Probiotics get a ton of positive press from a multitude of sources (including here ). It’s one of those areas of nutrition that receives approval from pretty much every camp out there, like fish oil . (Even Dean Ornish supports the usage of fish oil; just about the only type of fat  he seems to approve of.) I’m firmly in the “For” column as well if you haven’t already guessed. I feel so strongly about probiotics and their integral role in gut and overall health that, as you may know, I’m coming out with a probiotic supplement in a few weeks. But there’s another aspect to the manipulation of beneficial gut flora. I briefly mentioned them last time , and today I’m going to really gut the whole beast, so to speak.
What Are Prebiotics?
Prebiotics are, quite simply, indigestible food ingredients that stimulate the growth and maintenance of beneficial gut microbiota. I suppose “indigestible by humans” is more accurate, because they are being digested – just not by our host digestive system (about 90% of prebiotic fiber makes it through the small intestine intact). Instead, it’s those oft-thankless, microbial workhorses of our colons doing all the work while we reap the benefits. They are getting free meals, so don’t feel too bad about putting them to work.
So you could say that prebiotics are food for your flora, those living organisms that contribute to our health and well being. For our intents and purposes, prebiotics are classified as soluble fiber . Conversely, insoluble fiber is the stuff that the human digestive system – neither host cells nor gut flora – cannot process nor digest, instead acting as a lubricant stimulant for our bowels (literally tearing our insides up, prompting the release of natural lubricant to speed up processing and limit damage). When most people discuss the so-called benefits of dietary fiber, they’re talking about insoluble fiber’s effect on stool passage and volume. Yeah, it speeds up the process and makes for bigger toilet paper bills. But I’m not interested in mere bulking agents. I’m interested in soluble fiber, in the type of fiber that our gut flora can actually consume and ferment. I’m talking inulin and oligofructose, along with fructooligosaccharides (FOS), galactooligosaccharides (GOS), and other oligosaccharides. Pectin, too, appears to have some prebiotic potential, but inulin and oligofructose are the big ones. Of course, all of these can be obtained by adhering to the Primal Blueprint Food Pyramid  and eating a few servings of vegetables and fruits each day.
Researchers distinguish between long chain, short chain, and full spectrum prebiotics. Inulin is a long chain prebiotic fiber; long chain prebiotics contain 9-64 links per saccharide molecule and are digested more slowly, providing food for bacteria in the left side of the colon. Oligofructose is a short chain prebiotic, containing 2-8 links per saccharide molecule and fermenting in the right side of the colon, considerably faster. A full spectrum prebiotic supplement would be something like oligofructose-enriched inulin (OEI), which contains all possible saccharide links.
Prebiotic foods were certainly consumed by our ancestors, when and where they were available. The strongest evidence consists of cave deposits in North America in which remains of inulin-and-oligofructose-rich agave, sotol, wild onion, and camas bulbs have been found , along with massive cooking stones and vast (around twelve feet in diameter) ovens. Since similar cooking pits have been found the world over, from Australia to Europe, with some dating as far back as 30,000 years ago, it seems likely that these disparate sites were also used to roast the occasional fibrous tuber. None come with hard evidence of fibrous root or tuber remains, but that’s to be expected. Vegetable matter doesn’t last that long. We do know that wild roots, tubers, and other fibrous foods are available almost everywhere and that they are eaten where available by local populations. Take this account  (PDF) of the Hadza of Central Tanzania, where the “tubers are continuously available throughout the year” and “all of their tubers have high fiber content.” Or there’s the Maori, who used nearly every part of the cabbage tree, prized for its inulin content (which partially converts into fructose when steamed) and touted as a natural cure for colic, diarrhea, and other gastrointestinal disorders (maybe they were on to something?). If these types of fibrous, wild foods were widely eaten – and it seems likely that they were – early humans got a fair amount of prebiotic fiber in their diets.
In my opinion, one of the most compelling arguments for the importance of prebiotics in the human diet is the presence of galactooligosaccharides in human breast milk . Even the most ardent detractor of the viability of Paleolithic nutrition couldn’t deny that the macronutrients and micronutrients present in the only food specifically and expressly “designed” for human consumption – breast milk – are necessarily suitable for human consumption (unless there’s some creepy, nutty vegan extremist offshoot claiming breast milk promotes the suffering/subjugation of women and the consumption of too much dangerous saturated fat  that I don’t know about). Breast milk contains both probiotics (bifidobacterium, mostly) and prebiotics for the bacteria to feed on. Since it’s in breast milk, there is a precedent for prebiotics in the human diet by design. I don’t think it’s a stretch to assume they can continue to have a role in the human adult diet.
What About the Health Benefits?
It’s all well and good to establish that prebiotic fiber was available to and even regularly consumed by many early human populations, but to build a case for inclusion in our modern diet requires some more recent evidence.
Dietary inulin and oligofructose increase fecal bifidobacteria .
Dietary inulin and oligofructose increase magnesium and calcium absorption . Most interestingly was the fact that calcium uptake apparently increases with inulin intake only when calcium intake is low or calcium requirement is high, suggesting a modulating (rather than blind) effect.
Prebiotics (inulin, oligofructose, and xylooligosaccharides) exhibited inhibitory effects on precancerous colon lesions in rats . Xylooligosaccharides increased gastrointestinal flora more than inulin and oligofructose, indicating possibly greater effectiveness.
An eight week regimen of 4g daily xylooligosaccharides reduced fasting glucose, HbA1c, oxLDL, LDL, and apolipoprotein B levels  in Type 2 diabetes patients.
Prebiotics increase the production of short chain fatty acids in the human colon, including the supremely beneficial butyric acid , given a great summary by Stephan.
Prebiotics combined with probiotics (called synbiotics) was more effective at gut modulation than either alone.
Matt Metzgar guesses  that the supposed health benefits of whole grains stem from their prebiotic content, and that it was the widespread consumption of refined flour free of soluble fiber that made an already poor grain-based diet even more damaging. Sure, you could eat whole grains and get a bit of prebiotic fiber (along with loads of insoluble fiber, lectins, gluten, phytic acid and starch), but why go through the trouble when you could get even more from some crisp jicama sticks, Jerusalem artichoke, garlic, onion, or leeks with less effort, less roughage, and fewer antinutrients? To get the recommended six grams of inulin from chicory root, you’d have to eat a third of an ounce; to get the same six grams from whole wheat flour, you’d have to consume a quarter of a pound!
How Much Should We Be Consuming?
Jeff Leach, of PaleoBiotics Lab, recommends upwards of 135 g per day of prebiotic fiber, based on research into archaeological evidence from the Northern Mexican desert . That seems really extreme to me. I guess extreme conditions (“semi-arid region” with “limited rainfall and poor soil conditions”) necessitate extreme diets (“plant-based diet”). Those northern Chihuahans weren’t getting much dietary fat, so they probably had to rely on their gut flora to convert the prebiotic fiber into short chain fatty acids. That’s how the gorillas do it : they end up with a high-fat, moderate-protein, low-carb diet simply by consuming and fermenting an incredible amount of indigestible fibrous plant matter. We moderns have access to real animal fat  and protein , so I doubt we need anywhere near 135 g of inulin and oligofructose.
I do think prebiotics are important. I’ve never really made a point of consuming them specifically (seeing as how I’m generally pretty down on fiber), but all my research on gut flora and probiotics leads me to believe soluble, prebiotic fiber (as opposed to insoluble fiber, the stuff seniors pop like candy, bran muffins, etc.) is actually quite important. Gut health is much more than just the small intestine. It’s almost as if there’s an entirely different digestive system playing out in the colon. The human colon may not be as robust and expansive as the gorilla colon, but it has the potential to do some damn fine work all the same – provided it gets the prebiotic fuel it needs. I suggest you provide that fuel by eating several servings of Primal prebiotic-rich foods each week, if only as a short experiment. Give it a couple weeks, at least until the sometimes explosive (but totally normal and expected) gaseous reactions subside, and monitor your digestive health.
What Foods Contain Prebiotics?
Let’s take a look at some more examples. In parentheses is the prebiotic fiber content by weight, followed by the amount of food required to obtain 6 g prebiotic fiber:
- Raw chicory root (64.6%) – 1/3 oz
- Raw Jerusalem artichoke (31.5%) – 3/4 oz
- Raw dandelion greens (24.3%) – 1 oz
- Raw garlic (17.5%) – 1.2 oz
- Raw leek (11.7%) – 1.8 oz
- Raw onion (8.6%) – 2.5 oz
- Cooked onion (5%) – 1/4 lb, or 4 oz
- Raw banana (1%) – 1.3 lb
Inulin/oligofructose content  (per 100g raw)
- Chicory root – 41.6 g/22.9 g
- Jerusalem artichoke – 18 g/13.5 g
- Dandelion greens – 13.5 g/10.8 g
- Garlic – 12.5 g/5 g
- Leek – 6.5 g/5.2 g
- Asparagus – 2.5 g/2.5 g
- Banana – 0.5 g/0.5 g
Inulin content is altered by cooking, but not a lot; some of it is even converted into fructose (that’s how agave nectar is made, in fact). As Jeff Leach shows, traditional-style oven roasted chicory root (356 degrees F) lost about 10-20% of its inulin content , while cooked/fried onions lost only 10%. It’s safe to assume that cooking will preserve most of the prebiotic fiber in other foods, too.
As for getting all the other prebiotics researchers are beginning to explore through whole foods, it isn’t clear how available, say, xylooligosaccharides are in the context of a normal Primal eating plan. From what I can see, most of the newer, more obscure prebiotic supplements are obtained by processing oats or corn or some other cheap, readily available food source. They undoubtedly occur in other veggies, too, but it probably doesn’t make financial sense to reduce several dozen tons of broccoli or asparagus when you could just as easily process a bunch of cheap grains. Supplementation seems pretty safe all around, especially in the context of a probiotic-rich diet. On the other hand, sticking with the Primal Blueprint Food Pyramid  and eating a variety of fresh vegetables (leafy greens like kale, chard, and spinach, as well as broccoli, cauliflower, and any other vegetable that contains soluble fiber) will get you plenty of soluble, prebiotic fiber in all its forms and is definitely safe.