A Simple Blood Metabolite Could Detect Colorectal Cancer Before Surgery

Colorectal cancer rates are climbing globally, and the tools available for early detection remain invasive, expensive, or both. A small but methodologically careful new study from researchers at San Carlos Hospital in Madrid suggests that a group of gut-derived metabolites called short-chain fatty acids, measurable in a standard blood draw, may carry diagnostic information that nobody has fully mapped yet. The findings are preliminary, but the signal is specific enough to warrant attention.

Serum valerate achieves 0.813 AUC for colorectal cancer detection

The team, led by Vicente-Valor J and colleagues, recruited 36 patients scheduled for curative colorectal cancer surgery and 20 cancer-free controls at San Carlos Hospital between 2021 and 2023. All samples were collected before any treatment began. Using high-performance liquid chromatography coupled to triple-quadrupole mass spectrometry, they quantified seven short-chain fatty acids (SCFAs) in fasting serum: acetate, propionate, butyrate, valerate, isobutyrate, isovalerate, and 2-methylbutyrate.

The headline result: serum valerate showed the best diagnostic accuracy for distinguishing CRC patients from controls, with an area under the receiver-operating curve of 0.813. Colorectal cancer patients had significantly lower valerate concentrations than cancer-free participants. Butyrate was also reduced in the cancer group. On the other side of the ledger, total SCFAs, acetate, and a cluster of branched SCFAs (isobutyrate, isovalerate, and 2-methylbutyrate) were all significantly elevated in CRC patients. After statistical adjustment for age, gender, and BMI, butyrate and valerate remained protective variables, while total branched SCFAs, isobutyrate, and 2-methylbutyrate were associated with higher CRC risk.

Short-chain fatty acids have drawn growing interest as cancer biomarkers precisely because they are produced continuously by gut bacteria fermenting dietary fiber and amino acids, making them accessible through non-invasive sampling. Straight-chain SCFAs like acetate, propionate, and butyrate come primarily from fiber fermentation; branched SCFAs like isobutyrate and isovalerate arise from bacterial degradation of amino acids.

Why these blood-based signals matter for CRC patients

The gut microbiota's complex metabolite output contributes critically to colorectal cancer initiation, progression, and metastasis, and SCFAs are among its most studied effectors. Butyrate, for instance, is generally considered protective: it fuels colonocytes, inhibits histone deacetylases (enzymes that control gene expression), and supports intestinal barrier integrity. Its consistent reduction in CRC patients in this study is consistent with that protective framing.

The staging associations add another layer. Serum total SCFAs and acetate concentrations were both lower in patients who already had lymph node invasion (N1 or N2 disease) compared to node-negative patients. Separately, total branched SCFAs and isovalerate were reduced in the two metastatic cases in the cohort. The authors are appropriately cautious: the metastatic group contained only two patients, making those particular numbers illustrative rather than definitive. Still, the directional consistency between staging severity and SCFA reduction is a pattern worth tracking in larger studies.

Branched SCFAs like isobutyrate have also been shown in other work to modulate immune activity against tumors, shifting T-cell profiles toward greater anti-tumor activity by increasing PD-1 and IFN-gamma expression while reducing FoxP3-positive regulatory T cells. The direction of that immunological lever may explain why elevated branched SCFAs in early disease but reduced levels in advanced disease looks paradoxical at first glance.

How the team measured serum metabolites against gut bacterial abundance

Beyond the diagnostic question, the study asked whether blood SCFA levels actually reflect what SCFA-producing bacteria are doing in the gut. The answer, put simply, is: not very well. The researchers cross-referenced serum SCFA concentrations against the relative abundance of known SCFA-producing bacterial genera in both fecal and serum 16S rRNA sequencing data, then used PICRUSt2 bioinformatic inference to identify SCFA-related biosynthesis pathway activity.

Correlations between serum metabolite levels and bacterial abundance were scarce and inconsistent across both sample types. Interestingly, the total relative abundance of SCFA-producing bacteria was lower in the feces of CRC patients but higher in their serum samples, a reversal that itself needs explanation. Meanwhile, pathways linked to butyrate and acetate biosynthesis showed significantly higher predicted counts in CRC patients' fecal and serum microbiota, even though serum butyrate levels were lower in those same patients.

A prospective study that tracked serum SCFAs over 30 years before colorectal cancer diagnosis using similar mass spectrometry methods has previously demonstrated that the relationship between circulating SCFAs and CRC risk is longitudinally detectable, which adds credibility to the idea that blood-based measurement is a viable approach. The current study's decoupling finding suggests the mechanism is not straightforward: colonocytes consume butyrate as their primary fuel before it reaches the portal circulation, hepatic first-pass metabolism further transforms what remains, and systemic levels ultimately reflect a composite of production, absorption, utilization, and host metabolism rather than bacterial output alone.

Open questions about sample size, confounders, and mechanistic validation

The investigators are direct about the study's boundaries. Thirty-six cancer patients and 20 controls is a modest cohort, and the two metastatic cases in the M1 group are too few to draw firm conclusions about SCFAs in advanced disease. Beyond sample size, the researchers lacked data on medication use (other than antibiotics), dietary habits, and lifestyle factors that independently influence both gut microbiota composition and SCFA metabolism. The CRC and control groups also differed in age and other clinicopathological features, which the statistical adjustments address only partially.

Methodological variability and the absence of standardized validation protocols remain a recognized problem across SCFA research broadly, making replication in independent cohorts with harmonized collection and measurement protocols a prerequisite before any clinical application.

Larger validation cohorts and microbiota modulation strategies ahead

The study's authors frame their results as hypothesis-generating: serum valerate and butyrate warrant testing in larger, prospectively designed cohorts with matched dietary and lifestyle data. Restoring SCFA levels through microbiota modulation has been proposed as a prevention and treatment strategy for colorectal cancer, and understanding which blood-based metabolite signatures reliably track disease state is a necessary precondition for that therapeutic direction.

For now, the clearest takeaway is that blood SCFAs are not a simple readout of what gut bacteria are producing. They appear to carry independent diagnostic information, particularly valerate and the branched SCFA cluster, but deciphering that signal will require mechanistic work alongside the epidemiological validation. The 0.813 AUC for valerate is a meaningful starting point, not a finish line.

Evidence rating

INVESTIGATIONAL — 75/100

Caveats:

• Small study: 36 CRC patients and 20 controls — results are preliminary and require validation in larger independent cohorts.
• Cross-sectional design: cannot determine whether altered SCFA levels are a cause or consequence of colorectal cancer.
• The CRC and control groups differed significantly in age, sex, and BMI, which are known to influence SCFA levels; residual confounding cannot be excluded despite logistic regression adjustment.
• No dietary, lifestyle, or medication data (beyond antibiotic exclusion) were available, limiting interpretation of what drives the observed SCFA differences.
• Functional pathway findings are based on bioinformatic inference (PICRUSt2), not direct metabolic measurement or experimental validation.
• Funding source was not disclosed by the authors.

Single-center cross-sectional study comparing serum SCFA profiles between 36 colorectal cancer patients and 20 cancer-free controls. The design establishes associations, not causation, and the modest sample size limits the robustness of diagnostic performance estimates. Findings are directionally consistent with at least one prior plasma SCFA study in CRC, but independent replication in a larger, well-characterized cohort is needed before clinical translation can be considered.

References

1. Juan Vicente-Valor, Sofía Tesolato, María Paz Lorenzo, et al. (2026). Serum Short-Chain Fatty Acids in Colorectal Cancer: Diagnostic Performance and Decoupling from Gut Producer Abundance. Cells. https://doi.org/10.3390/cells15121096
2. Petra Chalova, Anton Tazky, Ludovit Skultety, et al. (2023). Determination of short-chain fatty acids as putative biomarkers of cancer diseases by modern analytical strategies and tools: a review. Frontiers in Oncology. https://doi.org/10.3389/fonc.2023.1110235
3. Wanru Zhang, Yaping An, Xiali Qin, et al. (2021). Gut Microbiota-Derived Metabolites in Colorectal Cancer: The Bad and the Challenges. Frontiers in Oncology. https://doi.org/10.3389/fonc.2021.739648
4. Erikka Loftfield, Roni T Falk, Joshua N Sampson, et al. (2022). Prospective Associations of Circulating Bile Acids and Short-Chain Fatty Acids With Incident Colorectal Cancer. JNCI Cancer Spectrum. https://doi.org/10.1093/jncics/pkac027
5. Tomasz Sylwestrzak, Michalina Ciosek, Krzysztof Pastuszak, Tomasz Jastrzębski (2025). Fecal Short-Chain Fatty Acids in Colorectal Cancer Patients Versus Healthy Controls: A Systematic Review and Meta-Analysis. Journal of Clinical Medicine. https://doi.org/10.3390/jcm14248949
6. Boobalan Thulasinathan, Kanve N. Suvilesh, Sumanas Maram, et al. (2025). The impact of gut microbial short-chain fatty acids on colorectal cancer development and prevention. Gut Microbes. https://doi.org/10.1080/19490976.2025.2483780