I won't say that every human condition can be fixed by diet, but certainly a large percentage of vitamin or mineral deficiencies can. If you're lacking a nutrient, it's either because A) your body can't absorb it, in which case you should see a doctor to find out why, or B) you're not eating it. When I began losing weight, experiencing brain fog, starving for carbs, and being cold all the time, I realized I was lacking a very crucial macronutrient - protein - and specifically the eight different "essential" amino acids it's made of. Plant foods have varied kinds of amino acids, and animal sources have all eight. I was eating very little of either. Instead of doing the math on which amino acids I lacked and spending a fortune on individual shots of 5-HTP or a sketchy bottle of "branched chain amino acids" from a bodybuilding supplement store, I bought a carton of eggs and ate them like it was going out of style. Three days later I feel like a different person. Food trumps pills.
Checking the ingredient list of every single supplement you're taking can be exhausting and overwhelming, as I realized while stocking shelves. Nobody's diet is perfect, and sometime you have to take a synthetic whatever to live on in relative health (hence my taking Vitamin D pills knowing they come from somewhere weird). But in the interest of not ingesting the same chemicals as are in your antiperspirant, consider keeping an eye out for the following:
1) Avoid ingredients that sound like chemicals. You don't go to the grocery store to buy a cup of Polysorbate 80 for baking, after all. If you don't recognize it, do a quick Google search before you start ingesting it. Despite what we'd all like to think, vitamin companies don't necessarily care how good for us their product is, as long as we buy it.
2) Do not ingest heavy metals. Good for the ears, but not good for the gob. Be wary of anything that sits on the Periodic Table. Some minerals are totally harmless, but others can be extremely detrimental to your health. Eat nothing containing lead, mercury, cadmium, chromium, titanium, silver, gold, or arsenic, and be cautious of anything with iron, copper, zinc, aluminum, beryllium, manganese, and/or cobalt.
4) Avoid shopping at the drugstore. This isn't to say that organic food shoppes are automatically better; I've gotten excellent-quality omega 3 fish oils at the drugstore where I work. The thing is, drugstores stock cheaper brands of supplements because their clientele is unlikely to want to spend much money on pills that won't relieve their symptoms right away. Always check to see what's in your vitamin and let it play a factor in how much you spend on it. As in the case of the probiotics I bought for GF, I would prefer to spend a little more for fewer tablets that were just germs and capsules than more tablets chocked full of strangely-named and unnecessary milk powders.
5) If it looks or tastes like candy, it's not helping. Almost all of the Vitamin C and multivitamin capsules you'll see marketed to kids will be either gummies or chewables, usually formed into fun shapes (and in some cases include temporary tattoos and stickers). Take one look at the ingredients of these and you'll see a list that goes, "Sugar, sugar, Splenda, sugar, chemical colour, Vitamin A, sugar, weird gummy emulsifiers, preservatives." Not only are these full of things you shouldn't be feeding to an already hyperactive three-year-old, but they're a waste of money. You could spend $15 on a tub of Flintstone vitamins to make sure your child gets her Vitamin C, and unintentionally pump her full of chemicals and mood-altering artificial colours, or you could buy a box of nectarines for $5. There are cases in which a fun-tasting vitamin may be the only option - with a picky or unwell senior, for example - but try to find the best bang for your buck and remember #3.
6) Liquids > everything else. If you can find a liquid fish or flax oil, I guarantee it'll work ten times better than "horse pill"-sized tablets for getting your omega EFAs into you. This one I can attest to personally.
7) You should feel a difference! I take four specific supplements every morning, and I notice when forget to. Instead of feeling awake and well, I'm overly anxious, easily depressed, broody, and unfocused. While vitamins are not stimulants and shouldn't have you feeling wired, they are meant to help - if you don't feel like it's doing anything, it's probably not. Stop wasting your money.
8) Alligators Don't Eat Fat Kids. Vitamin A (alligators), Vitamin D (don't), Vitamin E (eat), Omega Essential Fatty Acids (fat), and Vitamin K (kids) are all fat-soluble substances that our bodies store and therefore need less often than water-soluble vitamins. Taking large quantities of these can actually make you sick. Remember this when shopping for any multivitamin, unless you've been specifically told by a naturopath or other licensed practitioner to take more. The less of these a multivitamin contains, the better.
In the end, the most important thing is to listen to your body. You know you better than anyone else!