Writing in the journal Naunyn-Schmiedeberg’s Archives of Pharmacology, researchers from the Hannover Medical School in Germany compared the climate in which vitamin C supplements are sold in that country and the United States. The goal was to determine what consumers can expect when taking the available products, especially regarding the transparency of vitamin dosage and safety information.
“More than three quarters of products exceed the recommended daily dose for vitamin C-containing dietary supplements and nearly 40% of all preparations transgress a potential upper safety level,” the researchers wrote. “The daily doses taken in the form of vitamin C supplements are often high, and consumers are not informed properly.”
Vitamin popularity
Supplement use in Germany is popular as one in three people in the country take a nutraceutical. In the United States, the number is higher. According to the National Center for Health Statistics, more than 57% of U.S. residents take supplements regularly. In 2023, Grand View Research valued the global dietary supplement market at $177.5 billion.
Vitamin C is one of the most consumed dietary supplements, considered the third most popular in Germany. For American consumers, it is the sixth most popular supplement among all supplements taken. The demand for the vitamin rises during winter and cold season because of the common, yet unsubstantiated notion that it can quickly stymie colds and flu-like conditions.
Also known as ascorbic acid, vitamin C is water soluble and consequently has limited storage ability in the human body. People cannot synthesize the vitamin by themselves and need to meet their daily vitamin C requirement via diet. The vitamin is critical for many vital bodily functions, serving as an antioxidant.
“Apart from this, vitamin C is a co-factor for the synthesis of collagen, carnitine and certain hormones like adrenaline and noradrenaline,” the researchers explained. “By affecting gene transcription, vitamin C also plays a role in epigenetic processes.”
Vitamin C is additionally needed for iron absorption, reducing ferric iron in the gastrointestinal tract. It plays an important role by directly influencing the amount of iron absorbed through diet.
According to the study, the U.S. Food and Drug Administration and Germany’s Bundesinstitut für Risikobewertung recommend 90 to 110 mg of vitamin C daily for optimal biological function. As indicated by the European Food Safety Authority and the U.S. National Institutes of Health, the upper intake level for vitamin C ranges from 1,000 to 2,000 mg, though bioavailability occurs at doses of 100 to 400 mg.
Study details
The researchers analyzed 106 supplements sold by online stores in Germany (n = 66) and the United States (n = 40). A limitation of the study was that researchers did not examine a greater number of vitamin C supplements.
Monopreparations were typically more popular among consumers than combination preparations, though the researchers found that more than one-third of all supplements contained active ingredients other than vitamin C.
“Combination preparations can pose a health risk in two ways,” the researchers wrote. “Firstly, some additives may be added at a dosage that exceeds the safe upper tolerable level of the respective ingredient. Fat-soluble vitamins such as vitamin A, D and E possess a higher risk of overdosing, as they can accumulate inside the human body. Secondly, since the additives are not the focus of the supplement, some consumers might not be aware of the fact that they are taking them at all. Even if each additive is included at a safe dose individually, if multiple dietary supplements are consumed at once, this can add up to a dosage that exceeds safe levels.”
They indicated that mean daily doses of the products studied will likely not lead to adverse outcomes among healthy individuals but could be unsafe for people who have a history of kidney stones.
Only three out of the 106 products had a warning about a potential laxative effect. When it came to product labeling, 5 to 20% of products were missing warnings that were either recommended or legally required.
Also, listed dosages were sometimes different than what the product contained.
“Of all products included in this analysis more than three quarters transgress the recommended daily dose for dietary supplements… It is worth noting that no U.S.-American institution made a recommendation for supplements specifically, but instead only listed the upper limit of vitamin C as 2,000 mg,” the researchers noted. “This can explain why U.S.-American products generally possess higher doses and more frequently pass a daily dose of 1,000 mg.”
Source: Naunyn-Schmiedeberg’s Archives of Pharmacology. doi: 10.1007/s00210-025-04248-y. “Market analysis of vitamin C-containing dietary supplements in Germany and the USA: Consumer information and risks and benefits”. Authors: J. Decke, R. Seifert
