Why do carbohydrates acidify urine instead of protein?

The Myth of Protein and "Acidic Urine"

Proteins do indeed produce acidic residues (mainly sulfuric and phosphoric acids) during metabolism, but this is a natural load to which the kidneys are adapted. With sufficient alkaline minerals (potassium, magnesium, sodium) and in the absence of insulin resistance, urine remains at a normal pH.

Moreover, a protein-rich diet usually increases fat intake, which does not activate aldosterone and does not cause sodium loss.

However, an excess of carbohydrates systematically acidifies urine:

Carbohydrates, insulin, and sodium. High carbohydrate intake stimulates insulin release. Insulin affects the kidneys: it reduces sodium excretion, promoting its reabsorption in the nephron tubules.

However, with prolonged and chronically high levels of insulin (for example, in insulin resistance), the kidneys' sensitivity to sodium is impaired. In this case, despite increased insulin, sodium is retained poorly, and the body begins to lose it in urine.

Sodium loss and activation of the renin-angiotensin-aldosterone system (RAAS). When the body loses sodium, it is perceived as a threat of dehydration and hypovolemia. In response, the RAAS is activated, and the adrenal glands release aldosterone—a hormone that stimulates the kidneys to enhance sodium reabsorption and excrete potassium and hydrogen ions (H⁺).

Aldosterone and urine acidification. Aldosterone activates proton pumps in the distal tubules of the kidneys, increasing the secretion of H⁺ ions into the urine. This makes the urine more acidic. Thus, increased aldosterone → increased H⁺ excretion → urine acidification. At the same time, there is a loss of potassium and a shift in the acid-base balance towards acidosis.

Additional contribution of glucose. At high levels of glucose in the blood and/or urine (glucosuria, characteristic of prediabetes or diabetes), glucose pulls along water and electrolytes (osmotic diuresis), which also leads to additional sodium loss, enhancing aldosterone activity.

Additionally, a high-carbohydrate diet reduces the level of citrates in the urine—these are the main natural buffers that protect against acidification. A low level of citrate is associated with the risk of stone formation and metabolic acidosis.

Conclusion

It is not protein per se that acidifies urine, but an excess of carbohydrates through hormonal and electrolyte mechanisms.

Protein foods can only slightly shift pH if consumed in the context of a mineral- and water-deficient diet. In contrast, carbohydrates—especially refined ones—systematically and deeply acidify urine, causing a cascade of reactions through insulin, aldosterone, sodium loss, and electrolyte disturbances.