What is the difference between high blood sugar and a blood sugar spike?
Chronically high blood sugar and a blood sugar spike are two different things that each require their own approach. If your blood sugar is reasonably under control but you still want to reduce spikes, making meals lower in carbohydrates or exercising more can help.
High blood sugar (also known as chronic hyperglycaemia) means that your blood sugar level is structurally too high, even after you have not eaten for hours. A blood sugar spike is something different: it is the temporary rise that occurs after you eat something, measured as the difference between your blood sugar before and after a meal. After that spike, the value drops back to its starting point. The two things are therefore not measuring the same thing.
In people with good blood sugar regulation, blood sugar rises on average about 2 mmol/L above the starting value after a meal. In type 2 diabetes, that rise is almost three times as large (on average 5.9 mmol/L) and it also takes much longer for blood sugar to return to normal: blood sugar remains elevated for about 14 hours per day, compared with 8 hours in people without diabetes. The spike is therefore both higher and more prolonged.
The well-known HbA1c test, which your doctor uses to assess your average blood sugar over the past months, does not pick up blood sugar spikes well. Especially when someone's average blood sugar is still reasonably under control (HbA1c below 7.5%), it is precisely those post-meal spikes that cause the most damage. With poorer control, background blood sugar (also too high when fasting) plays a greater role. These two problems therefore call for different approaches.
Blood sugar spikes can be effectively influenced through diet. Fewer carbohydrates per meal, foods with a lower glycaemic index and more soluble fibre demonstrably lower the spike. This has been investigated in multiple randomised studies in people with type 2 diabetes. Physical activity also helps: six months of regular walking training reduced both fasting insulin levels and the blood sugar response after a meal in people with impaired glucose tolerance.
Medication can also influence spikes independently of the daily average. People who use prednisone after a kidney transplant experience a higher peak blood sugar with a single daily dose than when the same dose is spread across two moments. This illustrates how spikes and the overall level can vary independently of each other and can be managed separately.
All claims are based on one primary study (PMID 16759538) for the core figures on spike magnitude and duration, supplemented by a review on diet in T2D (PMID 32471238), a narrative review in gestational diabetes (PMID 33036170), an RCT in kidney transplant patients (PMID 24009292) and a study in stroke patients (PMID 17702957). The quantitative comparison between spike and chronic hyperglycaemia rests on one central study; this limits the certainty of the exact figures.