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Dear Mark: What’s the Deal with Fiber?

Last week’s guest post [7] from Konstantin Monastyrsky, author of Fiber Menace [8]generated a lively, boisterous, and at times combative comment section. I use these descriptors in the best sense possible, mind you; debate is healthy and necessary, even – nay, especially – if it’s impassioned. So right off the bat, I want to thank everyone who wrote in. I also want to thank Konstantin, whose views on fiber forced me to reconsider my own way back when I first encountered him over five years ago. Without his input last week, we wouldn’t be having this discussion, and many people would still be sitting on whatever side of the fiber fence they prefer, never even considering that another side exists. I know I might still be catching up if I’d never read his book all those years.

Many of you asked whether I endorsed the views espoused in the guest post. You wondered whether I’d shifted my stance on the Big Ass Salad [9]. You wanted my take on the whole fiber thing, basically. So without further ado, let’s discuss fiber.

It’s often said that fiber is indigestible, that it serves no nutritive purpose – and that’s partially true. Humans can’t digest fiber. Our digestive enzymes and endogenous pancreatic secretions simply have no effect on roughage. Our gut flora, though? Those trillions of “foreign” cells residing along our digestive tract that actually outnumber our native human cells? To those guys, certain types of fiber are food to be fermented, or digested. That we feed our gut flora these prebiotic fibers [10] is important for three main reasons:

1. Because the health (and composition) of the gut flora helps determine the health of the human host (that’s us!). It’s difficult to name a physiological function or health parameter that is not impacted by the gut microbiome, including but not limited to digestive [11], cognitive [12], immune [13], emotional [14], psychological [15], metabolic [16], and liver [17] health. Our microbiota [18] depend on fermentable fibers for food. It’s not clear what exactly constitutes “healthy gut flora,” and we’re still teasing out exactly how it affects the various physiological functions, but we know we need them and we know they need to eat something to even have a chance at helping us.

2. Because the short chain fatty acids that are byproducts of fiber fermentation, including butyrate, propionate, and acetate, improve our health in many ways. Butyrate in particular has been shown to have beneficial effects on insulin sensitivity [19]colonic transport [20]inflammation [21], and symptoms of Crohn’s disease [22]. It’s also the preferred fuel source for our native colonic cells. Basically, without enough butyrate (and, by extension, fermentable prebiotic fiber to make it), our colons don’t work as well as they should. This can lead to digestive impairments and perhaps even cancer. Mucin-degrading bacteria predominate in colorectal cancer [23] patients, for example, while butyrate-producing bacteria rule the roost in healthy patients without cancer. Populations with lower rates of colorectal cancer also tend to have higher levels of butyrate [24]Propionate is helpful [25], too, though not to the extent of butyrate.

3. Because by feeding and bolstering the populations of “good bacteria,” we reduce the amount of available real estate for “bad bacteria” to set up shop. Gut bacteria don’t just float around in there. They cling to surfaces, nooks, crannies, and crevasses. They’re impossibly small, but they do take up space. After antibiotic treatment where both good and bad gut flora are indiscriminately targeted and wiped out, pathogenic obesity-promoting bacteria take advantage of the open space [26]. That’s a worst-case scenario, but it shows what can happen when the harmony of the gut [18] is disturbed (whether by antibiotics or lack of fermentable fibers).

Overall, because the health of our gut community is inextricably tied to the health of our minds and bodies, I think attaining fermentable fiber through the fruits and vegetables we eat is incredibly important. Heck, even the only food that’s actually expressly “designed” to feed humans – breast milk – contains prebiotic compounds [27] whose main purpose is to feed and cultivate healthy gut flora in infants, which suggests that the need for prebiotics is innate. Coprolite (read: ancient fossilized stool) studies certainly indicate that our ancestors may have consumed a significant amount of prebiotics [28], and we even have receptors and transporters built-in to handle and accept the butyrate produced from fermentable fiber [29]. Or, if you want to say that humans haven’t evolved a dietary requirement for fiber, that’s fine. But we have evolved to rely on gut flora to help our bodies work best, and that gut flora has evolved to require a steady, varied source of fermentable, prebiotic fiber. That can’t be denied.

have been suspicious of fiber in the past, though. Like Konstantin, I’ve discussed the folly of loading up on the kind of fiber [30] whose only purpose is to rend the intestinal walls, doing enough damage to induce mucus secretion which acts as lubricant [31]. I’m talking about insoluble fiber, of course.

Insoluble fiber is a bulking agent. You know how weight lifters swear by whole milk [32] and beef [33] for adding mass [34]? Insoluble fiber is like that, only for poop. It makes for extremely impressive toilet bowl displays and potentially expensive plumber fees, I’ll admit. And some people “need” to feel like they’ve done something down there. They like to take a peek after a bowel movement and let the distinct sensation of accomplishment wash over them. But for digestive health? I’m unconvinced, and there’s not much evidence in favor of it. Optimally, stool is made up of mostly water and bacteria [35] – not undigested food.

The health claims just don’t add up.

For one, insoluble fiber doesn’t ferment very well. That’s why neither we nor our gut microbes can digest, say, cellulose-rich grass – we don’t have the hardware, and neither do our gut flora. No fermentation, no short chain fatty acid production.

How about constipation? Bulking up your stool is supposed to improve symptoms of constipation, right? That’s why almost every doctor will tell you to “eat more fiber” upon hearing that you’re constipated. It’s gotta be evidence-based advice! Well, the actual evidence is rather weak. A recent meta-analysis [36] concluded that while increasing dietary fiber does increase the frequency of bowel movements, it does nothing for stool consistency, treatment success, laxative use, and painful defecation. So it will make you poop more often, sure, but each bowel movement is going to hurt and you’re still going to need laxatives to do it. Another recent study [37] found that stopping or reducing dietary fiber intake reduced constipation.

Or cancer? One recent study [38] compared the fiber intakes and gut flora composition of advanced colorectal cancer patients to healthy controls. Healthy controls who ate high-fiber tended to have more butyrate-producing microbes than low-fiber healthy controls and high-fiber cancer patients, suggesting that it’s not “fiber” that protects against cancer but “fermentable fiber.” The cancer patients who ate high-fiber were likely eating insoluble, cereal-based fiber, which was not protective. This jibes with an older study’s results: while both fruit and vegetable fiber were associated with lower risks of cancer, cereal fiber – which is mostly insoluble – was associated with a slightly higher risk [39]. Another study [40] found similarly protective links between fruit and vegetable fiber and stomach cancer, but not grain fiber.

If you desperately need to execute a double decker at the home of a sworn enemy, load up on insoluble fiber beforehand. Otherwise, stick to what insoluble fiber you’ll get as a byproduct of eating real fruits and vegetables.

How many fruits and vegetables should I eat, you might be wondering? And does that mean soluble, fermentable fiber, and lots of it, is fair game?

It depends. I know that’s not a sexy, easy answer, but it’s the right one. Allow me to explain.

The Sub-Saharan farmer who’s spent his life handling and milking goats, picking up and distributing manure, working the fields, plunging his bare hands into fresh loamy soil to plant a seed or pull a weed, taking his meals with soil still underneath his fingernails, eating lots of fiber-rich vegetation (often without washing it), and encountering not a single dose of antibiotics [41] is going to have a more robust, varied gut microbiome and greater capacity to handle fiber than the suburban pencil pusher (perhaps that term needs updating – let’s go with keyboard rattler or desk jockey [42]) who’s spent his childhood mostly indoors wearing a perma-sheen of hand sanitizer and sunscreen while eating a diet of peanut butter and jelly on white bread, mac and cheese, hot dogs, and pizza, and his later years wracked by chronic low level stress [43] that disrupts his gut flora and alters his digestion [44].

Should you fear fruits and vegetables because of the fiber? Has your modern upbringing ruined your digestive capability forever? No, I don’t think so. It may have temporarily impaired your ability to handle fermentable fibers – increasing numbers of people are reporting trouble with the class of fermentable carbohydrates known as FODMAPs (check the FODMAP list at this PDF [45]) largely because they don’t have the right levels/populations of gut flora – but it isn’t permanent. You just need to be aware of the complex, delicate interplay between the food fiber we eat, the composition and health of our gut flora, and our digestion. You should pay attention to your own digestion and how fiber affects it. You should introduce foods rich in soluble, fermentable fiber gradually and even cautiously. Allow time for your gut flora to adjust to the new food source. Expect flatulence.

But you should definitely introduce them.

Eventually, you’ll be able to fish a sample out of the toilet, snap a shot of it with your iPhone camera, and have your entire gut microbiome analyzed on the spot, complete with dietary fiber recommendations for optimal butyrate production and minimal flatulence, but that’s a long while off. Scientists are still figuring out which gut flora are best, which species are good and which are bad, what kind of fiber source they like, and how often and how much we should feed them. In other words, we’ve reached the stage of knowing enough to know that we know very little. In the meantime, we know “gut flora are important.” We have vague ideas of which populations are “good” based on correlative studies that link certain species with diseases. We know we need gut flora because of their endless interactions with the host, and that they need food. We know that the plants (and breast milk) we eat provide that food.

And that’s about it.

It’s enough to get started, though. I’d say between 75 and 100 grams of carbs from mostly vegetables and some fruits [46], plus the occasional emphasis on plants particularly heavy on the prebiotic fiber [47] – stuff like raw onion and garlic, leeks [48], jerusalem artichokes, dandelion greens [49], raw plantains and green bananas – should provide sufficient food for your gut. If you have too much, you’ll know it.

Oh, and the Big Ass Salad is definitely here to stay.

That’s what I’ve got, folks. What say you?