High doses of vitamin C can pose a threat to the kidneys.

      In autumn, pharmacy shelves are filled with packages of effervescent vitamin C in dosages that exceed the daily norm by several times. A 1000 mg tablet is now not uncommon, but rather a standard offering. But are these "shock doses" as beneficial as marketing convinces us they are?

      The physiological need for an adult is significantly more modest. Men need about 90 mg per day, while women need around 70 mg. This norm can easily be met with a regular diet: just eat a bell pepper, a serving of broccoli, or a handful of black currants. The upper acceptable level, according to international recommendations, is 2000 mg per day—and exceeding it without a doctor's supervision is not advisable.

      The main feature of vitamin C is that it is water-soluble. The body does not make strategic reserves of it. Absorption occurs through intestinal transport systems, which quickly become saturated. At a dose of 200 mg, nearly the entire volume is absorbed, while at around a gram, less than half is absorbed. The remainder is excreted in urine, and the concentration of vitamin in plasma reaches a ceiling and does not increase further. Simply put, additional intake does not create "super protection," but only burdens the excretory system.

      The main danger to the kidneys is that excess ascorbic acid is metabolized into oxalates. These compounds are excreted by the kidneys, and with prolonged abuse of high doses, the risk of forming oxalate stones increases. Men, patients with already diagnosed urolithiasis, and those with reduced kidney function should be particularly cautious. For them, mega doses are a direct contraindication without medical supervision.

      In addition to the kidneys, the gastrointestinal tract can also suffer. Doses above two grams often provoke nausea, bloating, cramps, and diarrhea. Ascorbic acid irritates the mucosa, especially on an empty stomach, which is dangerous for people with gastritis or reflux. One should not forget about tooth enamel: soluble and chewable forms with high acidity gradually destroy it with daily intake.

      The influence on laboratory diagnostics deserves special attention. Being a strong reducing agent, ascorbic acid at high concentrations interferes with the chemical reactions underlying many tests. The result is a false-negative stool test for hidden blood, distorted urine indicators for glucose, bilirubin, and nitrites, as well as inaccuracies in the operation of home glucometers. Therefore, doctors recommend stopping the intake of high-dose forms at least a day before tests.

      Experts agree: vitamin C is necessary daily, but in amounts that the body can utilize. Everything extra is simply excreted. In autumn, it is wiser not to increase doses randomly but to pay attention to those deficiencies that are truly common in our latitudes and are asymptomatic—vitamin D, iron, vitamin B12.

      The key principle remains unchanged: before buying supplements, it is worth obtaining objective data and discussing them with a doctor. Mega doses of vitamin C purchased "just in case" are not health insurance but rather a reason to think.