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So much focus in recent years has landed on the gut, and rightly so. It houses around 70% of the body’s immune cells, hosts more microorganisms than any other site in the body, and acts as a central hub connecting nearly every major system, from the gut-brain axis to the gut-skin and gut-muscle axes.¹
The consumer side of the story is also becoming clear. Women have led the gut health category for years, but men are increasingly recognizing its importance, particularly as they age. And for the longevity-minded consumer in mid-life (or older), the gut moves from a wellness curiosity to a real priority. Gut function declines with age, as do the related functions of nutrient absorption and immune resilience.
Which raises a question worth digesting — pun intended. So much of the longevity conversation still centers on what to take, whether that is creatine, collagen, spermidine, or NAD+ precursors. But every one of those compounds must pass through the gut first. If the gut isn’t working well, the rest of the longevity stack is operating with a handicap.
What changes in the aging gut, and why it matters
Two shifts stand out. Both are measurable, and both respond to diet.
The first is microbial alpha diversity: the breadth and balance of microbial species living in the gastrointestinal tract. Diversity tends to decline with age, and higher diversity has consistently been associated with healthier aging and a lower biological age relative to chronological age.²
The second is short-chain fatty acid (SCFA) production. SCFAs are metabolic byproducts — well-known ones being butyrate, propionate, and acetate — that gut bacteria produce when they ferment dietary fibers in the colon, since humans lack the enzymes to break those fibers down ourselves.³
It is a quiet, symbiotic arrangement: we provide the substrate, the bacteria do the chemistry, and their resulting metabolites do real work for us. A balanced SCFA profile helps maintain the gut barrier, regulates immune signaling, and dampens the chronic low-grade inflammation often described as “inflammaging.”
Healthy SCFA levels have also been linked to improved insulin sensitivity, more stable blood glucose regulation, and stronger intestinal barrier function.⁴ Butyrate in particular is the primary fuel source for colonocytes, the cells lining the colon, which makes it central to maintaining a healthy gut wall and preventing "leaky gut."⁵
As diversity and SCFA production decline with age, the downstream effects compound. Nutrient absorption becomes less efficient, inflammatory signaling rises, and overall resilience weakens. But these shifts aren’t destiny. Diet remains one of the most consistent and scalable levers we have for shaping gut composition, diversity, and SCFA output.
The gut as the gateway
Many longevity interventions underdeliver because they treat the gut as a passive pipeline rather than an active participant. Everything that enters the diet, from protein and creatine to spermidine and NAD+ precursors, must first pass through the gut lining. As we age, that lining can become compromised, reducing absorption efficiency, while a less diverse microbiome can change how compounds are metabolized and ultimately used by the body.
In active nutrition, muscle protein synthesis, nutrient uptake, recovery, and immune resilience are all mediated — at least partly — through gut function. Gut health isn’t a wellness-adjacent consideration. It is the central control system through which every other longevity intervention must operate.
Not all fibers behave equally
Fiber sits at the center of gut health, but fiber isn’t one thing.
Different fibers ferment, or are eaten by microbes, at different speeds in different regions of the colon. There are two broad categories — soluble and insoluble — both of which the diet needs. Soluble fibers dissolve in water to form a thick gel in the digestive tract, showing health benefits like lowering LDL cholesterol and promoting satiety. Insoluble fibers do not dissolve in water and add bulk to stool, showing health benefits like supporting healthy bowel movement.
Soluble fibers like inulin ferment rapidly in the upper colon. Beta-glucan also ferments quickly, primarily in the proximal-to-mid colon. Both show prebiotic effects, meaning a beneficial shift in gut microbiome composition and the production of SCFAs.
However, soluble fibers behave differently from insoluble fibers like chitin and chitosan present in fungal mycelium, which ferment more slowly and reach further into the lower colon. That distribution matters, because where and how a fiber ferments shapes which microbes it feeds and which metabolites it produces.
Put simply: the type of fiber matters as much as the amount.
Introducing Fermotein®: A whole-food approach to gut-first longevity
Fermotein® is an example of a whole-food ingredient designed to support a gut-first approach to longevity.
The nutrient-dense mycelium powder is minimally processed and naturally contains around 30% fiber unique to fungi, including chitin and chitosan. These fibers are distinct from plant-derived fibers in both structure and fermentation behavior, and that distinction shows up in scientific data.
Our external in vitro fermentation work with TNO shows that Fermotein®, which is rich in chitin and chitosan, increased microbial alpha diversity relative to its control. (Inulin, by contrast, reduced alpha diversity in a similar study design.)⁶ In other words, the fungal mycelium moved the microbiome in the direction associated with healthier aging.
The SCFA picture was equally distinctive. Inulin produced a narrower response: a strong increase in butyrate but a reduction in propionate. Fermotein® produced a broader, more balanced increase across all three metabolites: butyrate, propionate, and acetate.⁶ Rather than amplifying a single pathway, Fermotein® supported different SCFAs, which is relevant when the goal is overall gut and metabolic resilience.
Beyond fiber, Fermotein® delivers a complete protein with all essential amino acids, while retaining the natural fiber matrix, which is typically removed during isolate production. As amino acid availability becomes increasingly important for aging consumers — for both muscle maintenance and gut barrier — protein quality matters even more. And because Fermotein® is one of the highest food sources of spermidine, a single whole-food ingredient ends up addressing multiple layers of the longevity stack at once: protein, fiber, and cellular health.
A food-first model for healthspan
Across this series, we have followed a single thread. In the first post, we looked at longevity through the lens of cellular health and the role of spermidine in supporting autophagy. In the second, we looked at how performance nutrition needs to evolve for the aging athlete, where fiber and complete protein together support muscle renewal and repair. Here, we have stepped back further, to the gut-first system that underlies all of it.
Gut health isn’t just another category within health and wellness. It is the biological foundation that helps every other system function well. As longevity continues to move into the mainstream, the most credible approaches will be the ones that work with that reality: food-first, holistic, and built for healthier years.
Interested in further insights, or trying Fermotein® in your own product?
(1) Wiertsema SP, van Bergenhenegouwen J, Garssen J, Knippels LMJ. The interplay between the gut microbiome and the immune system in the context of infectious diseases throughout life and the role of nutrition in optimizing treatment strategies. Nutrients. 2021;13(3):886. doi:10.3390/nu13030886. On the gut–brain axis, see Cryan JF, et al. The microbiota–gut–brain axis. Physiol Rev. 2019;99(4):1877–2013. doi:10.1152/physrev.00018.2018.
(2) Wilmanski T, Diener C, Rappaport N, et al. Gut microbiome pattern reflects healthy ageing and predicts survival in humans. Nat Metab. 2021;3(2):274–286. doi:10.1038/s42255-021-00348-0. See also Tuikhar N, et al. Comparative analysis of the gut microbiota in centenarians and young adults shows a common signature across genotypically non-related populations. Mech Ageing Dev. 2019;179:23–35. doi:10.1016/j.mad.2019.02.001.
(3) Mann ER, Lam YK, Uhlig HH. Short-chain fatty acids: linking diet, the microbiome and immunity. Nat Rev Immunol. 2024;24(8):577–595. doi:10.1038/s41577-024-01014-8.
(4) Canfora EE, Jocken JW, Blaak EE. Short-chain fatty acids in control of body weight and insulin sensitivity. Nat Rev Endocrinol. 2015;11(10):577–591. doi:10.1038/nrendo.2015.128. See also Parada Venegas D, De la Fuente MK, Landskron G, et al. Short chain fatty acids (SCFAs)-mediated gut epithelial and immune regulation and its relevance for inflammatory bowel diseases. Front Immunol. 2019;10:277. doi:10.3389/fimmu.2019.00277.
(5) Donohoe DR, Garge N, Zhang X, et al. The microbiome and butyrate regulate energy metabolism and autophagy in the mammalian colon. Cell Metab. 2011;13(5):517–526. doi:10.1016/j.cmet.2011.02.018.
(6) The Protein Brewery / TNO Health & Work. Predigested Fermotein® modulates human colonic microbiota activity in vitro. TNO Restricted Report No. 2026 R15116. Leiden, NL: TNO; 30 March 2026.