Low Fiber: A Low-Carb Casualty

This post is updated and adapted from a chapter in my Essential Thyroid Cookbook, an article I wrote for Susan Blum, MD, and an article I wrote for Tastebud Magazine.

Fiber. It’s not a very…sexy topic. Just the word conjures up images of Metamucil commercials with the sandy-looking granules swirling in a glass and promises of being “regular.” But the benefits of a whole food, high fiber diet are vast and certainly extend beyond the water closet.

Most Americans are fiber deficient—some experts in the functional medicine community claim that it’s the most clinically important dietary deficiency largely due to the Standard American Diet (SAD), which doesn’t favor whole, nutrient-dense foods like fruits and vegetables (complex carbs) and is chock full of sugar and processed carbohydrates in the form of “junk flour” from conventional bread, pasta, bagels, etc. (aka simple carbs).

[You can learn about healthful fat, protein, and carb sources, how to easily balance these macronutrients, and the distinction between simple and complex carbs in my ebook, Balance Your Blood Sugar, Balance Your Life.]

Low carb, low fiber?

You could consider this post Part 2 of my low-carb rant (go here for Part 1) because we can’t discuss the important role of fiber and sidestep the carb conversation, given that so many gut-healing, immune modulatory diets (like AIP or Paleo) eschew—or largely eschew—carbs and that fiber-rich foods are largely carbohydrates.

You may be thinking…What??I thought I was supposed to be low-carb to heal my gut and reverse my autoimmune condition. Read on!

A low-carb diet, with its overreliance on fat and protein and under-reliance on grains, legumes, and starchy vegetables, tends to be low-fiber. Fiber not only helps reduce the risk of heart disease and diabetes, but also helps with weight loss. According to Monica Reinagle, licensed nutritionist, “Trying to lose weight on a low-fiber diet is like parallel parking without power steering.”

Unlike macronutrients and micronutrients that our bodies break down and absorb, dietary fiber (also known as roughage or bulk) isn’t actually digested. It comes in two forms, soluable and insoluable; both are essential and can be obtained from a wide variety of delicious, high-fiber foods like true whole grains, fruits, vegetables, nuts, seeds, and legumes, which indeed, are mostly carbohydrates, although nuts, seeds, and legumes have their share of protein.

Fiber and the gut

The importance of fiber in the diet is indisputable and has a profound impact on our digestive health and microbiome, our 100 trillion organism-strong “mini ecosystem.” An equally important consideration for those with autoimmunity and the concomitant digestive concerns is our gut microbiome, “the forgotten organ.”

Justin Sonnenburg, PhD and Associate Professor of Microbiology and Immunology at Stanford, is the author of The Good Gut, co-authored with his wife, Erica Sonnenburg, PhD. The Sonnenburgs are considered some of today’s preeminent experts on digestive health and in the science of the fibers found in grains and their role in providing an important fuel source for the microbiome.

Justin says, “You have to ask the question of what it means when we’re consuming 15 grams of dietary fiber per day instead of 150—a 10-fold decrease in the foods that feed our gut microbe.” (Source)

In their book, the Sonnenburgs state, “Increasing dietary fiber is essential to cultivating diversity in the microbiota. Microbes in the gut thrive on the complex carbohydrates that dietary fiber is primarily composed of. But rather than ‘dietary fiber,’ we prefer ‘microbiota accessible carbohydrates,’ or MACs. MACs are the components within dietary fiber that gut microbes feed on. Eating more MACs can provide more nourishment to the microbiota, help gut microbes thrive, and improve the diversity of this community. Our family eats what we jokingly refer to as a ‘Big MAC diet.’ This diet is rich in complex carbohydrates from fruit, vegetables, legumes, and unrefined whole grains, and is designed to create and maintain diversity within the gut microbiota.”

Dr. John Douillard states, “While fiber is linked to heart health, it is also critical for the protective health of the intestinal skin. If the intestinal skin breaks down, the beneficial gut microbes disappear.”

If you’re still unconvinced that grains and legumes can be part of a gut healing protocol, Dr. Susan Blum mentions quinoa, amaranth, millet, teff, buckwheat, various types of rice, and legumes in her book, The Immune System Recovery Plan, and incorporates these foods in several of her recipes. She calls them “foods to include” and states, “Fit lots of fiber onto your plate in the form of veggies, low sugar fruit like berries and apples, whole gluten-free grains, and legumes—to feed the good bacteria of the gut.”

If you have autoimmunity and you’re shocked at the suggestion to consider whole grains and/or legumes, please check out my posts, In Defense of Grains and In Defense of Legumes. You can also check out my post, AIP for Autoimmunity: Is it ALWAYS the Answer?

Because of the common fiber-constipation association, many may not be aware of the many other benefits of adequate fiber intake:

  • Promotes weight loss: Fiber may be your best friend if you’re trying to lose weight, as foods rich in fiber are filling, which means you eat less and stay fuller longer.
  • Lowers risk of digestive conditions such as irritable bowel syndrome
  • Lowers risk of heart disease by getting rid of digestive debris and environmental toxins and keeping bad cholesterol in check
  • Lowers risk of diabetes: Researchers are now finding that the fiber in grains specifically lowers the risk of diabetes. A September 2018 study from The Journal of Nutrition that followed 55,000 middle-aged women and men for 15 years discovered that those who consumed the most whole grains had the lowest risk of Type 2 diabetes.
  • Lowers risk of stroke
  • Promotes healthy estrogen levels, which can improve thyroid function, as there’s an important relationship between estrogen and thyroid hormones.

It’s easy to get enough

If you think you may be fiber deficient, slow and steady wins the race. Going overboard and increasing your intake with gusto can cause gut distress. And drink plenty of water.

Regardless of any deficiency, we all need regular fiber in our diet. At the end of the day, if you’re committed to a whole foods diet rich in color and variety, you’re likely getting the fiber that you need.

  • Add nuts, seeds, and flaxseeds * to whole grain cereals, salads, soups, and smoothies
  • Snack on raw veggies
  • Beans and lentils play well with others; use them in soups, salads and many of your other favorite dishes
  • Choose whole grain products vs. refined, like crackers, cereal, bread, tortillas, and pasta, although I’m not categorically against these foods (Beware, as many so-called whole wheat products are made with white flour. Avoid “unbleached flour” and “flour,” both of which are merely white flour.)
  • Eat fruit as a dessert or snack; the skin and/or seeds is where you’ll get the most fiber. Berries are your best choice, as they’re low on the glycemic index. Apples and pears are the best skin-on fruits.
  • Incorporate all kinds of vegetables into every meal. In general, the darker the veggie, the higher the fiber.
  • Get all that you can—it all counts!

* Speaking of flaxseeds, both ground flax and ground psyllium seed are excellent sources of supplemental fiber. Psyllium seed is the primary ingredient in products like Metamucil, although I don’t recommend these products, as they don’t provide the vitamins, minerals, and other disease-fighting phytonutrients and antioxidants that high fiber, whole foods do. Yet some people may need a pure psyllium supplement, especially if they have certain medical conditions, such as irritable bowel syndrome. It’s also important to note that there may be a risk of drug interactions. Always check with your doctor before taking fiber supplements.


I am surprised you suggest eating lentils. Lentils are sprayed an exorbitant amount of times from when they first appear out of the ground. Organic or not that is definitely one thing I would never consume. 
Didn't know you could eat  grains for Hashimotos. How would the gut heal?


Hi Joyce, can you provide me with information that organic lentils are sprayed? By definition, they can't be labeled organic if they're raised with pesticides, etc. As for grains and Hashimoto's, see links above for my posts: 'In Defense of Grains' and 'AIP for Autoimmunity: Is it Always the Answer?' I've worked with the Hashimoto's community since 2008 and only a handful have been AIP, by their own choosing. 

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