Blood sugar can go up during fasting because your liver is “letting sugar out of storage” under the influence of hormones that rise when you’re not eating, especially if you have insulin resistance or diabetes.

Why does blood sugar go up when fasting?

The basic idea

When you stop eating, your body still needs fuel. Instead of getting glucose from food, it:

  • Pulls stored glucose (glycogen) out of the liver and releases it into the blood.
  • Makes new glucose from protein and other substrates (gluconeogenesis), especially during longer fasts.
  • Uses hormones that oppose insulin (called counter‑regulatory hormones) to keep blood sugar from dropping too low.

If those “glucose‑raising” forces are stronger than your insulin response, your blood sugar can actually rise instead of fall.

Key hormones that drive the rise

When you fast, insulin drops and several hormones increase:

  • Glucagon – tells the liver to break down glycogen and release glucose.
  • Cortisol – stress hormone that promotes glucose production in the liver.
  • Adrenaline/noradrenaline – increase blood sugar so your body has quick energy.
  • Growth hormone – supports fat burning and also pushes the body to make glucose.

These are called counter‑regulatory hormones because they counter insulin and push blood sugar up.

Role of the liver and insulin resistance

In people with insulin resistance or type 2 diabetes, the liver often behaves like it is “deaf” to insulin’s signal.

  • The liver keeps releasing glucose even when there’s already enough in the blood.
  • During fasting, the counter‑regulatory hormones tell the liver to release more sugar, and because insulin’s signal is weak, there is nothing to “brake” the liver.
  • Fatty liver (excess fat stored in liver cells) can make this even worse by increasing glucose output.

So instead of a gentle decrease, fasting blood sugar can drift higher, especially overnight or during long fasting windows.

The dawn phenomenon

Many people notice higher blood sugar first thing in the morning even after not eating all night. This is called the dawn phenomenon.

  • Between roughly 3–6 a.m., the body releases hormones (cortisol, growth hormone, adrenaline) to prepare you to wake up.
  • These hormones push the liver to release glucose, which raises blood sugar.
  • In people without diabetes, insulin quickly balances this, so levels stay normal.
  • In those with diabetes or prediabetes, the response is weaker, so fasting readings can be high even though no food was eaten.

Other possible reasons fasting sugar is high

Health professionals also point to several other contributors when blood sugar rises during fasting:

  • Under‑dosing or timing issues with diabetes medications or insulin.
  • High‑fat or very large dinners that slow digestion and worsen insulin resistance overnight (think heavy pizza or takeout).
  • Stress, poor sleep, illness, or surgery, all of which raise cortisol and blood sugar.
  • Hormonal/endocrine conditions (like Cushing’s or acromegaly) and certain medications such as steroids or estrogens.

These factors can make “no food” periods look surprisingly high on your glucose meter.

Does this mean fasting is bad?

Not necessarily—but it’s individual.

  • In many people, fasting or intermittent fasting lowers average glucose and improves insulin sensitivity over time.
  • In others (especially with uncontrolled type 2 diabetes, significant insulin resistance, or on glucose‑lowering medications), fasting can cause temporary spikes or unsafe swings.
  • Some coaches and clinicians note that longer or more aggressive fasts can initially worsen morning readings before improving as liver fat and insulin resistance improve.

Because of this, any change in fasting pattern should be discussed with a doctor, especially if you use insulin or diabetes medications.

Simple example

Imagine your body as a hybrid car:

  • Food = the plug‑in charger (easy energy).
  • Liver glycogen = the backup battery.
  • Counter‑regulatory hormones = the computer that turns on the backup battery.
  • Insulin = the brake that stops the battery from over‑discharging.

During fasting, the computer turns the battery on. If the brake (insulin) is weak or the system is glitchy (insulin resistance), the battery over‑delivers power and blood sugar rises.

When to talk to a doctor

You should get medical advice if:

  • Your fasting blood sugar is repeatedly high (for example, above targets set by your clinician).
  • You feel unwell during fasting (thirst, frequent urination, blurry vision, fatigue).
  • You have diabetes or prediabetes and are planning longer fasts or big changes in meal timing.

A professional can help adjust medications, check for other causes, and suggest a safer plan tailored to you.

Information gathered from public forums or data available on the internet and portrayed here.