There are a lot of interesting things to look at!
Cardio is what you want. Weight lifting has benefits for other things, but if you want to specifically target BG, it would definitely be cardio.
Your body will use multiple fuel sources at the same time, and use them in different amounts for whatever conditions make sense at the time.
Since you are looking at something specifically for fasting times, and for longer times at night, you want to do something with an intensity that makes the most use of muscle glycogen, rather than fat metabolism or glucose that is in your blood at the time of the exercise. Ideally you would do something that causes your body to pull glucose from your blood during the nighttime in order to replace muscle glycogen.
Any exercise will help you to some degree, but if you burn a higher amount of fat, it will not accomplish this as effectively. If you do something that is too low of an intensity, your body will use more fat. You don’t want that.
If you do it right after you eat, your body will also use more of the immediately available glucose, and that reduces the amount of muscle glycogen you will use.
Ideally, you want something before dinner, and long enough after lunch so that your body’s choice of ideal fuel is muscle glycogen.
You want your intensity to be more muscle glycogen related (as reference, see how-does-my-body-fuel-exercise ).
Since you have time constraints, you want to try to do a steady state cardio event that lasts the duration that fits into your schedule.
So if you only have 30 minutes and you wanted to run, that time should be a fairly consistent effort. Aim for steady state heart rate for the whole time. The intensity or pace should make you feel this way:
- “I could to this for 10 or 15 minutes with no problem!”
- “30 minutes? Well, I can do that with some effort. It won’t be too hard, but not exactly easy either…”
- “60 minutes? There is no way I could do this for an hour. I am glad it is only 30 minutes…”
Keeping a steady state heart rate with hills is tough, because you have to run down the hill so much faster to keep the same heart rate that you had running up! If your heart rate spikes and drops too much, you will increase the release of liver glycogen to fuel the run, which won’t be as effective for the long nighttime lowering of BG that you are looking for.
Another alternative would be a stationary bike or treadmill. Are you in the Bay Area? There are some nice flats in Golden Gate Park. Also along the Embarcadero and Fisherman’s Wharf are flat. And my favorite place to run in San Francisco is across Golden Gate bridge which is also relatively flat.
Long winded answer, so my quick summary:
steady state run, moderate intensity, before dinner
Another side note. That cracked me up! I mean, that’s like me saying, “If I never test my BG, I am never low or high!”