Rapid bg drop

I’m having a problem, and I hope someone can help me out. I use Lantus in the morning and Novolog during the day. Just last week my husband and I walked 2.5 miles to dinner. Before I left the restaurant my CGM said I was at 190 and rising. We walked 1 mile and it had dropped to 70 and was still falling. It was very windy, so I know the walk was hard, but that’s an awful fast drop. It has happened to me before, too, but not this bad. Since my husband works during the day, we often walk after dinner. My CGM says I’m high before we leave, but by the time I get home, or within an hour or so, I start to drop. Any ideas??

I drop like a stone if I walk any substantial distance right after eating.


That does seem like a lot. It sounds like both readings were from your CGM, but did you sanity-check either one with a finger-stick? Particularly when you’re having a rapid change a CGM can be less reliable. I know mine tends to undershoot my BG as compared with fingerstick when I’m exercising, though more from hard exercise than from the kind of thing you’re describing.

Another consideration: exercise dramatically increases your insulin sensitivity, so a 2mi walk with just your basal insulin running is one thing, but the same walk with a bolus on board, even if you’ve just eaten a meal, can have a more dramatic effect than you expect. That’s something I’ve experienced fairly often.

So, what can I do to fix it?

I didn’t do a finger-stick at the time, but I could tell it was getting low. Since I ate a full meal, I had to give myself some insulin. Should I wait until after I eat, so the food has time to kick in before the insulin? I don’t use a pump.

I appreciate the input.

Honestly, the same thing used to happen to me all the time when I did anything aerobic like walking, hiking or dancing. What saved me is going on a pump. You can set a lower temporary basal (baseline) rate starting about an hour before you start your exercise, and once you’re done you can set a different temporary basal for a few hours afterwards, depending on how your body tends to respond. Have you considered a pump? They are truly a godsend for stuff like this, as well as for PMS or illness days.

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Everyone is different, but as a general matter, I think the opposite usually obtains–that carbs usually go into effect much more rapidly than the insulin and it takes a while for it to catch up. I generally find I need to take a bolus 45min to an hour before eating if I’m going to get a reasonably flat post-prandial curve. Everyone’s metabolism is different of course.

This stuff can be really hard to figure out. Everyone has had times when they thought they’d calculated everything correctly and suddenly had that “Uh oh…” feeling. I’ve definitely been taken by surprise when walking, particularly. I think because it seems like no big deal, as opposed to going out for a run or bike ride or lap swimming. In those cases you KNOW you’re going to exert and you have to plan ahead, whereas walking seems like just… walking. 2.5 miles isn’t a strenuous day hike but it’s not totally trivial either. Maybe a combination of that plus erring a little on the high side with your bolus could explain it. Assuming the CGM was perfectly accurate, though that’s a big “if.” A lot of people have said they find their CGM gets more inaccurate the farther high or low it goes.

[quote=“Joan3, post:5, topic:51825”] Should I wait until after I eat, so the food has time to kick in before the insulin?

If you were at 190 before you started dropping, I think just the opposite strategy may be worth trying… By taking the insulin earlier before you eat you might be able to better match it to the absorption of the food and have a smoother trend overall.

Exercising when insulin is at the peak of its action curve really supercharges the dose, I’ve found. I agree with @Sam19 that a longer prebolus time might help prevent that post meal hyperglycemia. It seems to me that blood sugar lows often follow blood sugar highs, almost as certain as night follows day.

In addition, you could back off some on your meal dose in anticipation of a brisk walk. This will take some experimentation, of course, but eating and walking are both fun in my book. You might start out by only taking 90% of your usual dose.

Like others have said, with a pump you could back off on the basal rate an hour or two before you walk, but you don’t have that option. I think backing off a little on the meal dose might do the trick. Another fun option would be to indulge in a nice dessert. Maybe share a dish of ice cream with your husband.

I usually do one or more of the following -

I will wait until the middle of the meal to give myself insulin

I will also reduce my bolus by 1/2 to a full unit

I will go very low carb for the meal and take a very small bolus.


Thanks everyone, for all of the input. I’m still not ready to go on a pump, but may be there soon. I really appreciate all of the help!

Hi, @Joan3. In The Diabetic Athlete’s Handbook (by Shari Colburg), MDI users are advised to reduce mealtime insulin 0-20 percent for slow/shorter walks and 20-50 percent for fast/longer walks. Colburg also recomends taking in an extra 10 - 15 grams of carbs per hour for the longer walks. She also notes that:

“Exercising right after a meal, even breakfast, will result in greater use of blood glucose, whether you take insulin or your body releases it. In addition, if you exercise after your meal-related insulin is mostly gone (two to three hours later), you’ll need fewer adjustments to keep your blood sugars normal.”

Hope that helps!

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Thanks, this helps a lot. I’m going to pick up that book.