After you correct your diet you must then use your judgment whether any experimental supplementation is necessary. For example, in the cases of Manganese and Selenium (in some parts of the world), it is easy with food alone to get to levels of these nutrients which, in supplements, would be considered dangerously high. Always use caution with supplementation of the minerals and the fat-soluble vitamins (A, D, E, K), because the body pools of these substances change only very slowly, and you may be unable to recognize any difference until toxicity begins. Vitamins A (retinol), D3, and K2 all have to be present in the body for each of these vitamins to work right. Vitamin K2 is the animal version of Vitamin K and the conversion from vegetable-sourced K1 to K2 can be very poor for humans. The USDA tables have very little data on Vitamin K2, only for some foods and only for menaquinone-4. For Vitamin D, the Daily Value equals about 40 seconds of sun exposure and is not an adequate allowance in the absence of sufficient sun exposure. For Vitamin A, the Daily Value can mislead because not everyone is good at converting beta-carotene and the rest of the carotenoids to Retinol, the real Vitamin A, and Retinol only comes from animal foods. And lastly but not leastly, Vitamin E is weird. It works great when you get it from food (check out high-oleic sunflower oil!), but in my experience it is counter-productive to supplement it because while it almost immediately lowers inflammation, it seems to promote future inflammation somehow--you start to feel worse instead of better. So when I developed some inflammatory thing and I felt I had to do something about it, I took one dose of alpha-tocopheryl succinate, but no more, because continued supplementation always produced a worse result.
The Daily Value for Sodium is a suggested upper limit with no scientific basis and is not a minimum daily requirement. Although NUT does not mention it, be aware that you need reliable sources of Iodine, such as seafood and iodized salt. The Daily Values for Potassium and Calcium seem excessive to me, and my experiments show 2600 mg. is adequate for Potassium and 1000 mg. is adequate for Calcium.
The Daily Value limits on Saturated Fat have no scientific basis and will be found to be counter-productive to vitamin and mineral intake in several ways, such as limiting the intestinal absorption of minerals and fat-soluble vitamins, increasing the Omega-6 to Omega-3 ratio, and reducing vital nutrients only obtainable from animal foods. The Daily Value for Cholesterol is totally meaningless and has been debunked for 70 years, but somehow it remains in the standard because the demonization of cholesterol is necessary to sell the drugs that lower cholesterol and pay off the physicians who ignorantly prescribe them to patients whose real problem is hyperinsulinemia and insulin resistance from too much carbohydrate and high Omega-6 industrial oils, foods that, of course, don't have cholesterol.
With the water-soluble vitamins, the "B"s and C, the experiments are much easier because these nutrients are in a constant daily flux, and a single substantial dose can frequently tell you within a day whether anything beneficial is happening. Vitamin B-6 is reported to cause irreparable nerve damage in large doses, but usually you need only a few times the Daily Value to make an informed judgment whether supplementation is useful. When one is not deficient, supplementation can cause unwanted effects, such as headaches or jumpy feelings from B vitamins, or the weirdest effect that I saw in my experiments: excessive Vitamin B-6 supplementation always made me dream of feces, apparently because B-6 is the characteristic intestinal signature of voluminous bacteria. I advocate that you buy all the B vitamins in individual supplements and apply the foolproof test: Take one right before bed when you do not have to be well-rested the following day. Those B vitamins that make you toss and turn and keep waking you up during the night are those B vitamins that you are probably not deficient in. The B vitamins that feel pleasant, give you a restful sleep, or appear to do absolutely nothing are the B vitamins which are your main candidates when a symptom shows up and you don't know which nutrient is missing. The one exception is B-12, which I would test during the day because it affects consciousness if you are deficient. And yes, you will end up with a zillion bottles of vitamins where you decided you didn't need it. But you will also have an invaluable mental picture of all of them that you can sort through to immediately determine the most likely candidate when something is wrong.
I followed this previous advice and found it instructive. However, there is much to be said for the "shovel in some B vitamins to pep me up" school. Here's how it works. You need three things taken together. You need a B complex formula, preferably time-release so you feel less jumpy when you take it. If you take it three times a day you may notice some additional enthusiasm as now the body has the means to do all the repairs and metabolism steps that make you feel good, but you may feel anxious and need the second thing, a concurrent 100 mg. Thiamin (B1) tablet to make energy production go more smoothly for a relaxed, calm disposition. Finally, add the crown jewel, a sublingual 1000 mcg. (1 mg.) Vitamin B12. I get the best results from cyanocobalamin as opposed to the highly-touted methylcobalamin. The B12 adds an inspirational and courageous note to the proceedings. Critics will compare your dosages to the recommended daily allowances and decry expensive urine and reckless disregard for what is good and true, but if you feel really good from this regimen, you will ignore these fools. However, they may get the last laugh if the reason you need to be pepped up is hepatic insulin resistance and fatty liver disease.
Megadoses of Vitamin C can cause me to feel cold as if the Vitamin C is turning off thermogenesis somehow, so that I am frequently adjusting the thermostat in the room to a higher temperature to compensate. I use this effect to determine the maximum size of a dosage, and it's about one gram per meal.
Don't take vitamin and mineral supplements unless you can see they are doing something and you can see what they are doing--then find the minimum amount of the supplement that accomplishes the task. Whenever I made a major change to my diet I always dropped all the supplements and made them prove their utility once again. The idea that you cannot know in the present that you are deficient in a vitamin or mineral is just a marketing ploy and guarantees that almost everything you take will be a placebo. Taking supplements as "insurance" doesn't insure you against anything because you don't know what you need, and if they left the nutrient you need out of the pill, you wouldn't know enough to be able to tell. But precise supplementation of exactly what you need can make a huge difference. And get a pill splitter--sometimes the whole tablet is just too much!