Vitamin C, also known as ascorbic acid, is a water-soluble nutrient essential for various bodily functions. It supports the immune system, aids in collagen formation for healthy skin and connective tissues, and acts as an antioxidant, protecting cells from damage. While excess is typically excreted in urine, consuming extremely high doses can sometimes lead to adverse effects.
Recommended Intake and Upper Limits
The Recommended Dietary Allowance (RDA) for adult men is 90 milligrams per day, while adult women require 75 milligrams daily. Pregnant women have a higher need at 85 milligrams per day, and those who are breastfeeding require 120 milligrams daily. These amounts are usually met through a balanced diet rich in fruits and vegetables.
The Tolerable Upper Intake Level (UL) for vitamin C in adults is 2,000 milligrams (2 grams) per day. This UL represents the maximum daily intake unlikely to cause adverse health effects for most healthy individuals. Reaching this upper limit through diet alone is highly improbable; concerns about excessive intake primarily arise from high-dose supplements.
Symptoms of Excessive Intake
When vitamin C is consumed in amounts significantly higher than the body can absorb, it can lead to symptoms, most commonly affecting the digestive system. Mild gastrointestinal disturbances such as diarrhea, nausea, stomach cramps, and abdominal discomfort are frequently reported. These symptoms occur because unabsorbed vitamin C creates an osmotic effect in the intestines, drawing excess water into the digestive tract, leading to loose stools or diarrhea.
Some individuals may also experience heartburn or flatulence with very high doses. While these gastrointestinal effects are generally not serious and typically resolve once intake is reduced, they can be uncomfortable. A more serious, though less common, concern is an increased risk of kidney stone formation in susceptible individuals. This risk arises because vitamin C is partly metabolized into oxalate, a substance that can form crystals. These crystals can combine with calcium to form calcium oxalate kidney stones, particularly when urinary oxalate levels become elevated.
Populations at Increased Risk
Certain individuals may be more vulnerable to the adverse effects of high vitamin C intake due to underlying health conditions. People with hemochromatosis, a genetic disorder causing excessive iron accumulation, should approach high vitamin C doses with caution. Vitamin C enhances the absorption of non-heme iron, which is the type of iron found in plant-based foods. In individuals with hemochromatosis, this enhanced absorption can exacerbate iron overload, potentially leading to tissue damage in organs like the liver or heart.
Individuals with existing kidney disorders or chronic kidney disease (CKD) also face heightened risk. Their kidneys may be less efficient at filtering waste products, including oxalate, from the blood. This reduced clearance can lead to an accumulation of oxalate, increasing the likelihood of kidney stone formation or worsening existing kidney conditions. Careful monitoring of vitamin C intake is generally advised for those with compromised kidney function.
Another group at risk includes individuals with glucose-6-phosphate dehydrogenase (G6PD) deficiency, a genetic condition that affects red blood cells. In these individuals, high doses of vitamin C can act as a pro-oxidant, leading to the generation of reactive oxygen species. Since G6PD-deficient red blood cells have a reduced ability to protect themselves from oxidative stress, this can trigger hemolysis (premature breakdown of red blood cells).