Thanks so muchFirst take a look at the Glycolysis pathway... After a cascade of changes Glucose--->Pyruvate which is key to your question... But to really understand why giving Thiamine first is so important you really should understand this next part..
There are three important reactions in the body that require the following Co-factors...
*Thiamine pyrophospate (TPP)
The Co-factors work together with the Enzymes to allow the following reactions to work...
1. Pyruvate----PDH-->Acetyl CoA
2. AlphaKetoglutarate---AlphaKetoglutarate Dehydrogenase-->Succinyl CoA
3. Leucine, Isoleucine, Valine---BranchChain Ketoacid Dehydrogenase-----> AcetylCoa, PropionylCoA
If you do not have the Cofactors the Enzymes in the reactions either don't work or work way to slow..
For your question you need to worry about all these reactions but i tend to focus on Reaction #1.. After glycolysis gives you Pyruvate there are two different pathways Pyruvate can go...
When A DRUNKard comes into your office missing thiamine. The Pyruvate--->Acetyl Coa reaction wont work and the only pathway that is left is Pyruvate---->Lactate... So if you were to give the drunk just glucose they would drop dead because there is only one Pathway to take which is Pyruvate---->Lactate causing lactic acidosis and eventual death... When you give (Thiamine) first it allows the body to replenish the Cofactors so that the Pyruvate--->acetylCoA pathway can be used instead of making all that lactic ACID... Obviously there a few other reactions that do not work without the CoFactors but I thought this one is the easiest to see the direct consequences...
Sorry for the long winded explanation but i was having difficulty with this concept and this is how i was able to sort things out. Make sure to checkout the other reactions that need those Cofactors they are all important for the step... Hope this helps!
If anyone catches any big flaws please comment...