Furosemide = Improved Performance?

Hello Summarians!

Furosemide may not just prevent lung bleeding in racehorses. It may also make them run longer by creating a mild alkalosis that delays fatigue. That is a meaningful distinction for anyone who regulates the sport.

Also this week: gut microbiome markers in healthy puppies are largely stable after eight weeks, meaning abnormal results are more likely disease than normal maturation. And intravenous magnesium during disk surgery in dogs produced no measurable pain relief.

Three studies. The first one matters beyond the barn.

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Puppies GI Maturation

This study evaluated how fecal microbial and metabolic markers change during the first year of life in healthy puppies. Researchers followed 23 client-owned dogs from 8 to 12 weeks of age through 1 year, collecting 62 fecal samples. They measured the canine dysbiosis index (DI), selected bacterial populations using quantitative PCR, and several metabolic products, including bile acids, long-chain fatty acids, and sterols.  

Overall, the intestinal markers were remarkably stable. The DI and most bacterial populations remained within established reference ranges and did not change significantly with age. Bacteroides was the primary exception, with lower abundance at 8 to 12 weeks compared with 6 months. Clostridium perfringens was commonly detected in healthy puppies at every age, demonstrating that its presence alone does not necessarily indicate gastrointestinal disease. The enterotoxin gene of C. perfringens became numerically more common with age, but this increase was not statistically significant. Clostridioides difficile was not detected.  

Fecal bile acid metabolism was also stable. Secondary, predominantly unconjugated bile acids were already dominant by 8 to 12 weeks. One bile acid, 12-oxo-lithocholic acid, differed between 16 weeks and 1 year, but this finding lost statistical significance after adjustment for multiple comparisons. Long-chain fatty acids, cholesterol, other sterols, and their metabolic products showed no significant age-related changes.  

These results suggest that many clinically relevant intestinal microbial and metabolic functions have largely matured by approximately 8 weeks of age. Therefore, abnormal DI scores, bacterial populations, bile acid patterns, or lipid metabolites in puppies older than 8 weeks may be more likely to represent gastrointestinal disease than normal maturation. Important limitations include the small sample size, different diets among dogs, incomplete sampling at every age, and use of targeted PCR rather than comprehensive microbiome sequencing. Overall, the study provides useful baseline values for interpreting fecal microbiome and metabolomic testing in young dogs. 

Birk, D., Weidinger, A., Schmidt, T., Bachmann, S., Spring, I., Felten, S., Meunier, S. M., Blake, A. B., Suchodolski, J. S., & Unterer, S. (2026). Targeted quantitative polymerase chain reaction–based bacterial and fecal metabolite markers show early stability in healthy puppies after 8 weeks of age. American Journal of Veterinary Research https://doi.org/10.2460/ajvr.26.06.0245

Bottom line — Seems to be mature at 8 weeks.

Magnesium And Spinal Surgery

This prospective, randomized, double-blind clinical trial evaluated whether intravenous magnesium sulfate (MgSO₄) given during spinal surgery could improve postoperative pain control and reduce inflammation in dogs with thoracolumbar intervertebral disk extrusion (IVDE). Thirty dogs undergoing hemilaminectomy were divided equally between a magnesium group and a saline placebo group. The magnesium group received a 20 mg/kg loading dose followed by a continuous infusion of 20 mg/kg/hour until surgical closure. All dogs received standardized anesthesia and postoperative fentanyl analgesia.  

Pain was evaluated using the Glasgow Composite Measure Pain Scale–Short Form immediately after recovery and at 4, 8, 12, and 24 hours. Researchers also measured interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) as indicators of inflammation. There were no statistically significant differences in pain scores between the magnesium and control groups at any time point, and magnesium did not significantly reduce the need for additional analgesia.  

Similarly, IL-6 concentrations did not differ significantly between groups. TNF-α concentrations were below detectable levels in most dogs, preventing meaningful statistical analysis. Magnesium concentrations increased significantly immediately after surgery and at one hour in treated dogs but generally returned to normal by 24 hours.  

One potentially beneficial finding was that dogs receiving magnesium experienced smoother anesthetic recoveries. Some magnesium-treated dogs developed bradycardia, including one with second-degree ventricular block, although bradycardia also occurred in control dogs. No major adverse effects were identified.  

Overall, intraoperative magnesium sulfate did not provide clinically significant additional pain relief or measurable anti-inflammatory benefits in dogs undergoing hemilaminectomy for IVDE. Although it appeared relatively safe and may improve anesthetic recovery quality, the study does not support routinely adding IV magnesium sulfate to postoperative analgesic protocols for these patients.

Mallet, G., Gaitero, L., Sanchez, A., James, F. M. K., Wood, G., Monteith, G., & Samarani, F. (2026). A randomized controlled trial of intravenous magnesium sulfate shows no analgesic or anti-inflammatory benefit in dogs undergoing thoracolumbar disk herniation surgery. American Journal of Veterinary Research https://doi.org/10.2460/ajvr.26.05.0198

Bottom line — Produced no measurable benefits.

Furosemide And Exercise In Horses

This study investigated whether furosemide, a diuretic commonly administered to racehorses to reduce exercise-induced pulmonary hemorrhage (EIPH), alters acid-base balance in a way that may improve athletic performance. Six fit Thoroughbred horses participated in a randomized, blinded crossover trial. Each horse received either intravenous furosemide at 0.5 mg/kg or saline placebo four hours before running on an inclined treadmill at approximately 115% of maximum oxygen consumption until fatigue. Researchers measured blood gases, electrolytes, lactate, bicarbonate, pH, and other indicators of acid-base balance.  

Furosemide produced approximately 2% body-weight loss through diuresis and caused a mild hypochloremic metabolic alkalosis. Compared with placebo, treated horses had lower chloride concentrations and higher bicarbonate levels, strong ion difference, and blood pH. Mixed venous carbon dioxide pressure was also lower at fatigue and immediately afterward. Sodium, potassium, and lactate increased substantially during intense exercise, but these changes were similar with furosemide and placebo. Maximum oxygen consumption and peak carbon dioxide production were not significantly different between treatments.  

The most important finding was improved exercise duration. Horses receiving furosemide ran an average of about 15 seconds longer, representing approximately a 17% increase in time to fatigue. Despite similar lactate accumulation, furosemide-treated horses maintained a slightly higher pH, suggesting that the mild alkalosis may delay the acid-base disturbances associated with muscular fatigue. Weight loss may also improve performance by increasing oxygen consumption relative to body mass.  

The authors concluded that furosemide may enhance supramaximal exercise performance through a combination of diuretic weight loss and an alkalinizing effect, rather than solely through control of EIPH. However, only six horses were studied, EIPH severity was not measured, and treadmill exercise does not perfectly reproduce actual racing. Therefore, the results demonstrate improved treadmill performance but cannot establish how much these effects influence real-world racing performance. 

Meyer, H. L., Tang, Y., Wang, Y., Kingston, J. K., Brown, J. A., Sides, R. H., & Bayly, W. M. (2026). Horses undergoing supramaximal fixed-speed treadmill exercise after administration of furosemide experience less severe acidosis and slower onset of fatigue. American Journal of Veterinary Research https://doi.org/10.2460/ajvr.26.03.0143

Bottom line — Seems to enhance maximal performance.

Just putting things in perspective …