IOB: the second number

Same number, different afternoon

My brother Trey and I both have type 1. One afternoon we decide to walk to a new bakery 2 miles away, about 40 minutes on foot. Before we leave, we both check: 130 mg/dL, flat arrow. On a CGM screen, we look identical.

Trey can lace up and go. If I walk the same way, the insulin I already took plus 40 minutes of walking can pull me low before we get there. An hour after lunch, my Humalog is heading into its peak, so it would be working hardest right while we walk. Same number, same arrow, different plan. The difference is IOB.

What insulin on board is

Insulin on board is the estimated amount of insulin you've already taken that hasn't finished working. Rapid-acting insulin keeps lowering glucose for several hours after a dose, so the effect of a shot or bolus lasts long after you give it.

Pumps, automated systems, smart pens, and apps estimate IOB using a setting called duration of insulin action, often 3 to 5 hours, plus a curve for how fast insulin works. The result is an estimate, not a measurement.

What gets counted varies by device:

• Many pumps count bolus insulin only, not basal.
• Automated systems handle basal changes and auto-corrections in different ways, and some can show negative IOB after insulin was held back.
• On injections, IOB is only tracked if a smart pen or app logs each dose. Otherwise, it's mental math.

Why it's the second number

Blood sugar shows what's happening now. IOB shows what's still coming. IOB is one of the newest features in Zukka, the app I use to read my sensor, so now I see it right next to my glucose. How I read the rest of my charts is on Reports.

1 2 3 4

1. 2.15 units still working. That's the second number.
2. Glucose looks calm, between about 95 and 140.
3. Each blue triangle is a dose. I can't take any of them back.
4. The shaded curve is IOB. It jumps with each dose, then fades.

Insulin I've already taken is a commitment, so every decision after a dose starts with how much is still active.

Where it matters most: exercise

Exercise is where IOB matters most, because moving makes active insulin hit harder. Insulin sensitivity can stay raised for up to 24 hours after exercise¹. Experts at an ADA session said you need to know how much insulin is on board when you start².

Exercise lowers blood sugar on its own. Working muscles pull glucose out of the blood, so exercise can bring blood sugar down even without insulin on board³. That's why Trey still drifts down a little on the walk. Insulin on board adds to that drop, which is why mine falls so much faster.

Back to our walk:

Two ways to fuel differently

1. Eat for the insulin that's there. With high IOB, take carbs before and during the activity. For longer aerobic sessions on injections, the 2017 consensus suggests about 0.5 g of carbs per kg of body weight per hour, along with a smaller meal bolus¹⁴.
2. Take less insulin up front. If you know a workout is coming, a smaller meal bolus means less IOB when you start. The 2020 CGM exercise position statement suggests cutting the pre-exercise meal bolus by 25 to 50%, depending on the type and intensity of exercise⁵⁶.

Good to know: those numbers are for injections and standard pumps. Automated systems have their own exercise settings, and the 2020 guidance says not to apply its steps to them⁵. Low IOB lowers the risk but doesn't remove it, so Trey still carries fast carbs.

Before and during: prevent, then treat

Preventing a drop and treating one call for different food.

Protein's effect on glucose is slow, showing up 3 to 5 hours after eating⁷. That's too late to help during the walk. For a low, foods with fat or protein take too long to work⁸. The ADA's 15-15 rule is 15 g of fast carbs, then a recheck in 15 minutes, repeated if you’re still under 70 mg/dL⁹.

Why the croissant can't be the fix. If I get to the bakery low, a croissant from the case won't treat it. It's mostly fat, and fat slows the carbs down, so my blood sugar keeps falling while it digests⁸. So I take fast carbs to treat the low and eat the croissant too, then bolus for it late rather than skip that bolus¹⁰. When I bolus late, a quick walk afterward helps soften the peak, since working muscles pull glucose from the blood³.

For our walk, that means a carb snack for me before we leave and glucose tabs in both our pockets. CGMs can lag during a fast drop, so if I feel low but the screen looks fine, a fingerstick settles it¹⁰.

Always check before you…

Check IOB alongside your glucose before any of these.

Before you…	Why IOB matters
Exercise	The biggest one. Moving makes insulin that’s already working act faster and harder, so a steady number can drop quickly. Plan for it with carbs before you start or a smaller bolus at the meal before. Your body can stay more sensitive to insulin for up to a day afterward, so check again at bedtime1.
Drive	A low behind the wheel is dangerous. Active insulin means the number you see may not hold for the trip.
Go to sleep	Insulin from dinner or a late correction keeps working overnight while you can't react.
Correct a high	Insulin still working may already bring it down. Correcting again too soon stacks doses.
Eat	IOB changes how much more insulin the meal needs, if any.
Drink alcohol	Alcohol can lower glucose for hours, adding to what active insulin is already doing.

Where IOB misleads

IOB is only as good as the doses and settings behind it.

• Duration setting is off. Too short, and IOB drops to zero while insulin is still working, which invites stacking.
• Doses aren't logged. A manual injection or a missed pen entry won't show up.
• Absorption changes. Heat, exercise, a bad site, or injecting into scar tissue can make insulin work faster or slower than the estimate.
• Food isn't counted. IOB ignores carbs still being absorbed, so it shows only half the picture without carbs on board.

How to use it

Make it a two-number habit: before you act, check glucose, then IOB.

1. Glance at glucose and the trend arrow.
2. Check IOB, and ask how long that insulin will keep working.
3. Decide: go, wait, eat something, or adjust.

If your IOB doesn't seem to match what your glucose actually does, the settings may need a look. Review your insulin duration and other settings with your care team rather than changing them on your own.