While most carbohydrates, like sugars and starches, are efficiently broken down by the body's digestive enzymes into glucose for energy, a special class of carbohydrates resists this process. These non-digestible carbohydrates are fundamental to a healthy diet and play a vital role in our health, particularly in the health of our digestive system and gut microbiome.
What is the type of carbohydrate that does not digest?
The short and most direct answer is dietary fiber. The human digestive system does not produce the enzymes required to break down fiber's complex carbohydrate structure, allowing it to pass through the stomach and small intestine largely intact. In the large intestine, however, this undigested fiber is not inert. It becomes a crucial food source for the trillions of beneficial bacteria that reside in our gut, known as the gut microbiome.
Dietary fiber is often discussed in two main categories based on its properties, though many fiber-rich foods contain both types.
The two types of dietary fiber: Soluble vs. Insoluble
Soluble Fiber
- What it is: This type of fiber dissolves in water, forming a gel-like substance as it moves through the digestive tract.
- Function: This gel slows down digestion, which can help to regulate blood sugar levels by moderating the absorption of glucose. It can also help lower "bad" LDL cholesterol by binding to it and carrying it out of the body.
- Food sources: Oats, peas, beans, apples, citrus fruits, barley, and carrots.
Insoluble Fiber
- What it is: Unlike its soluble counterpart, insoluble fiber does not dissolve in water. It remains largely unchanged as it passes through the gastrointestinal tract.
- Function: It adds bulk to the stool and helps to move food through the digestive system more quickly, promoting regularity and preventing constipation.
- Food sources: Whole-wheat flour, wheat bran, nuts, beans, cauliflower, green beans, and potatoes.
The importance of resistant starch
In addition to the well-known types of fiber, another key player in non-digestible carbohydrates is resistant starch. As the name suggests, this is a type of starch that 'resists' digestion in the small intestine.
- Formation and Sources: Resistant starch is found naturally in certain foods, like green bananas and legumes, but it can also be formed when cooked starchy foods, such as potatoes, rice, and pasta, are cooled. When a starchy food is cooked and then cooled, some of its digestible starch turns into resistant starch.
- Function: Like fiber, resistant starch ferments in the large intestine and nourishes the beneficial bacteria in the gut. This process is crucial for producing short-chain fatty acids (SCFAs), particularly butyrate, which is the preferred fuel source for the cells lining the colon.
A comparison of digestible vs. non-digestible carbohydrates
| Feature | Digestible Carbohydrates (e.g., Starches, Sugars) | Non-Digestible Carbohydrates (Fiber, Resistant Starch) |
|---|---|---|
| Primary Function | Immediate energy source for the body. | Supports digestive health and feeds gut bacteria. |
| Digestion Location | Broken down in the mouth, stomach, and small intestine. | Passes through the small intestine largely intact. |
| Absorption | Absorbed as glucose into the bloodstream. | Not absorbed, instead fermented by gut microbes. |
| Energy Yield | Provides 4 calories per gram. | Provides minimal or no direct energy to the body. |
| Impact on Blood Sugar | Can cause rapid spikes, especially simple carbs. | Regulates blood sugar and prevents sharp spikes. |
| Gut Microbiome | Minimal impact on large intestine bacteria. | Crucial for feeding and maintaining a healthy gut microbiome. |
The gut microbiome and undigested carbohydrates
The fermentation of non-digestible carbohydrates by gut bacteria is a cornerstone of good health. This process produces beneficial byproducts, most notably SCFAs, which play a major role beyond simply nourishing the colon. SCFAs, especially butyrate, have been linked to improved immune function, anti-inflammatory effects, and a potential reduction in the risk of certain chronic diseases. A diet rich in non-digestible carbohydrates fosters a diverse and robust microbial ecosystem, which is increasingly recognized as a key factor in overall well-being.
How to increase your intake of non-digestible carbs
Incorporating more fiber and resistant starch into your diet is a simple but effective strategy for improving your health. Here are some actionable steps:
- Choose whole grains over refined grains. Opt for whole wheat bread, brown rice, quinoa, and barley. A serving of oatmeal or high-fiber cereal in the morning is a great way to start.
- Eat more legumes. Incorporate beans, lentils, and chickpeas into salads, soups, and main dishes. These are excellent sources of both fiber and resistant starch.
- Embrace fruits and vegetables. Eat a variety of fruits and vegetables with the skin on whenever possible, and aim for a wide array of colors to maximize your nutrient intake. Pears and apples with their skin are good examples of sources for both soluble and insoluble fibers.
- Add nuts and seeds. Sprinkle ground flaxseeds or chia seeds on your cereal or yogurt. Almonds and walnuts are excellent high-fiber snacks.
- Eat cooled starches. Make a potato salad with cooled potatoes or enjoy some leftover pasta. Cooling these starchy foods increases their resistant starch content.
- Don't forget unripe bananas. A slightly green banana contains more resistant starch than a fully ripe one.
Conclusion
Ultimately, the concept of a carbohydrate that does not digest is not a weakness of our system but a vital part of a healthy nutritional strategy. By focusing on incorporating plenty of dietary fiber and resistant starch from whole foods, you can nourish your gut microbiome, improve digestive function, and support overall health. These carbohydrates may not provide a direct energy boost like simple sugars, but their long-term benefits for your body are far more significant. For further reading on this topic, consider exploring resources from reputable health organizations like the Mayo Clinic.
