Many people skip breakfast and drink black coffee instead to cut calories, assuming that a sugar-free beverage will not affect their blood sugar, Yahoo Taiwan reported.
However, Japanese physician Kainuma Meng warns that drinking coffee on an empty stomach can cause sharp fluctuations in blood glucose levels in some people.
Blood sugar levels are not influenced by food alone. Hormones also play a role. Upon waking, the body releases cortisol and adrenaline to increase alertness, raising blood pressure and blood sugar in the process.
Drinking coffee at this time further stimulates the sympathetic nervous system. Caffeine, combined with cortisol, prompts the liver to release more glucose into the bloodstream, potentially raising blood sugar even without food intake.
The extent of these fluctuations varies depending on individual health and metabolic function. Meng cited the case of a man in his 40s who regularly skipped breakfast and drank only black coffee in the morning. By noon, he often felt extremely hungry and irritable, followed by severe drowsiness after lunch.
Although routine tests showed normal fasting blood sugar levels, continuous glucose monitoring revealed sharp spikes after his morning coffee, followed by steep declines.
Meng attributed the fluctuations to work-related stress, insufficient sleep and impaired insulin secretion, which made the man particularly sensitive to caffeine.
Another case involved a woman in her 50s who skipped breakfast and drank only black coffee to lose weight. She frequently experienced trembling hands, cold sweats and sugar cravings.
Her symptoms were not simply caused by hunger. Instead, coffee triggered a rise in blood sugar, prompting excessive insulin release and causing glucose levels to fall sharply afterward, Meng said.
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Drinking black coffee on an empty stomach can cause blood sugar spikes, particularly in people who are stressed, sleep-deprived or have metabolic problems. Photo by Bao Bao |
Meng identified four groups particularly vulnerable to blood sugar fluctuations after drinking black coffee on an empty stomach: people under prolonged stress, those who sleep poorly, individuals with a family history of diabetes, and those with impaired insulin secretion or who fast for extended periods.
Experts say coffee is not inherently harmful, but its effects depend on individual tolerance.
People who get enough sleep, experience little stress and have healthy metabolic function are less likely to be affected. Those with impaired blood sugar regulation, however, may experience sharp fluctuations when drinking coffee before eating.
To minimize these effects, experts recommend three precautions. First, avoid drinking coffee immediately after waking up. Wait at least 30–60 minutes before having the first cup of the day.
Second, eat a small snack beforehand, such as nuts, yogurt or half a boiled egg, to reduce metabolic stimulation on an empty stomach.
Third, pay attention to how the body responds. Those who experience palpitations, a rapid heartbeat, anxiety or fatigue after drinking coffee should stop and switch to warm water.
Drinking black coffee on an empty stomach is not suitable for everyone. People who frequently experience morning fatigue, rapid heartbeats, trembling hands or mood swings despite normal health check-up results should reconsider the habit and consult a doctor.