Guys, please have a look at this video, at least the first 10-15min:
Here's how it goes basically:
you're probably not B1 deficient in a purely diet-oriented sense (you're getting enough from food), and you're probably fine if your standard B1 levels were checked by a doctor. HOWEVER: B1 happens to be the co-factor (the main helper) for an enzyme (a worker of the body) that's needed for energy production from your glucose. Chronic disease, imbalance, exposure to chemicals etc, whatever, can make your enzyme lose its affinity for the co-factor. In other words the worker is no longer friends with B1 as much. Result is the worker can't work properly, to help you make glucose into energy in the mitochondria (the powerhouse of the cell, all over your body).
And megadosing on B1 (standard recommendation is 0.1 mg, but here by mega-dosing, you could take up to 1500 mg, even more) makes those broken enzymes work again, so they can get back to making glucose into energy.
An experiment is brought up here: rats with traumatic brain injury had low ATP (so energy) production and that special mitochondrial enzyme I mentioned was broken. Also brain inflammation. Then, another group of rats, pre-trauma, were given mega-doses of B1, and THEN had the brain injury: and they noticed that special enzyme was fine, no reduction of ATP production in the cells. And no brain inflammation.