Ascorbic acid, more commonly known as Vitamin C, is a water-soluble nutrient found in many fruits and vegetables. Because the human body cannot produce it, it must be obtained through diet or supplementation. The presence of ascorbic acid in urine is a normal finding, reflecting the body’s process of managing this vitamin. Its appearance in urine is directly tied to consumption and how the body handles substances it cannot store in large quantities.
The Excretion of Ascorbic Acid
As a water-soluble vitamin, ascorbic acid is absorbed from food and transported through the bloodstream to be used by various tissues. Unlike fat-soluble vitamins, which can be stored in fatty tissues and the liver, water-soluble vitamins have no major storage system. The body takes what it needs for immediate use, and any surplus is removed.
This removal process is handled by the kidneys, which filter the blood. Blood passes through filtering units called glomeruli, where waste products and excess substances, including ascorbic acid, are separated. The kidneys are designed to reabsorb what the body needs, and for ascorbic acid, this happens in the renal tubules, but this system has a limit.
When the blood concentration of ascorbic acid is low, the kidneys reabsorb almost all of it. However, when the blood concentration surpasses a threshold of around 1.4 mg/dL, the reabsorption mechanism becomes saturated. Any amount of ascorbic acid that exceeds this capacity is not reabsorbed and is passed into the urine.
Factors That Influence Urinary Levels
The most significant factor determining the amount of ascorbic acid in urine is dietary intake. Consuming foods rich in Vitamin C, such as citrus fruits, strawberries, bell peppers, broccoli, and kiwi, will increase its concentration in the blood. This elevation, in turn, leads to greater excretion by the kidneys once the body’s needs are met and the renal threshold is exceeded.
Supplementation with Vitamin C has a more pronounced effect on urinary levels. Taking high-dose supplements can quickly saturate the body’s tissues and the blood, causing a sharp increase in the amount filtered out by the kidneys. Doses of 500 mg or more result in detectable levels for several hours post-ingestion, while doses around 250 mg may not lead to significant urinary excretion.
Hydration status can also influence the concentration of ascorbic acid in a urine sample. When a person is dehydrated, their urine is more concentrated, making the level of ascorbic acid appear higher. Interpreting the results of a urine test requires consideration of how much fluid a person has consumed.
Interference with Urinalysis Results
The presence of high levels of ascorbic acid in urine can interfere with the results of a standard urinalysis test strip, also known as a dipstick test. This interference is due to the chemical properties of ascorbic acid, which is a strong reducing agent. A reducing agent is a substance that donates electrons to another substance in a chemical reaction. Many dipstick tests rely on oxidation reactions to produce a color change that indicates the presence of a specific substance.
Ascorbic acid can prevent these necessary oxidation reactions from occurring, leading to a falsely low or false-negative result for several markers. The primary tests affected are:
- Glucose: High levels of ascorbic acid can inhibit the enzymatic reaction used to detect glucose, potentially masking signs of glucosuria, a common indicator of uncontrolled diabetes.
- Blood (heme): It can interfere with the test strip’s reaction to hemoglobin, producing a false negative and masking signs of kidney disease, urinary tract stones, or bladder cancer.
- Bilirubin: Interference with this test can hide evidence of liver disease or bile duct obstruction.
- Nitrite: A false negative for nitrite can conceal a bacterial urinary tract infection, as many common bacteria convert nitrate to nitrite.