The Answer is Insoluble Dietary Fiber
When you eat plant-based foods, the cellulose contained within the plant cell walls is referred to as dietary fiber. More specifically, it is a type of insoluble fiber, meaning it does not dissolve in water. This is distinct from soluble fiber, which forms a gel-like substance when it mixes with water in the digestive tract. While all fiber is considered beneficial, insoluble fiber plays a unique role in your digestive system, primarily by providing bulk and aiding the movement of waste. Many foods, especially whole foods, contain a mix of both soluble and insoluble fiber, and it is important to consume a variety of plant foods to get a balanced intake.
Why Humans Can't Digest Cellulose
As a complex carbohydrate, cellulose is made up of long chains of glucose molecules. However, the glucose units are linked by beta acetal bonds, which human digestive enzymes are incapable of breaking down. Our bodies produce enzymes like amylase, which can break the alpha-bonds in starch, but lack the specific enzyme, cellulase, required to break the beta-bonds found in cellulose. Because our digestive system cannot extract energy from cellulose, it is not considered a source of calories for humans. In contrast, some animals, like ruminants (e.g., cows), have specialized bacteria in their digestive tracts that produce the necessary enzymes to break down cellulose.
The Digestive Journey of Cellulose
Upon consumption, cellulose travels through the stomach and small intestine largely unchanged. It retains its fibrous structure and bulk, passing into the large intestine. Once there, it is not completely inert. Intestinal bacteria in the colon can ferment some components of dietary fiber, including certain types of insoluble fibers, though to a much lesser extent than soluble fibers. This fermentation process produces short-chain fatty acids, which can be beneficial for colon health. However, the bulk of the cellulose remains intact and performs its most significant function: acting as roughage to facilitate bowel movements. It helps to sweep waste through the digestive tract and out of the body.
Types of Dietary Fiber: Soluble vs. Insoluble
To better understand the role of cellulose, it helps to distinguish between the two main types of dietary fiber.
| Feature | Insoluble Fiber (contains cellulose) | Soluble Fiber |
|---|---|---|
| Water Solubility | Does not dissolve in water | Dissolves in water, forms a gel |
| Effect on Digestion | Adds bulk to stool, speeds up food transit | Slows down digestion, attracts water |
| Health Benefits | Promotes regularity, prevents constipation, reduces risk of diverticular disease | Helps lower cholesterol and blood glucose, prolongs fullness |
| Common Sources | Whole wheat flour, wheat bran, vegetables (e.g., cauliflower, green beans), nuts, skins of fruits and potatoes | Oats, peas, beans, apples, carrots, barley |
Health Benefits of Consuming Insoluble Fiber
Despite being indigestible, insoluble fiber offers several important health benefits:
- Promotes Regular Bowel Movements: By adding bulk and softening stool, insoluble fiber helps prevent constipation and promotes regularity.
- Supports Bowel Health: A diet rich in insoluble fiber can help reduce the risk of developing hemorrhoids and small pouches (diverticula) in the colon. Some studies suggest it may also lower the risk of colorectal cancer.
- Aids in Weight Management: The bulk added by insoluble fiber helps you feel full, which can reduce overall food intake and assist with weight management.
- Contributes to Gut Microbiome Health: While less fermentable than soluble fiber, insoluble fiber still provides some substrate for gut bacteria. The passage of undigested fiber also contributes to a healthy gut environment.
Common Food Sources of Cellulose (Insoluble Fiber)
- Whole Grains: Wheat bran, whole wheat products, brown rice, quinoa.
- Vegetables: Leafy greens (kale, spinach), cauliflower, green beans, root vegetables like carrots.
- Fruits: The skins of apples and pears, as well as berries and kiwi.
- Legumes: Lentils, beans, and peas.
- Nuts and Seeds: Almonds, walnuts, and flaxseeds.
Cellulose as a Food Additive
It is also worth noting that cellulose is not only consumed naturally through plant foods but is also added to many processed products under various names. Powdered cellulose, cellulose gel, or carboxymethylcellulose are used as thickening agents, emulsifiers, and anti-caking agents in foods ranging from shredded cheese to ice cream and diet products. While these additives are generally recognized as safe, it is always best to prioritize whole, unprocessed food sources for the most nutritional benefits. Learn more about dietary fiber and its types from authoritative sources like the Harvard School of Public Health's nutrition resource.
Conclusion
To answer the question, what we call cellulose when we eat it is insoluble dietary fiber. This indigestible, fibrous component of plant foods is not broken down by the human body but is absolutely vital for maintaining digestive health. By adding bulk to stool and promoting regularity, insoluble fiber is a crucial part of a healthy diet, helping to prevent constipation and potentially lowering the risk of more serious digestive issues. Consuming a wide variety of whole plant foods is the best way to ensure you are meeting your daily fiber needs and reaping the full range of health benefits.
