Vitamins A, D and E: What you need to know before taking them long-term

"After taking 15,000 IU of vitamin D every day for a year, my body fell apart."

On July 11, GOD member Park Jun-hyung said on YouTube that he had been diagnosed with arrhythmia, adding that he had "taken vitamin D3 for his health but ended up suffering side effects instead."

It is not just vitamin D — vitamins A, E and K can also cause serious harm when taken in high doses over a long period. All of these are fat-soluble vitamins. Unlike water-soluble vitamins, they accumulate in the body when consumed in excess, and that buildup is the problem.

Wipada Saerao, senior director of nutrition education and training for Asia-Pacific at global wellness company Herbalife, said "fat-soluble vitamins (A, D, E and K) are stored in the liver and fatty tissue after absorption, so taking high doses over a long period can cause them to build up in the body and become toxic."

She added that "water-soluble vitamins such as vitamin C and the B vitamins are mostly excreted in urine," and stressed that "for fat-soluble vitamins, it is important to stick to the recommended intake unless a medical professional advises otherwise."

Vitamin D is a nutrient involved in bone health, muscle function, the nervous system and immune regulation. Its key role is helping the intestines absorb calcium more effectively. Because it boosts calcium absorption, prolonged high-dose intake can cause blood calcium levels to rise abnormally — a condition known as hypercalcemia.

Hypercalcemia can trigger arrhythmia — an irregular heartbeat — as well as nausea, vomiting, excessive thirst, lethargy and fatigue. Anyone taking vitamin D who notices palpitations or a racing heart should consider whether these side effects may be the cause.

The Ministry of Food and Drug Safety recommends a daily vitamin D intake of 10 micrograms (400 IU) for adults. The upper tolerable intake level (the maximum amount) is 100 micrograms (4,000 IU).

Even water-soluble vitamins require caution when taken in excess. Wipada said "vitamin C is a water-soluble vitamin, but consuming too much of it daily can cause gastrointestinal symptoms such as diarrhea, abdominal cramps and nausea," adding that "people prone to kidney stones may face a heightened risk." She went on to say that "for healthy adults, taking around 100 to 500 milligrams per day is sufficient to maintain adequate vitamin C levels."

Vitamin C is one of the most commonly taken supplements in South Korea. According to the Korea Disease Control and Prevention Agency's dietary supplement usage statistics from 2020, vitamin C was the most consumed single-vitamin product in the country. Under Ministry of Food and Drug Safety guidelines, the upper tolerable daily intake of vitamin C for adults is 2,000 milligrams.

Some people report difficulty sleeping when they take water-soluble vitamin B late in the afternoon. Wipada said there are reports that B vitamins can affect sleep depending on when they are taken, but "the research available so far is not sufficient to conclude definitively that B vitamins disrupt sleep." She went on to say that "if you find it hard to fall asleep or feel discomfort during the night after taking B vitamins, you might consider taking them in the morning or during the day instead."