Is That Vitamin D Really Necessary? A Warning Against Blind Faith in Supplements from 25 Product Tests in Germany

"Vitamin D is prone to deficiency, so let's just take it for now."

If you're interested in health and beauty, you might have thought this at least once.

Vitamin D supplements are easily accessible at drugstores and online, ranging from a few hundred IUs per day to high-dose products offering several thousand IUs in one pill.

However, a consumer product test conducted in Germany is questioning the sense of security that comes from thinking "as long as you supplement with vitamins, you're safe."

The German consumer product testing organization Stiftung Warentest examined 25 types of supplements with vitamin D as the main ingredient. The results showed that only two products could be recommended unconditionally. Furthermore, five products were clearly advised against, as the health risks of long-term use might outweigh the benefits.

At first glance, this might seem like news that "23 out of 25 products are dangerous."

However, that's not where the real focus should be.

The test highlights the issue that the simple notion of "Vitamin D is necessary for the body, so taking more will make you healthier" does not hold true.

And this is not an issue that can be ignored in Japan, where the supplement market is expanding.

The Meaning of "2 out of 25 Products"

According to information released by Stiftung Warentest, the 25 products tested included various types such as tablets, capsules, liquids, and drinks, with daily intake ranging from 200 IU to 4,000 IU.

The evaluation also included products combining vitamin D with other ingredients like vitamin K2, magnesium, selenium, and zinc.

The evaluation considered not only whether the product contained the stated amount of vitamin D but also the latest medical evidence, standards from public institutions and professional societies, ingredient amounts, labeling, and long-term safety. The actual content of vitamin D, K2, zinc, selenium, and magnesium was also tested.

According to the original article, the two products deemed "suitable" were GSE's "Vitamin D compact" and Rotkäppchen's "Vital Vitamin D." The former had a relatively low dosage of 200 IU per day, and the latter 400 IU.

There is a very interesting point here.

In the world of supplements, larger numbers like "1,000 IU," "2,000 IU," or "5,000 IU" tend to appear more potent and effective.

However, nutrients are not about competing for potency like medicine.

While they serve their purpose by supplementing what is deficient, taking more than necessary does not proportionally increase the effect.

In fact, with vitamin D, excessive intake requires caution.

Vitamin D is Not a "Flush it Away" Vitamin

Vitamin D is an important nutrient deeply involved in calcium absorption and bone formation.

Unlike water-soluble vitamins like vitamin C and some B vitamins, vitamin D is fat-soluble.

Therefore, you cannot simply think, "Excess will just be excreted in urine, so it's fine."

The Ministry of Health, Labour and Welfare's eJIM indicates that excessive intake of vitamin D can increase calcium levels in the blood, potentially causing nausea, muscle weakness, loss of appetite, dehydration, frequent urination, intense thirst, and kidney stones. In extreme cases, it can lead to kidney failure or arrhythmia.

Information from Japan's National Institute of Health and Nutrition also explains that excessive intake can raise calcium and phosphorus levels in the blood, leading to calcium deposits in the kidneys and muscles and calcification of soft tissues.

The issue is not

"Is vitamin D good or bad?"

but rather

"Is the amount necessary for that person?"

that is the point.

Is Germany's "800 IU" Contradictory to Japan's "4,000 IU"?

There are numbers to be cautious of when reading this article.

The German Federal Institute for Risk Assessment (BfR) recommends choosing products that provide about 20 micrograms, or approximately 800 IU, of vitamin D per day when supplementing. This is considered an amount unlikely to cause health concerns even with long-term intake and additional sources from food.

On the other hand, what about Japanese adults?

The current Japanese dietary intake standards suggest a "reference intake" of 9 micrograms per day for adults aged 18 and over, regardless of gender. Meanwhile, the "tolerable upper intake level" is set at 100 micrograms per day, equivalent to 4,000 IU.

Looking at this alone,

you might think, "If 4,000 IU is safe in Japan, isn't 800 IU too little?"

However, these two numbers have different meanings.

The "reference intake" of 9 micrograms is a suggested intake to maintain a certain nutritional status.

Meanwhile, the "tolerable upper intake level" of 100 micrograms is not a number that should be actively pursued.

It is set as an upper limit considered unlikely to cause health problems from habitual excessive intake for most people.

It's similar to how "a speed limit of 100 km/h" is not the same as "the speed at which a car runs most efficiently and safely."

The upper limit is not a recommended amount.

This distinction is crucial when choosing supplements.

Japanese People Also Need to Be Cautious of Vitamin D Deficiency

On the other hand, fearing only excessive intake and thinking "all vitamin D supplements are unnecessary" is also extreme.

Vitamin D deficiency cannot be ignored in Japan either.

Vitamin D can be obtained from fish and mushrooms and is also produced in the skin when exposed to ultraviolet rays.

Therefore, the body's status can easily change depending on diet, season, latitude, time spent outdoors, clothing, sun protection measures, and age.

The National Institute of Health and Nutrition also points out that vitamin D deficiency can occur not only due to insufficient dietary intake but also when there is insufficient exposure to sunlight. Long-term deficiency is associated with osteomalacia in adults and decreased bone density and fracture risk in the elderly.

Especially in modern Japan, indoor living has progressed.

People work in offices from morning to evening, commute by subway, and spend weekends at home or in commercial facilities.

Additionally, many people use sunscreen, hats, and long sleeves to avoid the effects of ultraviolet rays on the skin.

Considering such living environments, interest in vitamin D is not unnatural.

The problem is

jumping from the fact that

"some people are deficient"

to the conclusion that

"everyone should take high-dose supplements."

On Social Media, "High-Dose Faction" and "Cautious Faction" Clash Directly

The evaluation by Stiftung Warentest became a topic on social media and online communities.

 

Germany's public broadcaster WDR's news account introduced that 25 types of vitamin D supplements were tested, and many were evaluated as "unsuitable or not sufficiently suitable."

However, the reception by net users is not uniform.

On Reddit, there are posts harshly criticizing Stiftung Warentest's vitamin D evaluation, saying "I can't agree with this test," while others suggest, "You should look at individual evaluation items, not just the overall score," and "It's better to check the criteria important to you."

In another thread about vitamin D itself, the confrontation is even clearer.

"If you're not deficient according to a blood test, it's better not to take it."

"Since it's fat-soluble, you should be cautious of excessive intake."

"It's better to consult a doctor."

While there are cautious voices, there are also testimonials from people who take high doses regularly and opinions suggesting aiming for higher blood levels.

It's important to note that social media posts are not medical evidence.

Even if a testimonial like "I took it and felt better" is true, it's unclear whether the change is genuinely due to vitamin D, a change in other lifestyle habits, or a placebo effect.

Conversely,

a testimonial like "I took a lot and had no problems"

is not proof of safety.

The Consumer Affairs Agency advises not to trust only advertising catchphrases or user testimonials about health foods but to confirm the safety and efficacy of the ingredients.

How True is the Theory that "Taking K2 Together Makes it Safe"?

If you're interested in vitamin D, you might have come across the combination "Vitamin D3 + K2."

On social media,

the opinion that

"You should take K2 along with vitamin D"

is not uncommon.

A randomized clinical trial published in JAMA Cardiology in 2026 reported that administering 360 micrograms of MK-7, a type of vitamin K2, daily to 180 people with coronary artery disease for two years suppressed the progression of coronary artery calcification compared to a placebo.

This is an intriguing result.

However, it cannot be concluded that "taking K2 can prevent heart attacks."

The researchers themselves stated that larger prospective trials are needed to confirm whether there are benefits for actual clinical events like heart attacks or death.

Furthermore, this study did not prove that "taking high doses of vitamin D with K2 makes excessive intake of vitamin D harmless."

On social media, explanations of mechanisms can quickly lead to

the conclusion that "therefore, taking D3 + K2 in large amounts is okay."

In science, "theoretically possible" and "proven safe and effective in humans" are different things.

In Japan, It's Important Not to View Supplements as "Medicine"

When Japanese people read this German news, the positioning of supplements is particularly important.

In Japan, there is no clear legal definition of what is called "health food," and it broadly includes foods used with the expectation of contributing to health maintenance and promotion.

The Consumer Affairs Agency also explains that health functional foods are not the same as pharmaceuticals and are not taken to treat or prevent diseases.

This is an area where consumers can easily misunderstand.

Tablets in a white bottle.

Labels detailing ingredient names and contents.

Instructions to take "one pill a day."

Visually, they resemble pharmaceuticals.

Therefore,

some might unconsciously feel that "they are sold after rigorous examination like medicine."

However, food and medicine are not under the same system.

Especially in online shopping, product pages feature attractive words like "health," "immunity," "bones," and "aging," and influencers' or users' testimonials often become the deciding factor for purchase.

If you combine multiple supplements to "enhance the effect," you might unknowingly consume the same nutrient redundantly.

The Consumer Affairs Agency also warns that tablet and capsule products are prone to excessive intake and advises caution when using multiple products simultaneously.

"A Blood Test Will Solve Everything" is Not the Case

From this topic,

the question arises,

"Should everyone get a blood test for vitamin D?"

When evaluating the body's vitamin D status, the blood level of 25-hydroxyvitamin D is generally used as an indicator.

However, this does not mean that all healthy individuals need to be tested regularly and aim for high numbers.

What's important is to consider diet, outdoor activities, age, medical history, bone condition, medications being taken, and any deficiencies noted by a doctor comprehensively.

Especially, it's not the same for someone with confirmed deficiency and a healthy person deciding on their own to take high doses to "increase the numbers."

There are cases where doctors prescribe amounts exceeding the usual upper limit for a certain period as a treatment for deficiency.

Seeing this,

thinking "since hospitals use large amounts of vitamin D, I can take the same amount"

is dangerous.

Therapeutic administration and long-term self-judged intake need to be considered separately.

What Japanese Should Check When Choosing Supplements Is More Than "IU Size"

So, what should you look for when purchasing vitamin D supplements in Japan?

The first thing to check is not "how many IUs are in the product."##HTML_TAG_276