Giving your kid a daily multivitamin supplement can certainly bring some peace of mind. But here’s the catch: Most children don’t need to take them, despite nearly a quarter of U.S. kids and teens taking vitamin supplements, according to the Centers for Disease Control and Prevention.

Experts say that children who are deficient in some vitamins—picky eaters, for example— would be better off just taking those specific vitamins over a multivitamin supplement.

“It’s really a case-by-case basis,” Vanessa Hurley, a registered dietitian with expertise in pediatric nutrition, tells Fortune. She suggests “working closely with the registered dietitian or other licensed healthcare professional to do a complete assessment to see if, in fact, it’s a requirement for them to supplement their current diet.”

There are also certain tolerable limits to certain vitamins and minerals—such as vitamin D, which has an upper limit of 25 mcg to 100 mcg for kids 0 to 18, depending on their age, according to the Office of Dietary Supplements. It is why it’s so important to check dosages on bottles—and why, when it comes to multivitamins for kids, Hurley says, “I wouldn’t recommend them broadly.”

Jill Castle, a pediatric nutritionist and registered dietitian, agrees. “Not all kids need them,” she says. “Ideally, children get their nutrients from food. And between food fortification—things like cereal, where we can get vitamin D and calcium—and…a wide variety of all the different food groups, we can meet their needs for micronutrients.”

To figure out what a child might be missing, Castle says, she does a “dietary recall,” talking with families about what their kids eat throughout a typical day and what bothers parents about the diet. “Parents are very good at saying, ‘Well, they never eat any meat.’ Especially if it’s a picky eater, they always seem to be very attuned to what’s going on with food intake. So part of my role is to determine: Is it adequate? Is there a lack of nutrition or a lack of nutrients?”

And sometimes, there is.

Why some kids might need supplements

Castle says she’d be concerned about proper brain development if a child is “cutting out whole food groups,” such as meat or vegetables. “Particularly in a young toddler who’s 2 or 3 years old, where things like iron and zinc and vitamin B12 are still really important to the brain-development phase,” she says.

And then there are the “growing number of extremely picky eaters,” says Castle, referring to kids with Avoidant Restrictive Food Intake Disorder (ARFID). “They don’t eat any fruits and vegetables, or they don’t eat any meat, or they have no fish in their diet, or they don’t drink any milk or have any dairy products.” For these kids, she would suggest either a multivitamin or a selection of supplements, such as vitamin D, Omega-3 fatty acids, or vitamin C.

Vitamins that are most likely to need supplementing

Vitamin D

“Liquid vitamin D is necessary for breastfed infants,” says Hurley, making note of one of the earliest supplemental needs for kids.

Castle likes to extend that beyond infancy. “I have certain nutrients that I almost always want kids to be on. Vitamin D is one of them,” she says. “It’s a nutrient that’s very hard to get from food alone. And even though we can activate it in our skin by being outside in the sunshine, a lot of kids are watching screens. And kids who live in the Northeast don’t have as much exposure to the sun as kids in the south or on the west coast. So it’s a nutrient that oftentimes we find is on the low end in children, and because they’re developing their bones…it’s an important nutrient.”

For kids who don’t eat meat or animal products, Hurley is also a proponent of supplementing vitamin D—along with calcium, iron, and B12.

Omega-3 fatty acids

Says Castle: “I tend to work with either underweight, picky-eater type kids, or kids in larger bodies who are carrying extra body fat. And both of those subsets are typically not consuming fatty fish twice a week, or walnuts, or some of the other foods that we know are contributing Omega-3 fatty acids.”

This group of polyunsaturated fats—mainly found in fish but also in plant sources including walnuts, flaxseed, and chia seeds— support heart, eye, and brain health. “They’re helpful even for kids with ADHD, as they’ve been shown to be helpful with brain function and reducing inflammation,” Castle adds.

It’s the one supplement Hurley gives to her own two kids.

“They’re not big fish eaters,” she says. “It’s hard to find a good one that doesn’t taste fishy, so I try to incorporate hemp seeds, ground flax seed, and other sources into their diet… But Omega-3 fatty acid, if there’s not a lot of sources in your diet, can be something beneficial for everybody.”

Vitamin C

Hurley suggests that kids who are “just eating pasta with butter,” for example, might need C, which is an important vitamin for one’s immune system. That’s especially true if there are no vegetables in their diet. “However,” she says, “it’s important to look at the entire day, the entire week.” If kids are eschewing veggies but eating lots of fruits, they’re likely covered. “But if there’s not a lot of variety in there—not a lot of fruit or whole grains, and if there’s a lot of processed foods—then a supplement may be recommended.”

When you think about vitamin C deficiency, Castle adds, “you think scurvy, and sailors in the 1800s, but these things are surfacing here and there in our current culture.”

Fiber

Another supplement to consider if your kids are not eating whole-grain foods, fruits, or vegetables, is fiber. “Especially if they’re not having regular bowel movements and they’re a little constipated,” Hurley says.

It’s why aiming to get nutrients from food over supplements is so important. “There are so many other benefits to food versus through a liquid or a pill form—especially the fiber, which you don’t get from a multivitamin,” she says. “You also don’t get things like antioxidants and phytochemicals, and the child is not learning how to develop an appreciation for different textures and flavors if you’re relying on a supplement.”

This sounds like a lot of work. What if a blanket multivitamin is all I can handle?

“I don’t have a problem if a family says, ‘Just for comfort, I want to make sure I’m covering everything,’” Castle says, especially if it’s a multivitamin made for children and they follow dosage guidelines.

“My advice is always, like, if you want to do that, that’s fine,” she says—as long as it’s not at the expense of “still working on exposing your children to more vegetables and fruits and dairy foods or non-dairy substitutes. You really still want to be offering this balanced diet that introduces kids to a variety of foods.”

Nearly a quarter of U.S. kids take vitamin supplements. Here’s why some kids probably need them—and why most do not