Carbohydrates are sugar compounds that plants make when they’re exposed to light. (The name means “carbon plus water.”) This process of making sugar compounds is called photosynthesis, from the Latin photo-, meaning “light,” and synthese, meaning “put together.”
This chapter shines a bright light on the different kinds of carbohydrates, illuminating all the nutritional nooks and crannies to explain how each contributes to your vim and vigor — not to mention a tasty daily menu.
Checking Out Carbohydrates
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
What makes simple carbs simple is, well, simple. These relatively small molecules have only one or two units of sugar, which makes them easy to digest — which is why they provide a fast energy lifter.
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 considered pretty simple but its real name is 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
Complex carbohydrates, which are also known as polysaccharides (poly = many), 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. In fact, it’s an oligosaccharide (with more than 10 glucose units) built of many units of glucose.
Because complex carbohydrates may have anywhere from three to several thousand 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
Dietary fiber is a term used to distinguish the fiber in food from the natural and synthetic fibers (silk, cotton, wool, and nylon) used in fabrics. Dietary fiber is a third kind of carbohydrate. Like the complex carbohydrates, dietary fiber (cellulose, hemicellulose, pectin, beta-glucans, and 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 isn’t like other carbohydrates. The bonds that hold its sugar units together can’t 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 isn’t considered a source of energy. (For more about fatty acids, see Chapter 7.)
Carbohydrates and Energy: A Biochemical Love Story
Your body runs on glucose, the molecules your cells burn for energy. (For more information on how you get energy from food, check out Chapter 3.)
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, also known as “insulin secretion.”
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. (See the section “Some people have problems with carbohydrates,” later in this chapter.)
Most healthy people 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, the fact remains that for most people, a carb is a carb is a carb, regardless of how quickly the sugar enters the bloodstream. You can find the complete story in Diabetes For Dummies, by Alan L. Rubin, MD (Wiley).
For info on why the difference between simple and complex carbs can matter for athletes, see the later section “Some people need extra carbohydrates.”
How glucose becomes energy
Inside your cells, 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.
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 don’t 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? Because too much lactic acid makes your muscles ache.
How pasta ends up on your hips when too many carbs pass your lips
Your cells budget energy very carefully. They don’t store more than they need right now. Any glucose the cell doesn’t 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 4 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.
Other ways your body uses carbohydrates
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.
Carbohydrates also
Regulate the amount of sugar circulating in your blood so all your cells get the energy they need
Provide nutrients for the friendly bacteria in your intestinal tract that help digest food
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.
The National Academy of Sciences Institute of Medicine (IOM) recommends that 45 to 65 percent of your daily calories come from carbohydrate foods, such as grains (bread, cereals, pasta, and rice), fruit, and vegetables, foods that 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 4 calories. To find the number of calories from the carbohydrates in one food serving, multiply the number of grams of carbohydrates by 4. 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 doesn’t 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.
Some people have problems with carbohydrates
Some people have a hard time handling carbohydrates. For example, people with Type 1 (“insulin dependent”) diabetes don’t 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 these people 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 avoid milk as an ingredient. (Quick! Name one native Asian dish that’s made with milk. No, coconut milk doesn’t count.) To get the calcium their bodies need, these people simply substitute high-calcium foods such as greens or calcium-enriched 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 of which are created by adding friendly bacteria that digest the milk (that is, break the lactose apart) without spoiling it. Other solutions include lactose-free cheeses and enzyme-treated milk.
Some people need extra carbohydrates
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, 6 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 would start pulling energy first out of stored 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 found another way to leap the wall: 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, PhD, 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 isn’t 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 plain table sugar (candy, honey) because it’s hydrophilic (hydro = water; philic = loving), which means that it pulls water from body tissues into your intestinal tract. This can increase dehydration and trigger nausea. Getting the sugar you want from sweetened athletic drinks, which provide fluids along with the energy, is safer, especially since the athletic drink also contains salt (sodium chloride) to replace the salt that you lose when perspiring heavily. Turn to Chapter 13 to find out why this is important.
Dietary Fiber: The Non-Nutrient in Carbohydrate Foods
Dietary fiber is a group of complex carbohydrates that aren’t a source of energy for human beings. Because human digestive enzymes can’t break the bonds that hold fiber’s sugar units together, fiber adds no calories to your diet and can’t 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!
Defining the two kinds of dietary fiber
Nutritionists classify dietary fiber as either insoluble fiber or soluble fiber, depending on whether it dissolves in water.
Insoluble dietary fiber includes cellulose, some hemicelluloses, and lignin found in whole grains and other plants. This kind of dietary fiber is a natural laxative. It absorbs water, helps you feel full after eating, and stimulates your intestinal walls to contract and relax. These natural contractions, called peristalsis, move solid materials through your digestive tract.
By moving food quickly through your intestines, insoluble fiber may help relieve or prevent digestive disorders such as constipation or diverticulitis (infection that occurs when food gets stuck in small pouches in the wall of the colon). Insoluble fiber also bulks up stool and makes it softer, reducing your risk of developing hemorrhoids and lessening the discomfort if you already have them.
Soluble dietary fiber includes pectins (found in most fruit), and beta-glucans (found in oats and barley), and gums such as guar, fenugreek and locust bean gums derived from seeds and used as thickeners in various prepared food products. Soluble dietary fiber seems to lower the amount of cholesterol circulating in your blood (your cholesterol level), which is why a diet rich in fiber appears to lower cholesterol levels and thus offer some protection against heart disease.
Here’s a benefit for dieters: Soluble fiber forms gels in the presence of water, which is what happens when apples and oat bran reach your digestive tract. So, like insoluble fiber, soluble fiber can make you feel full without adding calories.
Ordinary soluble dietary fiber can’t be digested, so your body doesn’t absorb it. But in 2002, researchers at Detroit’s Barbara Ann Karamonos Cancer Institute fed laboratory mice a form of soluble dietary fiber called modified citrus pectin. The fiber, which is made from citrus fruit peel, can be digested. When fed to laboratory rats, it appeared to reduce the size of tumors caused by implanted human breast and colon cancer cells. The researchers believe that the fiber prevents cancer cells from linking together to form tumors. Today, modified citrus pectin is being sold as a dietary supplement, but the American Cancer Society notes on its website that this fiber’s effects on human bodies (and human cancers) remain unproven.
Getting dietary fiber from food
You’ll find absolutely no fiber in foods from animals: meat, fish, poultry, milk, milk products, and eggs. But you will find lots of dietary fiber in all plant foods — fruits, vegetables, and grains.
A balanced diet with plenty of foods from plants gives you both insoluble and soluble fiber. Most foods that contain fiber have both kinds, although the balance usually tilts toward one or the other. For example, the predominant fiber in an apple is pectin (a soluble fiber), but an apple peel also has some cellulose, hemicellulose, and lignin.
Table 8-1 shows you which foods are particularly good sources of specific kinds of fiber. A diet rich in plant foods (fruits, vegetables, and grains) gives you adequate amounts of dietary fiber.
Table 8-1 Food Sources of Different Kinds of Fiber
According to the U.S. Department of Agriculture, the average American woman gets about 12 grams of fiber a day from food; the average American man gets about 17 grams. Those figures are well below the Institute of Medicine (IOM) recommendations that I conveniently list here:
25 grams a day for women age 19 to 50
38 grams a day for men age 19 to 50
21 grams a day for women older than 50
30 grams a day for men older than 50
The amounts of dietary fiber recommended by IOM are believed to give you the benefits you want without causing fiber-related unpleasantries. Unpleasantries? Like what? And how will you know if you’ve got them? Trust me: If you eat more than enough fiber, your body will tell you right away. All that roughage may irritate your intestinal tract, which will issue an unmistakable protest in the form of intestinal gas or diarrhea. In extreme cases, if you don’t drink enough liquids to moisten and soften the fiber you eat so it easily slides through your digestive tract, the dietary fiber may form a mass that can end up as an intestinal obstruction. (For more about water, see Chapter 12.)
If you decide to up the amount of fiber in your diet, follow this advice:
Do it very gradually, a little bit more every day. That way, you’re less likely to experience intestinal distress. In other words, if your current diet is heavy on no-fiber foods such as meat, fish, poultry, eggs, milk, and cheese, and low-fiber foods such as white bread and white rice, don’t load up on bran cereal (35 grams dietary fiber per 3.5-ounce serving) or dried figs (9.3 grams per serving) all at once. Start by adding a serving of cornflakes (2.0 grams dietary fiber) at breakfast, maybe an apple (2.8 grams) at lunch, a pear (2.6 grams) at midafternoon, and a half cup of baked beans (7.7 grams) at dinner. Four simple additions, and already you’re up to 15 grams dietary fiber.
Always check the nutrition label whenever you shop. (For more about the wonderfully informative guides, see Chapter 17.) When choosing between similar products, just take the one with the higher fiber content per serving. For example, white pita bread generally has about 1.6 grams dietary fiber per serving. Whole wheat pita bread may have as much as 7.4 grams.
Get enough liquids. Dietary fiber is like a sponge. It sops up liquid, so increasing your fiber intake may deprive your cells of the water they need to perform their daily work. (For more about how your body uses the water you drink, see Chapter 12.) That’s why the American Academy of Family Physicians (among others) suggests checking to make sure you get plenty of fluids when you consume more fiber. How much is enough? Back to Chapter 12.
Table 8-2 shows the amounts of all types of dietary fiber — insoluble plus soluble — in a 100-gram (3.5-ounce) serving of specific foods. By the way, nutritionists like to measure things in terms of 100-gram portions because that makes comparing foods at a glance possible.
Provisional Table on the Dietary Fiber Content of Selected Foods (Washington, D.C.: U.S. Department of Agriculture, 1988)
To find the amount of dietary fiber in your own serving, divide the gram total for the food shown in Table 8-2 by 3.5 to get the grams per ounce, and then multiply the result by the number of ounces in your portion. For example, if you’re having 1 ounce of cereal, the customary serving of ready-to-eat breakfast cereals, divide the gram total of dietary fiber by 3.5; then multiply by 1. If your slice of bread weighs 1/2 ounce, divide the gram total by 3.5; then multiply the result by 0.5 (1/2).
Or you can just look at the nutrition label on the side of the package because it lists the amounts of the nutrients per serving.
Finally, the amounts in Table 8-2 are averages. Different brands of processed products (breads, some cereals, cooked fruits, and vegetables) may have more (or less) fiber per serving.
Dietary fiber isn’t like other carbohydrates. The bonds that hold its sugar units together can’t 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 isn’t considered a source of energy. (For more about fatty acids, see Chapter 7.)
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, also known as “insulin secretion.”
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.
This carb-loading diet isn’t for everyday use, nor will it help people competing in events of short duration. It’s strictly for events lasting longer than 90 minutes.