The discovery of prebiotic fibers as a main nutritional source for human bowel flora, I believe, is among the most important health discoveries of the last 20 years. The recognition that the intestinal microbiome has nutritional needs distinct from that of their Homo sapiens host is a huge finding, on a par with such things as the internal combustion engine or the recognition of iodine deficiency as the cause for goiters and hypothyroidism.
But I find that the issue of prebiotic fibers is often regarded as fluff, an afterthought in diet when, in reality, it should be at the top of the list in dietary considerations, way before any discussions about fat, calories, or carbs. Prebiotic fibers trigger the proliferation of healthy bacterial species, prevent the adverse effects some species are capable of inflicting, and yield metabolic byproducts that lead to substantial health advantages for the human host. But remember: If you are intolerant of prebiotic fibers, the problem is SIBO, not prebiotic fibers. Ignore SIBO and you are setting yourself up for a host of health problems—don’t ignore it.
In the Wheat Belly lifestyle, we aim to obtain 20 grams of prebiotic fibers per day, as this is the intake associated with maximum beneficial effects. There is no harm in obtaining more (it’s not uncommon for people in some primitive cultures to obtain over 100 grams per day), but benefits appear to plateau at this level of intake. Recall that, by obtaining prebiotic fibers, we are trying to mimic the behavior of primitive humans who dig in the dirt with sticks, stones, or bone fragments and consume the roots and tubers they uncover, sometimes raw, sometimes roasted on a stick over a fire. You, as a modern member of our species, probably have no interest in walking through the forest or field to forage for such things, as you have a job, family responsibilities, and may live in a region where the ground may be frozen part of the year. We therefore mimic the behavior of primitive foraging humans by turning to sources of prebiotic fibers that are accessible to us in a modern setting.
Given their huge importance to your health and welfare, here is an updated list on the prebiotic fiber content of various foods. Not all foods have been studied to quantify prebiotic fiber content, nor is there full agreement on exactly how to measure them. Nonetheless, here are the quantities of prebiotic fibers (and net carbs, where relevant) in the most prebiotic-rich foods according to the best evidence we have to date:
Green bananas and plantains: 10.9 grams in one medium (7-inch) banana (0 grams net carbs)
Raw white potato: 10-12 grams per ½ medium (0 grams net carbs) (Avoid any raw potatoes with green skin, as this is a fungus. If encountered, cut off skin.)
Jerusalem artichokes: 1 gram per 1/2-cup sliced (Slice thinly and add to salads. 12 grams net carbs per cup)
Inulin and/or FOS powders: 4 grams per teaspoon (0 grams net carbs)
Konjac (glucomannan): 4 grams per teaspoon
Chia seeds: 1.5 grams per 1/4 cup
Flaxseed: 2.5 grams per 1/4 cup
Dandelion greens: 1 gram per cup (uncooked)
Hummus or chickpeas: 8 grams per 1/2 cup (13.5 grams net carbohydrates in 1/2-cup)
Lentils: 2.5 grams in ½ cup (11 grams net carbohydrates)
Beans: 3.8 grams in 1/2 cup. White beans are the richest with twice this quantity. (12 grams net carbohydrates)
Peas: 1.3 grams per 1/2 cup (7 grams net carbs)
Acacia fiber (“gum”): 2 grams per teaspoon
Onions, garlic, leeks: not yet quantified
Jicama: not yet quantified
Apple: 1 gram
Parsnips: not yet quantified (8.5 grams net carbs per 1/2 cup)
Turnips: not yet quantified (5.0 grams net carbs per 1 cup)
Unmodified potato starch, green banana flour: <1 gram per tablespoon (10 grams net carbs per tablespoon)
Psyllium seed: not yet quantified
Nuts are also proving to be, in preliminary studies, a source of unique forms of prebiotic fibers and other compounds. Walnuts, almonds, pistachios, cashews, pecans, and hazelnuts are therefore sources of prebiotic fibers that, though precise quantification is lacking, add to your daily prebiotic fiber intake.