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.” )

● Vegetables:

• 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. )