Happy Saturday, PWDs (people with diabetes)!

Welcome back to our weekly advice column, Ask D'Mine, hosted by veteran type 1, diabetes author and educator Wil Dubois. This week, we've got Wil humming that Rolling Stones tune Time Is On My Side as he answers a question about insulin activity and pre-bolusing for meals. Care to hum along as you take a read...?

{Got your own questions? Email us at AskDMine@diabetesmine.com}


Susan, type 1 from California, writes: Hey Wil, I wanted to ask you about carb ratios and that ethereal time component. Let's say my carb ratio is 1 unit for every 12 grams. Why then, if I take that dose 30 minutes early, do I need more than 12 grams to correct it? Similarly, if I take that dose accidentally 30 minutes late, why do I need more than 1 unit to correct the resulting high? This non-linearity kills me and I'd love a physiological rationale for it.  

Wil@Ask D'Mine answers: You actually answered half of your own question, I think, without even realizing it. You called it the "ethereal time component," which I find borderline poetic. We tend to think about insulin-to-carb ratios as a two-component equation, so much insulin for so much food. But that's not correct. There's a third part of the formula—ethereal time. And a fourth part that I'll call allocation of resources.

Let's start with time. You're right, it's not linear. It's more bell-shaped. Well... I guess more sand dune-shaped. It's a curve that slopes steeply upwards, rolls over at the crest, then tapers back downwards at a 45-degree angle.

What the hell am I talking about? Action, baby.

I'm talking about the action curve of the insulin. Taking insulin for carbs isn't like mixing a whiskey sour where all you need to do is take the right amount of a good ol' Kentucky bourbon and the right amount of low-carb sweet and sour mix, place them in a crystal glass and stir and—voilà—you've got the perfect drink! No, it's more like baking a cake. You need to mix up the ingredients in the correct proportions, but at that point you don't have a cake—you only have batter. You still need to add heat for a certain length of time to get the batter to become a cake.

Insulin is like cake. (Talk about things you never expected to say.)

Here's why: Neither insulin nor carbs hit instantly. Even instant mashed potatoes aren't instant. At least not in their glycemic response, anyway.

If you eat a meal and forget to bolus, what happens? Your sugar goes up, right? It might even go up pretty fast, but it won't go up instantly. As your body digests the food and moves the glucose into the blood stream, without some insulin on board, the blood sugar rises. How fast and how high depends on what you eat.

In a similar way, insulin is not on the job as soon as you shoot it up, either. Whether delivered from syringe, pen, or pump, insulin has a commute to get to work. In medical parlance, this commute is called "onset." For Apidra, Humalog, and Novolog onset is 10-15 minutes, which as commutes go, I think we can all agree is pretty great. But it means there's a good quarter of an hour between when you take insulin and when it even starts to work. Then there's a second element of time, which we could compare to the time between when you arrive at your place of employment and the point at which you've had enough coffee to be able to actually do your job well. With insulin, the end of this caffeining-up period is called the "peak" action. It's when the insulin is having the strongest effect. Depending on your body, this hits anywhere between one hour and two hours after injection.

But after that point, insulin starts to get tired. Run down. Anywhere between 3 and 5 hours after arriving at work it's poor little face is flat on its desk and it's fast asleep on the job.Waiting (Here's a useful chart of onset, peak, and duration of action.)

So, darn it! Carb ratios are not a simple matter of just getting the right amount of insulin to match the right amount of food. We also have to synchronize the timing. You want the insulin's peak to line up with the peak carb absorption in your gut. There's actually quite a bit of research showing that the old-fashioned pre-bolus makes modern "rapid-acting" insulins more effective. One study sets the magic number at 20 minutes, basically making sure that the insulin has at least arrived at work and is pouring its first cup of coffee before you start throwing food at it.

If a pre-bolus is so great, why don't we all do it? Well, there are dangers, like what if the meal gets delayed? And of course, simply organizing your life enough for a pre-bolus is a bigger trick that it sounds like.

That takes care of time, but it doesn't explain everything, does it? Even if the timing is off, shouldn't it all work out right in the end? If we were mixing whiskey sours, not baking cakes, yes. If we were in the realm of pure chemistry, it wouldn't matter when the various ingredients were added, it would all even out: A given volume of insulin would dissolve a given volume of sugar. But this isn't chemistry. It's home economics. The allocation of resources I talked about in the beginning.

So let's compare it to a bank account: If I earn $3,000 a month, and my bills are $3,000 a month, I'd be a broke bloke, but it would work out perfectly, right? It's a balanced equation.


Why? Because the timing will be off, and there are unexpected consequences to bad timing. The stupid car payment will clear the bank before my paycheck does. I'll go below my minimum balance, or worse yet, overdraw my account. Then what? Well, then the bank hits me with all kinds of fees, services charges, and taxes. Now I have an outbound cash flow of $3,500 but I've still only earned $3,000. I just suffered an economic hypo, haven't I? Hypo-cashemia, I think it's called. So you can see that economics isn't chemistry. Simple math is out the window when new variables are introduced.

The same thing happens in your body. You can overdraw your insulin or your blood sugar. It doesn't matter if the checkbook would have balanced at the end of the month if the cash flow doesn't play out right.

If you bolus late, the blood sugar gets ahead of the insulin. Now you have two problems. You still need a set amount of insulin for the food, but your delay bought you a high blood sugar. You changed the equation. You overdrew your bank account. The food will still soak up the insulin, but you don't have enough to cover the new high blood sugar the delay brought on. Taking insulin late is kinda like taking a meal bolus without a correction when you are high before the meal. The best you can hope for is to return to the same level you were at before you started: High.

On the other hand, if you bolus early, well, too early, it's possible to drop low before the carbs from the food hit. The low blood sugar will soak up some of the meal's sugar now, but the full amount of insulin is still hanging around, and you'll need yet more carbs to soak it up.

So, bummer for us, the insulin-to-carb ratio is not as simple as X units cancels Y carbs. It's X units cancels Y carbs when timing is perfect and nothing unexpected happens.

And when was the last time you recall nothing unexpected happening?

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.

Disclaimer: Content created by the Diabetes Mine team. For more details click here.


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.