Three clear fizzy drinks. One of them is a dessert.

Three glasses on the counter.

All fizzy. Two clear, one dark.

All three are things people reach for when they are being good.

I drank them on three separate mornings. Empty stomach, same time, monitor on.

Before you scroll, pick the one you think moved my blood sugar most.

Got one? Keep going.

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The answer

The tonic water. It was not close.

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

The scoreboard

Why tonic catches people

Tonic water is clear.

It sits next to the soda water in the shop. It gets poured in small glasses. And it has a bitter edge that reads as grown up rather than sweet.

Read the label on a Schweppes tonic and it says 33 grams of sugar in a 12 ounce serving.

The second ingredient is high fructose corn syrup.

For comparison, a regular Coke has 39.

In other words, a gin and tonic is a gin and a small Coke.

The quinine is what makes it taste bitter. The corn syrup is what stops the bitterness being unbearable.

Both things are true at once. And the bitter half is doing a very good job of hiding the sweet half.

And the two that did nothing

Three points and two points. Both inside the range my monitor wanders on its own.

There is one real difference between those two cans, and it is not sugar.

A Diet Coke carries 46 milligrams of caffeine. That is more than the 34 milligrams in a regular Coke.

Caffeine can blunt how well you handle glucose. A study at the University of Bath gave 29 adults about 300 milligrams of it as black coffee before a sugar drink, and their glucose response came out roughly 50 percent higher.

But look at that dose again. Three hundred milligrams is more than six cans of Diet Coke, drunk at once, on an empty stomach.

One can is nowhere near that, and my monitor agrees.

If you like a gin and tonic, there are slimline tonics with the sugar taken out, and they pour the same.

Tomorrow, the comparison this whole week has been walking towards. A regular Coke against the zero sugar one, side by side.

— Stay steady