The Undigestible Power of Fiber
When people ask, "Which of the following nutrition cannot be digested by humans?" the most prominent answer is dietary fiber. Fiber is a complex carbohydrate found in the cell walls of plants that our bodies cannot break down into absorbable sugar molecules. While most carbohydrates like starches are easily digested by enzymes in our saliva, stomach, and small intestine, fiber remains largely intact as it travels through our digestive tract. This indigestibility is not a flaw in our design but a feature that provides numerous health benefits.
There are two primary types of dietary fiber, and both are indigestible by humans but serve different purposes in the body:
- Soluble Fiber: This type of fiber dissolves in water to form a gel-like material. It can help lower blood cholesterol and glucose levels by slowing the rate of digestion and absorption. Good sources include oats, beans, apples, and carrots.
- Insoluble Fiber: This fiber does not dissolve in water and remains relatively unchanged as it passes through the body. It adds bulk to the stool, helping to promote regular bowel movements and prevent constipation. Excellent sources include whole grains, nuts, and the skins of many fruits and vegetables.
The Scientific Reason Behind Indigestibility
The core reason we cannot digest fiber lies in our lack of a specific enzyme called cellulase. Fiber, particularly cellulose, is a polysaccharide made of long chains of glucose units linked by beta-glycosidic bonds. The digestive enzymes in our bodies are equipped to break down alpha-glycosidic bonds, which are found in starches and glycogen, but are unable to process the beta-bonds in cellulose. This anatomical difference is what separates humans, as omnivores, from herbivores like cows and other ruminant animals, which possess specialized gut bacteria that can produce the necessary enzymes to break down cellulose.
Other Nutrients and Substances We Can't Fully Digest
Beyond dietary fiber, other substances are not fully broken down by the human body or are indigestible for certain individuals:
- Resistant Starch: Found in foods like unripe bananas, legumes, and cooked and cooled potatoes or pasta, this type of starch behaves like a fermentable fiber. It resists digestion in the small intestine but is later fermented by bacteria in the large intestine.
- Lactose (for some): Individuals with lactose intolerance lack the enzyme lactase, which is necessary to digest lactose, the sugar in milk. The undigested lactose travels to the colon, where gut bacteria ferment it, leading to symptoms like bloating and gas.
- Artificial Sweeteners: Many non-nutritive, or artificial, sweeteners are not fully absorbed by the body. They pass into the large intestine, where they can be fermented by bacteria and potentially cause digestive discomfort.
- Some Seeds and Kernels: The outer casing of many seeds and kernels, like those found in corn, are made of indigestible fiber. This is why you may notice corn kernels appear whole in your stool. However, the nutrients inside are often digested.
A Comparison of Digestible vs. Indigestible Carbohydrates
| Feature | Digestible Carbohydrates (e.g., Starch) | Indigestible Carbohydrates (e.g., Fiber) |
|---|---|---|
| Enzyme Required | Amylase (salivary, pancreatic) | Cellulase (which humans lack) |
| Chemical Bond | Alpha-glycosidic bonds | Beta-glycosidic bonds |
| Breakdown | Broken down into simple sugars (glucose) | Passes through the digestive system largely unchanged |
| Energy Contribution | Provides calories and energy | Does not provide direct energy |
| Effect on Digestion | Fuels the body directly | Adds bulk and aids bowel regularity |
| Source | Bread, rice, pasta, potatoes | Whole grains, vegetables, fruits, legumes |
The Unexpected Benefits of Indigestible Nutrients
Even though we cannot digest cellulose and other forms of fiber for energy, these nutrients are vital for maintaining good health. Their journey through the digestive system is far from useless. As fiber passes through the large intestine, it is fermented by beneficial gut bacteria, acting as a prebiotic. This process supports a healthy gut microbiome, which has been linked to numerous health benefits, including reduced inflammation and a stronger immune system. The bulk from fiber also helps scrape waste from the intestinal walls and ensures the efficient movement of food through the digestive tract, preventing issues like constipation. The indigestible nature of fiber is a necessary component of a balanced diet, proving that what our bodies can't break down is just as important as what they can.
Conclusion
While the human body is a highly efficient machine for extracting energy from most nutrients, it is intentionally not designed to digest certain substances, most notably dietary fiber like cellulose. This is due to our lack of specific enzymes, an evolutionary trade-off for an omnivorous diet. However, this indigestibility is not a hindrance but a cornerstone of digestive health. Fiber adds bulk, supports a thriving gut microbiome, and ensures the smooth passage of waste. The next time you eat a leafy green or a handful of nuts, remember that the part you can't digest is doing some of the most important work for your gut and overall well-being. For further reading, consider exploring the NIH's detailed resource on carbohydrates.
Frequently Asked Questions
Question: Why can't humans digest cellulose? Answer: Humans cannot digest cellulose because our bodies do not produce the enzyme cellulase, which is required to break the unique beta-glycosidic bonds that link its glucose units.
Question: Is it bad that humans can't digest fiber? Answer: No, it is not bad; in fact, it is beneficial. The undigested fiber acts as roughage, which is essential for proper bowel function, preventing constipation, and supporting a healthy gut microbiome.
Question: Are there any animals that can digest cellulose? Answer: Yes, many herbivores and ruminant animals, such as cows and sheep, can digest cellulose. They have specialized digestive systems and symbiotic bacteria that produce the cellulase enzyme necessary for this process.
Question: What happens to fiber when we eat it? Answer: When you eat fiber, it passes through your stomach and small intestine largely intact. In the large intestine, gut bacteria can ferment some of it, while the rest adds bulk to your stool and is eventually eliminated from the body.
Question: Does fiber provide any calories or energy? Answer: No, fiber does not provide direct calories or energy because it cannot be broken down and absorbed by the human body in the same way as digestible carbohydrates. The energy-yielding components are mostly in the food's digestible parts.
Question: What is resistant starch? Answer: Resistant starch is a type of starch found in certain foods that, as its name suggests, resists digestion in the small intestine. It then functions as a fermentable fiber in the large intestine, feeding beneficial gut bacteria.
Question: What are some common foods that contain indigestible fiber? Answer: Common foods with indigestible fiber include whole grains like brown rice, bran, nuts, seeds, legumes, and the skins of vegetables and fruits. Corn is a well-known example where the fibrous outer shell is not digested.
