Why a morning jog kept one man’s blood sugar stubbornly high

A Japanese endocrinologist says switching from dawn running to short walks after meals helped one patient bring his HbA1c down from above 8% to 6.6%.

A diabetes specialist in Japan has highlighted how a well-intentioned exercise routine can sometimes push blood sugar in the wrong direction, at least at certain times of day. In the case described by endocrinologist Hiroyuki Yano, a company owner in his fifties had spent years trying to improve his health by cutting carbohydrates and jogging first thing in the morning, yet his HbA1c remained above 8% for six years. HbA1c reflects average blood glucose over roughly one to two months, so Yano used a continuous glucose monitor to look at how the man’s levels changed through the day. The result was striking: his blood sugar was already elevated on waking, then climbed further after his morning run before staying high through the day. According to Yano, the pattern suggested a combination of the dawn phenomenon, when blood sugar rises early in the morning because of hormonal changes, and the added stress of vigorous exercise.

Mayo Clinic says the dawn phenomenon usually occurs between 4 a.m. and 8 a.m. and can be worsened by too little insulin, the wrong medication dose or eating carbohydrates late at night. Cleveland Clinic also notes that exercise timing matters, with physical activity after meals often helping to lower glucose, while intense efforts can briefly raise it because stress hormones such as adrenaline are released. In Yano’s case, he believed the morning jog was adding to the hormonal rise already under way. Once the patient stopped running at dawn and instead walked for 15 minutes after each meal, his overnight glucose fell from around 140 mg/dL to about 100 mg/dL, and his HbA1c improved to 6.6% after three months.

The episode is a reminder that diabetes management is not only about working harder, but about working more intelligently. The Centres for Disease Control and Prevention says people with diabetes should monitor blood sugar regularly, understand what pushes it up or down and combine diet, activity and medication consistently. Mayo Clinic likewise advises people to check glucose before, during and after exercise, especially when starting a new routine. Yano stressed that the man’s family history of diabetes also mattered: his father and brother had the condition, so the long-term high HbA1c could not be blamed on exercise alone. He also cautioned that medicine, diet and alcohol intake can all affect the result, making it impossible to attribute improvement to one change in isolation.

The broader lesson, Yano argued, is that healthy habits can still backfire if they are not matched to the individual’s physiology and timing. That applies not just to exercise, but to other routine choices that are often assumed to be harmless, such as vegetable juice, low-fat eating or regular over-the-counter painkillers. In the full article, he examines three cases in which people believed they were doing the right thing, only to discover that their daily habits were nudging their test results in the wrong direction before they finally saw their numbers improve.