Overdoing vitamin supplements could have side effects like diarrhea or hair loss, according to a nutritionist

• Taking vitamin supplements you might not need is not only pointless, but can be dangerous.
• If you're not deficient in a nutrient, your body can't use it as a supplement, registered nutritionist Rhiannon Lambert told Insider.

Vitamin supplements can be useful for filling nutritional gaps in our diets. 31% of Americans are at risk of at least one vitamin deficiency, research suggests.

But there are potential problems with taking vitamin supplements your body doesn't need, registered nutritionist Rhiannon Lambert explains in her upcoming book, "The Science of Nutrition."

If you take a vitamin supplement and your body isn't deficient in that nurtient, in some cases, toxic build-up can occur, which can lead to issues like nausea, diarrhea, stomach cramps, hair loss, gastrointestinal upset, fatigue, and mild nerve damage, Lambert told Insider.

Water-soluble vitamins are excreted, but fat-soluble ones build up

Some vitamins are water-soluble, such as vitamin C and the B vitamins, which means they are excreted through urine if not needed, which is why you might see bright yellow urine after taking certain vitamin tablets.

Others are fat-soluble, such as vitamins A, D, E, and K, which the body can't get rid of.

"They don't leave the body and toxicity can build up over time," Lambert said, which can lead to overdose, and in extreme cases, a visit to the emergency room.

Taking too much vitamin C can cause digestive issues

Overdoing water-soluble vitamins is less dangerous than overdoing fat-soluble ones, but overdosing on vitamin C, for example, while rare, can still cause nausea, diarrhea, and stomach cramps, according to Lambert.

Vitamin C has many benefits, including keeping the immune system healthy, but most people don't need a supplement if they're eating a decent amount of fruit and vegetables, she said.

An orange or a cup of strawberries, chopped red pepper, or broccoli provides the recommended daily 65-90 milligrams of vitamin C, according to the Mayo Clinic.

Lambert and other experts recommend a "food first" approach to nutrition, because it's easier for your body to absorb vitamins from food, rather than in supplement form.

Some capsules don't disintegrate quickly enough for the body to use the nutrients, especially if the coating is shiny or waxy, she said.

It's 'pointless' taking supplements your body doesn't need

Unless you are deficient in a certain nutrient, it's "pointless" taking a supplement, Lambert said because, "the body will only take what it needs."

Many people opt for multivitamin tablets, which can be counterproductive because the vitamins and minerals compete and it's too much for the body to use in one go, Lambert said.

"It can really disrupt your digestive system," she said.

Lambert said she often sees clients who are struggling with stomach cramps and think they have IBS, but once they stopping taking their multivitamin, the problem goes away.

Supplement needs are individual

There are specific circumstances and individual differences that can affect someone's vitamin requirements, for example, if you're pregnant, elderly, or have been advised by your doctor.

In many parts of the world it's recommended to take a vitamin D supplement, especially in the winter months when there's less sunlight.

"You're never really going to get enough vitamin D from your diet, especially in the UK, which is why people should be supplementing 10 micrograms a day all year round," Lambert said.

The only real way to know if you're deficient in a certain nutrient is to see your doctor and have a blood test.

If you experience severe hair loss, a burning sensation in the feet or tongue, slow-healing wounds, bone pain, irregular heartbeat, or poor night vision, see your doctor as these can all be signs of nutrient deficiency, according to Rush University Medical Center.