The Awakening of the Sixth Nutrient: The Definition, Classification, and Evolution of Dietary Fiber

Previously, Japan's nutritional problems stemmed from addressing insufficient protein and vitamin intake. Now, the primary concern is preventing obesity and hyperlipidemia caused by overnutrition. Fiber, once considered unhealthy, is now recognized as vital for health and believed to play a significant role in preventing adult-onset diseases. According to the *Kojien* dictionary, fiber is ① a fine, filamentous substance that makes up living organisms, and ② a generally fine, filamentous substance. We usually imagine fiber as long and thin. However, dietary fiber differs from this commonly understood concept of fiber.

Dietary fiber refers to substances in food that are difficult to break down by digestive enzymes and difficult to absorb after being ingested. Dietary fiber includes not only the previously mentioned crude fiber, such as cellulose and hemicellulose, which are insoluble in water and commonly referred to as fiber, but also water-soluble substances like pectin found in apples, which are not commonly considered fiber. Dietary fiber is not limited to plant sources; it also includes animal sources, and even substances that are difficult to digest, such as chitin, a component of shrimp shells.

Dietary fiber cannot be broken down by digestive enzymes. Even herbivores like cows cannot digest dietary fiber using their own digestive enzymes. Cows rely on enzymes that break down cellulose to digest and absorb it, using it as energy. A cow's stomach is weakly alkaline, suitable for microbial growth. Conversely, a human stomach is strongly acidic, so most bacteria cannot survive. In humans, most indigestible dietary fiber is sent to the large intestine. As dietary fiber moves from the stomach to the small and large intestines, its physicochemical properties, such as water retention, cation exchange, gel formation, and adsorption, affect nutrient digestion and absorption.

Some dietary fiber is broken down into fatty acids and carbon dioxide by enzymes produced by microorganisms in the large intestine. Fatty acids are absorbed and used as a source of energy. Among the various functions of dietary fiber, its role in preventing constipation was the first to be noticed. This effect is rapid. As early as ancient Greece, the famous physician Hippocrates used wheat bran to treat constipation. However, the Japanese, from eating brown rice and flour to refined rice, while not excessively so, have increasingly favored refined foods, thus gradually reducing their intake of dietary fiber.