Increased urination, or polyuria, is a common and expected physiological response when a person begins fasting. This change in fluid output often starts within the first day. This frequent need to urinate is not a sign of a problem, but rather the result of the body making necessary metabolic adjustments. The body switches from burning stored carbohydrates to utilizing fat for fuel, which causes two distinct mechanisms that shed stored water and minerals.
Initial Cause: Glycogen Depletion
The first and most immediate cause for increased urination relates to the body’s primary energy reserve: stored carbohydrates called glycogen. Glycogen is stored mainly in the liver and muscles, and the body taps into these stores when fasting begins. A significant amount of water is bound to glycogen; for every gram stored, the body holds three to four grams of water. As the body breaks down these reserves for fuel in the first 12 to 48 hours, this associated water is released into the bloodstream. The kidneys filter and excrete this sudden influx of water, leading to a noticeable increase in urine volume.
This initial water loss accounts for the rapid early weight reduction observed during fasting. The fluid loss is compounded by a drop in insulin levels. Lower insulin causes the kidneys to excrete more sodium, and since water follows sodium, this contributes to the initial diuretic effect.
Sustained Effect: How Ketones Increase Urination
Once glycogen reserves are depleted, the body shifts into fat burning, producing ketone bodies. This metabolic state, called ketosis, provides a sustainable fuel source during a prolonged fast. The production and excretion of these ketones maintain increased urination beyond the initial phase. Ketone bodies are organic acids the body must eliminate to keep blood acidity balanced.
These acidic compounds are filtered by the kidneys and excreted in the urine (ketonuria). Ketones are osmotically active, meaning they draw water out of the body and into the urine collection system. Furthermore, these ketone acids are often paired with minerals, particularly sodium and potassium, to maintain electrical neutrality before being flushed out. This obligatory water and electrolyte loss acts as a continuous diuretic, resulting in a sustained increase in urine output while the body remains in ketosis.
Maintaining Hydration and Electrolyte Balance
Understanding the mechanisms behind increased urination highlights the importance of proactive fluid and mineral management during fasting. The body actively sheds electrolytes necessary for nerve signaling, muscle function, and fluid balance. Failing to replace lost minerals—primarily sodium, potassium, and magnesium—can lead to symptoms such as fatigue, headaches, and muscle cramps. To counteract this, increase water intake and focus on mineral replacement. Consuming electrolyte-infused water or mineral-rich liquids like bone broth helps replenish the essential salts being lost, as drinking plain water alone can dilute remaining electrolytes and worsen symptoms.