Coke Zero & caffeine: for me it’s not the sweetener – it’s the caffeine

The trigger: a new sweetener analysis

The trigger was a new analysis from Tufts University (June 2026): across 21 randomised trials, the researchers found on average higher fasting insulin and higher long-term sugar (HbA1c) with calorie-free sweeteners compared with water or placebo, plus a trend to poorer insulin sensitivity.¹ A change in gut flora is discussed as a possible mechanism.

Sounds worrying – but the devil is in the detail. The authors expressly emphasise: no harm was proven, the effects differ by sweetener, and sweeteners remain the clearly better choice than sugar when you use them to replace large amounts of sugar.¹ It’s an average trend across studies, not a fixed rule for the individual.

My observation: the sweetener moves nothing

Evidence level: single-case observation (n=1)

I typically drink Coke Zero at the weekend. When I look at the CGM curve, it stays flat after a Coke Zero – no acute rise like the one that comes immediately after regular cola (the comparison is in “Ordered Zero, got regular cola”, where I measured the sugar content of regular cola). That fits the physiology: the usual sweeteners in Coke Zero (aspartame, acesulfame-K) provide no usable carbs that would drive the acute glucose curve.

What really works for me: the caffeine

Evidence level: plausible (matches the evidence)

The actually interesting effect comes not from the sweetener but from the caffeine. And that’s well documented scientifically: caffeine can temporarily lower insulin sensitivity and amplify the blood-sugar response.² For me, though, the effect isn’t always the same – it depends strongly on the time of day.

In the morning my body is in a state of higher insulin resistance anyway – the dawn phenomenon: cortisol and growth hormone push blood sugar up towards morning, without any food.³ If caffeine, which additionally dampens insulin action, comes on top, two effects add up. No wonder I see something most clearly in the morning.

Late in the evening a second factor joins: caffeine disrupts sleep – and poor sleep raises blood sugar the next day (more on this in Sleep & blood sugar). So a late caffeinated cola works doubly for me: directly via insulin sensitivity and indirectly via poorer sleep.

How I handle it

• Coke Zero at the weekend is okay for me – the curve stays flat.
• Caffeine timing-aware: early in the morning and late in the evening I’m more careful, because that’s where the effect is largest.
• Late at night rather caffeine-free, so as not to burden sleep and the next day.
• I don’t judge the cola across the board, but separate sweetener and caffeine – two different things in one glass.