Thiamine and Wernicke's: The Vitamin You Give Before Glucose

A vitamin is a coenzyme in disguise

The B vitamins do not build tissue — they run the machinery that does. It helps to see the whole B group as a single idea: these are not structural nutrients but cofactors — small molecules that clip into enzymes and make them work. Without the cofactor, the enzyme is a lock with no key. Thiamine is the cleanest example. Inside the cell it is phosphorylated into thiamine pyrophosphate (TPP), and TPP is the working part of three pivotal enzymes of carbohydrate metabolism. The vitamin, in other words, is not the point — the coenzyme it becomes is. This same logic runs through the rest of the family, and the figure below lays out the whole set; here we follow thiamine, the lead example, all the way to the bedside.

Three enzymes, one fuel line

TPP sits on pyruvate dehydrogenase — the gateway that feeds glucose-derived pyruvate into the Krebs cycle; on alpha-ketoglutarate dehydrogenase — a key step inside the Krebs cycle itself; and on transketolase — the enzyme of the pentose phosphate pathway that generates the building blocks for nucleic acids and NADPH. The first two are the direct pipeline from sugar to ATP. Take TPP away and pyruvate cannot enter the cycle; it piles up and is diverted to lactate, so cellular energy production stalls just as the tank runs dry. The organ that feels this first and worst is the brain, which burns glucose almost exclusively and has little metabolic flexibility. A tissue this dependent on aerobic glucose metabolism cannot tolerate a broken TPP-dependent step for long — which is exactly why thiamine deficiency announces itself as a neurological disease.

Wernicke's encephalopathy: the neurological emergency

The classic triad is confusion, eye signs and ataxia — but you must not wait for all three. Acute thiamine deficiency in the brain produces Wernicke's encephalopathy, taught as a triad: an acute confusional state (global confusion, apathy, disorientation); ophthalmoplegia and nystagmus (weakness of the eye muscles and jerking eye movements); and ataxia (a broad-based, unsteady gait). The trap is that the full triad appears in only a minority of patients — most present incompletely, with just one or two features, and the diagnosis is missed precisely because the textbook picture is incomplete. Wernicke's is a clinical diagnosis and a medical emergency: the damage falls on structures around the third and fourth ventricles — the mammillary bodies, thalamus and periaqueductal grey. Treated promptly it is largely reversible. Left untreated it progresses to something that is not.

That irreversible endpoint is Korsakoff syndrome: a profound, permanent memory disorder. The patient loses the ability to lay down new memories (anterograde amnesia) and struggles to retrieve old ones, and — the signature feature — fills the gaps with confabulation, inventing plausible false memories without any intent to deceive. Wernicke and Korsakoff are two ends of one process (the Wernicke–Korsakoff syndrome): Wernicke's is the acute, treatable phase; Korsakoff's is the chronic scar left when treatment came too late. The gap between them is measured in hours of clinical decision-making — which is why the threshold to give thiamine is deliberately low. This whole story is picked up again in the Central Nervous System and Toxicology sections, in the chapter on alcohol and the ethanol-withdrawal syndromes.

Beriberi: when the deficiency hits the body

Outside the brain, chronic thiamine deficiency causes beriberi, which comes in two classic forms. Dry beriberi is a symmetrical peripheral neuropathy: tingling, burning and weakness that begins in the feet and legs, with wasting and loss of reflexes — the nerves, like the brain, starved of the energy TPP unlocks. Wet beriberi is a cardiovascular disease: a high-output cardiac failure. Here the peripheral blood vessels dilate and the heart is driven to pump ever harder, until it fails with a paradoxically high cardiac output — the patient develops oedema, breathlessness and a dilated, failing heart. It is one of the few reversible causes of heart failure, and it connects directly to the Cardiovascular section, where high-output failure is otherwise a rarity. A fulminant cardiac form ("shoshin" beriberi) can kill within days and responds dramatically to thiamine.

Who is at risk — and why alcohol leads the list

Deficiency lives wherever intake, absorption and demand fall out of balance. Chronic alcohol use is the classic setting, and for several reasons at once: heavy drinkers often eat poorly, alcohol impairs thiamine absorption across the gut and its conversion to the active TPP form, and hepatic stores are depleted. But the risk is far wider. Any prolonged malnutrition or starvation qualifies, as does persistent vomiting — most notably hyperemesis gravidarum in pregnancy, a well-recognised and preventable cause of Wernicke's. Patients after bariatric surgery are increasingly important: reduced intake and altered absorption can precipitate deficiency months or years later. Critically ill and post-operative patients on prolonged intravenous fluids without vitamins are at risk, and so is anyone being refed after starvation — the crossover with the Refeeding-syndrome chapter, where thiamine must be replaced before feeding begins. The common thread is simple: low intake, poor absorption, or a sudden surge in metabolic demand for a coenzyme the body barely stocks.

Treatment: cheap, safe, and urgent

For suspected Wernicke's, the treatment is prompt, high-dose parenteral thiamine — given intravenously or intramuscularly, because oral absorption is unreliable in exactly the patients who need it most and cannot keep pace with the deficit. In practice this is a B-vitamin preparation such as Pabrinex given several times a day for a few days, then continued as oral thiamine. The governing principle is a low threshold: if Wernicke's is even possible, treat. The downside of unnecessary thiamine is essentially nil — it is water-soluble and the excess is excreted — whereas the downside of withholding it is permanent brain damage. Prophylactic oral thiamine is also given to at-risk drinkers and malnourished patients to prevent the emergency ever arising. Beyond these classic syndromes, thiamine deficiency is worth remembering in unexplained lactic acidosis (pyruvate cannot enter the Krebs cycle and spills to lactate) and as a reversible contributor in some cases of heart failure.