How is Wernicke's encephalopathy precipitated after a carbohydrate load in thiamine deficiency?

Wernicke's encephalopathy involves damage to multiple nerves in both the central nervous system (brain and spinal cord) and the peripheral nervous system (the rest of the body).
It may also include symptoms caused by alcohol withdrawal. The cause is generally attributed to malnutrition, especially lack of vitamin B1 (thiamine), which is common in those with alcoholism.
Heavy alcohol use interferes with the break down of thiamine in the body, so even if someone with alcoholism follows a well-balanced diet, most of the thiamine is not absorbed.
Korsakoff syndrome, or Korsakoff psychosis, tends to develop as Wernicke's symptoms do away. This involves damage to areas of the brain involved with memory. Patients often attempt to hide their poor memory by creating detailed, believable stories about experiences or situations. This is not usually a deliberate attempt to deceive because the patient often believes what he is saying to be true.
Please see the web pages for more details on Wernicke's encephalopathy and Wernicke-Korsakoff syndrome

Thiamine (vitamin B1), when combined with phosphoric acid, constitutes thiamine pyrophosphate (TPP). TPP, the physiologically active form of thiamine, has a wide range of activity in intermediary carbohydrate metabolism. It acts as a coenzyme for pyruvate dehydrogenase (PDH) and alpha-ketoglutarate dehydrogenase (alpha-KGDH), which are responsible for the decarboxylation of pyruvate and alpha-ketoglutarate, respectively. In addition, TPP functions as a coenzyme for transketolase, an enzyme of the NADPH-producing hexose monophosphate shunt. Because of its central role in these processes subserving carbohydrate metabolism, the requirement for thiamine is greatest when carbohydrate is the principal source of energy, such as alcoholic patients or patients receiving large quantities of parenteral glucose. The brain, which relies exclusively on carbohydrates for immediate energy demands, is particularly vulnerable to thiamine deficiency.