Seems that there can be lots of confusion about vitamins- what they are and how much of them we need. The general term that we see with RDA, which is recommended dietary allowance. These numbers are derived from EAR's (estimate Average requirements) and are assumed that these numbers will meet the needs of 97% of the population. The are set to help individuals avoid deficiencies in any given nutrient. What about the other 3% of the population that doesn't fall into that category or what if you have other circumstances?
There will be other acronyms that you will see in books or on supplement bottles, AI which is adequate intake meaning that it will meet the needs of most healthy individuals. You will also see UI, mean tolerable upper intake level. UI's were established with the onslaught of enriched and fortified foods that are available on the market as it is too easy to consume too much of a nutrient when eating these foods, especially if you are taking nutritional supplements as well.
ODI's and SONA's Optimum daily intake and Suggested optimal Daily nutritional Allowances These have been set as a way to not only avoid deficiency but to promote optimum health benefits as well. ODI's are set in ranges for men and women and SONA's are set generally in mg's(micrograms) or IU's(international units)
In times when your body is under stress or healing you may need more of certain nutrients than in times of health. Nutrients work much like a symphony, a balance is needed for optimum health. Too much of some nutrients may negate the effects of others. For example, vitamin D helps with the absorption of calcium in the small intestine, and decreases it's excretion through the urine. If one is deficient in Vit D despite adequate calcium intake our bodies may not be getting its requirements. Calcium also works with magnesium to be absorbed so adequate intake of magnesium is also necessary; however, to much can inhibit absorption as well. Also,phosphorus can inhibit calcium absorption. Sodas are high in phosphorus, so individuals drinking a lot of soda's may have issues with this. This doesn't mean that we should not have any phosphorus intake, just that a well rounded diet will ensure proper absorption of needed nutrients.
One of the best ways to ensure that we have proper nutrient intake is to eat a large variety of plant based organic(when possible) foods. Look for fresh grown produce as foods lose their nutrient value from storage as well as from cooking. Local is always a great way to go as it is probably as fresh and nutrient dense as it can be. If you still don't think you have adequate intake, consider a quality multivitamin supplement. (talk to your health care provider)
I wish you all the best of health on your journey with good nutrition.
Nutritional Sciences-Michelle McGuire, The Real Vitamin and Mineral Book-Shari Lieberman, Textbook of Natural Medicine, Hawthorn University Lecture