Hi Friends, and happy holidays!
In this issue: A New Year’s Resolution for more fiber!
For anyone new—this is a monthly newsletter where we address new, or older but relevant, research findings, as well as summarize recent gut-related news. Here, “gut-related” is broadly interpreted, so we will be covering anything that might affect the gut, both “top down” and “bottom up.” This means brain/mind things (“top down”) such as psychology of stress, resilience, and emotion regulation, as well as body-based things (“bottom up”) including inflammation and typical comorbidities of gut disorders.
Dr. Lisa Goehler
Table of Contents
An Overview: Fiber, the Gut, and Why It Matters
The end of the year is a time when many people take stock of the previous year and make resolutions to do better in the next year—“New Year’s Resolutions.” For this month’s newsletter, I have a suggestion for a good New Year’s resolution: Get more fiber in your diet!
Why is this a great resolution?
The benefits of getting enough fiber involve quality of life and overall health, including improved gut health and function, reduced risk of insulin resistance and type 2 diabetes, and improved control of appetite and inflammation (Ariyarathna 2025, Bai 2022, Guan 2022, Kim 2020, Omary 2025, Weickert 2018, Zheng 2025). Fiber is critical for supporting the microbes in our guts that provide energy and regulate metabolism and immune system function (e.g., Cronin 2021, Guan 2021, Yuksel 2025).
But most people in Western/“developed”/“industrialized” countries don’t get enough fiber in their diets (Quagliani 2016).
How much is enough?
Good question! U.S. dietary guidelines indicate that age and gender influence how much fiber we need (2020). They recommend 31 grams of fiber for men aged 31–50, but only 28 grams for men over 50. Similarly, women aged 31–50 are recommended to get 25 grams of fiber, but older women (over 50) only 22 grams. Why do women and older people “need” a few grams less? No idea… it isn’t made clear, and I can’t think of a reason on my own. But at the least, these numbers give us a good “ballpark” estimate of how much fiber we should be getting.
How to get enough?
For much of my life, when I heard “fiber,” I thought “roughage”—things like bran and fibrous parts of vegetables such as broccoli. What I was thinking of is called insoluble fiber, and it is made of plant parts that do not dissolve in water. But there is another type of fiber called soluble fiber that does dissolve in water (but to me doesn’t seem very “fiber-y”).
Other fibers we might encounter are not in foods but are refined from plants or manufactured for supplements. Examples are guar gum, psyllium, xanthan gum, or alginate. These are typically soluble fibers (Weigert 2018).
What kind of fiber is best?
I’ve been unclear on what kind is best because the literature on that has been confusing. Historically, most studies of fiber benefits have studied soluble fiber, but most fiber-containing foods contain both. And as it happens, we need both kinds of fiber.
Soluble fiber includes beans, lentils, chickpeas; grains—especially wheat bran, barley, and oats; most fruits, especially apples, avocados, and bananas; and green leafy vegetables, especially cabbages.
Insoluble fiber is found in green leafy vegetables, intact grains and bran, and fruit and vegetable skins.
I find all foods with fiber to be delicious, but some are my favorites: grains, greens, and beans.
Fun Facts About Grains
Grains are underappreciated generally, and even vilified by some. But they are key sources of both soluble and insoluble fiber.
Grains are grass seeds, from the grass family Poaceae (for plant nerds). This includes wheat, rice, maize, oats, rye, barley, triticale, sorghum, and millet.
The seeds have three parts: bran, germ, and endosperm. Processing grains removes the bran and germ, which leaves only the starchy endosperm and some proteins. This dramatically reduces the grain’s nutritional composition (Ariyarathna 2025), and therefore flours are often “enriched” to replace some of the nutrients.
What counts as “whole grain”? “According to the Cereals and Grains Association, whole grains consist of the intact, ground, cracked, flaked, or otherwise processed kernel, excluding inedible parts such as the hull and husk. In whole grains, all the anatomical components, including the endosperm, germ, and bran, must be present in the same relative proportions as found in the intact kernel” (Ariyarathna 2025). This description implies that foods we buy that are labeled “whole grain” may actually have been reconstructed from processed, refined grains. This is why there are recommendations for consuming “intact grains” as much as possible, such as cracked wheat.
A paradox: even though “insoluble fiber” is supposedly not fermented (although it almost certainly is, albeit slowly), it has reliable and consistent unexplained benefits for insulin resistance and type 2 diabetes (Ariyarathna 2025, Omary 2025, Weikert 2018).
The traditional Mediterranean diet, which many studies have shown is beneficial for a wide range of conditions, contains nearly 50% grains (Capurso 2021).
Fun Facts About Greens
I overhear so many people talking about how they hate greens, and how they always avoid eating gasp a salad. I see memes on the internet disparaging salad, and… I’m sad, because green leafy vegetables are super important for gut and brain function.
Greens are sources of cellulose, which has been shown to increase fecal butyrate and improve gut inflammation (Kim 2020).
Diets high in leafy greens are associated with better cognitive function than diets low in green leafy vegetables (Morris 2018, Agarwal 2023, Liu 2025).
Fun Facts About Beans and Other Legumes
Beans are excellent sources of fiber.
Concern about “lectins” has inspired advice to avoid eating them, but beans and other legumes don’t have that many more lectins than other vegetables, and the lectins they do have are denatured by cooking. Who eats raw beans anyway?
Beans, along with grains, are a staple of the Mediterranean and other traditional diets that are highly nutritious and associated with good health.
Bottom Line
A diet that is low in refined grains (white flour, especially from ultra-processed food) but rich in intact grains, beans, fruits, mushrooms, and vegetables will provide enough fiber.
Getting fiber when gluten-free
Since we are focusing on fiber, it is worth noting that gluten-free diets tend to be low in fiber (Taetzsch 2018), and thus extra effort should be made to help incorporate more fiber into a gluten-free diet.
What foods are gluten-free but good sources of fiber? There are lots of choices, including the following:
Avocados have about 14 grams per whole one (celiac.com), and yes—I for one can eat a whole one.
Beans range from 11–17 grams per cup, and they are also very good sources of protein.
Nuts can have 4–12 grams per ounce!
Figs have 16 grams of fiber per cup.
Passion fruit was a surprise to me—it reportedly has 24.5 grams of fiber per cup (celiac.com). Wow!
New Guidelines on Fiber for Inflammatory Bowel Disease
Historically, diet recommendations for inflammatory bowel disease (IBD) have excluded fiber, for reasons that have now been discredited. Fortunately for IBD patients, there are now enough basic and clinical studies showing that recommendations have shifted to include fiber—even for people who have ileostomies or have recently had surgery (Gold, et al.).
Fiber improves constipation because it increases bulk, making feces move through the colon easier.
I see this claim all the time, everywhere, when fiber and constipation are discussed. It has always mystified me, as someone who has struggled with constipation. I’ve usually found that “bulk” was more of a problem than a solution. Where this claim came from, I have no idea. We have known for many decades that fiber is beneficial for “regularity,” and can help manage constipation. I’m guessing that someone thought this up as an explanation, and in the absence of a better one, it became dogma.
What are the real ways that fiber helps regulate gut motility?
The principal way that fiber helps is by supporting gut microbes that modulate the gut nervous system, and thus motility. Zheng and colleagues (2025) recently published a review of what we know about the effects of microbes on constipation. They highlight the key role of certain groups of microbes that ferment fibers in normal bowel motility. The mechanism mostly relies on interactions between microbial products—such as short-chain fatty acids (like butyrate)—and serotonin, which regulates gut motility.
So, while fiber can help stool pass by absorbing water and softening it, the major benefits of fiber rely on the products of specific gut microbes, which in turn rely on dietary fiber.
Every month, we pick a published (and peer reviewed) article to highlight and discuss.
This month’s selected article
Title
A core microbiome signature as an indicator of health.
Authors
Wu G, Xu T, Zhao N, Lam YY, Ding X, Wei D, Fan J, Shi Y, Li X, Li M, Ji S, Wang X, Fu H, Zhang F, Shi Y, Zhang C, Peng Y, Zhao L.
Publication
Cell. 2024
I think this paper is worth a read! Check it out →
A key challenge for me, when trying to advise about microbes and gut health, has been making sense of the literature. Most studies investigate only one species, often use subjective outcomes, and show low consistency across findings. For example, a study may report that “microbe X improves symptom Y,” followed by another showing that microbe X worsens symptom Y, and then another showing no effect at all. It’s understandable why I’ve become skeptical of many clinical studies on gut microbes.
Microbial populations in our guts are highly complex, and their health and behavior depend on each other, our diet, and poorly understood genetic factors. The study by Wu et al. (2024) addresses this complexity by using sophisticated genetic analyses of gut microbiota in response to a high-fiber diet in people with type 2 diabetes. It is not an easy read, but the findings are important.
Highlights include:
The authors describe groups of unrelated microbes that interact and carry out similar functions (such as producing beneficial short-chain fatty acids like butyrate), which they call “guilds.” I like this term—it echoes medieval guilds such as bakers or brewers, groups organized around shared functions.
The study identified two guilds that responded differently to a high-fiber diet. One was considered “beneficial” and produced butyrate, which supports immune regulation, metabolism, and gut function. The other “detrimental” guild expressed antibiotic resistance and virulence factors that could promote inflammation.
The balance between these guilds was strongly influenced by dietary fiber. High fiber intake increased the beneficial guild and suppressed the detrimental one.
The authors propose thinking of beneficial microbes as “housekeeping microbes,” analogous to housekeeping genes that keep cells functioning properly. These microbes help regulate inflammation, support gut cells, and maintain overall physiological balance.
Tips for getting fiber:
I like to throw ½–1 cup of baker’s bran, or my new favorite, coarse cracked wheat, into breads and doughs to add texture and boost fiber. My gut thanks me for this.
Other favorites to consider when shopping or planning meals include farro, an intact heritage wheat that’s delicious in Mediterranean-style grain salads in summer and hearty casseroles in winter, as well as barley.
And mushrooms!
Unexpected sources of fiber: Condiments, garnishes, and appetizers can contain a surprising amount of fiber. Guacamole is a great example, with about 14 grams of fiber per avocado. This is one of the ways I know I can eat a whole avocado. On a recent trip to a favorite restaurant, I may have eaten closer to two whole avocados—wow! That’s a day’s worth of fiber in one appetizer. Bean dips, both spicy red bean and Mediterranean-style white bean dips, can also be excellent sources of fiber.
Do you have a tip for us? Let us know!
Every month we will highlight an easy to make, gut-healthy dish that we are eating now!
Mom’s Dark Fruitcake
(adapted from Grandma Thelma’s recipe)
This recipe is from my grandmother, Thelma Williams, passed to me from my mother, June Williams Goehler, with some modifications. (Comments in parentheses are mine!)
In keeping with the fiber theme, here’s what we’re eating now: my favorite fiber-filled Christmas treat—fruitcake!
Yes! Fruitcake. This homemade, more traditional recipe is loaded with fiber-rich dried fruit, including prunes and apricots. Surprisingly, the prunes are delicious in it. Every year I think I should add even more. The recipe is low in added sugar, relying on fruit for sweetness, balanced by the tartness of apricots and cherries. Walnuts add crunch.
Ingredients
3 cups seedless raisins (mixed black & yellow)
3 cups chopped dates
2 cups chopped walnuts
1 cup sherry (or a little more, depending on how dry the fruit is)
1 lb dried fruit, chopped (any you like—tangy is good; we use tart cherries, prunes, apricots; roughly 2 cups total)
Combine the above in a large bowl and marinate overnight.
1 cup soft butter
1 cup brown sugar
Cream together in a separate bowl.
4 eggs
Add one at a time to the sugar mixture, beating well.
Then add to the fruit mixture.
Combine:
2 cups flour
1 tsp cinnamon
1 tsp cloves (we use a generous interpretation; we also add cardamom and ginger)
Pinch of salt
Pinch of baking soda
Add to fruit mixture and mix well.
Brandy
This makes enough for two wax (parchment)-paper–lined medium loaf pans (9½ × 5½ × 2½ inches).
Place a pan of water on the floor of the oven.
Bake at 275°F for 2+ hours—check with a toothpick. (The top starts to crack when done.)
When done, remove from pans while still warm and brush with warm glaze (or skip this—it’s still good with just the brandy).
Glaze:
½ cup brown sugar
⅓ cup light corn syrup
⅓ cup water
2 Tbsp lemon juice
Boil for 2 minutes (makes plenty of glaze).
(You can also use just lemon juice and brandy instead of glaze—it’s very nice.)
When cool, sprinkle about 1 Tbsp brandy on top of each loaf.
Repeat this every week or so (this fruitcake is very tasty with coffee—we find it doesn’t last long in our house).
Wrap in aluminum foil to store.
Enjoy!
Community Affairs
Aesthetic changes
My website and branding is undergoing a bit of a change (new colors!), so if you were confused by the orange and purple, that’s why! These changes will be incorporated into next months email newsletter as well.
About the Author
Lisa E. Goehler, Ph.D. is a neuroscientist and expert in the science and treatment of psychological stress, chronic inflammation, and gut-related disorders. She pioneered the study of how GI-tract related bacteria can interact with the brain to lower mood and increase anxiety. Throughout her career, she authored over fifty publications and contributed to peer review for scientific journals and funding agencies, including the National Institute for Health.
References
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