Carb Sources and Functions

Carb Still Life

Carb Sources and Functions

In order to understand carb sources, it helps to define carbs.

Sugars, starches and fibers are the three types of the organic compounds popularly called carbs, which is short for carbohydrates. Another way of saying this is that we have simple carbs (sugars) and complex carbs (starches and fibers).

Sugars can be monosaccharides (“one sugar”) or, when they are composed of two monosaccharides, they are called disaccharides (“two sugars”). Starches are polysaccharides(“many sugars”) and are composed of long chains of monosaccharides.

The Simple "-oses"

The most common monosaccharides are glucose, fructose and galactose, and the most common disaccharides are maltose (alcohol sugar), sucrose (table sugar) and lactose (milk sugar).

Of all of these sugars, glucose is the most important, because it is fuel for the brain and, when the body stores it as glycogen, fuel for the muscles. You may hear glucose referred to as “blood sugar” because it circulates in the blood supplying energy to the brain and fuel to your muscles.

The Complex carb sources

The complex carbs (polysaccharides) are starches and fibers. Starches are stored by the plants that we eat, and so, come almost exclusively from plant foods. Fibers make up the structures of plants and are classified as soluble fiber and insoluble fiber. Both types of fiber are essential to a healthy eating lifestyle.

Glycemic Index

There is another carb issue that has become popular in recent times. This is something called the Glycemic Index. This is a way of defining foods in terms of how fast the sugar from carbs gets into your bloodstream. This may be important both in weight loss and control of diabetes, because of its affect on insulin, the hormone produced by your pancreas that allows the glucose to enter the cells and provide energy. Generally, the simpler the carb, the more quickly it is broken down and becomes glucose in your bloodstream.

Fun Fact for Carb Sources and Functions: Your body can convert polysaccharides to glucose through the process of digestion. If you hold a piece of bread or cracker in your mouth, it will start to taste sweet as the saliva begins to breakdown the complex starch and make it into simple sugar.

Are there healthy carb sources?

Carbs have gotten a bad name in recent years, but, without them, you will not feel well. It is important to know that you can be healthy eating carbs. You need them for energy, and good quality carb foods contain many of the other nutrients, such as vitamins, minerals and fiber that are part of a healthy diet.

You can tell which foods are carb sources since they are mostly the plant foods you eat. There are the healthy carb foods, such as

Foods from whole grains: Breads… Rolls… Pasta… Cereal… Bagels… Rice…

Fruits: Apples… Oranges… Pears… Bananas… Grapes… Berries… Peaches… Watermelon… Pineapple… Kiwi… Grapefruit…

Vegetables: Lettuce… Broccoli… Carrots… Potatoes… Peas… Corn… Onions… Beans… Spinach… Squash…

and there are less healthy (some would say un-healthy) choices, such as

French fries… Doughnuts… Chips… Pies… Cakes… Cookies… Products made from all white flour.

It's important that you choose most of your foods from the first list rather than the second. In addition to being poor carb foods, the foods in the second list are loaded with fat, particularly saturated fat and trans-fats, which have been implicated in the rise of heart disease and cancer.

Starchy and less starchy carb sources

Within the vegetable group, a distinction can also be made between those with a significant amount of carbs, and those with not much carb at all. 

Starchy vegetables: Carrots, Potatoes, Winter Squash, Corn, Peas, and Sweet Potatoes

Less-starchy vegetables: Broccoli, Lettuce, Spinach, Green Beans, Peppers, and Summer Squash

Although both groups offer good nutrition, if weight loss is an issue for you, choose vegetables from the less starchy group more often than from the starchy group.

Protein foods that are carb sources

Certain plant foods that are carb foods, also supply a significant amount of protein.

Lentils… Split peas… Kidney beans… Pinto beans… Black beans… Soybeans… Garbanzo beans… Navy beans… Peanuts… Peanut Butter… Rice… Wheat…. Barley… Oats… Peas…

When you eat these foods, you will enjoy the advantages of both a good source of carbs and also a significant source of protein. Most of them also have the added benefit of being high in fiber.

Conversely, although most carbs come from plants, one animal product, dairy, contains the carb lactose. There also is some carbs found in liver, but it is not considered a significant source.

Fun Fact for Carb Sources and Functions: There are carbs in liver, because the liver is where glycogen (storage form of glucose) is kept as an extra fuel supply and ready to be converted to glucose, when needed.

How much is enough?

There is a great deal of discussion about how much carb is needed in the diet. Since carbs encompass the Bread, Cereal, Rice and Pasta Group, as well as the Fruit and Vegetable Group, and even shares some foods with the Protein group, it is difficult to make an across-the-board recommendation for carbs. Most health experts agree that you should be eating more of your calories (some say as much as 55-60%) from carbs than from the Protein and Fats groups.

This would mean that if you eat 2500 calories a day, 1375-1500 of them should come from carbs.

2500 calories x 55-60% = 1375-1500 calories.

40%-30%-30% Rule

Another way that some nutritionists look at carb consumption is the 40%-30%-30% rule. This says that you should get 40% of your calories from Carbs, 30% from Protein and 30% from Fats. These percentages vary somewhat, depending on the source, but the idea generally is to eat a balance of the three with a slightly higher percentage from the Carb category.

This would mean that if you eat 2500 calories a day, about 1,000 of them should come from carbs.

2500 calories x 40% = 1,000 calories

The problem with these approaches is that they do not take into account the type of carbs you are eating. If you are eating 1,000 or 1500 calories of white flour products  each day, you are meeting the requirement, but missing out on the benefits of the other types of healthy carbs, such as fruits, vegetables and whole grains.

So what do you do?

I know you don't much like counting calories! You want to enjoy your food, not analyze it, and you definitely don’t want to think of it only in terms of percentage of daily requirements. In addition, many of the foods you eat do not fall into only one category.

As with protein, if you live in a developed nation like the U.S., you are probably getting enough carbs. Even in less developed countries, carbs are generally more common in the food supply than protein.

The right stuff

The big concern with carbs is not whether we are getting enough, but rather, which carbs are we eating. Are you eating white flour products and missing out on the benefits of whole grain foods? Are you eating a lot of bread and skipping vegetables and fruits? Do you concentrate on starchy vegetables, such as potatoes, and say no to broccoli and dark leafy greens? Do you go for deep-fried vegetables rather than eating them lightly cooked or raw?

Chances are you are getting enough carbs, but are they the right kind?

Fun Fact for Carb Sources and Functions: The pigments in colorful carbs--deep greens, dark oranges, bright yellows, deep purples, and vivid reds--seem to provide some protection to your body from oxidative stress. Take advantage of this rainbow brigade by choosing fruits and vegetables that are colorful. (Orange frosting on your doughnut doesn’t count!)

What is a serving?

The serving size for carbs varies according to type. 

For vegetables, a serving would be ½ cup cooked or raw or 1 cup leafy greens and for fruits, a serving would be 1 medium for most fruits, 1 melon slice, ½ grapefruit or ½ c. berries or canned fruit. A serving of vegetable or fruit juice is ¾ cup.

For the Bread, Cereal, Rice and Pasta Group, a serving would be 1 slice of bread, ½ cup cooked pasta, rice or cereal, 1 small roll, biscuit or muffin, ½ bagel or bun or 3 small crackers.

Smaller than you think

For most people, a portion of any food is generally smaller than they think. If you drink juice from a 12-oz. tumbler, and you fill it up, you will be getting 2 servings of fruits or vegetables. If you eat a large delicious apple or large navel orange, you are probably getting 2-3 servings of fruit.


The rule of thumb is that a medium fruit is a little larger than a tennis ball, and one cup of cooked vegetables or rice is about the size of a fist. It's probably not necessary to be extremely precise, but you should not kid yourself about serving sizes either. If you cannot visualize it, then, you may want to actually measure it, just once, so that you will get a picture in your head for future reference.

Whole foods

The other thing to remember is that the recommended servings are based on choosing more whole grains and unprocessed fruits and vegetables, whole wheat bread over white bread, baked potato over French fries and fresh fruit over sweetened, canned fruit.

Are there "bad" carbs?

In addition, there is some evidence that too many of the wrong carbs, simple sugars and refined starches, can lead to cavities, insulin resistance and even malnutrition.

Too much fiber in the diet can lead to gastrointestinal distress and can limit the absorption of important nutrients.

Caveat for Carb Sources and Functions: It is generally when a person is eating too many high-fat and high-sugar processed foods that carbs become a problem for them.

The important thing

Eat carbs from a variety of sources. Concentrate on whole grains and colorful fruits and vegetables. Be aware of your portions, especially of the starchy carbs. Just for fun, be adventurous and try out some new carb foods.

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