Perhaps you’re feeling generally run down and could use a boost. You’re living a stressful life and not eating as well as you could be. So, you might think that taking a multivitamin would be a good idea, just to cover your bases. But a multivitamin may not be the answer.
Problem 1: Multi-Vitamins Don’t Address Your Individual Needs
In my opinion, before you start supplementing you should get a blood test to really figure out if you have vitamin deficiencies and exactly what you should be taking. If you blindly take a multi-vitamin, you put yourself at risk for getting to much of one particular nutrient, while depriving yourself of another.
For instance, if you’re vitamin D deficient (as many of us in the northern hemisphere are) you may need to take 5000 to 10,000 IUs a day to get your levels to an optimal state. In a typical multivitamin you’ll get maybe 400 IUs–not enough to raise your levels or really see any benefit from it. (You can get tested at your doctor’s office or try a test kit like this.)
Many multivitamins will contain selenium, which can actually end up being harmful. For many years we looked at selenium as a cure-all for many conditions. It’s a vital mineral for thyroid function, reproductive health, cancer prevention, brain health and so much more. Researchers saw low selenium in so many diseases so people started supplementing with it. However, high levels of selenium have been found to increase LDL cholesterol levels, (Source)(Source)(Source) and while some studies have shown that selenium can aid in supporting glucose control, higher selenium levels have been associated with increased diabetes risk. (Source)
A review article  that comprehensively investigated the association between selenium and type 2 diabetes concluded that this complicated relationship may be explained by the possible harm that occurs both below and above the physiological range for optimal activity of some or all selenoproteins. The present study and a previous one  were both conducted on a population with mean dietary selenium intake close to the RNI, but the association in a population with very low dietary selenium intake may be different. Previous review articles suggested that the association between selenium and diabetes might be U-shaped: selenoproteins both below and above the physiological range might become a risk factor for diabetes [1,3,34].(Source)
In other words, both high and low levels of selenium can be harmful.
While low selenium has been tied to risk for thyroid conditions, and selenium is an essential trace mineral for enzymatic function within the thyroid (source) it was found that selenium supplementation was not helpful for improving thyroid conditions. (Source) Perhaps due to the forms in the supplementation or the higher doses. This recent review found that there was not significant evidence to prove that selenium supplementation helped prevent or cure cancer and further highlighted the idea that while low selenium levels have been found in people with certain illnesses, we’re still not sure if it’s more of a symptom or a cause of the illnesses.
Zinc is another nutrient that’s vital for health and is touted as a benefit in many men’s multivitamins. However a 2009 study found that long-term supplementation with zinc was associated with a higher prostate cancer risk.
Long-term iron supplementation has been found in many studies to create oxidative stress in the body as the iron deposits accumulate in the body’s tissues and artery walls, potentially leading to atherosclerosis. (Source)(Source)
Problem 2: Heavy Metals
Another problem with multi-vitamins is that they can contain heavy metals. Many times vitamin companies will use minerals that are the products of mining. Because lead and other heavy metals occur naturally in the earth’s crust, these minerals can contain trace amounts of heavy metals. In 2007 the FDA tested a number of multivitamin products and found them to have levels of lead that were above the recommended daily exposure levels.
In 2013 a study of multivitamins distributed in Lebanon found elevated levels of cadmium, copper (an essential mineral, however in higher doses can be toxic) and arsenic. The study found that the levels of cadmium were increased with higher doses of calcium and zinc, while elevated levels of arsenic corresponded with the levels of iron and manganese in the product. Similar studies have not been done on vitamins distributed in the U.S., however the researchers suspect that these correlations had to do with how these heavy metals were attracted to particular minerals in the earth’s crust.
Problem 3: The Right Forms
Many times while looking for a multivitamin, you may be tempted to look at the RDA percentages and just try to pick the product with the highest percentages of as many vitamins as possible. After all, why not get as many things in the highest amount for your money? However, this is not necessarily a reflection of how the product will benefit you (and in cases of some nutrients, high levels are toxic).
In order to benefit from your supplements, you need forms of vitamins that your body can absorb and utilize. Cyanocobalamin, for instance, is used in many supplements as a form of vitamin B12. However in the digestive tract it must be converted to methylcobalamin or adenosylcobalamin in order to function in the body. Many people have a hard time with this process, either due to a genetic variation called MTHFR or impairments in the gut. (Source)
Sodium selenate and sodium selenite are commonly used forms of selenium in multivitamins and they are absorbed well in to the body. However, they’re not retained and quickly excreted in the urine, thus their efficacy is questionable. (Source)
One form of synthetic vitamin E commonly found in multi-vitamins was actually found to reduce the levels of vitamin E in the blood. (Source)
Problem 4: Labeling Credibility
In the U.S., there is little oversight in the big business of vitamins and supplements. Manufacturers don’t have to register with the FDA, test products, or have any manufacturing requirements. There are guidelines that companies are suggested to follow, but unless there’s a major issue with a product, the FDA doesn’t inspect or intervene.
However, in 2010 Consumer Reports tested 15 multivitamins and had fairly good results–only one of the products had significant variance from what was stated on the label and none had worrisome amounts of heavy metals. However, there is no independent agency that tests vitamins on the market for potency or safety.
Choosing a Multi-Vitamin
When looking for a multi-vitamin you may have the tendency to look for one that’s more natural-sounding and “pure,” with the least amount of additives. However, sometimes there are vitamins that can’t be derived from plants reliably (like vit D3), and sometimes the best, most absorbable forms of vitamins have long, scary-sounding names.
There is no one multivitamin that’s right for everyone and I always recommend eating nutrient-dense foods instead of a poor diet and supplementing with a multi. Ultimately, though, the allure for a one-pill-does-all solution to your supplement needs is strong. Plus, sometimes there are absorption problems or nutrients that your body just isn’t getting. I am not affiliated with this company in any way, but here is one company that offers multi-vitamins and does testing for potency and heavy metals with independent labs and offers a selection of multivitamins based on different needs.