[Column] Myths and truths about nutritional supplements’ interactions

Drug interactions are important.

They can lead to big problems and even death. The same can happen with nutraceuticals and dietary supplements.

However, compared to drug-to-drug interactions, nutritional and dietary supplement interactions are usually not as serious. Many nutraceutical interactions introduced on YouTube and other media are exaggerated or overblown. These media prioritize view counts, so unimportant interactions are often presented with sensational headlines.

One common example is concerns about minerals and malabsorption.

Taking calcium and iron together reduces their effectiveness. That's right. Minerals like calcium, magnesium, iron, and zinc are absorbed into the body through similar pathways in the intestine. When taken together in high doses, they can compete and reduce absorption. People who have been diagnosed with anemia and are taking iron supplements should be cautious.

However, this interaction is negligible if you're taking multivitamins because you're worried about nutritional imbalances. Some interactions can also occur when minerals are consumed through food rather than pills.

For instance, milk contains calcium and magnesium. Magnesium is present in much smaller amounts—about 10 percent of the calcium content. However, you don’t have to worry that the magnesium in milk will be less absorbed because of the calcium. A 2000 Austrian study found that drinking a glass of milk absorbs 75 percent of the magnesium in milk.

Multivitamin pills often combine multiple minerals, not because pharmaceutical companies are unaware of their interactions, but because deficiencies are uncommon.

Few people are deficient in zinc, calcium, iron, and magnesium, so they must take separate pills. Suppose a multivitamin contains 210 mg of calcium, 5 mg of iron, and 11 mg of zinc. That’s 30 percent of the daily recommended dietary allowance (RDA) for calcium, 42 percent for iron, and 129 percent for zinc.

When taking this amount, you don't have to worry about reduced absorption. It's better to take it as a single pill than to try to take each ingredient separately and get frustrated and give up.

Interactions are primarily a concern when taking nutraceuticals and supplements alongside prescription medications.

Minerals like calcium and iron can bind to thyroid hormone medications and interfere with their absorption. Therefore, these mineral supplements should be taken at least four hours apart from thyroid hormone medications.

You should also avoid taking thyroid hormone medications with caffeinated beverages such as coffee and energy drinks. Caffeinated beverages can reduce the medication's absorption by up to 55 percent, reducing its effectiveness.

Similarly, if you're taking osteoporosis medication, you should be wary of caffeine interactions. When looking at health information, always make sure it's meaningful to you.

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 Feb. 13, 2025. – Ed.