCK acts as a very direct message and indicator of your fullness. (Remember, leptin is more of a long-term indicator of your satiety; CCK provides a very short-term, intense message.)
After the food spends some time in your stomach, it will slowly leave that reservoir and go into the small intestine via the duodenum, the first part of your intestines that comes right after the stomach. That’s when CCK puts up a digestive detour sign, in a very clear physical signal that makes you feel full. It causes the pylorus—the opening at the end of the stomach—to slam shut; that keeps food from moving into the small bowel. That’s how your stomach gets full physically and how you feel full mentally. One interesting note: High-saturated-fat diets lead to less CCK sensitivity, so you do not feel as full as you should after eating a steak.
After the stomach, your food enters the small intestine and has a head-on collision with bile. Bile is the thick green digestive fluid that’s secreted by the liver, stored in the gallbladder, and released into the small intestine. (CCK also has a third effect: It’s what causes the gallbladder to contract.) After fat is broken down into smaller particles by substances called lipases, which are released by the pancreas, these tiny particles interact with bile to form a compound that is easily absorbed by the cells of your body. Bile surrounds the fat in our meal like soap surrounds grease on our hands so it can be scrubbed from the intestinal wall and better digested and absorbed.
Once it reaches the bloodstream, food continues to influence how hungry you may feel. Elevated blood sugar sends your brain the message that it’s time to take your plate to the sink and hit the couch. When your blood sugar is low, that’s what stimulates hunger and causes you to feed like a rat in the Kraft aisle.
Many of us get into trouble when we eat foods with simple sugars (think soft drinks, jelly, cake). Simple sugars create a rebounding effect. You’re feeling blah, so you eat a 3 Musketeers. That sugar surge works like an electrical jolt, and you instantly feel more energy. But less than two hours later, that energy surge (in the form of elevated blood sugar levels) plummets, and then you feel blah again. Your conclusion? You must need another Musketeer. That rebound effect (combined with the desire for the taste that’s stimulated by the pleasure center in your brain) can put your body in biological turmoil, where you eat to feel better, though what you’re eating is actually making you feel sluggish, so you swirl and swirl around, always feeling like you need to eat.
The Word on GERD
Fat doesn’t just pose problems for your belly and subway turnstiles; it also can mess with your throat. About half of obese people have the chest-burning condition called GERD (gastroesophageal reflux disease). The thinking is that extra fat in the belly pushes down on your stomach, thereby opening the angle of the GE junction and pushing it toward the chest. (Remember, it’s at an acute angle to keep food from going back up your throat every
