By Michael Ormsbee, PhD, Florida State University
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily
Carbohydrates are often maligned in contemporary diets, such as Atkins™ and ketogenic diets, but when we understand how our body breaks down carbs, we don’t necessarily have to ditch carbs to lose weight. Dr. Ormsbee explains how the chemical structure of carbohydrates influences how our body processes them.
Carbohydrate Structure
Common carbohydrate-rich foods include grains, pastas, potatoes, rice, fruits, and vegetables. Sugar itself is also a very simple form of carbohydrate, meaning that your body rapidly breaks it down and absorbs it. These foods are all considered carbohydrates due to their similar chemical structure—composed of carbon, hydrogen, and oxygen atoms in a 1:2:1 ratio.
The breakdown of these atoms ultimately provides adenosine triphosphate (ATP), or the energy needed for all human functions. The carbohydrates in the foods we eat can range from quite simple, like sugar, to more complex forms, like starches and fibers. This ultimately determines how quickly we can digest, absorb, and use them to fuel our activity and to store them as fat in our muscles, liver, and fat tissue.
We classify carbohydrates by their structure, from simplest to most complex, and these include monosaccharides, disaccharides, oligosaccharides, and polysaccharides. The term saccharide means sugar, from the Greek word saccharum.
The prefixes mono, di, oligo, and poly refer to how many sugars are linked together. Mono is one sugar, di is two sugar molecules, oligo is three to nine molecules, and poly is 10 or more sugar molecules bound together.
The simplest form of carbohydrate is the monosaccharide, which consists of only one sugar molecule. This includes household names like glucose and fructose, which are derived from fruit sugars.
When monosaccharides are combined, a disaccharide is formed, such as sucrose—fructose and glucose bound together—also known as table sugar. Longer chains of monosaccharides can also combine to form oligosaccharides, which are commonly found in vegetables, and even longer chains called polysaccharides. Starch, glycogen, and fiber contain polysaccharides.
Rate of Carbohydrate Processing
The length of the carbohydrate chain is one factor that determines the rate of carbohydrate breakdown. The shorter the chain, the faster this rate becomes.
In terms of overall health, you actually want a slower rate of breakdown to avoid any large variations in blood sugar and insulin concentrations. Insulin is one hormone responsible for inhibiting our ability to use fat as a fuel.
Another interesting factor that determines the rate of carbohydrate breakdown is the shape of the carbohydrate. Starch, which is a carbohydrate from plant sources, can have two basic forms: amylose and amylopectin.
Amylose is a straight, long chain molecule, which is digested slowly. Amylopectin, on the other hand, is a highly branched and rapidly digested molecule due to the increased surface area of the molecule’s branched structure. Glycogen is also highly branched, and it’s in the form of carbohydrates that we store in our bodies.
Benefits of Fiber
One source of carbohydrate that has clear health and body composition benefits is fiber. Dietary fibers are from plants and are not digested or absorbed by humans, so having these in your diet will slow the rate of carbohydrate digestion, which is usually a good thing. The only exception would be for some athletes during exercise—this is when an athlete needs food that can be digested and absorbed quickly.
Dietary fibers are found in foods like bananas, oatmeal, beans, whole grains, and dark leafy greens. Fiber may help you feel full or satiated for a longer period of time, lower your blood cholesterol levels and blood fat levels, and improve overall gut health. That means you might eat less by including fiber in your meals.
The minimum recommendation of fiber from the Institute of Medicine is about 14 grams per every 1,000 calories. This is about 25 grams per day for women and 38 grams per day for men.
However, these numbers were set to prevent health issues and not necessarily to live optimally. A banana has five grams of fiber, and one cup of baked beans has about 15 grams of fiber.
Process of Carbohydrate Break down
The process of carbohydrate break down starts in our mouth with an enzyme called salivary amylase. It breaks the long sugars apart into smaller subunits to be absorbed, and these small, simple carbohydrates move through the cell lining of the small intestine and into the blood in capillaries that lead to the portal vein.
The portal vein takes the blood to the liver, and the liver takes what glucose it needs for its own energy requirements and also what it needs to store as glycogen. The remaining glucose continues to circulate in the blood, and no matter which carbohydrate you eat, ultimately it will be in your blood as glucose because your body favors glucose for production of energy.
The fructose you eat is stored as liver glycogen, and once the liver stores all the glycogen it needs from fructose, fructose then serves to increase fat synthesis. Clearly, this may not be the best for overall health. The end result of carbohydrate break down is glucose being directed into circulation and ultimately to the cells that need it throughout your body.
Edited by Kate Findley and proofread by Angela Shoemaker, Wondrium Daily
Michael Ormsbee is an Associate Professor in the Department of Nutrition, Food, and Exercise Sciences and Interim Director of the Institute of Sports Sciences and Medicine in the College of Human Sciences at Florida State University. He received his MS in Exercise Physiology from South Dakota State University and his PhD in Bioenergetics from East Carolina University.
