[Column] What you need to know about vitamin D

Adequate blood levels of vitamin D are linked to a significantly reduced risk of death, according to a new study. Published in the September issue of the international journal Clinical Nutrition, the National Institute of Health’s study tracked 18,797 men and women aged 40 and older in rural Korea over 14 years. The findings are noteworthy. However, there's a confusing aspect to consider.

The study compared death risks among individuals with varying blood levels of vitamin D. Those with the lowest levels (under 30 nmol/L) served as the baseline group. Findings revealed that the overall risk of death decreased by 18 percent, 26 percent, and 31 percent as vitamin D levels increased to 30-50 nmol/L, 50-75 nmol/L, and above 75 nmol/L, respectively. Specifically, the risk of cancer-related death was 37 percent lower in the 50-75 nmol/L group and 45 percent lower in those with levels above 75 nmol/L. However, there was no significant difference in cardiovascular-related death risk across the groups.

As with any study, there are a lot of numbers. It's complicated and confusing. When you get your blood vitamin D levels tested at the doctor's office, the results are often given in ng/mL. However, the units used in this study are nmol/L. It needs to be converted by multiplying it by 0.4.

In this study, the lowest level of vitamin D in the blood, less than 30 nmol/L, is converted to 12 ng/mL. In other words, anything below 30 nmol/L is classified as vitamin D deficiency in the published study, but anything below 12 ng/mL is considered deficient when translated into a doctor's office test.

The study analyzed the overall risk of mortality for each 1 nmol/L increase in blood vitamin D levels and found a significant decrease in mortality risk from low levels up to about 50-60 nmol/L (equivalent to 20-24 ng/mL), with a modest decrease thereafter. Again, this study shows correlation, not causation. In 2024, a clinical trial published in the U.S. demonstrated that vitamin D supplementation reduced the risk of cancer by 7 percent compared to placebo, but increased the risk of heart disease by 6 percent. The optimal level of vitamin D for Koreans has not been determined yet.

Vitamin D is produced when your skin is exposed to sunlight. If you've spent enough time outdoors during the summer, you likely have sufficient levels. However, if your vitamin D levels are too low, it may be beneficial to boost your vitamin D intake through food or supplements during the fall and winter months. Good dietary sources of vitamin D include fish, egg yolks, beef liver, mushrooms, and vitamin D-fortified products like dairy and cereals.

For modern individuals, checking the units after a number is crucial. Just like the value of money can vary significantly depending on whether it's in dollars or won, understanding the units of measurement is essential. It may seem complicated, but it's important to know the difference.

Jeong Jae-hoon is a food writer and pharmacist. He covers a variety of subjects, including trends in food, wellness and medications. This column was originally published in Korean in Joongang Ilbo on Sept. 26, 2024. – Ed.