Metabolomics Reveals How Starch Overload Reshapes the Cow Gut and Blood

High-grain diets have long been the engine of modern dairy production, delivering the starch and energy that Holstein cows need to sustain extraordinary milk yields. But that same nutritional power carries a hidden metabolic cost. A new open-access study in the journal Metabolomics, led by Ezequias Castillo-Lopez of the University of Veterinary Medicine Vienna and colleagues, has now mapped, in unprecedented chemical detail, what happens inside the rumen, the hindgut and the bloodstream when cows are pushed onto a high-concentrate ration and when starch escapes the rumen to ferment further down the gut. The results reveal two distinct acidotic insults with separate chemical signatures, and they point to a surprising candidate biomarker that could one day help farmers detect gut acidosis before it undermines herd health.

The research team worked with nine rumen-cannulated Holstein cows in their third lactation, using a changeover design with two three-week experimental periods separated by a three-week washout. The animals were assigned to four treatments: a control diet containing 40 percent concentrate, the same control diet plus a daily abomasal infusion of three kilograms of starch to induce hindgut acidosis, a high-concentrate diet containing 65 percent concentrate, and the high-concentrate diet combined with the starch infusion. Infusing starch directly into the abomasum, the cow’s true stomach, was a clever experimental trick: it bypassed the rumen entirely, allowing the researchers to isolate the effects of starch fermenting in the hindgut from the effects of a starch-rich ration fermenting in the rumen.

The physiological measurements confirmed that both insults did what they were designed to do. Cows on the high-concentrate diet spent an average of 249 minutes per day with ruminal pH below the critical threshold of 5.8, compared with just 69 minutes per day for cows on the control diet, and their mean ruminal pH dropped from 6.26 to 6.09. Meanwhile, the abomasal starch infusion lowered fecal pH from 6.58 to 6.33, a hallmark of hindgut acidification, and shifted hindgut fermentation toward greater propionate and lactate production. In other words, the study successfully created subacute ruminal acidosis and hindgut acidosis in the same animals, sometimes simultaneously, setting the stage for a metabolomic showdown.

To read the chemical story, the team analyzed rumen fluid, feces and blood serum using high-performance liquid chromatography coupled to tandem mass spectrometry, with anion exchange chromatography linked to high-resolution mass spectrometry reserved for carboxylic acids, sugar-related compounds and nucleotides in serum. The data were processed with rigorous quality controls, including blank-to-sample filtering and drift correction, and analyzed in MetaboAnalyst with false-discovery-rate adjustment. The statistical power of the design, estimated at 78 to 95 percent with an average of 85 percent, gave the researchers confidence that the differences they detected were real rather than noise.

The ruminal results were strikingly one-sided. Only the diet mattered here: the high-concentrate ration reshaped the ruminal metabolome, while the abomasal starch infusion left it essentially untouched. Cows on the high-concentrate diet showed elevated concentrations of adenosine, histidine, inosine, guanine, citrulline, arginine and a lipid-related compound, 1,2-dilinoleoyl-3-palmitoylglycerol. Pathway analysis revealed enrichment of purine metabolism, the urea cycle, aspartate metabolism, methyl histidine metabolism, and arginine and proline metabolism. The authors interpret this as evidence of intense microbial turnover: when ruminal pH falls, microbes die and lyse, releasing their nucleic acids and proteins, while the extra energy and roughly 700 additional grams of daily protein in the high-concentrate ration fuel faster microbial growth and protein synthesis.

Some of these ruminal shifts carry warnings for long-term health. The enrichment of the urea cycle suggests a heavier workload for the liver and kidneys, which must convert microbial ammonia into urea for recycling or excretion. If ammonia is not efficiently detoxified, the authors note, chronic subclinical ammonia overload could impair nervous system function, hormonal activity, reproduction and blood pH regulation. The increased citrulline, produced by rumen bacteria converting arginine under low-pH conditions, is itself a survival strategy: the reaction generates ammonia, which helps buffer the acidic rumen environment that threatens the bacteria. It is a vivid example of the microbial community fighting back against the chemistry its host’s diet has created.

The fecal metabolome told a different story, one in which both dietary treatments left fingerprints, but the abomasal starch infusion was by far the stronger force. Starch infusion altered 48 fecal metabolites, including increases in linoleic acid, ethanolamine, oleoyl ethanolamide, linoleoyl ethanolamide, xanthine, alpha-linolenic acid, stearoyl ethanolamide and pentoses. The high-concentrate diet affected 10 metabolites, several of them overlapping with the infusion effects. Enriched pathways included bile acid biosynthesis, arginine and proline metabolism, linoleic acid and alpha-linolenic acid metabolism, glycine and serine metabolism, purine metabolism, the urea cycle and biotin metabolism. The bile acid finding is particularly intriguing, because bile acids are not merely fat-digesting detergents but powerful signaling molecules that regulate glucose and lipid metabolism; their enrichment may reflect the activation of energy homeostatic mechanisms in response to the starch overload reaching the hindgut.

Among all the fecal metabolites, one stood out as a potential diagnostic prize: ethanolamine. This compound rose under both high-concentrate feeding and abomasal starch infusion, meaning it marks acidity in either the rumen or the hindgut. Ethanolamine is derived from phosphatidylethanolamine, abundant in the membranes of shed intestinal cells, so its accumulation likely reflects gut lining damage and dysbiosis, consistent with the elevated fecal lactate observed in the companion microbiome study. More troubling still, ethanolamine can serve as a nitrogen source for pathogenic bacteria, including enterohemorrhagic Escherichia coli O157:H7, and research has shown that Salmonella enterica and E. coli can exploit it to outcompete beneficial commensal microbes. Chronic accumulation may also damage gut permeability, promote inflammation and disrupt glucose metabolism. A simple fecal marker that flags dual-site acidosis would be a genuinely valuable tool for dairy veterinarians.

The blood metabolome delivered the study’s most counterintuitive result. Despite the dramatic changes in the gut, hindgut acidosis left the serum metabolome essentially unchanged, and the high-concentrate diet produced no major overall profile shift, only specific decreases in hippuric acid, 3-phenylpropionic acid, 2-hydroxybutyric acid, acetic acid and 3-hydroxybutyric acid. The authors suggest this is because hindgut fermentation products are only partially absorbed and partly excreted, whereas ruminal fermentation end-products are extensively absorbed into portal circulation. Yet the pathways enriched in the blood favored the control diet: amino sugar metabolism, aspartate metabolism, fatty acid biosynthesis, propanoate metabolism, branched-chain amino acid degradation and, notably, the malate-aspartate shuttle, the most efficient mechanism for shuttling cytosolic NADH into the mitochondrial electron transport chain. The authors speculate that high-concentrate diets may force cows into less efficient cellular energy use, with potential consequences for feed efficiency.

Taken together, the study draws a clear chemical map of two acidotic conditions that dairy nutritionists have long struggled to disentangle. High-concentrate feeding dominates the ruminal metabolome through nitrogen and purine metabolism, both treatments converge on the fecal metabolome with ethanolamine as a shared signature, and only diet reaches the bloodstream in a meaningful way. The work also carries honest caveats: the washout period was relatively short, milk data lacked sufficient statistical power, ruminal ammonia was not measured, and increasing concentrate changes more than just starch. Even so, the findings emphasize that maintaining adequate gut health is not simply about preventing a drop in pH; it is about protecting an entire interconnected metabolic network that links rumen microbes, hindgut fermentation, liver function and cellular energy economy. For an industry in which subacute ruminal acidosis affects an estimated 10 to 26 percent of commercial dairy herds, a fecal biomarker like ethanolamine could transform how silently sick cows are found, long before production losses become visible.

Subject of Research: Metabolomic impacts of high-concentrate feeding and hindgut starch flow on rumen, fecal and blood metabolism in Holstein dairy cows

Article Title: Metabolome analyses of the rumen, feces and blood reveal separate and simultaneous impacts of increased dietary concentrate and hindgut starch flow in Holstein cows

Article References: Castillo-Lopez, E., Aigensberger, M., Atif, R. M., Biber, P., Hartinger, T., Raykova, S., Guerrero-Lavin, A., Reisinger, N., Schwartz-Zimmermann, H. E., Berthiller, F., & Zebeli, Q. (2026). Metabolome analyses of the rumen, feces and blood reveal separate and simultaneous impacts of increased dietary concentrate and hindgut starch flow in Holstein cows. Metabolomics, 22(5), Article 163. https://doi.org/10.1007/s11306-026-02519-0

Image Credits: AI Generated

DOI: 10.1007/s11306-026-02519-0

Keywords: dairy cattle, metabolomics, ruminal acidosis, hindgut acidosis, Holstein cows, starch fermentation, rumen, fecal biomarkers, bile acids, ethanolamine, animal nutrition, gut health