Your stressy horse may be quietly running low on one vitamin

How thiamine works: Energy, nerves, and keeping calm under pressure

Thiamine does its work in its active form, thiamine pyrophosphate (TPP), acting as a helper molecule for some of the most important enzymes your horse’s body relies on: pyruvate dehydrogenase, alpha-ketoglutarate dehydrogenase, and transketolase. These enzymes keep energy production running smoothly, feeding into the pathways that make ATP, the fuel that powers every cell, and keeping the mitochondria doing their job properly.

So why does thiamine keep coming up when we talk about horses in work? Think about what’s happening inside your horse’s body when he’s cantering across a field or competing. As he exercises, carbohydrates break down into a molecule called pyruvate. Normally, pyruvate dehydrogenase, which depends on thiamine to function, takes that pyruvate and converts it into acetyl-CoA, feeding it into the energy cycle. If thiamine is running low, that conversion slows, pyruvate piles up, and the body shunts it towards lactic acid instead. That lactic acid builds up in the muscles, and that’s when things start to feel hard. Human studies confirm it directly: when thiamine levels in cells drop, maximal oxygen use, anaerobic threshold, and the point at which blood lactate starts to climb all fall measurably. In mouse research, a thiamine derivative improved endurance, grip strength, lactate clearance after exercise, markers of muscle damage, and glycogen storage in both liver and muscle, all pointing to the same underlying picture.

Thiamine also has an important job in the nervous system that sits quite separately from energy metabolism. Research using thiamine-blocking compounds showed it’s needed for proper nerve-to-muscle signalling, specifically something called post-tetanic potentiation, which is simply the way a nerve strengthens a signal after repeated firing. When horses were experimentally made thiamine-deficient using a thiamine antagonist called amprolium, they developed bradycardia (a slowed heart rate), ataxia (loss of co-ordination), and muscular fasciculations (muscle twitching), alongside raised lactate and pyruvate in the blood. You see the same pattern in bracken fern poisoning, where an enzyme called thiaminase in the plant actively destroys thiamine.

There’s a calmer, subtler dimension to thiamine too, one I think deserves more attention than it gets. In rodent studies, thiamine deficiency reduced exploratory behaviour and novelty-seeking; essentially, the animals became more withdrawn. Supplementation has been shown to bring down anxiety-like behaviours, lower oxidative stress, restore serotonin levels, and reduce pro-inflammatory markers including IL-1β, TNF, Cox-1, and Cox-2. In human clinical work, thiamine at 250 mg per day successfully addressed anxiety disorders, along with chronic fatigue, poor sleep, and loss of appetite, in people whose cells simply weren’t using thiamine efficiently. When their levels came back to normal with supplementation, their symptoms improved too. If you’ve ever had a horse who seems tense, spooky, or hard to settle, this line of research is worth knowing about.

Finally, thiamine plays a genuine anti-inflammatory and antioxidant role, reaching into the NFκB/p38 MAPK/AMPK signalling pathways that help the body manage oxidative damage and inflammation. When thiamine runs seriously low in the central nervous system, the knock-on failure of pyruvate dehydrogenase and alpha-ketoglutarate dehydrogenase causes lactic acid to accumulate in localised brain areas, triggering oxidative stress, excitotoxicity, and ultimately cell death. That’s why a genuine, prolonged deficiency isn’t just about tiredness, it produces real neurological signs. Your working horse likely isn’t anywhere near that point, but understanding the mechanism tells you exactly why keeping thiamine topped up matters.