A week of holiday junk food could derail your gut microbiome—but there’s hope

Treat your gut to some roughage.
Eat delicious foods like pizza, but add fibrous veggies to them. Pixabay

We’re right between Christmas and New Years—that time of the year when we often allow ourselves to indulge in deliciously sweet and savory treats. For one week out of the whole year, that’s absolutely fine. Moderation is key to maintaining a healthy diet all year round.

But it’s important not to completely neglect your fruits and vegetables, and not just for the vitamins and other nutrients they provide. Yup: we’re talking about fiber.

The amount of fiber in a person’s diet can affect weight, blood sugar, insulin regulation, and gut health. But scientists still don’t completely understand how fiber makes all this happen. A pair of 2017 studies in the journal Cell attempt to understand why fiber is so important. In a study with mice, researchers found that just three days on a low fiber diet can change gut microbe diversity, and alter the protective mucus-y layer that forms a sort of seal between your intestinal cells and the surrounding food and bacteria.

Humans can’t actually digest and use any of the fiber we eat. It just passes on through. So why is it so good for us?

Most of the food we eat is quickly broken down in the high-acidity environment of the stomach, then absorbed there and in our small intestines. But fiber survives this entire process and makes its way to the colon unscathed, where it’s feasted upon by resident bacteria of the large intestine. Consequently, the more fiber we eat, the more energy we can provide our microbes, and the more abundant and diverse they become. Our microbes then, in turn, do all sorts of good things for us—like help us digest certain foods, regulate our metabolism and blood sugar, and maintain a healthy weight. Research suggests that the healthiest gut microbiome is one full of a diverse flourish of flora, so it follows that high doses of fiber help keep us healthy.

But what happens if you take a few days—or even a week—off your fastidious fiber foraging in favor of more frivolous feasts? Researchers found that after just three to seven days on a low-fiber diet, mice microbiomes changed and shrank, becoming less diverse as various bacterial species died off. Additionally, the key layer of mucus that protects the lining of the intestine started to break down. That allows bacteria to reach the gut lining, triggering widespread inflammation which can contribute to metabolic diseases like diabetes and obesity.

So-called Western diets, featuring more heavily processed foods that are thus lower in fiber, have long been associated with a slew of health issues like diabetes, obesity, and inflammatory diseases (like IBD). And research indicates that many people with these conditions have altered microbial diversity. The new study shows just how quickly these microbes start dying off—just one week after a dietary shift.

When researchers switched the mice onto diets with lots of a fiber called inulin, the change did create a more diverse and microbe-friendly gut. Unfortunately, their mousy microbiomes never quite reached the level of diversity seen at the start of the study.

What does this mean for me?

Fiber is really, really important. Although it’s fine to indulge in foods that are not considered the healthiest, we should probably never completely neglect fiber. Anxiety over packing on a pound or two might have you counting calories and denying yourself cookies, but you’d be much better off focusing your efforts on keeping fruits and veggies in the mix as you enjoy treats in moderation.

And while you’re at it, keep high-fiber diets in your heart every day of the year. Americans, on average, consume just 15 grams of fiber daily, which is something like half the recommended amount. Even if you meet that recommended threshold, you can do better: a study done this year analyzing the diets of the Hadza, a hunter-gatherer tribe in Africa, found that they eat, on average, 100 grams of fiber a day. They tend to live long and healthy lives, and they definitely have healthier guts.

Of course, the entire world can’t go back to a hunter-gatherer lifestyle. Instead, researchers are figuring out which microbes help our guts (and how) so we can harness them to treat and reverse certain diseases. But in the meantime, you should definitely eat more fruits and vegetables.