Fiber’s structural integrity keeps plants strong – and its indigestibility keeps your digestive system healthy
If you’re over the age of 10, the World Health Organization recommends that you consume at least 25 grams of fiber every day. The best fiber-containing foods come from plants: fruits, vegetables, nuts, seeds, whole grains and legumes.
While it’s sometimes overshadowed by other nutrients, such as protein, fiber plays a significant role in gastrointestinal health, digestion and nutrient absorption.
As a biochemist and someone who enjoys eating all types of foods, I find it remarkable that the structure of fiber, which is so similar to other carbohydrates, gives it all these unique functions. A tiny difference in the bonds that hold the molecules together allows your body to process a bagel differently from a raspberry.
Structure dictates function
In the biochemistry classes I teach, I emphasize that structure dictates function. If you’re about to cross a bridge but notice the braces are falling off or the wood is rotting, you’ll probably avoid stepping on it because the structure looks fragile.
This concept is true in the food you eat as well. The structures of the molecules that make up your food require them to be broken down in different ways in order to produce the energy that fuels your body.
Some foods contain vitamins and minerals that your body absorbs and uses for multiple functions. Other foods can keep your digestive system healthy and help your body absorb nutrients.
Food molecules consist of proteins, fats and carbohydrates. Each of these classes of food molecules has unique structures that allow your body to process them differently. For example, fats are long chains of carbon atoms that do not dissolve in water, while proteins have large amounts of nitrogen due to their amino acids. In addition, subclasses of biomolecules have even more specializations in their structures and functions.
Carbohydrate structure
Carbohydrates, or sugars, are biomolecules made up of carbon, oxygen and hydrogen atoms. Simple carbohydrates include single sugars, such as glucose or fructose, and two sugars linked together, such as sucrose – table sugar – or lactose, milk sugar. These simple carbohydrates exist mostly in rings, although they sometimes open up into a linear form.
Complex carbohydrates, on the other hand, have a lot – hundreds to thousands – of sugar molecules linked in large sheets called polysaccharides. These carbohydrates are linked only in their ring forms.
Plants connect sugar molecules in two types of polysaccharides – starch and fiber. These molecules have similar structures because they contain only one type of sugar – typically glucose – linked together many times.
However, one tiny difference in the chemical bonds within starch and fiber translates to very different functions for the molecules.
Starch, also called amylose and amylopectin, is a polysaccharide with glucose molecules that are linked using alpha bonds. Fiber, which consists primarily of cellulose, is a polysaccharide with glucose molecules that are linked using a beta bond.
The types of bonds refer to how certain parts of the molecules are oriented. These slight differences mean the two polysaccharides’ overall three-dimensional structures differ at the locations of these bonds.
The starch molecule is branched and doesn’t pack together very tightly. Plants use starch as a long-term store of glucose that they can break down to use for energy. The fiber molecules, due to their beta bonds, pack together very tightly. The glucose molecules in fiber form the support structures of a plant’s leaves, seeds and stems. The bonds of the starch enable the plants to easily break down the sugar so they can access quick energy, while the bonds of the fiber are designed to last and add strength to the plant’s architecture.
Fiber’s dietary function
The structural differences between starch and fiber mean these molecules have different dietary functions when you consume them.
Human bodies easily digest starch. Your body uses an enzyme called amylase that breaks the alpha bonds to release glucose molecules, which cells break down further to use for energy. Starch’s structure is perfect for fueling your cells. High starch foods include potatoes, pasta, rice, corn and bread.
Human bodies, on the other hand, cannot digest the beta bonds in the fiber we consume. They do not make enzymes that can release glucose molecules from the fiber, so most fibers pass through your digestive tract without being digested and absorbed. They contribute no energy to your diet. Foods with large amounts of fiber include peas, broccoli, oatmeal and pears.
The fiber does offer other health benefits. Fiber promotes bowel health by keeping your stools soft and moist, which reduces the risk of constipation, hemorrhoids and diverticulosis. Since the fiber stays intact in your gut, it gives your muscles something to push on to make it easier to eliminate stools, reducing pressure and inflammation in your intestines.
Some research has shown that consuming fiber reduces risk of inflammatory bowel diseases and protects against cardiovascular diseases. Fiber binds to bile acids that are excreted into your intestines, which helps with fat digestion. The fiber molecules interact with bile acids and dietary cholesterol, causing them to be excreted more easily and lower blood cholesterol levels.
Eating a high fiber diet also helps people feel more full. The fiber absorbs water and expands in your intestine, slowing the movement of food through the digestive system.
Learning about the structures of the carbohydrates you consume can help you understand their function in the body. Even though I think about biochemistry every day, I’m still amazed by how profoundly one bond can change the function of a biomolecule.