Before taking it just because "it's winter." The reason why too much vitamin D supplement can actually be dangerous.

The Atmosphere Where "Vitamin D in Winter" Has Become Common

As it gets colder and the days get shorter, posts like "Started taking Vitamin D" and "It's essential in winter" increase on social media timelines. Indeed, Vitamin D is said to be involved in bone health, muscles, and immunity. In seasons with little sunlight, it's easy to worry, "Am I getting enough?" 

However, the issue is not only about whether it's necessary but also about "how much to take." According to German reports (dpa distribution), **many over-the-counter Vitamin D supplements are "too high in dosage,"** prompting caution. The trigger was a **test by Stiftung Warentest (German consumer testing magazine) in their January 2026 issue**. Offenbach Newspaper

First premise: Vitamin D is "hard to fulfill through diet alone," but "not everyone is deficient"

Vitamin D can be obtained from fatty fish like salmon and mackerel, dairy products, and vegetable oils, but diet only covers "a part of the whole"—the article introduces expert opinions suggesting about "10%." The rest is synthesized in the skin when exposed to sunlight (ultraviolet rays). 

On the other hand, it's not a simple matter of "everyone is deficient in winter." The article cites data from the RKI (Robert Koch Institute), showing that a certain number of adults in Germany are "sufficiently supplied," and suggests that additional supplements offer little benefit for this group. It also mentions that the effectiveness of supplements in preventing cancer and cardiovascular diseases is not sufficiently substantiated. Offenbach Newspaper

"Who Should Take It" Actually Varies Significantly

The article clearly identifies those at "high risk of deficiency." For example, people who have lifestyles with little sun exposure (bedridden, rarely go out, or have extremely little skin exposure). Additionally, **elderly people, those with obesity, people with darker skin, and certain conditions (kidney, digestive, etc.)** may have an increased need. This is particularly noticeable from October to March.

What is important here is that "whether you have enough" can only be determined by a blood test. The article introduces the process of consulting a doctor for measurement, the possibility of insurance coverage depending on the situation, and cost estimates if self-funded (with a note that measurements at the end of winter can be difficult to interpret due to reduced storage). Offenbach Newspaper

The Main Controversy: "Quantity"—Many Products Were "Overloaded"

This is the central topic of discussion. According to the article, many of the 25 products checked by Stiftung Warentest were high-dose, and some were deemed "inappropriate" as they were at levels suggested to pose a risk of kidney damage with long-term intake. 

So, what is the appropriate amount? The article advises, based on the BfR (Federal Institute for Risk Assessment) in Germany, to limit supplement-derived intake to 20 µg (800 IU) per day. This guideline is repeatedly shown on consumer information pages. 

Furthermore, in the test, only two products were deemed "suitable," with specific product names and daily amounts (200 IU/400 IU) mentioned. It is important to emphasize that this is merely the "evaluation of Warentest at that time," and if readers choose based on the same criteria, "checking the dosage first" becomes the top priority. Offenbach Newspaper

Is the Weekly "Depot Type" Dangerous?—"Taking the Wrong Dose" Leads to Overconsumption

A common sentiment on social media is, "It's a hassle to take it every day, so I switched to one that's okay once a week." However, the article points out that depot types like weekly doses are prone to "accidental" mistakes. If taken too frequently by mistake, it can quickly lead to overconsumption. Offenbach Newspaper

On this point, the BfR also warns that "high-dose bolus intake" at intervals like daily or weekly can pose health risks. The concept of safety (UL = Tolerable Upper Intake Level) is based on "daily" continuous intake, and high-dose bolus intake is a separate issue. Bundesinstitut für Risikobewertung

Reactions on Social Media (Reconstructed Based on Common "Posting Trends" / Not Quoting Specific Posts)

From here, let's align the "internet arguments" that this article is likely to resonate with, as typical reactions on social media.

1) "Just Take It" Group

• "I take ◯◯IU every day in winter. I feel like I don't catch colds."

• "I haven't had a test, but everyone is taking it, so I am too."
  → While many continue based on "personal experience," the article emphasizes that "some people are already sufficient." Offenbach Newspaper

2) "Thought Quantity Was Key" Group (Surprised)

• "Wait, aren't supplements more effective the more you take?"

• "I thought 5000IU was normal..."
  → The article presents the reality of "many high-dose products" and the upper limit guideline (800IU). Offenbach Newspaper

3) "Get Tested First" Group

• "Isn't it better to take a blood test and then take it if deficient?"

• "Consulting a doctor is cost-effective in the end."
  → The article also organizes that it can only be determined by a "blood test." Offenbach Newspaper

4) "Food is Enough" Group vs. "That's Impossible" Group

• "Eating fish is enough."

• "They say you can't get enough from food alone."
  → The point that "diet-derived is only a part" often becomes the landing point of the debate. Offenbach Newspaper

5) Counter to "Depot Type is Convenient"

• "The once-a-week one is actually easier to remember."

• "No, taking the wrong dose is scary."
  → The article warns of the path from "taking the wrong dose→overconsumption." Offenbach Newspaper
  
  

The Conclusion to Take Away from This Article is Simple

• Don't start just because "it feels like winter." First, check if you are in a risk group for deficiency. Offenbach Newspaper

• If you take it, prioritize the dosage. A guideline of "up to 800 IU (20 µg) from supplements" is indicated. Offenbach Newspaper

• Weekly high doses are "convenient" but also "mistakes are costly." Designing how to take them is crucial. Offenbach Newspaper

• If in doubt, blood test→consultation. The necessary amount for those who need it. Offenbach Newspaper

*This article is for informational purposes and is not a substitute for treatment or medication. If you have a chronic illness, are pregnant, or are taking medication, consult a healthcare professional before starting or increasing dosage.*