A Japanese clinic case suggests that strenuous exercise before breakfast may worsen morning blood sugar for some people, while short walks after meals helped one patient bring his HbA1c down from above 8% to 6.6%.
A man in his 50s who prided himself on his discipline had done almost everything he thought would help his blood sugar. He rose early to run, cut back on carbohydrates and kept close watch on his annual health checks. Yet, as Japanese diabetes specialist Hiroyuki Yano described in a clinic account, his haemoglobin A1c, or HbA1c, had stayed above 8% for six years. Harvard Health says HbA1c reflects average blood sugar over the previous two to three months, which is why it is such an important marker when daily readings do not tell the whole story.
When Yano and his team used a continuous glucose monitor to follow the patient’s levels through the day, the pattern was not what he expected. His blood sugar was already elevated on waking and climbed further after his morning jog, reaching close to 200 mg/dL. Once breakfast followed, the level rose again and remained high through the morning. The American Diabetes Association says morning highs can happen for several reasons, including the dawn phenomenon, when blood sugar rises in the early hours, while Mayo Clinic notes that exercise can have different effects depending on intensity and timing.
In this case, the likely explanation was a mix of the body’s natural morning hormone surge and the added stress of a strenuous run. Diabetes UK says moderate activity often helps reduce blood sugar, but more intense exercise can sometimes push it up instead. Yano then changed the routine, replacing the early jog with 15-minute walks after breakfast, lunch and dinner. After that adjustment, the patient’s overnight readings fell from around 140 mg/dL to about 100 mg/dL, and his HbA1c improved to 6.6% within three months. The findings also fit with research published in Diabetologia, which found that afternoon exercise improved glucose control more than morning exercise in people with type 2 diabetes.
Yano stressed, however, that the lesson is not that morning running is harmful for everyone. The patient had a strong family history of diabetes, and other factors such as diet, alcohol intake and possible medication use can also shape HbA1c. Even so, the case underlines a broader point echoed by diabetes organisations: what matters is not simply how hard someone tries, but whether the routine matches the person’s physiology. A habit that looks healthy on paper may still be working against the numbers.
Disclaimer: This content is for informational purposes only and is not intended to be a substitute for professional medical judgment, advice, diagnosis, or treatment.
