Wil Dubois

Happy Saturday! Welcome to Ask D'Mine, our weekly advice column hosted by veteran type 1 and diabetes author Wil Dubois. This week, Wil's talking minutes, which may be much more impactful in daily diabetes care than you might think.

{Got your own questions? Email us at [email protected]}


Jim, type 2 (and D-dad to a type 1) from Michigan, writes: How quickly does food or insulin begin to affect a meter reading? If I start to eat and remember I hadn’t tested yet, can I still get an accurate reading? OR, if I got ‘out of sequence’ and hadn’t tested before taking insulin or meds, if tested then, how accurate would it be?

[email protected] D’Mine answers: Aw hell. That reminds me that I forgot to test my blood sugar this morning… as we all do from time to time. So that’s a great question, Jim. Thanks for taking time to write, and I think you’ll be surprised that, in this case, being on time is later than you might think.

Let’s talk insulin first, then food.

The onset of insulin action for most modern fast-acting insulins in most people (not that I’m hedging my bets or anything… oh… wait… I am) is 20 minutes. Generally that means that for the first 20 minutes after taking a shot, the insulin isn’t even on the job. And once it starts working it’s slooooow, not reaching “peak” action for a good two hours — half-way through its life. This is the reason a half-hour pre-bolus gives a better blood sugar response: A pre-bolus ensures that the insulin is already starting to work when the food arrives.

So any meter reading taken in the first half hour or so after an insulin shot is unlikely to show any change. Nothing has happened yet to change the blood sugar.

OK, so what about food? How quickly does food change blood sugar? Food is a lot more variable than insulin because it depends on what the food is, how much of it there is, and how fast you eat it. Not surprisingly, a 64-ounce T-bone steak is going to be slower to impact your blood sugar (but it will) than a 64-ounce Slurpee drink.

Also throwing a monkey wrench in the works is your stomach. Different people digest at different speeds. In general, the longer you’ve had diabetes, the slower your stomach will digest, thanks to the radioactive effect of a lifetime exposure to sugar bombs on the nerve cells of your stomach.

Still, most carbs — even pure, organic non-GMO glucose — don’t have an immediate effect on blood sugar. That’s why we wait 15 minutes before we re-test our blood sugar after taking glucose following a hypo. And that’s also why so many people over-correct lows. At 15 minutes after a low, only the first signs of an upward change in blood sugar should be evident. Most people won’t be back into normal range at that point, but this fact is not well known, and many D-peeps keep popping Skittles until their meters clock 100 mg/dL, and then wonder why they’re at 300 two hours later.

So clearly, if you’re two bites into your meal, a quick fingerstick will still show you your pre-meal fasting level, or a number damn close to it. The food hasn’t had time to get from the digestive system into the blood stream, and the insulin isn’t a factor yet—as most of us can’t get our shit together enough to pre-bolus, and even if we do, our insulin is just barely starting to work.

OK, you say, but how much farther can I stretch it into the meal?

I’d say that for most people, if they’re 5 to 10 minutes into the meal when they remember that they forgot, they can go ahead and test and get an accurate pre-meal reading. If it’s more than 10 minutes, most people will record a rise in sugar over what their pre-meal baseline would have been had they checked it—although a few people will see a dip at the start of a meal if their meds kick in faster than their digestion.

What if you totally missed the boat and the meal is nearly over, should you still test? That depends on your goal for testing. A pre-meal test serves different purposes for different people. Type 1s like your kiddo and me generally test before eating to adjust our meal insulin. If we’re running a little “sweet” before the meal, we can add a correction dose to the shot for the food; if we’re coasting a bit low, we can reduce the meal shot to avoid a downstream hypo. Type 2s often test before meals as a measure of how effective their fasting basal control is. A late test is worthless for either of these goals; but it can still serve a different purpose.

Because when it comes to blood sugar, I don’t think there’s such a thing as worthless information.

For instance, you might discover that you shoot crazy-high in the hour following a meal. If you only checked before the meal and two hours later, you’da never known that. Can shooting crazy-high for a short time hurt you? As a matter of fact, it can. Or you might find that you dip dangerously low following a meal if your meds and your carbs are out of sequence in their action profiles.

Either of those little factoids are priceless information, even though they may not be the facts you originally set out to learn.

So no test is ever wasted. But to answer your main question: For most people, taking a pre-meal fingerstick in the nick of time means taking it within 15 minutes after the start of the meal.


This is not a medical advice column. We are PWDs freely and openly sharing the wisdom of our collected experiences — our been-there-done-that knowledge from the trenches. But we are not MDs, RNs, NPs, PAs, CDEs, or partridges in pear trees. Bottom line: we are only a small part of your total prescription. You still need the professional advice, treatment, and care of a licensed medical professional.
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This content is created for Diabetes Mine, a consumer health blog focused on the diabetes community. The content is not medically reviewed and doesn't adhere to Healthline's editorial guidelines. For more information about Healthline's partnership with Diabetes Mine, please click here.