Diabetes Forecast

The Science of Carbs

Sugars, starches, and fiber, from molecule to meal

By Erika Gebel, PhD ,

Carb counting, carb cutting, carbo-loading, low-carb: Carbs, short for carbohydrates, are a ubiquitous part of the diet vernacular, especially for people with diabetes. Focusing on the challenges of counting carbs and planning meals may have kept you from asking a basic question: What is a carbohydrate? Here's your chance to break down those carb-laden muffins, potatoes, lattes, and carrots into their basic carbohydrate components so you know just what you're eating—and how your body deals with it.

All of the foods you eat contain a mixture of carbohydrates, protein, and fat that your body converts to either energy or raw materials essential for life. Carbohydrates are particularly good at providing quick energy, and understanding them is key to managing diabetes because they are broken down directly into glucose, the sugar that people with diabetes have in excess in their bloodstreams.

As the word carbohydrate implies, these molecules are made up of carbon, hydrogen, and oxygen. Simple as that sounds, these three components can be joined together, like Tinkertoys, in many ways and serve many purposes. The body can both get carbohydrates from food and make its own carbs.

Isn't It Sweet?

Carbs that we eat fall into three main categories: sugars, starches, and fiber. They can be simple or complex: The simple ones are sugars, while starches and fiber are complex carbohydrates. Sugars are the basic units used to build all carbohydrates. For nutrition, the three basic sugars you should know are glucose, fructose, and galactose. Glucose, the body's main energy currency, is nutritionally the most important of these sugars. Fructose, the sweetest of the sugars, can be found in honey and fruit or in high-fructose corn syrup. Galactose is rarely found alone in nature but joins with glucose to make lactose.

These sugars combine, like links on a chain, to form larger sugars, the simplest of which are sucrose (glucose plus fructose), maltose (two glucoses), and lactose (glucose plus galactose). Sucrose is the "table sugar" in the 5-pound bags at the grocery store; brown sugar and powdered sugar are sucrose, too, but processed differently. Maltose is produced during alcohol fermentation, leading to malt liquors. Lactose is the sugar in milk; it makes up about 5 percent of milk's weight.

Increasing Complexity

As sugars join together in chains, they become the complex carbohydrates known as starches and fiber. Stringing sugars together turns out to be an efficient way to store energy in plants and animals; 10,000 glucose molecules, for example, can be packaged as one big molecule. Starch is our main dietary source of carbohydrates. Plants generate glucose through photosynthesis and store it as starch, which they can tap for energy. When people eat plants, they acquire rich stores of glucose from starch to help meet their own energy needs. Giant starch molecules are densely packed into grains (like rice, corn, and wheat), potatoes, and legumes (like peas, lentils, and beans). Starchy foods made from these sources include bread, cereal, and pasta.

Fiber is also made by plants, but this type of complex carbohydrate is generally a structural component, not a nutrient. Think of the strands that run along a stalk of celery: That's the fiber cellulose. There are two types of fiber: soluble and insoluble. Gums, pectins (a component of plant cell walls, used as a gelling agent in foods), and mucilages (a sticky plant substance used in some adhesives) are soluble fiber; they can dissolve in water, forming a gel. Cellulose, on the other hand, is an insoluble fiber; it doesn't mix with water and, like oil, remains separate

Soluble Fiber Insoluble Fiber
Found in: oat bran, barley, nuts, seeds, legumes (beans, lentils, peas), and some fruits (bananas,
blueberries) and vegetables (artichokes, carrots)
Found In: wheat bran, vegetables, and whole grains
Effect: slows digestion Effect: adds bulk to the stool
Benefit: lowers cholesterol Benefit: helps food pass more quickly through the stomach and the intestines

The big difference between starches and fiber for human nutrition is that people don't have digestive enzymes capable of breaking the bonds between the sugars in fiber. Since humans can't release the glucose, fiber doesn't directly supply us with energy or raise blood glucose. However, bacteria in your intestines can break down some fiber for use by the body. Eating fiber may have indirect effects on blood glucose and can benefit nutrition, health, and digestion.

A Carb We Make

Just as plants make starch to store their glucose, animals, including humans, have their own glucose storage unit of choice: glycogen, sometimes called animal starch. When glucose surges into the blood after a meal, the cells collect the glucose to use immediately for energy or to package as glycogen, a complex carbohydrate. To keep the body energized until the next meal, special enzymes can then break off bits of glucose from the glycogen molecules as needed. Little glycogen is found in the meat we eat (it rapidly breaks down after an animal is slaughtered); that's why meat isn't a significant source of carbs.

The liver stores extra glycogen to help buffer against high and low blood glucose. When blood glucose levels dip, such as overnight when we don't eat, the liver taps its glycogen stores to pump more glucose into the bloodstream, keeping levels high enough to fuel the brain and body. During periods of fasting or intense exercise, glycogen made from food sources can run out, but the liver has the unique ability to make glucose from scratch to keep blood glucose levels up.

When blood glucose is high, insulin—the hormone that signals the body is well fed—tells the liver to stop sending glucose to the blood. However, in type 2 diabetes, the liver doesn't respond to insulin normally; this is a form of insulin resistance. As a result, the liver thinks the body is fasting all the time and so keeps pumping out glucose even when blood glucose levels are already high.

While the need to count carbs may be a chore for people with diabetes, these storage chains of tightly packed but highly accessible energy are a miracle of nature's chemistry. Sugars, starches, and fiber make up the bulk of our daily diet and, with the help of glycogen stores, provide most of the glucose the body needs for fuel. So, the next time you're cursing carbs, remember: We couldn't live without them

Read on for a quick look at the carb content of everyday foods...

Or for a more detailed carb chart, click here.

Carbs at a Glance

Here's a quick look at carbohydrate content in foods. Just because a food is low in carbs isn't an excuse to eat it with abandon; low-carb foods may be high in fat, salt, or other threats to a healthy diet. You can meet with a registered dietitian to see how many carbs you should consume in a day and to learn more about carbohydrate counting and healthy eating.

No Carbs: Grilled meat or fish, plain coffee or tea, butter, mustard

Low in Carbs: Hard cheeses, mushrooms, spinach, cucumber

High in carbs/low fiber: Potatoes, pasta, English muffins, cornflakes

High in carbs/high fiber: Beans, barley, raspberries, oatmeal



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