Why More Protein Isn’t Always the Answer

Protein continues to be a hot topic in nutrition and longevity medicine. Much of that interest stems from the growing recognition that skeletal muscle is a longevity organ that influences metabolism, mitochondrial function, immune regulation, and cognitive health. As a result, maintaining muscle mass and function has become a cornerstone of healthy aging, and clinicians are increasingly encouraging patients to consume more protein.

Yet an important clinical question often goes unasked: what happens when the gut isn’t prepared to handle all that protein?

That question, and many others, were explored in a recent New Frontiers in Functional Medicine podcast episode with Dr. Tom Fabian, PhD in molecular biology and science advisor at Diagnostic Solutions Laboratory. One of the leading voices translating microbiome research into clinical practice, Dr. Fabian offers a compelling shift in how clinicians can think about GI function, protein metabolism, and muscle health.

Download the free clinical guide, Supporting Muscle Health Through the Gut, for a concise summary of key takeaways and clinical considerations from the full podcast episode.

 

Protein Utilization Is Not the Same as Protein Intake

The assumption embedded in most protein recommendations is that what a patient consumes is what their body actually uses. In many cases, that assumption deserves scrutiny.

Before increasing protein targets, it may be equally important to ask whether digestion, absorption, and the microbiome can support those recommendations.

Protein must be effectively digested in the stomach and small intestine, broken down into absorbable amino acids, and then delivered to the tissues that need them. That process depends on a well-functioning digestive system, and for a meaningful subset of our patients, particularly older adults, it is compromised.

Transit time also matters. Even when protein digestion is adequate, slow colonic transit allows microbes to exhaust available fermentable carbohydrates and shift toward greater protein fermentation. This makes constipation an often-overlooked contributor to excessive proteolytic metabolism and another reason bowel habits should be assessed before substantially increasing protein intake.

Common contributors to impaired protein digestion include:

• Hypochlorhydria
• H. pylori infection
• Reduced pancreatic enzyme output
• Impaired brush border function
• Chronic stress and HPA axis dysregulation
• Gastrointestinal inflammation

When any of these are present, simply increasing protein targets may not translate to better muscle outcomes. Instead, more undigested protein may reach the colon, increasing the potential for protein fermentation and its downstream effects.

 

Excess Protein in the Colon: What We’re Actually Measuring

Under normal physiologic conditions, dietary protein is absorbed in the small intestine, and very little reaches the colon. When digestion is compromised, or transit is slow, however, unabsorbed protein becomes substrate for microbial fermentation in the large intestine.

Some degree of colonic protein fermentation is physiologically normal. The clinical concern arises when it becomes excessive, particularly in the context of an already-imbalanced microbiome. In those conditions, fermentation of protein-derived substrates can produce a range of potentially problematic metabolites, including:

• p-Cresol and other phenolic compounds
• Ammonia
• Histamine and biogenic amines
• Hydrogen sulfide
• Trimethylamine (TMA)
• Inflammatory tryptophan metabolites

Evidence suggests these metabolites may contribute to intestinal permeability, immune activation, and systemic inflammation under conditions of excessive protein fermentation. The clinical irony here is real: a dietary intervention specifically intended to support healthy aging may, in the context of GI dysfunction, be generating the very inflammatory milieu we’re trying to prevent.

 

Fiber and Polyphenols as Protective Modulators

One of the more clinically actionable findings from microbiome research is that adequate fiber intake substantially offsets the downstream effects of excessive protein fermentation. Rather than simply increasing protein intake, clinicians should consider whether the diet also provides enough fermentable fiber to support a healthy microbial ecosystem.

The goal isn’t simply more protein. It’s a dietary pattern that provides adequate protein alongside fibers and polyphenols that help the microbiome process it effectively.

From a microbial perspective, the question is less whether protein fermentation occurs, which it normally does to some extent, and more whether it begins to dominate over carbohydrate fermentation. Beneficial gut microbes preferentially ferment dietary fiber to produce short-chain fatty acids such as butyrate, acetate, and propionate. These metabolites help maintain gut barrier integrity, regulate immune function, support mitochondrial health, and create an environment that discourages excessive protein fermentation.

One proposed mechanism is that a fiber-fed microbiome effectively acts as a metabolic “sponge,” incorporating excess amino acids into microbial growth rather than converting them into potentially harmful fermentation metabolites. Research also suggests that a diverse range of fermentable fibers is important, as different fibers are fermented at varying rates throughout the colon, helping to sustain carbohydrate fermentation and limit the shift toward protein fermentation.

Polyphenols appear to provide complementary benefits. Studies consistently show that polyphenol-rich diets can help reduce protein fermentation while supporting a healthier microbiome, further reinforcing the value of plant-rich dietary patterns. Interestingly, studies suggest that plant-based protein sources generally result in less protein fermentation than animal proteins. While the mechanisms are still being investigated, their naturally higher fiber and polyphenol content likely contributes to this effect.

The clinical implication is straightforward: high-protein dietary patterns that crowd out fiber and phytonutrient-dense foods produce a very different microbial environment than those that include them. Supporting muscle health isn’t simply about increasing protein intake; it’s about creating the gut environment needed to utilize that protein effectively.

 

The Gut-Muscle Axis: A Mechanistic Framework Worth Understanding

What makes this conversation particularly compelling from a clinical standpoint is the emerging mechanistic framework linking microbiome activity to skeletal muscle, an interconnected system that Dr. Fabian and others have termed the gut-muscle axis.

The key mediators appear to be microbial metabolites, specifically SCFAs and secondary bile acids, which help shape immune tone by supporting regulatory T cells (Tregs) activity. While these immune cells originate in the gut, they can migrate throughout the body and participate in tissue repair, including skeletal muscle regeneration following injury or exercise. This may help explain why the microbiome influences muscle health beyond simply affecting protein digestion and absorption.

This reframes muscle health in an important way. The question is no longer only how much protein a patient consumes, but whether their gut microbiome is generating the metabolites and immune signals that support muscle repair and recovery. Resistance training and adequate protein intake remain essential inputs, but they appear to operate within a broader microbial ecosystem that can either amplify or diminish their benefits.

We’ll be diving deeper into these concepts during our upcoming virtual Functional Medicine IS Longevity™ Masterclass, where microbiome expert Dr. Tom Fabian will join us to explore the emerging science linking the gut microbiome to longevity, how advanced stool testing can be used to assess microbial function and metabolite production, and strategies to support the production of longevity-promoting microbial compounds.

If you’re a clinician interested in applying these insights in practice, this is a conversation you won’t want to miss.

 

Clinical Assessment: When to Evaluate GI Function First

For most patients, protein intake recommendations should come with at least a basic assessment of digestive capacity. Certain presentations warrant a more systematic evaluation of GI function before significantly increasing protein targets.

Red flags worth investigating include:

• Early satiety or bloating following protein-rich meals
• Subjective sense that food “sits” in the stomach
• Constipation or slow transit
• Food sensitivities, particularly to high-protein foods
• Low fecal pancreatic elastase
• Evidence of dysbiosis or elevated opportunistic bacteria on stool testing
• Chronic or recurrent GI symptoms

In these patients, addressing digestive function before increasing protein intake may improve both tolerance and clinical outcomes. This is especially relevant for older adults on GLP-1 medications, given that appetite suppression and slowed gastric emptying can significantly compound protein digestion challenges.

For clinicians seeking a more comprehensive view of digestive function and microbiome activity, advanced stool testing can provide valuable insight.

Dr. Fabian highlighted the utility of StoolOMX™, an add-on test from Diagnostic Solutions Laboratory that evaluates short-chain fatty acids, branched-chain fatty acids, bile acid metabolism, and markers of protein fermentation.

These insights can help identify patients who may benefit from digestive support, microbiome interventions, or dietary modifications before significantly increasing protein intake.

Learn more about StoolOMX™ and other testing options from Diagnostic Solutions Laboratory here.

 

The Right Clinical Question

Protein requirements remain widely debated, and optimal intake ultimately depends on an individual’s age, activity level, health status, and clinical goals. Yet one important factor is often overlooked: whether the gut can effectively digest and utilize the protein consumed.

Rather than asking simply, “How much protein should this patient eat?”, clinicians may be better served by asking a different question: “Is this patient’s gut prepared to digest, absorb, and utilize that protein effectively?”

When digestion, microbiome balance, fiber intake, overall dietary pattern, and phytonutrient density are considered alongside protein quantity, clinicians create the physiologic conditions in which protein recommendations are more likely to translate into meaningful improvements in muscle preservation, metabolic function, and healthier aging.

To explore these concepts in greater depth, download the free clinical guide, Supporting Muscle Health Through the Gut, which summarizes the key scientific insights and clinical considerations discussed in the podcast episode.

For an even deeper dive, join the upcoming Functional Medicine IS Longevity™ Masterclass, where Dr. Tom Fabian will explore the emerging science linking the gut microbiome to healthy aging. Learn how advanced stool testing can assess microbial function and metabolite production, and discover evidence-based strategies to support the production of longevity-promoting microbial compounds.