Last week, while shopping at Costco, I found myself standing in front of the supplements section for quite a while. A large container of vitamin D was prominently displayed, and the people next to me were adding one to their carts.

As the weather cools down, the same question always arises: do we need to take vitamin D supplements since there is less sunlight in winter?

I was curious too, so I looked up official information and organized my findings. To cut to the chase, the evidence that everyone needs to take it is weaker than expected.

First, let's talk about sunlight. Vitamin D is produced when the skin is exposed to UVB rays, which are significantly reduced in winter due to the sun being lower in the sky.

A study published in 1988 found that in Boston, located at 42 degrees north latitude, very little vitamin D precursor was produced in the skin from November to February, even on clear days. In contrast, synthesis was confirmed even in mid-winter at locations around 34 degrees north latitude.

North Texas, where we live, is around 33 degrees north latitude. So, we can say our situation is better than that of northern states.

However, it is clear that the amount synthesized in winter is less than in summer. On cloudy days or for those with darker skin, more time in the sun is needed to produce the same amount of vitamin D.

In fact, a bigger issue than sunlight is our lifestyle. Moving from home to the garage and from the car to the office means we spend very little time in the sun each day.

So, how much do we actually need? The National Institutes of Health Office of Dietary Supplements (NIH ODS) recommends a daily intake of 600 IU for those aged 1 to 70, and 800 IU for those over 71.

When converted to micrograms, 600 IU is 15 mcg, and 800 IU is 20 mcg. Since both units are often mixed on supplement labels, it's helpful to know this conversion.

The upper intake level for adults is 4,000 IU per day. It's important to remember that this number is not a target but a limit not to be exceeded.

Among the products sold in stores, it's common to find those with 5,000 IU per capsule. I was honestly a bit surprised when I first saw that.

When looking at labels, you should also check whether the serving size is based on one capsule or two. If you casually take two capsules, you might end up consuming a higher amount than you realize.

Vitamin D is fat-soluble, meaning it accumulates in the body. The ODS warns that taking too much for too long can lead to elevated calcium levels in the blood.

So, should everyone get a blood test? Here's where it gets interesting.

The U.S. Preventive Services Task Force (USPSTF) concluded in 2021 that there is insufficient evidence to recommend screening for vitamin D deficiency in asymptomatic adults. A major reason is that there is no consensus on what level constitutes a deficiency.

The Endocrine Society's guidelines released in 2024 go a step further, stating that routine vitamin D level testing is not recommended for healthy individuals.

However, the same guidelines suggested empirical supplementation for adults over 75, citing a potential reduction in mortality risk, although they acknowledged that the evidence level is low.

Conversely, they do not recommend exceeding the recommended intake for healthy adults under 75, indicating that there is insufficient evidence to support high-dose supplementation for disease prevention.

Pregnant women, adults with prediabetes, and some children and adolescents were also mentioned as potential candidates for supplementation. In such cases, it may be worth discussing with a healthcare provider.

After organizing this information, I realized it was quite different from the impression I had from advertisements. There was a time when vitamin D was presented as a cure-all, but large-scale research results are not as glamorous.

There are also ways to obtain vitamin D through food. Fatty fish like salmon, egg yolks, and vitamin D-fortified milk and cereals are typical sources.

The amount of vitamin D in fortified foods varies by product. Developing the habit of checking the vitamin D section on nutrition labels can be more helpful than expected.

Mackerel and saury sold in Korean markets are also good options. Sometimes, a piece of grilled fish feels more reassuring than a supplement.

If you still choose to take supplements, I recommend checking a few things. First, see if your multivitamin already contains vitamin D, and also check for overlaps with calcium supplements.

If you are taking other medications, you need to be more cautious. Some medications can affect the absorption of vitamin D or calcium, so be sure to consult with your doctor or pharmacist before starting.

This article summarizes official information and is not a diagnosis or prescription. It's best to confirm your health status through medical consultation.

As for me, I decided not to buy a large container of high-dose vitamin D without thinking. I plan to focus on food and sunlight within the recommended range and ask my primary care physician about it during my next check-up.

I also intend to get back into the habit of walking around the neighborhood for about twenty minutes after lunch. I should take advantage of the many sunny days in Texas during winter.

Supplements are merely tools to fill in the gaps. Instead of following what others are doing, I believe the first step is to check how much I actually need.