Citric acid, like acetic acid in vinegar will lower BG levels. Quite a few type 2s have a teaspoon of cider vinegar and it does work as an emergency, get the level down quick type measure.
Lemon juice though is more pleasant and with water or with a calorie and carb free lemonade it is refreshing. If you include the yellow flesh too, it is a good source of dietary fibre which helps blood circulation. Use the flesh though, not the pith. That tastes bitter. Limes of course have the same effect.
The effect is not terribly long lasting but every little helps and in addition to the fibre it is a good source of vitamin C. I get a bag of lemons on most supermarket visits. One day I hope to source some
Amalfi lemons. You can eat those like oranges.