No you don't have to do a valsalva just make sure your back isn't in an awkward position.
I can't shed any light on the "blood sugar" issue...you might throw that question to Jordan over at Barbell Medicine.
I have an energy drink with me at the gym, and I make sure to have a big bowl of cereal prior to training. If I know That it's gonna be a long session, I'll bring a banana or some Tim Bits to the gym to knosh on.
Sorry to read about the back tweak. Here's hoping it resolves itself quickly.
8/21/18
Gym 1545
P 1.0: 90x5x5 (belt less). No back spasms, which is good.
BP: 182x5. Even though the Press poundage was light, the higher than usual volume didn't make BP easy.
Because of the low back strain, I decided to reset and LP my squat. I'll hold off on my belt until the squat gets to about body weight.
8/22/18
Gym 1700
SQ: 45x5x2, 95x5, 135x5, 165x5, 170x5x3
Pull-ups: BWx6
Chins: BWx6, 6