Fasting blood sugar swings between high and low in 45-year-old on the same diet, routine and medication: What’s going on?

A 45-year-old woman had a fasting blood sugar test that showed a reading of 216 mg/dL. Two days later, with almost the same diet and again eight hours of fasting, her fasting sugar was 87 mg/dL. There was no change in medication in between. She asked us if this drop or swing was normal and should she repeat her tests or go for the HbA1c test, which measures average blood sugar over three months to understand a tendency.

Many people living with diabetes are puzzled and worried when their fasting blood glucose readings vary sharply from one day to the next. A high value one morning, followed by a normal or even low reading on another, often raises doubts about test accuracy, diet, or whether treatment is working at all. While such fluctuations can be unsettling, they are not uncommon in diabetes and warrant careful interpretation rather than panic.

Fasting blood glucose reflects the balance between glucose produced by the liver overnight and the effect of medication and insulin. The balance can shift from day to day. Even when the dinner and fasting durations appear similar, subtle differences in sleep, stress, physical activity or late-evening snacks can influence morning readings.

Minor infections, pain, emotional stress, poor sleep, or dehydration can temporarily raise fasting glucose. Physical exertion the previous day may lower it. Alcohol intake, especially at night, can sometimes suppress fasting glucose the next day, but it will derange blood glucose control in the long run.

Timing of medications matters as well. A missed dose, a delayed tablet, or a change in injection timing can alter morning values even with similar food intake.

Technical elements play a role. Differences between laboratories, variation in testing methods or even issues with home glucose meters can occasionally contribute to surprising results. However, these explanations should be considered only after clinical factors are reviewed.

What appears to be the “same diet” is not always metabolically the same. Many commonly consumed foods contain hidden carbohydrates and fats that are not obvious by taste or portion size. Gravies, sauces, chutneys, bakery items, restaurant meals, processed foods and even seemingly healthy snacks may contain added sugars, refined starches or fats. High-fat meals, in particular, can delay gastric emptying and cause a rise in blood glucose several hours later, sometimes affecting fasting levels the next morning. These hidden components can explain unexpected fasting elevations despite perceived dietary consistency.

Diabetes control cannot be judged by a single fasting reading. Certain people might have normal fasting values but high post-meal glucose levels. Others may show the opposite pattern. This is why doctors look for trends instead of isolated numbers. An occasional high fasting reading does not mean treatment has failed. Similarly, a normal reading after a very high one does not guarantee good control.

HbA1c is the most useful test to assess overall blood glucose control. It reflects average blood glucose over the previous two to three months and smooths out day-to-day fluctuations. HbA1c helps distinguish between a transient spike and a sustained problem. Consistent checking of both fasting and post-meal blood glucose provides additional insight. In selected cases, continuous glucose monitoring can reveal overnight patterns and identify causes of fasting variability.

Repeated fasting values above target, frequent large swings or symptoms such as unexplained weight loss, fatigue or recurrent infections ought to prompt a review of treatment. Adjustments may involve changes in medication timing or dose, or lifestyle measures such as the composition of the evening meal, physical activity, and sleep routines.

Wide swings in fasting blood glucose are a recognised part of living with diabetes and often reflect the body’s response to daily stressors rather than a sudden worsening of the disease. The key is to focus on patterns, not isolated numbers. Regular follow-up, appropriate testing and informed interpretation help ensure that such variations are managed without anxiety.

(Dr Saptarshi Bhattacharya is senior consultant, Endocrinology, Indraprastha Apollo Hospitals, New Delhi)