Lunchbox duty.

Last Tuesday, I was packing the kids' school bags when my wife held up a little strawberry yogurt cup. The kind with the fruit sitting at the bottom.

“These are healthy, right?” she asked.

I said yes. Out of habit.

But I was wearing my glucose monitor that week. It's a small sensor on the back of my arm that reads my blood sugar all day and sends it to my phone.

So the next morning, I ate one of those yogurt cups on an empty stomach, with nothing else. Then I watched my phone.

This is one person's experiment, not medical advice. Everyone's body responds differently.

That yogurt cup raised my blood sugar about 42 points in 40 minutes.

Then, almost two hours after I ate it, my blood sugar dipped a little below where I started. I felt that dip. I was hungry again by 9:30.

Now, 42 points is not a disaster. But I expected a cup of yogurt to barely move my blood sugar at all… and this cup raised it three times as much as the yogurt bowl I made later that week.

● The Science

So what's going on inside that little cup?

I flipped it over and read the label. The cup has 20 grams of carbs and 15 grams of sugar. Twelve of those grams are added sugar. Cane sugar is the second ingredient, right after milk.

It has just 5 grams of protein.

Here's the thing. Plain yogurt is one of the gentler foods you can eat. The University of Sydney keeps the world's biggest list of glycemic index scores. In other words, it rates foods from 0 to 100 by how fast they raise your blood sugar.

Plain yogurts on that list average about 27. Sweetened yogurts average about 41.

Both of those scores are low. So why did my cup do more than I expected?

A nutrition researcher named Thomas Wolever looked into this in 2017. Sweetened yogurts carry about two and a half times more carbs than plain ones. And plain yogurt has a lot more protein for each gram of carbs. Protein slows down how fast your body absorbs sugar.

In other words, the sweet cup brings more sugar and less of the thing that slows sugar down.

It's revealing.

● Meanwhile… The Hack

Meanwhile, a new study out of Japan this spring made me want to build a better bowl.

Researchers gave 303 healthy adults a cup of plain yogurt every morning for 12 weeks. Everyone wore a glucose monitor. By the end, their average blood sugar had dropped about 4 points, and their blood sugar swung up and down less during the day.

Interesting. But there are two things to know about that study.

It had no comparison group, so we can't be sure the yogurt did all the work. And one of the authors works for Meiji, a big Japanese yogurt company.

Still, notice what everyone ate. Plain yogurt. Not strawberry cups.

So I made my own version at home. Plain Greek yogurt, a small handful of fresh blueberries, and a spoonful of chopped walnuts.

My Greek yogurt bowl raised my blood sugar just 14 points. The fruit cup raised it 42.

It's still yogurt and fruit. It just raised my blood sugar about a third as much.

Here's how to build it.

● The Bigger Picture

The fruit cup isn't the only thing in the yogurt aisle with a healthy-sounding label.

Here are three more yogurt foods that raised my blood sugar more than the word “yogurt” made me expect.

By the way, back in 2024 the FDA said yogurt makers could print a limited claim on their labels: eating yogurt often may lower your risk of type 2 diabetes. The FDA also said the proof behind that claim is limited.

And that research is about yogurt in general. It can't tell you what a cup with 12 grams of added sugar will do to your blood sugar on a Tuesday morning.

So I'm not banning the fruit cups. My kids love them, and I'm not fighting that battle at 7 a.m.

For me, the fix is simple. Check the added sugar line. Pick plain when I can. Add the fruit myself.

I don't know if your body will respond the way mine did. Everyone's numbers are a little different. But it's an easy test to try.

Stay steady