Does Fiber Make You Fat? : The Gut’s Effect on Weight and Metabolism

There is so much conflicting evidence about what is good and bad for us. Eggs: one day they cause heart disease, the next they are a supreme source of heart healthy fat. Coffee: it’s a super elixir that protects against dementia, and the next minute it’s toxic bean sludge. Fiber: too little? You won’t poop, you’ll develop diverticulitis, you’ll get cancer. Too much? You won’t poop, you’ll develop diverticulitis, you’ll get cancer. You can see how challenging it is to tease apart bad information from good! Regarding fiber, however, those following very low-carb/ketogenic diets sometimes feel that fiber, because it’s technically a carbohydrate, can increase the likelihood of weight gain. But wait….isn’t this the opposite of what we’ve heard all along, which is that fiber can help one LOSE weight? What are we to believe?

Generally, increased microbial diversity in the gut is beneficial for one’s health. The gut should be brimming with multiple species, as lively and lush as a rainforest, but instead, due to certain practices of modern living, we end up with a barren, deserted, gut wasteland. Some modern practices that alter the development of an optimal gut environment are things like antibiotics, being born via C-section, being formula-fed over breastfed, and not playing outside in the dirt often enough. And when there’s not a full, lively ecosystem of healthy bugs in one’s gut, pathogens can take hold and potentially wreak havoc. It’s obvious that an unhealthy microbiome is related to the development of irritable bowel syndrome and inflammatory bowel disease, and there’s now evidence that dysbiosis, or bacterial imbalance in the gut, is related to the development of autoimmune disease as well. But what about obesity? Obesity, like the others listed, is a disease of modern civilization that is on the rise- does an imbalanced gut contribute here? And, perhaps more importantly, can we potentially treat obesity by modifying the gut flora? Given that up to 90% of the genetic material in your body actually belongs to your microbiome, isn’t it at least plausible that your gut bugs help modulate weight and metabolism?

Let’s take a trip down to the large intestine! This organ houses about 99% of the bacteria that live in and on your body, and one of their primary jobs is to break down dietary fiber that you were unable to digest yourself. When your gut bugs do this important job, they provide you with beneficial short-chain fatty acids (SCFAs), such as the all-star butyrate. These SCFAs are helpful for several reasons including providing extra calories to you. You may not think you WANT extra calories, but from an evolutionary standpoint this was a good thing: hunter-gatherers wanted to get everything they could out of each meal! In modern society, however, these extra calories help by keeping you full for longer stretches, and SCFAs provide a localized anti-inflammatory effect on the gut. Though we know that SCFAs are extra food as a gift from your gut bugs to YOU, if you have an imbalance of gut flora due to some of those modern lifestyle practices discussed above, you may have too many bugs that extract and give back TOO MANY SCFAs, and presto! Weight gain can occur. Though it was previously thought that those bugs in the Firmicutes phylum seemed to extract more calories from food (aka, “fat flora”), and those from the Bacteroidetes phylum seemed more associated with leanness, current research does not show such an obvious and simple correlation. The bottom line, however, is that a balanced gut with appropriate diversity contributes to a healthy weight and metabolism.

You may be thinking at this point that you don’t have any obvious risk factors for gut imbalance. You were born vaginally, breastfed, spent your childhood outside, and have never taken an antibiotic. Well, if all of that’s true, you are a bit of a miracle, but I do want to remind you that unless you are eating 100% organic, antibiotic free meats, fish, dairy, and produce, AND drinking filtered water ALL of the time, you’re being exposed to low levels of antibiotics. Over time, this ultimately can cause some unhealthy shifts in your gut, and potentially, your weight. We already know that giving antibiotics to farm animals helps them fatten up before slaughter, so is it possible humans are at risk for this phenomenon as well Let’s check out the SCIENCE:

Several experiments have been performed looking at the effects of stool implants on weight and metabolism using germ-free mice (mice raised in a sterile environment that, as such, do not have their own microbiome). One study took stool from human twins, where one twin was lean and one was obese. When evaluating the stools of these donors, it was noted that there was less bacterial diversity in the obese donor’s stool than the lean donor’s stool. After implanting stool from the lean donor into the germ-free mice, the mice remained lean as they aged; however, when stool was implanted from the obese donor, the result was chubby mice. So researchers took this experiment a step further: they took the guys given lean stool and the guys given obese stool and then placed them in the same living quarters. They fed them all standard mouse chow, which is essentially high fiber/low fat pellets. Interestingly, due to a particular mouse habit, as the mice aged, they all ended up lean. What is this particular habit, you ask? It turns out, my friends, mice like to chow down on each other’s stool…nom nom? This habit allows them to share flora, and in this case, the sharing allowed a diverse microbiome to develop, resulting in a lean body type. The next experiment added one small twist- the mice given stool from the obese donor were also placed on a high (junky seed oil mixed with sugar) fat diet, which is a terrible idea for a mouse, and, even though all of the mice lived together, those given obese donor stool and a high fat diet still became obese. Even with access to greater bacterial diversity, the bad diet could not be overcome when the starting point was unhealthy flora.

Now lets add low doses of antibiotics to the mix.  Mice in the next experiment were given lean donor stool, but then fed low doses of antibiotics with their normal mouse chow.  The end result was a bigger, beefier, but not grossly obese, mouse with 15% more body fat on DEXA scanning, AND stool that was NOT terribly diverse, but WAS dominated by bugs that were VERY good at extracting calories from fiber and donating TOO MANY SCFAs back, leading to increased body fat.  Next, mice were given lean donor stool but this time were fed the high fat diet PLUS low doses of antibiotics.  The end result?  A disaster.  The male mice had a 25% increase in body fat, and the females a 100% increase.  Yikes…sorry ladies.

There are a few human studies looking at the metabolic effects of fecal transplants, and though they are promising, the number of studies and the number of participants within each study are limited- I mean, there just aren’t that many people signing up for poop transplants. Obviously. There are, however, studies looking at increased fiber consumption’s effect on weight and metabolic disease. In one study, the group of participants was divided in two: Group 1 added a prebiotic (aka bacteria food) fiber supplement to their daily regimen whereas Group 2 did not. After 12 weeks, on the same food plan, Group 1 showed slightly higher weight loss, improved blood sugars, and a decrease in ghrelin levels (the little hormone that shouts at you “I’m hungry!” when you haven’t eaten in a while). Another similarly designed study used supplemental kimchi (prebiotic and probiotic all in one!) instead of fiber alone, and found that those receiving the kimchi showed improved weight loss, less belly fat and lowered triglycerides over those that didn’t.

So why would a little bit of extra dietary fiber lead to measurable improvements in metabolic disease? Well, as we learned earlier, SCFAs help keep you full, so ultimately you eat less. AND we just learned that fiber can lower the ghrelin hormone, again which lessens hunger. But there’s MORE! Fiber up-regulates production of glucagon-like peptides 1 and 2, which are proteins produced in the gastrointestinal tract that help reduce appetite, improve insulin sensitivity, and slow gastric emptying, so that (you guessed it), you feel FULL LONGER. Fiber! So multifaceted!

There is still much to learn about fiber’s role in our weight and metabolism, but some things are clear:

  • A diverse gut is ultimately healthier, and if you have a diverse, balanced gut you are less likely to have too many “fat flora,” that can potentially lead to weight gain.
  • Fiber, though it is a carbohydrate, does not seem to have significant consequences when it comes to weight and metabolism, and in fact, current research demonstrates that additional fiber can even improve these issues.
  • As such, if you count your carbohydrates for weight loss or blood sugar management, it’s reasonable to try counting your NET carbohydrates instead of your TOTAL carbohydrates (which includes carbohydrates from fiber).
  • Increasing gut diversity through pre- and probiotics can be a tricky feat in our modern lives- we just don’t have the exposure to bugs in our contemporary world as actual hunter-gatherers do in theirs. Because we in modern society are at increased risk for dysbiosis over hunter-gatherers, increasing fiber and probiotics may cause some GI distress. If this occurs, consider working with a functional practitioner to achieve the healthiest gut possible for YOU. Remember that MORE is not necessarily BETTER for everyone when it comes to fiber.

So, can increasing dietary fiber, and therefore, shifting your gut bacteria help you lose weight? At this point, the evidence seems to point to…maybe! Consider this cool fact: researchers can predict with 90% accuracy whether someone is lean or obese by looking at their gut flora, whereas they can only predict if one is lean or obese with 58% accuracy by looking at the person’s own genes. That’s bonkers! The bottom line is that even if we don’t know if adding fiber in hopes of shifting our gut flora will help with SIGNIFICANT weight loss, we know that it won’t hurt, even though it’s a carbohydrate, and, more than anything, may help you maintain a full, healthy tummy. It all comes down to this, low-carb and keto peeps: Eat your veggies.