Editor: The following is an email from February 2012 that I wrote to a few friends after a discussion about the need for multivitamins. I have been meaning to post it for a while and the recent new research on vitamins is a great excuse to do so. The recent research does not address the primary rationale for taking a multivitamin, which is as a nutritional "insurance policy", not as a way to reduce risk of death or cancer. I do agree that taking individual vitamins or supplements without a medical justification (i.e. deficiency) is unwise. One friend was later diagnosed as deficient in Vitamin D by her doctor.
I think there are four nutrients with related research which suggests a particularly good medical rationale to take a daily multivitamin, even among those whose diet is very healthy and are asymptomatic. Just posting some quick quotes and links here rather than editorializing:
1. Folate (B9)
- RDA(^) for women age 19-30: 320mcg. Amount provided in a multivitamin(+): 400mcg. Tolerable upper limit(*): 1000mcg.
- "In view of evidence linking folate intake with neural tube defects in the fetus, it is recommended that all women capable of becoming pregnant consume 400 μg [mcg] from supplements or fortified foods in addition to intake of food folate from a varied diet." [1, page 2]
- "Timing of folate is critical: For folate to be effective, it must be taken in the first few weeks after conception, often before a woman knows she is pregnant." 
- "Those who drink may benefit the most from getting extra folate, since alcohol moderately depletes our body’s stores." 
2. Vitamin B12 (if you are not eating meat regularly)
- RDA for women age 19-30: 2.4mcg. Amount provided in a multivitamin: 6mcg. Tolerable upper limit: unknown due to lack of adverse effects.
- "Some people who eat little or no animal foods such as vegetarians and vegans. Only animal foods have vitamin B12 naturally. When pregnant women and women who breastfeed their babies are strict vegetarians or vegans, their babies might also not get enough vitamin B12." 
- "But even vegetarians who eat eggs and dairy products consume, on average, less than half the adult Recommended Dietary Allowance of 2.4 mcg of B12, notes the Health Letter."… "The Harvard Health Letter recommends that vegetarians and older people with atrophic gastritis take a multivitamin, eat fortified breakfast cereal, or both." 
- "It is prudent to advise all vegetarian and vegan patients, particularly if they are elderly or anticipating a pregnancy, to consume synthetic cobalamin daily, either by taking a supplement containing vitamin B12 or eating a serving of vitamin B12–fortified grain products." 
- RDA for women age 19-30: 18mg [for men it's only 8mg]. Amount provided in a multivitamin: 18mg. Tolerable upper limit: 45mg.
- "Iron deficiency is the most common nutritional disorder affecting about 20-25% of the world's population, predominantly children and women. There is emerging evidence that depletion of iron stores may have adverse consequences for adults even in the absence of anaemia."
- 9% of US women age 20-49 have iron deficiency. 
- "Iron from meat, poultry, and fish (i.e., heme iron) is absorbed two to three times more efficiently than iron from plants (i.e., non-heme iron)." 
4. Vitamin D
- RDA for women age 19-30: 15mcg (600 IU). Amount provided in a multivitamin: 1000 IU (25mcg). Tolerable upper limit: 100mcg (4000 IU).
- "In adults, vitamin D deficiency leads to osteomalacia, causing bone pain and muscle weakness." 
- Note: There is not consistent evidence of a link between Vitamin D and MS. [7, pages 173-174]
So, that is my rushed personal case for a generally recommended daily multivitamin for women. It was fun to research since I was analyzing the nutrients of my own multivitamin earlier today anyway. There are lots of good multivitamin summary articles as well, such as http://www.hsph.harvard.edu/nutritionsource/multivitamin/ -- thinking of a multivitamin as a "nutrition insurance policy". But at the end of the day you're only as healthy as you feel, as they say in Taxi Driver, and I am the one who's half-deaf [I was suffering from mold allergies at the time].
^ "Recommended Dietary Allowance": Meets or exceeds the daily dietary requirements for 97.5% of the US population.
+ Reference multivitamin: Bayer One-A-Day Women's Multivitamin. http://www.walgreens.com/store/c/one-a-day-women%27s-multivitamin/multim...
* "Tolerable upper limit": A Tolerable Upper Intake Level (UL) is the highest level of daily nutrient intake that is likely to pose no risk of adverse health effects to almost all individuals in the general population. (Dietary Reference Intakes (DRIs): Estimated Average Requirements. Food and Nutrition Board, Institute of Medicine, National Academies)
1. Recommended Intakes for Individuals -- http://fnic.nal.usda.gov/dietary-guidance/dietary-reference-intakes/dri-...
7. Dietary Reference Intakes for Calcium and Vitamin D, IOM 2010. http://www.nap.edu/catalog.php?record_id=13050