[Yonhap]
[Yonhap]

The iced Americano — known colloquially in Korea as "아아" — is a go-to drink for office workers seeking a low-calorie, low-sugar caffeine fix. But experts are warning that even plain black coffee with no sugar or syrup can raise blood sugar levels in some people.

Ahn Chul-woo, a professor of endocrinology at Gangnam Severance Hospital who runs the YouTube channel "Ahn Chul-woo's Hormone TV," recently explained the relationship between black coffee and blood sugar in a video titled "Why your blood sugar rises even when you only drink an iced Americano."

"A surprisingly large number of people ask me why their blood sugar goes up when they drink an iced Americano without any sugar," Ahn said. "The fact that blood sugar rises after black coffee does not mean the coffee contains a lot of sugar. You need to look at how your body responds to caffeine, not the sugar content of the coffee itself."

The key mechanism involves caffeine stimulating stress hormones. When caffeine enters the body, it triggers a state of alertness, prompting increased secretion of cortisol and adrenaline. As cortisol levels rise, glucose stored in the liver is released into the bloodstream — causing blood sugar to climb even without eating anything sweet.

The effect is compounded in people with low insulin sensitivity. Normally, insulin moves glucose from the blood into cells, bringing blood sugar back down. But in people with reduced insulin sensitivity, that process does not work smoothly.

"Low insulin sensitivity means the power of insulin has weakened," Ahn said, according to Financial News on Wednesday.

"When the balance between cortisol, which raises blood sugar, and insulin, which lowers it, is disrupted, people with lower insulin sensitivity will see their blood sugar rise much more easily from the same cup of coffee," he added. "Even if your blood sugar tests come back normal, your insulin sensitivity may already be declining."

The timing of coffee consumption also matters. Cortisol levels are naturally elevated on an empty stomach in the morning, and adding caffeine at that point can cause a sharper blood sugar spike. Coffee consumed late in the afternoon or evening can also degrade sleep quality, potentially affecting fasting blood sugar the following morning.

"When caffeine reduces sleep quality, it suppresses the secretion of melatonin, the sleep hormone," Ahn said. "Melatonin plays an important role in steadily lowering fasting blood sugar overnight, so when melatonin decreases and you fail to get a good night's sleep, your fasting blood sugar the next morning can rise."

Decaf coffee is not entirely safe either. Trace amounts of caffeine can remain in decaffeinated coffee, and adding milk, syrup or powder can directly raise blood sugar through the lactose and carbohydrates they contain.

"Even with decaf, adding milk, syrup or powder means the lactose and carbohydrates in those ingredients will directly raise your blood sugar, so caution is warranted," Ahn said.

Adding extra espresso shots also warrants caution. Higher caffeine intake increases cortisol secretion and can affect insulin sensitivity, so people who need to manage their blood sugar would benefit from reducing the number of shots or choosing a smaller size.

Drinking coffee before exercise is not always beneficial, either.

"There is a saying that coffee before exercise is good for blood sugar — that is half right and half something to be careful about," Ahn said. "Caffeine can help with exercise performance and alertness, but drinking strong coffee on an empty stomach and then immediately doing high-intensity exercise can overstimulate stress hormones and actually push blood sugar higher."

To reduce blood sugar fluctuations, Ahn recommended drinking water before coffee on an empty stomach and having coffee after a meal; choosing black coffee with no syrups, sugar or other additives; cutting back on the number of shots and total daily caffeine intake; avoiding coffee in the late afternoon or evening to protect sleep quality; and monitoring blood sugar before and after coffee consumption to gauge personal caffeine sensitivity.

Experts note that individual differences in caffeine metabolism and hormonal response mean blood sugar reactions can vary widely even when people drink the same amount of coffee. For those with diabetes, prediabetes or a need to manage blood sugar, considering not just the type of coffee but also the timing, quantity and any added ingredients can make a meaningful difference in blood sugar control.


rainbow@heraldcorp.com