We will do a three way video call to verify how I look, my voice, my current weight, etc. Then, at the end of this year, if I'm above 90 KG, the other bettor keeps the whole 1 BTC.
And how can the escrow check the scale was not rigged by you? Human eye may be fooled easily for 10kg difference and since the scale is also yours there's not reliable way to see the truth.
Also it's not that difficult to lose weight. I've done that myself. The much more difficult part is to remain at the lower weight (I've failed that part).