What Happens to Blood Sugar During Different Types of Workouts

During any physical activity, muscles need energy, and the main “fuel” becomes glucose from the blood and from glycogen stores in the muscles and liver.

During exercise,

muscles start taking up glucose more actively and partly without insulin, via

insulin‑independent mechanisms (translocation of the GLUT4 transporter to the

membrane and activation of intracellular pathways).

As a result:

· During the workout, blood sugar more often decreases, especially during prolonged aerobic exercise.

· After the workout, insulin sensitivity remains increased for 24–48 hours, so the same dose of insulin or tablets may work more strongly.

But different types of workouts affect blood sugar in different ways.

Aerobic Exercise: Walking, Running, Cycling

Aerobic exercise is continuous, rhythmic activity of moderate intensity in which many muscles are engaged and oxygen is actively used: walking, jogging, swimming, cycling, dancing.

What happens to blood sugar

· Muscles actively use glucose as the main fuel.

· Insulin sensitivity increases already during the activity.

· Under the influence of hormones (primarily glucagon and catecholamines), the liver releases glucose into the blood, but during moderate exercise the muscles manage to “take it up”.

· As a result, blood sugar levels most often decrease — sometimes quite noticeably, especially if the activity lasts more than 30–40 minutes.

Studies show that the most pronounced and rapid tendency toward lower glycemia is observed precisely during aerobic exercise.

The glucose‑lowering

effect persists after the workout: increased insulin sensitivity can last up

to a day, and sometimes up to two days.

In practice, this means:

· Walking, light running, cycling, and swimming most often “pull sugar down” during and soon after the workout.

· In type 1 diabetes, this increases the risk of hypoglycemia if the insulin dose is not adjusted in advance and/or carbohydrates are not added.

· In type 2 diabetes, regular aerobic activity helps lower average glucose levels and reduce the need for medications.

Strength and Interval Training: Not Always “Minus Sugar”

Strength training (lifting weights, bodyweight exercises, resistance bands) and high‑intensity interval training (HIIT) are anaerobic or mixed‑type activities.

What happens at the hormone and sugar level

· During high‑intensity exercise, catecholamine levels (adrenaline, noradrenaline) rise sharply — 10–15 times or more compared with rest.

· Catecholamines stimulate the liver and muscles to break down glycogen more actively and release glucose into the blood.

· At the same time, part of muscle glucose uptake occurs via insulin‑independent mechanisms, but the liver’s glucose release can “override” this effect.

Therefore, during strength or very intense exercise, two scenarios are possible:

· A short‑term rise in blood sugar (hyperglycemia) due to the hormonal response and glucose release.

· A more moderate decrease in glycemia or “swings” (first a rise, then a gradual fall).

An important nuance: in the long term, strength training also

improves insulin sensitivity and blood sugar control, even if sugar rises slightly during the

session itself.

Mechanisms

· Increased muscle mass → more “storage and usage sites” for glucose.

· Improved insulin signaling pathway (IRS‑1/IRS‑2, PI3K, etc.) in muscles → insulin starts working more effectively.

· Increased insulin sensitivity persists for 24–48 hours after the workout.

This is exactly why current recommendations for people with diabetes advise combining aerobic and strength training throughout the week.

How It Looks Over Time: Before, During, and After

Physical activity affects blood sugar not only during the workout, but also before and after it.

Simplified:

· During moderate aerobic exercise

o Blood sugar more often decreases smoothly: muscles actively “consume” glucose, insulin sensitivity increases, the liver compensates for the expenditure, but not completely.

· During intense anaerobic exercise

o Due to the release of adrenaline and noradrenaline, the liver may release more glucose than the muscles can take up, and blood sugar temporarily rises.

· In the first hours after the workout

o Muscles continue to actively use glucose to restore glycogen stores, insulin sensitivity remains increased — blood sugar may decrease even after the session ends.

· Within 24–48 hours

o Increased insulin sensitivity persists, so with the same dose of insulin/medications, blood sugar may be lower than usual.

For a person with diabetes, this means:

· Aerobic exercise — risk of hypoglycemia “during and after”.

· Strength/HIIT — risk of short‑term hyperglycemia during the session, followed by a decrease.

· It is important to track your own response (logbook: type of activity, duration, blood sugar before/after).

Comparison: Aerobic vs Strength

What happens	Aerobic (walking, running, cycling)	Strength and interval training
Main fuel	Glucose + fats	Glucose (glycogen)
Change in blood sugar during	More often a decrease	Possible rise or “swings”
Hormonal response	Moderate increase in catecholamines	Sharp increase in catecholamines
Effect after workout	Lower blood sugar, ↑ insulin sensitivity up to 24–48 h	Same: ↑ insulin sensitivity 24–48 h
Long‑term effect	Improved glycemic control, endurance	Muscle growth, strong improvement in insulin sensitivity

The optimal option for a person with diabetes is a combination: 150+ minutes per week of moderate‑intensity

aerobic activity plus 2+ strength sessions per week.

Why All This Matters in Diabetes

In diabetes (especially type 1), the body’s insulin response is “disconnected” from real needs: insulin is administered from outside and cannot automatically decrease or increase during exercise.

Because of this:

· During prolonged aerobic exercise on a background of a “usual” insulin dose, blood sugar may drop more than in a person without diabetes.

· During very intense strength or interval training, blood sugar may first spike, and then, against the background of still‑active insulin and increased muscle sensitivity, fall below the usual level.

Therefore, any regular training requires:

· Glucose monitoring before (and if necessary during/after) the session.

· Possible adjustment of insulin/medication doses and a carbohydrate snack — individually, together with a doctor.

· A logbook that shows how different types of activity affect your specific blood sugar.

Bottom Line: Movement as a Tool for Managing Blood Sugar

Aerobic workouts act like a gentle but powerful “vacuum cleaner” for glucose: they lower blood sugar during the session and enhance the action of insulin for the next day.

Strength and interval sessions may temporarily raise blood sugar due to the release of stress hormones, but in the long run they significantly increase muscle mass and insulin sensitivity, helping to better control glycemia between workouts.