Dietary Fiber: Types, Benefits, and Food Sources
Crude Fiber vs. Dietary Fiber: What’s the Difference?
Crude fiber is an analytical measurement, an empirical value representing the organic residue that remains after a defatted sample is boiled in dilute sulfuric acid and then in an alkaline solution. It is essentially the washed and dried remains of this process. Dietary fiber, on the other hand, is the undigested fraction that passes through the digestive system. Some dietary fiber is soluble but not digestible, such as pectin.
Understanding Crude Fiber
Crude fiber is an estimate of the carbohydrate (CH) structure in food. It is determined by extracting a sample with ether (for highly fatty compounds, more than 10%). The remaining non-nitrogenous organic substances do not dissolve after subsequent hydrolysis, first in an acid medium (simulating the stomach) and then in an alkaline medium (simulating the small intestine). The final total is subtracted from the weight of the ash, representing the carbohydrate fraction insoluble in successive acid-base digestions.
The main components of crude fiber are cellulose (90%), hemicellulose, and lignin. The amount of lignin affects the biological availability of cellulose and hemicellulose for microorganisms, thus impacting the nutritional value of plant substances for animals. These are considered bulky foods, whose primary purpose is to provide volume to the diet, ensuring proper digestive functioning, especially in ruminants.
Crude fiber is a component of what is defined as the mineral portion of food. After extracting fat, water, protein, soluble sugars, and starch from food, what remains is lignin, cellulose, and other complex carbohydrates. Cellulose is a crucial component of dietary fiber, as it is not broken down by enzymes secreted by mammalian tissues. Lignin, another portion of crude fiber, can decrease the digestibility of cellulose when present in high percentages. Although not a true carbohydrate, lignin is closely linked to cellulose in plants. The percentage of lignin increases as plants mature, providing stiffness to stems and branches.
Dietary Fiber: Types and Functions
Dietary fiber, or unassimilable carbohydrate ballast, plays a vital role in facilitating peristalsis and the evacuation of feces. There are two main types of dietary fiber:
Soluble Fiber
- Forms a gel-like lattice with water.
- Fermented to a greater extent in the colon.
- Absorbs water, increasing stool volume.
- Captures substances in the intestine, preventing their absorption (e.g., cholesterol).
- Slows glucose absorption.
- Prolongs gastric emptying time.
- Found in vegetables, most fruits, nuts, oilseeds, seaweed, and the white part of citrus fruits between the shell and the edible portion.
- Examples: Pectins, gums, and mucilages of fruit, oats, barley, and beans.
Insoluble Fiber
- Forms mixtures of low viscosity with water.
- Hardly fermented.
- Has a laxative and bowel-regulating effect by increasing intestinal transit rate.
- Found in whole grains, legumes, and to a lesser extent, certain vegetables like artichokes, spinach, chard, green beans, lettuce, carrots, and tomatoes.
- Examples: Cellulose, hemicellulose, and lignin found in whole grains, cereals, vegetables, and seeds.
Health Benefits of Dietary Fiber
Fiber-rich foods generally increase satiety, making a person feel fuller for longer and delaying hunger after eating.
Specific Functions of Dietary Fiber:
- Reduces transit time in the digestive tract, increasing the frequency of bowel movements (combating constipation).
- May help prevent bowel diseases such as diverticulosis, diverticulitis, and colon cancer.
- Helps excrete cholesterol.
- Reduces the elevation of glucose and insulin levels in the blood after meals, beneficial for individuals with diabetes mellitus.
- May aid in weight management by reducing the ingestion or absorption of other nutrients.
Potential Issues Related to Fiber Deficiency
A diet lacking in fiber can lead to several health problems, including:
- Delayed intestinal transit: Increased intraluminal pressure, potentially causing appendicitis and intestinal diverticulosis.
- Increased intra-abdominal pressure: This can contribute to hiatal hernias and hemorrhoids.
- Increased amount and activity of fecal carcinogens: This poses a risk factor for the formation of polyps, bowel cancer, and constipation.
- Increased nutrient absorption: Higher caloric intake can lead to obesity and, subsequently, ischemic heart disease.
- Impaired digestion and metabolism of bile acids: Decreased cholesterol excretion can increase plasma cholesterol levels, a risk factor for gallstones and coronary heart disease.
Important Considerations
While fiber is essential, excessive intake can have drawbacks:
- Increased nitrogen excretion, hindering protein digestion and absorption.
- Potential to slow the absorption of vitamin B12 and minerals like calcium, magnesium, iron, and zinc.
- In very large amounts, it can cause scaling of epithelial cells of the intestinal mucosa.
Therefore, a daily fiber intake of 15 to 20 grams is recommended. A good source is a mixture of 50% oat bran and wheat.
Fiber-Rich Foods
Foods rich in fiber include: Bran, artichokes, beans, asparagus, spinach, green beans, eggplant, chard, red cabbage, tomatoes, whole grains, and lettuce.
Methods for Determining Fiber Content
- Crude Fiber: Determined through acid digestion (using 1.25% H2SO4) followed by alkaline digestion (1.25% NaOH). The sample is filtered, dried, and then burned. The result is obtained by difference in weighing.
- Dietary Fiber: Based on enzymatic digestion using alpha-amylase to gelatinize the food, followed by protease and amyloglucosidase to remove protein and starch. The sample is then solubilized with alcohol, dried, and the residue is weighed. One sample is analyzed for protein, and the other is incinerated to determine ash content. Dietary fiber is calculated as: weight of residue – (weight of protein + ash).