October 2015 update: Scientists are continuing to identify various food sources of fermentable fiber, including sources of “pre-biotic” fermentable fiber–the kind that provides the healthy bacteria for your gut to ferment. I’ll be updating this list as new studies come out. If you haven’t read through it in a while, you might want to do so.
Just when you thought you could tell the differences among various kinds of fiber, scientists start dishing out a brand new term for our anti-cancer diets: fermentable fiber.
For years, researchers had us making distinctions between soluble fiber, the kind that forms a gel with water, slows your digestion and keeps your blood sugar even, and insoluble fiber, which bulks up your stool and prevents constipation. Now “fermentable “ is becoming the new buzz word (although scientists have known about it for a long while.)
Healthy bacteria in your gut feed on fermentable fiber and turn it into butyrate and other short chain fatty acids. In turn, those fatty acids control appetite and blood sugar, tamp down inflammation and improve your immune system. Butyrate also helps to quiet genes that drive cancer and to heal the intestinal lining, often damaged by chemo.
Some types of fermentable fiber go even further. You’ve heard of probiotics, no doubt? Meet prebiotics, the prequel. Fiber that’s prebiotic contains the raw material–that is, certain types of sugars or starches– to stimulate the growth of beneficial bacteria. (Some polyphenols in plants also act as pre-biotics, just to complicate matters–so be sure to eat berries and other colorful plants, even if they’re not on the upcoming list. )
So what foods are sources of fermentable and prebiotic fiber? The first part is simple. Fermentable fiber is present in virtually all fruits, vegetables and legumes. It’s also in nuts and seeds and grains.
But pre-biotic fermentable fiber is more selective, although the list seems to keep growing as research progresses. Here’s a list of sources of pre-biotic fiber (based primarily on research compiled by Australia’s Monash University and dietitian Jo Anne Hattner, author of “Gut Insights.” )
- Allium bulbs: garlic, leeks, onions, scallions, shallots
- Crucifers: cauliflower, Savoy cabbage, collards, kale, mustard greens
- Roots and tubers: Jerusalem artichoke, beets, burdock, cassava, chicory and dandelion roots, jicama, potato starch, sweet potatoes, taro, yacon, yams (but watch the sugars in those starchy veggies by limiting your portion size)
- Others: Globe artichoke, asparagus, bamboo shoots, butternut squash, celery, dandelion, green peas, mushrooms, okra, salsify, snow and sugar snap peas, spinach
● Fruits: apples, bananas, some berries (blackberries, wild blueberries, goji–and probably all berries, considering that they’re filled with polyphenols that act as pre-biotics ), grapefruit, guava, kiwi, pears, persimmons, pomegranate, stone fruits (apricots, peaches, dark plums, nectarines), seedless watermelon, most dried fruits (but watch out for sugars in dried fruits, too)
● Grains/pseudo-grains: amaranth, barley, buckwheat, corn, couscous, freekah, oats, brown rice, rye, spelt, whole wheat (but watch the arsenic in brown rice)
● Nuts and seeds, raw: almonds, cashews, chestnuts, flaxseed, hazelnuts, pistachios with skins (and don’t forget walnuts and pecans; they’re also rich in polyphenols that seem to act as pre-biotics. )
● Legumes: all dried beans, peas, lentils; fresh peas; hummus. (Keep in mind that with canned legumes, much of the prebiotic fiber– the sugars, starches– ends up in the liquids.)
● Other foods: agave, green tea, honey, tomato sauce (but I recommend avoiding honey and agave because they’re so high in fructose. For a sweetener, try Lo han instead. )
Thanks, that list just went up on the fridge door. We’ll make sure we include some in every meal. So those gassy oligosaccharides in soy are precursors of some pretty good stuff? For me, any changes are best included slowly over several days/weeks to give the bugs time to accommodate to their new digs.
Congratulations, Coacervate. Yes, I was wondering who would be the first to make the connection between gas and good stuff. And yes, making changes slowly is ideal.
Awesome article. Thanks for taking the time to share all these beneficial foods. Anything that can help prevention is going to be a huge benefit in life. We need to start focusing on foods such as these listed foods. Thanks for sharing@
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Does cooking these sources degrade the quality of the fermentable fiber or militate against the pre-biotic effect?
Great question, Baronp. After the holidays, I’ll contact an expert to see if s/he can answer that. My hunch is that the answer would depend on the source of the prebiotics. If polyphenols in plants are acting as pre-biotics, then yes, cooking would likely diminish the prebiotic quality. If the prebiotic quality is due to the presence of resistant starch (such as in legumes), then cooking seems fine, even desireable because it breaks down those thick cell walls. (Note that canned legumes have pre-biotic material in the liquids.) But if the source of the pre-biotic quality is the oligosaccharides (chains of sugars), such as the inulin in allium bulbs, I’m really not sure. I’ll get back to you when I figure out the answer to your important question.
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