Dietary fiber is a group of complex carbohydrates that are not a source of energy for human beings. Because human digestive enzymes cannot break the bonds that hold fiber’s sugar units together, fiber adds no calories to your diet and cannot be converted to glucose.
Ruminants (animals, such as cows, that chew the cud) have a combination of digestive enzymes and digestive microbes that enable them to extract the nutrients from insoluble dietary fiber (cellulose and some hemicelluloses). But not even these creatures can pull nutrients out of lignin, an insoluble fiber in plant stems and leaves and the predominant fiber in wood. As a result, the U.S. Department of Agriculture specifically prohibits the use of wood or sawdust in animal feed.
But just because you can’t digest dietary fiber doesn’t mean it isn’t a valuable part of your diet. The opposite is true. Dietary fiber is valuable because you can’t digest it!
The small amount of glucose in your blood and cells provides the energy you need for your body’s daily activities. The 400 grams of glycogen stored in your liver and muscles provides enough energy for ordinary bursts of extra activity.
But what happens when you have to work harder or longer than that? For example, what if you’re a long-distance athlete, which means that you use up your available supply of glucose before you finish your competition? (That’s why marathoners often run out of gas — a phenomenon called hitting the wall — at 20 miles, six miles short of the finish line.) If you were stuck on an ice floe or lost in the woods for a month or so, after your body exhausts its supply of glucose, including the glucose stored in glycogen, it starts pulling energy first out of fat and then out of muscle. But extracting energy from body fat requires large amounts of oxygen — which is likely to be in short supply when your body has run, swum, or cycled 20 miles. So athletes have to find another way to leap the wall. Here it is: They load up on carbohydrates in advance.
Carbohydrate-loading is a dietary regimen designed to increase temporarily the amount of glycogen stored in your muscles in anticipation of an upcoming event. You start about a week before the event, says the University of Maine’s Alfred A. Bushway, Ph.D., exercising to exhaustion so your body pulls as much glycogen as possible out of your muscles. Then, for three days, you eat foods high in fat and protein and low in carbohydrates to keep your glycogen level from rising again.
Three days before the big day, reverse the pattern. Now you want to build and conserve glycogen stores. What you need is a diet that’s about 70 percent carbohydrates, providing 6 to 10 grams of carbohydrates for every kilogram (2.2 pounds) of body weight for men and women alike. And not just any carbohydrates, mind you. What you want are the complex carbohydrates in starchy foods like pasta and potatoes, rather than the simple ones more prominent in sugary foods like fruit. And of course, candy. This carb-loading diet is not for everyday use, nor will it help people competing in events of short duration. It’s strictly for events lasting longer than 90 minutes.
What about while you’re running, swimming, or cycling? Will consuming simple sugars during the race give you extra short-term bursts of energy? Yes. Sugar is rapidly converted to glycogen and carried to the muscles. But you don’t want straight sugar (candy, honey) because it’s hydrophilic (hydro = water; philic = loving), which means that it pulls water from body tissues into your intestinal tract. Using straight sugar can increase dehydration and make you nauseated. Thus, getting the sugar you want from sweetened athletic drinks, which provide fluids along with the energy, is best. The label on the athletic drink also tells you the liquid contains salt (sodium chloride). Why? To replace the salt that you lose when perspiring heavily.
Some people have a hard time handling carbohydrates. For example, people with Type 1 (“insulin dependent”) diabetes do not produce sufficient amounts of insulin, the hormones needed to carry all the glucose produced from carbohydrates into body cells. As a result, the glucose continues to circulate in the blood until it’s excreted through the kidneys. That’s why one way to tell whether someone has diabetes is to test the level of sugar in that person’s urine.
Other people can’t digest carbohydrates because their bodies lack the specific enzymes needed to break the bonds that hold a carbohydrate’s sugar units together. For example, many (some say most) Asians, Africans, Middle Easterners, South Americans, and Eastern, Central, or Southern Europeans are deficient in lactase, the enzyme that splits lactose (milk sugar) into glucose and galactose. If they drink milk or eat milk products, they end up with a lot of undigested lactose in their intestinal tracts. This undigested lactose makes the bacteria living there happy as clams — but not the person who owns the intestines: As bacteria feast on the undigested sugar, they excrete waste products that give their host gas and cramps. To avoid this anomaly, many national cuisines purposely are void of milk as an ingredient. (Quick! Name one native Asian dish that’s made with milk. No, coconut milk doesn’t count.) Does that mean people living in these countries don’t get enough calcium? No. They simply substitute high-calcium foods such as greens or soy products for milk.
A second solution for people who don’t make enough lactase is to use a predigested milk product such as yogurt or buttermilk or sour cream, all made by adding friendly bacteria that digest the milk (that is, break the lactose apart) without spoiling it. Other solutions include lactose-free cheeses and enzymetreated milk.
The most important sources of carbohydrates are plant foods — fruits, vegetables, and grains. Milk and milk products contain the carbohydrate lactose (milk sugar), but meat, fish, and poultry have no carbohydrates at all.
In the fall of 2002, the National Academy of Sciences Institute of Medicine (IOM) released a report recommending that 45 to 65 percent of your daily calories come from carbohydrate foods. The Food Guide makes it easy for you to build a nutritious carbbased diet with portion allowances based on how many calories you consume each day in
- 6 to 11 servings of grain foods (bread, cereals, pasta, rice), plus
- 2 to 4 servings of fruit and
- 3 to 5 servings of vegetables
These foods provide simple carbohydrates, complex carbohydrates, and the natural bonus of dietary fiber. Table sugar, honey, and sweets — which provide simple carbohydrates — are recommended only on a once-in-a-while basis. One gram of carbohydrates has four calories. To find the number of calories from the carbohydrates in a serving, multiply the number of grams of carbohydrates by four. For example, one whole bagel has about 38 grams of carbohydrates, equal to about 152 calories (38 4). (You have to say “about” because the dietary fiber in the bagel provides no calories, because the body can’t metabolize it.) Wait: That number does not account for all the calories in the serving. Remember, the foods listed here may also contain at least some protein and fat, and these two nutrients add calories.
Providing energy is an important job, but it isn’t the only thing carbohydrates do for you. Carbohydrates also protect your muscles. When you need energy, your body looks for glucose from carbohydrates first. If none is available, because you’re on a carbohydrate-restricted diet or have a medical condition that prevents you from using the carbohydrate foods you consume, your body begins to pull energy out of fatty tissue and then moves on to burning its own protein tissue (muscles). If this use of proteins for energy continues long enough, you run out of fuel and die.
A diet that provides sufficient amounts of carbohydrates keeps your body
from eating its own muscles. That’s why a carbohydrate-rich diet is sometimes
described as protein sparing.
What else do carbohydrates do? They
- Regulate the amount of sugar circulating in your blood so that all your cells get the energy they need
- Provide nutrients for the friendly bacteria in your intestinal tract that help digest food
- Assist in your body’s absorption of calcium
- May help lower cholesterol levels and regulate blood pressure
Your cells budget energy very carefully. They do not store more than they need right now. Any glucose the cell does not need for its daily work is converted to glycogen (animal starch) and tucked away as stored energy in your liver and muscles.
Your body can pack about 400 grams (14 ounces) of glycogen into liver and muscle cells. A gram of carbohydrates — including glucose — has four calories. If you add up all the glucose stored in glycogen to the small amount of glucose in your cells and blood, it equals about 1,800 calories of energy. If your diet provides more carbohydrates than you need to produce this amount of stored calories in the form of glucose and glycogen in your cells, blood, muscles, and liver, the excess will be converted to fat. And that’s how your pasta ends up on your hips.
Inside your cells, the glucose is burned to produce heat and adenosine triphosphate, a molecule that stores and releases energy as required by the cell. By the way, nutrition scientists, who have as much trouble pronouncing polysyllabic words as you probably do, usually refer to adenosine triphosphate by its initials: ATP. Smart cookies!
The transformation of glucose into energy occurs in one of two ways: with oxygen or without it. Glucose is converted to energy with oxygen in the mitochondria — tiny bodies in the jellylike substance inside every cell. This conversion yields energy (ATP, heat) plus water and carbon dioxide — a waste product.
Red blood cells do not have mitochondria, so they change glucose into energy without oxygen. This yields energy (ATP, heat) and lactic acid. Glucose is also converted to energy in muscle cells. When it comes to producing energy from glucose, muscle cells are, well, double-jointed. They have mitochondria, so they can process glucose with oxygen. But if the level of oxygen in the muscle cell falls very low, the cells can just go ahead and change glucose into energy without it. This is most likely to happen when you’ve been exercising so strenuously that you (and your muscles) are, literally, out of breath. Being able to turn glucose into energy without oxygen is a handy trick, but here’s the downside: One byproduct is lactic acid. Why is that a big deal? Too much lactic acid makes your muscles ache.
Your body runs on glucose, the molecules your cells burn for energy. Proteins, fats, and alcohol (as in beer, wine, and spirits) also provide energy in the form of calories. And protein does give you glucose, but it takes a long time, relatively speaking, for your body to get it. When you eat carbohydrates, your pancreas secretes insulin, the hormone that enables you to digest starches and sugars. This release of insulin is sometimes called an insulin spike, which means the same thing as “insulin secretion” but sounds a whole lot more sinister.
Eating simple carbohydrates such as sucrose (table sugar) provokes higher insulin secretion than eating complex carbohydrates such as starch. If you have a metabolic disorder such as diabetes that keeps you from producing enough insulin, you must be careful not to take in more carbs than you can digest. Unmetabolized sugars circulating through your blood can make you dizzy and maybe even trip you into a diabetic coma.
What makes this interesting is that some perfectly healthful foods, such as carrots, potatoes, and white bread, have more simple carbs than others, such as apples, lentils, peanuts, and whole wheat bread. The Glycemic Index, developed at the University of Toronto in 1981, gives you a handle on this by ranking foods according to how quickly they affect blood sugar levels when compared to glucose (the form of sugar your body uses as energy), the glycemic indicator par excellence.
Most people who don’t have a metabolic disorder (such as diabetes) that interferes with the ability to digest carbs can metabolize even very large amounts of carbohydrate foods easily. Their insulin secretion rises to meet the demand and then quickly settles back to normal. In other words, although some popular weight loss programs, such as the South Beach Diet, rely on the Glycemic Index as a weight loss tool, the fact remains that for most people, a carb is a carb is a carb, regardless of how quickly the sugar enters the bloodstream.
Carbohydrates come in three varieties: simple carbohydrates, complex carbohydrates, and dietary fiber. All are composed of units of sugar. What makes one carbohydrate different from another is the number of sugar units it contains and how the units are linked together.
- Simple carbohydrates: These carbohydrates have only one or two units of sugar.
- A carbohydrate with one unit of sugar is called a simple sugar or a monosaccharide (mono = one; saccharide = sugar). Fructose (fruit sugar) is a monosaccharide, and so are glucose (blood sugar), the sugar produced when you digest carbohydrates, and galactose, the sugar derived from digesting lactose (milk sugar).
- A carbohydrate with two units of sugar is called a double sugar or a disaccharide (di = two). Sucrose (table sugar), which is made of one unit of fructose and one unit of glucose, is a disaccharide. _ Complex carbohydrates: Also known as polysaccharides (poly = many), these carbs have more than two units of sugar linked together. Carbs with three to ten units of sugar are sometimes called oligosaccharides (oligo = few).
- Raffinose is a trisaccharide (tri = three) that’s found in potatoes, beans, and beets. It has one unit each of galactose, glucose, and fructose.
- Stachyose is a tetrasaccharide (tetra = four) found in the same vegetables mentioned in the previous item. It has one fructose unit, one glucose unit, and two galactose units.
- Starch, a complex carbohydrate in potatoes, pasta, and rice, is a definite polysaccharide, made of many units of glucose. Because complex carbohydrates are, well, complex, with anywhere from three to a zillion units of sugars, your body takes longer to digest them than it takes to digest simple carbohydrates. As a result, digesting complex carbohydrates releases glucose into your bloodstream more slowly and evenly than digesting simple carbs. (For more about digesting carbs, see the section “Carbohydrates and energy: A biochemical love story,” later in this chapter.)
- Dietary fiber: This term is used to distinguish the fiber in food from the natural and synthetic fibers (silk, cotton, wool, nylon) used in fabrics. Dietary fiber is a third kind of carbohydrate.
- Like the complex carbohydrates, dietary fiber (cellulose, hemicellulose, pectin, beta-glucans, gum) is a polysaccharide. Lignin, a different kind of chemical, is also called a dietary fiber.
- Some kinds of dietary fiber also contain units of soluble or insoluble uronic acids, compounds derived from the sugars fructose, glucose, and galactose. For example, pectin — a soluble fiber in apples — contains soluble galacturonic acid.
- Dietary fiber is not like other carbohydrates. The bonds that hold its sugar units together cannot be broken by human digestive enzymes. Although the bacteria living naturally in your intestines convert very small amounts of dietary fiber to fatty acids, dietary fiber is not considered a source of energy.
Most of the cholesterol that you need is made right in your own liver, which churns out about 1 gram (1,000 milligrams) a day from the raw materials in the proteins, fats, and carbohydrates that you consume. But you also get cholesterol from food of animal origin: meat, poultry, fish, eggs, and dairy products. Although some plant foods, such as coconuts and cocoa beans, are high in saturated fats, no plants actually have cholesterol. Because plants don’t contain cholesterol, no plant foods are on this list. No grains. No fruits. No veggies. No nuts and seeds. Of course, you can juice plant food up with cholesterol if you really try: Butter in the bread dough, cheese on the macaroni, cream sauce on the peas and onions, whipped cream on poached peaches, and so on.
At one point, back in the dawn of the Cholesterol Age, like, say, five years ago, the “safe” upper limit for LDLs was assumed to be around 160 mg/dl. Now, the National Heart, Lung, and Blood Institute, American College of Cardiology, and the American Heart Association have all put their stamps of approval on the National Cholesterol Education Program’s (NCEP) recommendations for new, lower levels of LDLs based on the presence of the risk factors I list under “Cholesterol and heart disease.” You know — diabetes, high blood pressure, obesity . . . those risk factors.
For healthy people with two or more risk factors, the new goal is to push LDLs below 130 mg/dl. For high-risk patients with heart disease or blood vessel problems and more than two risk factors, it’s LDLs below 100 mg/dl. For very high-risk patients who are hospitalized with heart disease or have heart disease plus several risk factors, LDLs should be under 70 mg/dl. If necessary, the NCEP suggests using cholesterol-busting “statin” drugs such as atorvastatin (Lipitor).
