When it comes to vitamins, how much is too much?

Vitamins and minerals are essential for good health, but there’s a reliable rule that applies to everything we consume: More isn’t always better.

People taking vitamin supplements must be especially careful to avoid getting too much of a good thing—and, according to a study published in JAMA Network, that applies to a whole lot of us. Approximately half of all American adults take at least one dietary supplement, noted the authors, and those working in the medical industry are no exception. Roughly 28% of US nurses report using at least four dietary supplements. 

Other estimates are even higher: According to the Council for Responsible Nutrition, a record-high 77% of Americans take at least one dietary supplement. The number is even higher—81%—in adults aged 35-54 years.  

While taking too many vitamin supplements doesn’t always pose a serious threat to health, it can result in adverse health outcomes in some cases. Here’s a breakdown of the vitamins that deserve your attention and the consequences of overdoing it, according to health experts and the latest research.

Why you should exercise caution

If you’re eating a balanced diet and don’t have any underlying health conditions, you’re likely getting all the vitamins and nutrients you need from food.  

“When I think about vitamins, I think supplements,” wrote Renee Miranda, MD, in a blogpost. “The vitamins should be supplementing something that’s missing from your diet. If your nutrition is bad or you have a condition or disease that prevents your body from absorbing certain nutrients, then yes—you need to take supplements. But if your gut is working well and you’re eating a balanced diet, you’re probably getting all the nutrients your body needs and shouldn’t have to take a supplement.”

If you do take supplements and ingest more of a vitamin or mineral than your body needs, the excess will either be excreted or stored, depending on the type of vitamin, Dr. Miranda added. If the vitamin is water-soluble, it can be flushed out of the system via urination. However, if it is a fat-soluble vitamin, it will be stored in fat, and an accumulation of fat-soluble vitamins is more likely to result in health problems. Of note, for elderly people or those with kidney problems, even water-soluble vitamins may also stick around and can accumulate to toxic levels.

Here’s a look at the fat- and water-soluble vitamins to be aware of.

Fat-soluble vitamins

According to Mark Rosenbloom, MD, in an article on Medscape, the fat-soluble vitamins A, D, and E are worth watching out for. While symptoms like nausea, vomiting, diarrhea, and rash are common with any case of chronic overdosing, there are more specific symptoms for each vitamin.

Acute vitamin A toxicity can result in hair loss, weight loss, fatigue, insomnia, bone fractures, hyperlipidemia, hypercalcemia, anemia, and other symptoms, according to Rosenbloom. Government guidance from the National Academy of Medicine (NAM) states that adult males should aim to consume 900 μg of vitamin A daily, and adult females should aim to consume 700 μg.

Vitamin E toxicity can involve gastric distress, abdominal cramps, easy bruising and bleeding, inhibition of platelet aggregation, suppression of other antioxidants, and increased risk of hemorrhagic stroke, noted Rosenbloom. NAM states that adults of any sex should aim to consume roughly 15 mg of vitamin E daily.

The symptoms of vitamin D toxicity are similar to those of hypercalcemia and can include muscle weakness, bone pain, constipation, abdominal cramps, polydipsia, polyuria, backache, calcinosis, hypertension, and cardiac arrhythmias.

While rare, a recent case of vitamin D overdose was detailed in an article published in the Canadian Medical Association Journal. A 54-year-old man who had just spent 2 weeks sunbathing for 6-8 hours per day while on vacation was taken to a nephrology clinic for suspected acute kidney injury. His creatinine level was 376 μmol/L. 

After numerous urine and blood tests and other screenings, doctors found that his serum calcium and parathyroid hormone levels showed a non-PTH-mediated hypercalcemia.

Doctors subsequently discovered that the man had been prescribed high doses of vitamin D by a naturopathic specialist, and that he had been taking 8-12 drops of vitamin D daily for the past 2.5 years, for a total daily dose of 8,000-12,000 IU. 

The patient was told to stop taking all vitamin D supplements and avoid calcium-rich foods. He was also prescribed oral hydroxychloroquine 400 mg to be taken daily. After a year of this treatment, the man’s calcium and vitamin D levels returned to normal—but he was left with stage 3 chronic kidney disease.

NAM recommends that adults get 15 mcg (roughly 600 IU) of vitamin D daily, and 20 mcg for those 70 years or older.

Water-soluble vitamins

While excess levels of water-soluble vitamins are typically excreted from the body, they can still accumulate to toxic levels. According to Rosenbloom, vitamin C toxicity can result in renal colic, diarrhea, hemolysis, dental decalcification, increased estrogen levels, and occult rectal bleeding. NAM recommends that male adults aim to consume 90 mg of vitamin C daily, and that female adults aim for 75 mg.

We should also pay attention to our intake of vitamin B6. Too much can result in neurological symptoms, like tingling in the feet and numbness, according to Dr. Renee Miranda.

Additionally, research indicates that high doses of vitamins B6 and B12 may be bad for our bones. The aforementioned JAMA study examined data from a cohort of 75,864 postmenopausal women in the United States who were followed from 1984 to 2014. Researchers measured correlations between vitamin B6 and B12 intake and rates of hip fractures.

They found that the roughly 2,300 women who suffered hip fractures were more likely to have had a high intake of both vitamins B6 and B12. In fact, women with high doses of both vitamins were almost 50% more likely to have a hip fracture than those who reported low intake. The researchers noted that the average intake of women who experienced hip fractures was far higher than recommended daily doses. 

While a biological explanation for this is currently unclear, researchers hypothesized that high doses of vitamin B6 may accelerate bone loss by counteracting the modulating influence of estrogen. According to NAM, adults of any sex should consume around 1.3 mg of vitamin B6 and 2.4 μg of vitamin B12 per day.

For more guidance on minerals and vitamins, and the optimal time of day to take them, please click here. You also might consider reading up on five stress-busting supplements and vitamins that can help keep burnout in check.