I've been getting sick (again), and what I've always reached for, in addition to salt water and ginger honey tea, is "Emergen-C", a vitamin C powder.
Now we're not going to talk about how vitamin C actually has not been shown to improve cold symptoms (take zinc supplements instead), but we're going to talk about the popularity and necessity of vitamins/supplements.
Absorption of Vitamins/Supplements:
Your body can only absorb a limited amount of each micronutrient (vitamins, minerals, trace elements that all have a known function in your body).
One little packet of this vitamin C powder (one serving) contains 1000 mg of vitamin C - guess what? Your body can only absorb 250 mg, and 2000 mg is the upper limit (maximum amount you can consume without experiencing adverse health effects, hence the manufacturer's instructions to only consume one packet per day to eliminate the risk of exceeding the upper limit!).
Side note: the second ingredient in Emergen-C is fructose! I'm unsure about the source of this fructose - extracted from fruits since the powder is fruit flavored or from corn?
Where does the excess 750 mg go?
My brother was a big fan of The Big Bang Theory, and I remember watching this episode when I was flying to LA a few years ago. Sheldon Cooper is absolutely right about your body only being able to absorb so much of a micronutrient, and partially right about expensive urine.
We discussed about absorption, but let's talk about the expensive urine part.
The excess of any micronutrient is either stored in the liver (fat-soluble micronutrients, such as vitamins A, D, E, K, iron) or excreted due to a lack of a storage system (water -soluble micronutrients, such as the B vitamins and vitamin C).
So Sheldon was partially right - he addressed the water soluble vitamins spot on, but not the fat soluble vitamins. The fat soluble vitamins, in chronic excess, can potentially cause a variety of health issues. There's no need to worry about overconsuming vitamins when you're eating real food; excess consumption is a concern when you take vitamins and supplements. Natural compounds in real foods (ex: phytates), cooking methods, and other micronutrients can inhibit the absorption of a micronutrient - this is your body's natural mechanism to balance your micronutrient absorption!
Many vitamins must also be converted into absorbable/usable forms during digestion, which further limits the amount that is truly absorbed. On the other hand, synthetic forms of micronutrients, namely vitamins & supplements, are much more easily absorbed because they're often sold in the form that's absorbable.
Manufacturers pack in as much product as possible to make money, but your body only needs a certain amount. With water soluble micronutrients, if each tablet contains an amount that's beyond your body's absorbable amount, then you excrete the excess in your urine -- you're paying for expensive urine! With fat soluble micronutrients, you risk the health effects that come with overconsumption of the micronutrient(s).
So the bottom line - as long as you're eating a variety of whole foods that are prepared in diverse ways (raw and cooked - believe it or not, some nutrients are more bioavailable when the food source has been cooked!), you don't need to take vitamins/supplements!
Now there are two main exceptions: vitamin B12 and vitamin D.
Vitamin B12: the individuals at concern for a B12 deficiency are older adults, anemics, individuals with GI (gastrointestinal) disorders, and vegetarians/vegans. B12 is a crucial micronutrient (important in DNA synthesis, red blood cell formation, myelin sheath replenishment, energy metabolism!), so an adequate intake is of utmost importance.
First of all, I've been taught that there aren't really any natural, plant sources of B12 -- I have to take a closer look into this, because I do know that B12 is found in nutritional yeast (any dairy-free eater's/vegan's best friend). But aside from nutritional yeast, plant based sources are all fortified (not natural). Meat eaters/pescetarians do not have to worry because B12 is found in shellfish/seafood, liver, meat, and milk. Those who eat 100% plant based diets need to supplement B12 in their diet.
Second of all, natural forms ("protein-bound" forms) of B12 is absorbed by your body by the help of "R proteins" and special enzymes during digestion. Those who lack R proteins and these enzymes (older adults, anemics, individuals with GI disorders) should take B12 supplements as well.
Vitamin D: Many many people are deficient in vitamin D because there aren't very many naturally occurring food sources that adequately meet your body's needs. The most effective source of vitamin D is the sun, and most people do not get enough sun exposure or their bodies don't effectively convert the sun's vitamin D into your body's usable form of vitamin D (mostly "calcitriol"). The best way to tell whether or not you're vitamin D deficient is to ask your primary doctor to run a blood test.