Do your vitamin and mineral supplements actually do anything? Here's what experts say.

With cold and flu season in full swing, 'tis the season for many Americans to throw back one or more dietary supplements in the hopes of fending off illnesses. And it isn't just a winter habit; for many, they've become routine, with nearly 58% of people ages 20 and older reporting using at least one dietary supplement.

But do all those little pills — which make up a multibillion-dollar industry — actually do anything?

Supplements vs. food

Experts say that food trumps supplements as the best source of nutrients. Dr. Marilyn Tan, a clinical associate professor of medicine at Stanford University, explained the benefits of acquiring a nutrient gradually throughout the day rather than getting “a large chunk of it all at once” via pill.

“I think that if you can take it throughout the day — for example, in nutrients through food — it’s just absorbed better. Because there is a maximum amount that your body can absorb at one time,” she said. “For example, for calcium, if you take more than 500 to 1,000 milligrams, your body is just going to pee it out. And a lot of vitamins are that way, where you just cannot absorb such a large amount at once.”

Tan said most Americans are already getting the nutrients they need from food alone.

“Most people with the standard American diet, unless they’re on very restrictive diets, get adequate nutrients through their diet,” she said. “Vitamin deficiency can happen with certain conditions like malabsorption or pernicious anemia, for example, but for the average, otherwise healthy American, they get plenty of nutrients through the diet.”

Lisa Moskovitz, a registered dietician, CEO of NY Nutrition Group and author of "The Core 3 Healthy Eating Plan," told Yahoo News that for someone who is already eating a relatively healthy diet, supplements likely won't make much difference and "can be a waste of money and just really expensive urine," as your body expels all those excess nutrients. And for people who are already getting enough nutrients through their diet, adding a vitamin supplement won't necessarily give them the extra boost they may be hoping for.

“If you already have sufficient levels in your body and you’re taking supplements of B12, for example, you’re not going to feel more energy from taking B12 if you already had enough B12 in your system to begin with,” she said.

When might supplements be a good idea?

Experts emphasize a “food first” approach to nutrients, meaning supplements should do just that — supplement but not compensate for bad eating habits. They may help fill in nutrition gaps in certain instances, such as if you’re restricting your food intake for weight loss or if you adhere to a vegan diet, have limited access to healthy foods or have a certain vitamin deficiency, which can be diagnosed by your doctor with a blood test.

An iron deficiency, for example, is not uncommon, especially in menstruating women or people who have sources of blood loss. Iron can also sometimes be harder to get solely through food if you’re a vegetarian.

And for many people, vitamin D can also be difficult to get through diet alone. We get vitamin D mostly from sunlight, but if you wear a thick layer of sunscreen while in the sun or if you don’t get outside enough, you may not be absorbing as much. How dark or fair your skin is may also affect vitamin D absorption.

“Vitamin D is very difficult to get adequately from food. There are not that many dietary sources of it,” Tan said. “But for most other vitamins, we are able to get them in food.”

Vitamin B12 is another example, she said, in which a doctor may recommend an oral supplement if you have a mild deficiency, which becomes more common as people age.

And folic acid, a B vitamin, is one supplement that has wide support from public health experts, even among supplement skeptics. It has been proven to prevent serious birth defects of the baby's brain and spine, and since the benefits of folic acid are most pivotal in the early days and weeks of a fetus's development — before many women know they are pregnant — the CDC recommends that "all women of reproductive age should get 400 micrograms (mcg) of folic acid each day, in addition to consuming food with folate from a varied diet."

“The risk is too great to take the chance of a woman who thinks they’re getting enough folic acid [through their diet] but they’re not,” Moskovitz said. “It’s just because the research is so, so strong.”

So do supplements actually work?

While folic acid supplements have proven benefits, the jury is still out on the merits of most other supplements.

In 2013, researchers at Johns Hopkins published an editorial titled "Enough Is Enough: Stop Wasting Money on Vitamin and Mineral Supplements," with one of the editorial's authors saying he didn't recommend any supplements other than folic acid for women who may become pregnant.

Earlier this year, the U.S. Preventive Services Task Force issued updated guidance saying that vitamin, mineral and multivitamin supplements are unlikely to prevent cancer or heart disease, or to impact overall mortality.

“It doesn’t usually hurt to take a multivitamin, but many studies have looked at whether a multivitamin can help to improve mortality or quality of life or sense of well-being or things like that, and nothing’s been very conclusive,” Tan said. “There’s no great randomized control trial that shows significant health benefits to taking a multivitamin.”

Tan said that if you have a diagnosed deficiency that’s impacting your health — such as a B12 deficiency that’s impacting memory, for example — supplementing it can help. But taking supplements simply in the hopes of reaping health benefits down the road may not do much.

“Many studies have tried to examine, for example, whether vitamin D can help with heart disease, or help with infections such as COVID,” Tan said. “Studies have been mixed, but there has not been anything that’s definitively proven that a specific supplement of a vitamin will help you with longevity.”

When it comes to using supplements to treat or shorten the duration of illnesses like the common cold, results are also mixed. Zinc is a mineral that has been touted by some for its ability to possibly reduce the duration of a cold if taken in lozenge form within the first 24 hours of the onset of symptoms, but nothing has been definitively proven. While some studies have indicated that zinc may shorten a cold by a few days, other studies have concluded that zinc had no effect on cold duration or severity.

Most over-the-counter vitamin supplements are safe in limited amounts, so if they make you feel better, it probably doesn’t hurt to take them. But they are unlikely to cure your ailments, Tan said.

“Are they going to necessarily cure or reverse an infection? No, probably not,” she said. “They’re also not a substitute for any recommended treatment [from your doctor]. For example, if you have the flu and your doctor recommends taking Tamiflu because you’re high risk, taking vitamin C may help or taking zinc may help, but it’s not a substitute for whatever your doctor recommends.”

Too much of a good thing?

It’s also possible to have too much of a good thing, experts say. Excess water-soluble vitamins are usually excreted through urine, but excess fat-soluble vitamins can stay in your body and have adverse effects.

Long-term use of zinc in high doses, for example, can cause a copper deficiency; high doses of vitamin A shouldn’t be taken during pregnancy because it can hurt the fetus; and excessive vitamin D can lead to high, unhealthy calcium levels.

Some supplements may also interfere with medications.

“If you’re taking certain medications, you do want to be careful, especially with herbal supplements like ashwagandha [or] herbal supplements like St.-John’s-wort,” Moskovitz said. “Those can affect psychotropic medication, so antidepressants [or] antianxiety medication. Some can actually interfere with heart medications [or] blood thinners. So that’s why it’s also very important to check with a professional.”

How can you be sure you’re taking the right supplement?

Supplements aren't regulated by the U.S. Food and Drug Administration the way medications are; they're considered a subcategory of food, not drugs, so anything the manufacturer feels is safe can hit the market without prior FDA approval.

One way to get some assurance that the supplement you’re taking lives up to its claims is to look for ConsumerLab or United States Pharmacopeia seals on the label, which indicate that the product has been quality-tested and verified. And if a product is making “miraculous claims” that it can improve your health, take it with a grain of salt, Tan said.

You should also consult with your health care provider before taking any supplements, Tan and Moskovitz said, because chances are, you may not need them.

“For someone who is looking to add more supplements to their diet, who wants to explore that and see if they can benefit, it always helps to first talk to a professional doctor and dietitian, especially a doctor who can order blood work,” Moskovitz said. “Test your levels before you spend your hard-earned money on something that you might not need and might just be excreting it anyway.”