During harder efforts, your body interprets the demand as a form of physical stress. (File photo/AI-generated)A patient, who is a disciplinarian when it comes to maintaining his diabetes protocol, came to see me with furrowed brows. “Doctor, I did a 55-minute cycling routine on a stationary bike, and my blood sugar spiked 24 points (99 on waking, 123 about 45 minutes after the ride). I had a protein shake for breakfast, with 2 g of carbs. I did not expect a spike!” he exclaimed. “I thought cardio was supposed to drop the blood sugar, not raise it! Maybe I went too hard? Can such spikes impact my HbA1C (average blood sugar count of three months) levels?,” he asked.Clearly, his anxiety stemmed from the fact that cycling on a stationary bike is a high-efficiency, low-impact workout that boosts cardiovascular health, builds leg strength and burns 200-300+ calories in 30 minutes. We are often told that exercise lowers blood glucose, so a post-workout rise can seem like something has gone wrong.Why does blood sugar spike post-workout?In reality, a temporary increase after exercise, especially if the session was intense, is a normal and well-understood response. Waking at 99 mg/dL and seeing 123 mg/dL about 45 minutes after a 55-minute stationary bike ride worried my patient. That 24-point rise is noticeable, but it is still within a healthy range for most people. The key is understanding what your body was doing during that ride.When you exercise, your muscles need fuel. That fuel is largely glucose. If the session is steady and moderate, muscles steadily pull glucose from the bloodstream, and levels may fall. But if the intensity creeps up, your body shifts into a stress response, even if you do not feel stressed emotionally.The immediate spike: The fight or flight responseDuring harder efforts, your body interprets the demand as a form of physical stress. In response, it releases hormones such as adrenaline and glucagon. These hormones signal your liver to break down stored glycogen and release glucose into the bloodstream.This process is protective. It ensures your muscles have enough fuel to keep working. However, in the short term, the liver may release glucose faster than your muscles can use it. The result is a temporary rise in blood glucose. This can happen not only with strength training but also with cardio, if the intensity is high, you are pushing into intervals or near your limit or you are not yet fully adapted to that workload.Even with only 2 grams of carbs in your protein shake, the spike my patient saw was almost certainly driven by hormones, not food.Why cardio does not always lower blood glucose immediatelyStory continues below this adSteady, low-intensity cardio often lowers glucose during and after the session. But moderate to high intensity cardio can behave more like strength training in terms of hormonal response. If your 55-minute ride included resistance, higher cadence bursts or a sustained challenging pace, your body may have activated that same stress pathway. The glucose rise is not a sign of failure. It is a sign that your liver was doing its job.Usually, levels settle down within a couple of hours as muscles continue pulling in glucose, hormone levels normalize and insulin sensitivity increases.Long-term benefits: Building a glucose spongeAfter exercise, your muscles become more insulin sensitive. They are primed to absorb glucose efficiently for up to 24 to 48 hours. This happens through mechanisms that allow muscle cells to take in glucose even with lower insulin levels. Over time, regular training improves insulin sensitivity, increases muscle mass and expands glycogen storage capacity. Think of muscle as a storage tank. The more muscle you have, the more space there is to store glucose safely. That means less glucose circulating in the blood after meals.How much do spikes affect HbA1C?HbA1C reflects your average blood glucose over about three months. It is influenced by overall patterns, not short, isolated spikes. A brief rise from 99 mg/dL to 123 mg/dL after exercise is unlikely to meaningfully affect your HbA1C, especially if your overall daily readings are in range, spikes are short lived and you return to baseline within a reasonable time.Story continues below this adWhat impacts HbA1C more are frequent, prolonged elevations that last hours, not minutes. What matters more is the trend over time. If your overall control is improving and your recovery levels are good, then your exercise routine is helping, not hurting.(Dr Bhattacharya is senior consultant, Endocrinology, Indraprastha Apollo Hospitals, New Delhi) © The Indian Express Pvt Ltd