You know the feeling. Around three in the afternoon, focus drops, the eyes get heavy, and the vending machine starts to look reasonable. The instinct is to reach for something sweet and fast. The problem is that the something sweet and fast is what causes the next dip.
What is actually happening
A refined-sugar snack spikes your blood sugar quickly, then drops it just as quickly. That drop is the crash. You feel briefly better, then worse than before, and an hour later you are reaching for the next one. It is a loop, and it is built into the snack, not into you.
Breaking the loop
Protein and fiber slow everything down. They blunt the spike, which means there is no cliff to fall off afterwards. A snack with 14g of protein, 6g of fiber and only 2g of added sugar holds you level instead of sending you up and down. The afternoon stops being something you survive.