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What are Carbohydrates?

Carbohydrates, along with fats, are the body's primary sources of energy. Carbohydrates provide energy not only to your muscles, but also to all the other tissues and organs of your body, including your brain, which is the only organ in your body that is completely dependent on carbohydrates. In addition to providing energy to your body, a small amount carbohydrates are also used by your liver to help build lipoproteins (lipoproteins transport fats and cholesterol throughout your body via your bloodstream). Carbohydrates, along with proteins and fats, are macronutrients. That means that carbohydrates are nutrients that are required in relatively large amounts within the body, and that they are capable of providing caloric energy when consumed (as opposed to vitamins and minerals, which are micronutrients and do not provide any caloric energy when consumed).

wheatThe basic chemical unit from which carbohydrates are made is the CH2O molecule. Essentially, this basic carbohydrate unit is just a water molecule (i.e. H2O) with a carbon atom (i.e. C), attached. Hence the term "carbohydrates," with "carbo" indicating the carbon, or "C" atom, and "hydrates" indicating the water, or "H2O" molecule. There are many different types of carbohydrates, but all carbohydrates are built from the basic CH2O molecule. Glucose, for example, is a carbohydrate composed of six of these basic CH2O molecules attached together.

Categories of Carbohydrates

From a biological perspective the most important categories of carbohydrates are monosaccharides, disaccharides, and polysaccharides. Monosaccharides and disaccharides are also known as simple sugars, or simple carbohydrates, and polysaccharides are also known as complex carbohydrates.

Simple Carbohydrates: Monosaccharides and Disaccharides

The term "saccharide" means sugar, so monosaccharide literally means one (i.e. "mono") sugar (i.e. "saccharide") molecule. Monosaccharide sugar molecules are also referred to as simple sugars or simple carbohydrates. Glucose, also known as blood sugar, and fructose, also known as fruit sugar, are examples of monosaccharide sugar molecules.

A disaccharide is composed of two monosaccharides (i.e. two sugar molecules). Maltose, for example, is a disaccharide composed of two glucose monosaccharides, and sucrose (or table sugar) is a disaccharide composed of a glucose monosaccharide and a fructose monosaccharide.

Complex Carbohydrates: Polysaccharides

Polysaccharides, also known as complex carbohydrates, are made up of many ("poly" means many) glucose molecules. Examples of polysaccharides include starch, glycogen, and cellulose, all of which are composed of just glucose molecules but differ in the manner in which the glucose molecules are linked together.

What Happens to Carbohydrates in Your Body?

When you eat carbohydrates they are broken down within your digestive system into their basic monosaccharide units, which are primarily glucose. The glucose units are then absorbed into your bloodstream, where they are also known as blood sugar. Your brain gets first priority for use of blood glucose, because glucose is the only nutrient that the brain is capable of metabolizing for energy (except during starvation). If your brain has sufficient glucose to satisfy its needs, the other tissues of your body will then receive glucose to satisfy the energy requirements for their various activities. At this point, after the immediate energy requirements of all your body's tissues have been met, the excess glucose in your blood will be converted into a polysaccharide storage form of carbohydrate called glycogen, the majority of which can be found in your muscles and liver. Then, if all of your glycogen stores are full and you still have excess glucose in your bloodstream, the remaining glucose will be converted to fat and stored in your adipose tissue.

Carbohydrates in Your Diet

When deciding what carbohydrates to eat, don't worry about whether or not they are classified as simple or complex carbohydrates. Instead, try to ensure that you are getting your carbohydrates from minimally processed vegetable, fruit, bean, and whole grain sources. Carbohydrates from these sources are ideal because they have high vitamin, mineral, phytonutrient, and fiber contents, so they are not only providing your necessary caloric energy but they are also delivering a significant amount of additional healthy nutrients that are lost in the more refined and processed carbohydrates (i.e. in carbohydrate sources like white flour, table sugar, white rice, fruit juices, sodas, cookies, cakes, jams, etc...). Generally speaking, carbohydrates from vegetables, fruits, beans, and whole grains are digested slower, which allows you to feel satiated for a longer period of time and reduces spikes in blood glucose levels, which are associated with increased risk for diabetes and heart and weight problems. Some examples of how to choose your carbohydrates are as follows:

Carbohydrate Dietary Reference Intakes

The table below displays the Dietary Reference Intakes (DRIs) for carbohydrates amongst all age groups of both sexes.

Carbohydrate Dietary Reference Intakes
AgeCarbohydrates (grams/day)
0 to 6 months60 *
7 to 12 months95 *
1 to 3 years130
4 to 8 years130
9 to 13 years130
14 to 18 years130
19 to 30 years130
31 to 50 years130
50 to 70 years130
over 70 years130
9 to 13 years130
14 to 18 years130
19 to 30 years130
31 to 50 years130
50 to 70 years130
over 70 years130
Pregnant Women
less than 18 years175
19 to 30 years175
31 to 50 years175
Lactating Women
less than 18 years210
19 to 30 years210
31 to 50 years210

* For infants aged 0 to 6 months old and 7 to 12 months old, the value provided is an Adequate Intake (AI) value rather than a Recommended Dietary Allowance RDA value. The source of the above table makes the following comments about RDAs and AIs:

RDAs and AIs may both be used as goals for individual intake. RDAs are defined to meet the needs of almost all (97 to 98 percent) individuals in a group. For healthy breastfed infants, the AI is the mean intake.

When broken down into a percentage of total energy intake, your consumption of carbohydrates should represent 45% to 65% of your total energy requirements (for all age groups). Therefore, if you consume 2000 calories per day it is recommended that 45% to 65%, or from 900 calories to 1300 calories, are from carbohydrates. As discussed above, those carbohydrate calories should ideally come from vegetable, fruit, bean, and whole grain sources.

Trumbo P, Schlicker S, Yates AA, Poos M; Food and Nutrition Board of the Institute of Medicine, The National Academies. Dietary reference intakes for energy, carbohydrate, fiber, fat, fatty acids, cholesterol, protein and amino acids. J Am Diet Assoc. 2002 Nov;102(11):1621-30. Erratum in: J Am Diet Assoc. 2003 May;103(5):563.

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