explain why a lack of thiamine (vitamin B1) causes a build up of lactate in cells

Thiamine is a co-enzyme for pyruvate dehydrogenase (PHD) the enzyme that converts pyruvate into acetyl-CoA at the start of the Krebs cycle, the 2nd stage of respiration. Without thiamine the activity of PHD is reduced and so less pyruvate is converted and starts to accumulate within the cell. As its levels rise pyruvate is converted into lactate within the cell cytoplasm causing lactate levels to rise.