A recently published clinical guideline on vitamin and mineral supplements reinforces every other evidence-based guideline, research review, and consensus statement on this topic. The bottom line is that there is absolutely no substitute for a well-balanced diet, which is the ideal source of the vitamins and minerals we need.
The brief article, co-authored by nutrition guru Dr. JoAnn Manson, cites multiple large clinical trials studying multiple nutritional supplements' effects on multiple end points. The gist of it is, our bodies prefer naturally occurring sources of vitamins and minerals. We absorb these better. And because commercially available vitamins, minerals, herbs, etc. are lumped together as "supplements," the FDA doesn't regulate them. When we ingest processed, concentrated, and artificially packaged "supplements," we may be doing ourselves harm. They may be toxic, ineffective, or contaminated (all of which are not uncommon).
In other words: Most people who eat a healthy diet are unlikely to benefit from nutritional supplements.
Note the very important qualifiers. We're talking about most people (not all) who eat a healthy, well-balanced diet.
Does anyone need vitamin and mineral supplements? Well, yes
There are medical conditions that put people at high risk for certain nutritional deficiencies, and there are medical conditions that can be treated with certain nutritional supplements. This is important, and is why the authors support targeted supplementation. But who needs what and where to acquire these are important discussions to have.
There are guidelines for specific groups, such as pregnant women. Folic acid is especially important for healthy fetal development, and a deficiency can cause spina bifida, a neurologic condition. I advise my patients to start either a prenatal vitamin with folic acid, or at the very least folic acid itself, ideally before they begin trying to conceive. As pregnancy advances, mom needs to provide her growing fetus with everything, and so she will benefit from a prenatal vitamin (either by prescription or a well-vetted over-the-counter one) which contains things like iron and calcium.
Older adults can have difficulty absorbing vitamin B12, and I have a low threshold when checking this level; if someone is taking an acid-reducing medication, it is very likely that they will become deficient in B12, as well as iron, vitamin D, and calcium, among other things. These folks may very well benefit from a quality multivitamin.
Of course, there's a long list of medical issues that predispose people to vitamin deficiencies. For example, people who have had weight-loss surgery may require a number of supplements including A, D, E, K, and B vitamins, iron, calcium, zinc, copper, and magnesium, among other things. People with inflammatory bowel disease (like Crohn's or ulcerative colitis) may have similar requirements. People who have or are at risk for osteoporosis may greatly benefit from vitamin D and, depending on the quality of their diet and other factors, possibly also calcium supplements.
There are other medical conditions that can be treated with supplements. One that immediately comes to my mind is inflammatory arthritis (or other inflammatory conditions) and turmeric. While quality scientific studies are lacking, there are plenty of smaller studies as well as historical experience suggesting that turmeric has anti-inflammatory properties, and I see some of our rheumatologists routinely recommending this to patients for pain relief. Then there's prediabetes/diabetes and cinnamon, which has blood sugar-lowering properties. With these compounds, I advise that people use the regular spice in normal culinary amounts, not a processed/concentrated packaged supplement.
Not all vitamins are created equal
And here is another key point that bears repeating: Manson suggests choosing vitamins that have been tested by independent labs such as US Pharmacopeia, Consumer Lab, and NSF International, and certified to have the labeled dosage of the correct ingredient, and not have toxins or contaminating organisms. Many commercially available supplements here in the US will bear a label from one of these labs.
On that point, gummy vitamins are often not certified and often do cause cavities. Yes, everyone loves them, because they're basically candy. I do not recommend gummy vitamins for anyone, but especially not for pregnant women.
I'll also add a warning: I often hear about providers who are selling supplements or other products directly to their patients. This is a conflict of interest, and it's unethical, as well as fraught with all sorts of potential problems. Please use caution if purchasing anything directly from the provider who is prescribing it.
The bottom line
In summary, enjoy a varied, colorful, healthy diet, consider supplements when they may be needed or helpful, and choose your sources carefully.
Vitamin, Mineral, and Multivitamin Supplements for the Primary Prevention of Cardiovascular Disease and Cancer: U.S. Preventive Services Task Force Recommendation Statement. Annals of Internal Medicine, April 2014.
Dietary supplements and disease prevention — a global overview. Nature Reviews: Endocrinology, May 2016.