Fiber: The Fourth Macronutrient?

By Robert Luby, MD

Note: This article is not meant as a substitute for proper medical advice. Please consult with your medical practitioner before using any type of remedy, herbal or otherwise.

“Fiber” may be the most underappreciated “nutrient”.  The trio of fat, carbohydrate, and protein are traditionally considered to be the three “macronutrients”.  However, it could easily be argued that fiber is so important for human health that it might be considered the fourth macronutrient.  There are two types of fiber, soluble and insoluble, and they both impart a variety of health benefits.

The main distinction between soluble and insoluble fiber is that the former has the ability to dissolve in water, while the latter does not.  This results in both types of fibers being important for regular bowel movements.  Soluble fiber forms a gel-like substance in the presence of water which helps to move the contents of the bowels.  Insoluble fiber will not absorb water, thereby “bulking” up your stool and making it easier to pass.  The regular transit of intestinal contents is critical for the detoxification capacity of the body, and both types of fiber are essential for this function.  Additionally, soluble fiber has the ability to directly bind toxins and inhibit them from being absorbed into the body.

In addition to binding toxins, soluble fiber has the ability to deter the absorption of carbohydrates (sugars) and cholesterol.  This makes foods which contain soluble fiber wise choices for those with diabetes or elevated cholesterol levels.  Some types of soluble fiber also possess anticancer, antibacterial, and antiviral properties.  Finally, soluble fiber promotes the growth of beneficial bacteria in the intestines.  This is important for nutrient production and absorption, immune function, and detoxification.

For those looking to reduce caloric intake, insoluble fiber has the beneficial effect of inducing a sense of fullness.  Insoluble fiber is resistant to digestion by humans, but it nevertheless possesses a critical nourishing function.  It is fermented by intestinal bacterial resulting in the formation of short chain fatty acids which are the main nutrient and energy sources for the large intestine.

Online, you may easily access lists of foods which contain soluble and insoluble fiber.  But when you are shopping or gardening, how can you recognize foods which contain fiber?  It is helpful to remember that insoluble fiber functions as a protective covering for plants, or as a stiff structure to support plants and “resist gravity”.  The protective covering, or husk, of grains is known as the “bran”.   Therefore all forms of bran are high in insoluble fiber.  The skins of fruit serve the same function, and are therefore usually rich in insoluble fiber as well..  Plant parts which resist gravity (stalks/stems/leaves) such as celery, asparagus, and all leafy greens are good candidates for high insoluble fiber content.  Lest you begin to think that “fiber is no fun”, happily note that pure cocoa and popcorn are also good sources of insoluble fiber.

Soluble fiber is most often found in the plant world associated with the “flesh” of the seed of the plant, or in the portion of the plant which stores nutrients.  This explains why legumes (dried beans) are very high in soluble fiber.  Many fruits, especially pears and citrus, are also rich sources.  Root vegetables can be good choices for soluble fiber.  Finally, the cruciferous family of vegetables (which contains some stalks, leaves, flowers, and roots) is also a smart choice for soluble fiber intake.

So while fiber is technically not considered a macronutrient, its importance should not be underestimated. Fiber’s role in promoting healthy digestion, detoxification, and blood sugar balance put it in the running for the title of “fourth macronutrient”. For this reason, consider fiber content of whole grains and produce when shopping for and preparing healthy meals for yourself and your family.