Mary M. Fynn, Ph.D, RD, L.D.N



January 2016

Carbohydrate is the major source of energy for both cellular metabolism and one’s diet. Carbohydrates can be either “simple” or “complex”.  Simple carbohydrates– for which food science gives the more colloquial name “sugar”–are small in size, making them also sweet to the taste.  The two classes of simple carbohydrates are called monosaccharides and disaccharides. As indicated by their prefixes, a monosaccharide comprises 1 sugar unit, while a disaccharide comprises 2.

In contrast, complex carbohydrates contain more than two sugar units. For this reason, they are called “polysaccharides” and are primarily chains of glucose.  Unlike monosaccharides, they are larger in structure and not sweet to the taste.  Plants store energy in the form of complex carbohydrates or what we commonly call “starch.” Starch is only found in plant products and some of its more familiar sources include all grains, potato, legumes (beans), and many vegetables.

Two major dietary sources of carbohydrate are the food groups “fruits” and “vegetables”. Although both contain large amounts of carbohydrates, the ripening process for each is different. In fact, they are inversely related. So, for instance, when a fruit first buds on a plant, it is primarily a complex carbohydrate. As it ripens, however, it transforms into a simple carbohydrate.  On the other hand, vegetables at the start of the ripening process are simple and then ripen to complex.  Thus, when they are ripe, fruits taste sweet, whereas vegetables taste sweet when they are younger and not ripened.

The one type of carbohydrate and polysaccharide that human beings do not digest is called fiber. Because we do not digest it, carbohydrate as fiber does not supply energy. For instance, some fibers are metabolized by the bacteria found in the large intestines. In this way, they are thought to be advantageous to the gut bacteria.  The amount of fiber in food tends to increase with age of plant product. “Dietary fiber” is fiber found naturally in food. For a variety of reasons, fiber is also added to foods where it is not normally found. For example, manufacturers can add fiber to non-fat yogurt, which makes the yogurt thicker than non-fat yogurt without fiber.

Fiber can either be insoluble or soluble, depending upon how it interacts with water.  Insoluble fiber does not dissolve in water and is therefore not metabolized by the bacteria in the large intestines. In contrast, soluble fiber dissolves in water, which leads to swelling and is metabolized by the bacteria in the large intestines.

Diets containing large amounts of fiber have been associated with a lower risk of both heart disease and gastrointestinal disease. Nonetheless, studies testing fiber as a supplement have failed to show a decrease in the risk of diseases, such as colon cancer. Thus, it may perhaps be the case that some other component of the food item apart from the fiber is providing the disease protection, like the phytonutrient content of plant products.  A number of studies have shown whole grains to be associated with better health.

Monosaccharides Main dietary source
Glucose Fruits, sweet corn, corn syrup, honey
Fructose Fruits, honey, high fructose corn syrup
Galactose Not found free in nature
Disaccharides Monosaccharide component Dietary source
Sucrose Glucose + fructose Sugar cane, sugar beets (both used to make “table sugar”), molasses, maple syrup
Lactose Glucose + galactose Dairy (milk, cheese)
Maltose Glucose + glucose Beer, malt liquor

The effect of the different fiber classes on health:

Fiber type Action in intestines Dietary source
Insoluble Increase in stool size, weight, and frequency; decrease in transit time (i.e., decrease in constipation) Wheat bran (breakfast cereals with > 10 grams/serving are most effective); whole grains
Soluble Forms gels so it slows digestion and absorption of nutrients (glucose, cholesterol, and minerals), which can decrease blood levels of the nutrient

Metabolized in the large intestines; helps to keep intestinal wall health

Pectin containing foods (fruits, legumes), gums (e.g., carrageenan), oat bran, psyllium

While whole grain products do contain fiber, they also contain other nutrients, such as trace minerals and phytonutrients.  People who frequently eat whole grain products have also been shown to weigh less and gain less weight over time compared to people who eat primarily refined grains.   Your writer thinks this is because whole grains have a more distinctive taste than refined grains.  As creatures of appetite, we invariably eat for taste. When foods have a more noticeable taste, we tend to eat less of them. Therefore, those who eat whole grains may be eating less of them than those who eat refined grains. Additionally, it may be that whole grains are related to an overall healthier lifestyle.

The main form of carbohydrate in the blood is glucose.  What is typically measured in a lab test is “fasting blood glucose” or FBG.  A healthy level of FBG, which may also be expressed as “FBS”  (fasting blood sugar), is up to 100 mg/dl.  When FBS is >100 and < 126 mg/dl, this indicates that the person is “insulin resistant” and is an indicator of pre-diabetes.

Our bodies store carbohydrates as glycogen, which is found in the liver and in skeletal muscle.  Glycogen stored in the liver is used to maintain blood glucose. Because the liver and skeletal muscles can only store so much glycogen, we need to eat foods rich in carbohydrates on a regular basis in order to maintain blood glucose.  Otherwise, in the absence of sufficient store of carbohydrates, our bodies will begin to use the amino acids in skeletal muscle to make glucose in a process called gluconeogenesis. Gluconeogenesis is a survival mechanism, but one that could potentially lead to skeletal muscle wasting over time. Glycogen stored in a muscle does not leave that muscle so it is not available to maintain blood glucose. During exercise or any other type of physical activity, the glycogen in skeletal muscle is used to fuel the muscle.  The amount of glycogen stored on skeletal muscle can be increased with training the muscle and eating a diet sufficiently high in carbohydrate.

Functions of carbohydrate:

  1. provision of energy is the primary function of glucose. Glucose is a primary energy source for most cell types.  Carbohydrate is also the largest dietary component of most natural diets.
  2. sparing protein – if insufficient carbohydrate is consumed, amino acids will be used to synthesis glucose by gluconeogenesis. We do not store protein, so the amino acids would come from skeletal muscle.
  3. prevention of ketosis (incomplete fat catabolism). Approximately 100 grams of carbohydrate are needed to prevent ketosis in the prototypical 70 kg male.

The glycemic index (GI) is the blood glucose response to a carbohydrate containing food compared to a standard (glucose or white bread).  The GI is influenced by the fiber content of the food, any processing of the food (particle size), physical structure, and other variables.  The GI could also vary depending on the age of a plant product.  For example, when fruit first forms on the plant it is more complex carbohydrate compared to simple carbohydrate; fruit ripens to simple carbohydrate; the reverse happens for most vegetables.  This is why ripe fruit (“older”) tastes sweeter than newly formed fruit, while young vegetables taste sweeter than “older” vegetables.  So young fruit would have a GI that is lower than ripe fruit and young vegetables would have a GI higher than old vegetables.

The GI for a food is also different depending on whether it is tested on its own versus part of a meal.  For example, fat in the meal delays the stomach emptying, so the GI of carbohydrate foods in the meal will be lower if there is fat in the meal.

The essential amino acids in meat cause insulin release so the GI is lower with a meal containing meat.  A lower GI does not mean that the food is healthy.  By examining a table with the GI of common foods, one would find that the GI of a baked potato is 48, potato chips 8, chocolate 9, and jelly beans 21.  Your writer does not find the GI very useful.  My approach is to encourage the use of whole grains, unlimited vegetables and limit fruit to 3 to 4 servings a day.  As fruit tends to be sweet, it is easier to overeat.  Also, a serving of both fruit and vegetable is about ½ cup with fruit having 60 calories per serving and vegetables have 5 to 15 calories.  While fruit contains fairly healthy components, calories are calories and overeating fruit can contribute to weight gain.

Phytonutrients are compounds in plant products that protect them from their environment.  Although we do not require them for either growth or development, many studies have made a strong case for phytonutrients containing properties that reduce the risk of chronic diseases. Thus, phytonutrients may explain why a diet rich in plant products has been strongly related to both good health and a long life. The number of different phytonutrients is in the hundreds: they are as diverse as the plant products they are found in. Because their primary purpose is to offer protection, they tend to be concentrated on the outer part of the plant. For example, the outside kernel of grain contains many phytonutrients; this is at least partly why eating a whole grain product is healthier than eating refined grains.  Additionally, a diverse number of phytonutrients are produced when a plant is cut, crushed, or otherwise “injured.” There are many phytonutrients in red grapes, for example. But many more are created when the grapes are crushed, manipulated, and fermented in the wine-making process.