‘Glucose spikes’ and continuous glucose monitoring

The low carbohydrate diet movement has popularised the idea that the rise in blood glucose that follows a carbohydrate-based meal causes diabetes-like damage to our bodies. Health technology businesses have exploited public fear of these ‘glucose spikes’ to sell continuous glucose monitoring (CGM) to the non-diabetic public. This raises several issues that we wish to address.

Glucose spikes are normal physiology. Blood glucose rises after meals, and insulin levels rise in response. Glucose is stored in muscle, liver, and other tissues, and glucose and insulin fall back to their baseline levels. Damage to blood vessels and other tissues only occurs when blood glucose rises excessively and for many hours and baseline levels are elevated. Elevated glucose levels are also normal physiology during and immediately after intense exercise. (Inside Exercise podcast # 78)

Continuous glucose monitor (CGM) measurements are only accurate to within 20%.  Accuracy may be worse during times of rapid increases or decreases in blood glucose levels. A measurement error of 10-20% is acceptable for the purpose of managing insulin in type 1 diabetes, when combined with symptom awareness and finger prick glucose when needed. However, for short-term users of CGM without diabetes or prediabetes, inaccuracies in measurement will make it difficult to assess small differences in glycaemic response to different meals.

CGM-based dieting will not help you prevent diabetes. The whole foods that cause blood glucose levels to ‘spike’ are not the foods associated with the development of diabetes. It may seem paradoxical that eggs, processed meat, and red meat – foods that do not result in a glucose spike – are associated with the development of diabetes. This is because diabetes and prediabetes are caused by insulin resistance and the easiest way to induce insulin resistance in humans and animal models is with a diet high in saturated fat. Meat, eggs, and ultraprocessed food have other qualities that contribute to this effect.

The glucose response to a meal depends on what was consumed at previous meals. Eating high-fibre foods, particularly legumes, leads to a ‘next meal effect’. Subsequent carbohydrate meals result in a smaller rise in blood glucose. A meat-heavy low-carbohydrate diet has the opposite effect, causing insulin resistance and exaggerated ‘glucose spikes’ in response to subsequent carbohydrate meals. CGM zooms in on abnormal elevation of blood glucose, which is a symptom distracting from the real problem: insulin resistance.

A low fat, whole foods, plant-based diet (WFPB) combined with physical activity is highly effective for reversing insulin resistance (increasing insulin sensitivity). The very low fat content of WFPB, particularly saturated fats, reduces the fat in the muscle and liver cells that causes insulin resistance. Whole plant foods, in contrast to animal products and processed plants, have many other qualities that improve glucose regulation: dietary fibre, anti-inflammatory effects, more favourable amino acid profile, and weight loss due to lower calorie density of meals.

Continuous glucose monitors (CGM) enable people with type 1 diabetes to achieve near-normal blood glucose regulation. CGM has an emerging role in type 2 diabetes. In people without diabetes, CGM data may demonstrate the obvious: high-carbohydrate ultraprocessed foods lead to a rapid rise in blood glucose. However, CGM data may be misleading in identifying the best foods to eat for long-term blood glucose regulation and overall health. HbA1c remains the most straightforward and most widely available measure of blood glucose regulation.