Do Glucose Spikes Mean Diabetes? Understanding Blood Sugar in Healthy Endurance Athletes

There has been increasing confusion around elevated blood glucose (BG) readings in endurance athletes who do not have prediabetes or type 2 diabetes. With the growing use of continuous glucose monitors (CGMs), athletes may notice glucose spikes after high-carbohydrate meals or during intense training sessions. This often raises concern, does this mean diabetes? In most cases, the answer is no.

Understanding Diabetes

Diabetes is not a single condition and exists in several forms:

Type 1 Diabetes (T1D) is an autoimmune disease in which the pancreatic beta cells that produce insulin are destroyed. Individuals with T1D require exogenous insulin to regulate blood glucose.

Type 2 Diabetes (T2D) is initially driven by insulin resistance, not insulin deficiency. In insulin resistance, the pancreas continues to produce insulin, but the body’s cells do not respond effectively, leading to elevated blood glucose levels.

When food is consumed, carbohydrates are broken down into glucose. Insulin’s role is to move glucose from the bloodstream into cells for energy. Over time, if cells become resistant to insulin, glucose remains elevated in the blood (hyperglycemia).

Diagnostic Criteria

Prediabetes is defined by any of the following:

• Fasting BG 100–125 mg/dL (after an ≥8-hour fast)

• 2-hour BG 140–199 mg/dL following a 75-gram oral glucose tolerance test (OGTT)

• Hemoglobin A1c of 5.7–6.4%

Common risk factors include genetics, excess body weight (particularly abdominal fat), physical inactivity, aging, history of gestational diabetes, and certain ethnic backgrounds.

Type 2 diabetes is diagnosed if any one of the following criteria is met:

• A1c ≥6.5%

• Fasting BG ≥126 mg/dL

• 2-hour BG ≥200 mg/dL during OGTT

• Random BG ≥200 mg/dL accompanied by symptoms such as frequent urination, excessive thirst, or unexplained weight loss

 Why A1c Can Be Higher in Healthy Endurance Athletes

Hemoglobin A1c reflects average blood glucose over the previous 2–3 months. It measures glycation — the binding of glucose to hemoglobin in red blood cells (RBCs). Once glucose binds, it remains until the RBC is removed from circulation, typically around 120 days.

In endurance athletes, RBC lifespan may be longer than average. Aerobic training increases capillary density, mitochondrial efficiency, and reduces oxidative stress, allowing RBCs to remain in circulation longer. This extended lifespan increases cumulative glycation, which can result in a slightly elevated A1c despite normal glucose regulation.

Additionally, endurance training expands plasma volume. While this may make RBC counts appear lower, it is a normal adaptation. To compensate, the body may slow RBC turnover, extending lifespan by 10–20 days. As a result, highly trained athletes may show A1c values around 5.6–5.8% without meeting criteria for prediabetes.

The Role of Cortisol

Cortisol is released during physical stress, including training. It promotes glucose release from the liver to ensure fuel availability for muscles and the brain. Acute increases in cortisol, and temporary rises in blood glucose, are normal and necessary during exercise.

However, chronic elevation of cortisol due to inadequate recovery, excessive training load, or persistent stress can lead to more sustained elevations in BG and contribute to higher A1c values. This reflects training stress physiology, not metabolic disease.

Blood Glucose Spikes During Intense Exercise

High-intensity exercise can cause brief spikes in BG in athletes without diabetes. During vigorous training, the body temporarily reduces insulin sensitivity in the liver and fat tissue to prioritize glucose delivery to working muscles.

This short-term insulin resistance is a normal adaptive response, not a sign of metabolic dysfunction. Glucose may remain elevated for several hours post-exercise and does not negatively impact performance.

Restricting carbohydrate intake to prevent glucose spikes is not recommended. Under-fueling impairs energy availability, focus, and overall performance.

Continuous Glucose Monitors in Athletes Without Diabetes

CGMs are valuable tools for athletes with T1D or T2D to maintain safe glucose ranges during training. However, CGMs measure glucose in interstitial fluid, not blood, and readings lag behind rapid changes from food intake or exercise.

Accuracy is generally lower in individuals without diabetes, especially during exercise. Mean absolute relative difference is typically:

• <10% in individuals with diabetes

• <15% in individuals without diabetes during exercise

Elite endurance athletes may spend:

• 10–20% of time with BG >140 mg/dL

• 5–7% of time <70 mg/dL

These fluctuations are driven by carbohydrate intake, adrenaline, glucagon, and exercise intensity, not disease.

Symptoms often mistaken for hypoglycemia (shaking, sweating, dizziness) in athletes without diabetes are usually due to rapid glucose shifts, low glycogen, dehydration, heat stress, or high adrenaline, not true hypoglycemia.

Key Takeaways

Interpreting glucose data and A1c in healthy endurance athletes is complex. An elevated A1c alone does not diagnose diabetes. If fasting glucose, fasting insulin, and OGTT results are normal, the athlete does not have diabetes.

Relying solely on A1c can lead to misdiagnosis and inappropriate dietary restrictions, increasing the risk of low energy availability and impaired performance. Clinicians must consider training load, recovery status, stress physiology, and family history, not just laboratory cutoffs when evaluating glucose markers in endurance athletes.

References:

1. American Diabetes Association Standards of Care. Diabetes Care (January 2026) Vol.49, Suppl.1:S1-S371.

2. Baskhshi S. "The Paradox of Elevated HbA1c in Elite Endurance Athletes with Optimal Metabolic Health". Healthspan. (2025, July 26). https://www.gethealthspan.com/research/article/a1c-levels-of-endurance-athletes?srsltid=AfmBOopi0I2eA5O-xSPuL9iZ5Et4BXnbSaFRjjBODbVP08LJnOW3BLmp

3. Flockhart M and Larsen FJ. "Continuous Glucose Monitoring in Endurance Athletes: Interpretation and Relevance of Measurements for Improving Performance and Health". Sports Medicine (2024) 54:247-255.

4. Riddle MC, Skroce K, and Turner LV, et al. "Continuous Glucose Monitoring Use in Athletes Without Diabetes". Sports Science Exchange (2025) Vol.38, No. 263, 1-9.