The debate about the use (or danger) of synthetic vitamins has been going on for years, and depending on which vitamin we are talking about: there may be a difference, or none at all, in how your body deals with it. To clarify, a vitamin is a molecule that your body cannot make itself and has to get from outside.
Before we discuss this topic, it is important to mention that fruit and vegetables contain more than just vitamins (and frequently contain many different, or all, vitamins in varying doses), they also contain different micronutrients and all amino acids (the building blocks of proteins which themselves are the building blocks of all life).
Plants, unlike animals, do not (generally) consume other life forms for energy but instead use light, and thus need to be able to create all amino acids on their own. The amino acids we cannot create on our own (de novo) are called “essential amino acids,” and we have no choice except to get them through our food. In general, a well-rounded diet is always preferable to supplements, but not because the supplements are toxic (at the right dose).
If you want to know which synthetic vitamin you likely shouldn’t take, our first stop is vitamin A . Preformed vitamin A can, in high doses, display a significant toxicity, which is not associated with beta-carotene. Beta-carotene, a precursor of vitamin A, is available in many foods (especially those that are orange), and vitamin A is particularly concentrated in the liver of all mammals. A vitamin A overdose, whether from supplements or too much liver, has toxic consequences on your body. The use of genetic modification (like with golden rice or the new beta-carotene bananas), has sought to increase availability of natural sources of beta-carotene (essentially vitamin A), but opposition of genetic modification in general has complicated this process.
Vitamin C, also known as ascorbic acid, has been shown equally bioavailable (absorbable) from fruits and veggies as from synthetic sources (Carr & Visser, 2013). It is important to note that the only form we can readily absorb is L-ascorbic acid, which is molecularly identical in fruit as it is in supplements. At doses over 200 mg, your body is able to absorb less of the vitamin C, so several smaller doses is preferable to taking a lot at once, and it is safe to consume up to 2000 mg in a day. Vitamin C is an important co-factor in many cellular processes, and is important to immune function. A lack of vitamin C leads to “scurvy” which was a common disease among old-world sailors.
Vitamin E is not as bioavailable in synthetic form as from natural sources. It is important for immune function as well as DNA expression, and can be found in many foods such as seeds and nuts. Dorea Reeser at Scientific American sums up the comparison of synthetic and natural sources well, writing;
“naturally derived Vitamin E is called d-α-tocopherol and synthetic Vitamin E is called dl-α -tocopherol. The difference between the two is that the “dl” refers to a mixture of both d- and l-α-tocopherol. There is no evidence that the “l” version is harmful to the human body at all, but it is about 1.4 times less effective than naturally derived Vitamin E”
Vitamin K is actually a few different molecules, and a normal diet and healthy gut flora should be enough to prevent you from ever needing a supplement (which was concluded from the analysis of the livers of dead patients who had been taking antimicrobials or not). It is necessary for blood clotting (which can be interfered with by too much vitamin E), and can be found in avacadoes and brocoli.
Vitamin B6 is important for nerve function and coordination, and like all B vitamins your body is unable to store it (meaning you need to renew your supply frequently). It is recommended to intake only about a milligram a day, so supplements run the risk of quickly overstepping the limits. It is obtained easily from eating bananas, nuts, and whole grains. Many sources that contain vitamin B6 also contain B12.
Vitamin B12 is an important co-factor for DNA regulation (methylation and transcription, necessary for methionine synthesis, which is the first amino acid in essentially all proteins). You don’t need much, and eating a variety of different fruits and vegetables should fulfill your needs easily. Synthetic vitamin B12 at 1 microgram (again, make sure you are not getting too much of anything: pharmacology is toxicology in lower doses) is only approximately 56% absorbable.
The other B vitamins (B1: thiamin, B2: riboflavin, and B3: niacin) can all be found in dairy products and nuts. You need about 1 mg a day for thiamin and riboflavin, but a little more than 10 mg a day of niacin. Together these vitamins are very important to your metabolism (your body’s ability to transform food into energy, so to speak). Also in the group of B vitamins which your body cannot store are pantothenic acid (B5) and biotin (B7), which are sensitive to both heat and oxygen, making them also difficult to store as supplements. Like the other B vitamins, pantothenic acid and biotin can be obtained from legumes, and dairy (among other sources).
The last essential vitamin on our list is folate (folic acid or B9), which is important to DNA replication and cell division. Humans are not commonly deficient in folate, and it can be obtained from a wide variety of foods (almost all fruits and vegetables). Its relevance in cell division makes it very important for pregnant women, although a healthy diet should warrant any supplements unnecessary.
I deliberately neglected to discuss vitamin D in this article, but primarily because people tend to know about it (click the hyperlink if you don’t, though).
The take home message is that you need lots of things, including many different minerals and micronutrients not discussed within this article. Although in most instances synthetic versions of natural vitamins are, well, exactly the same, several vitamins are absorbed more readily in their natural state (or do not keep well in storage). Taking a multivitamin can put you over the healthy threshhold for vitamins you are already getting enough of in your diet.
Eating fresh fruits and vegetables, as well as those which are cooked (but keep in mind some vitamins are heat-sensitive), as part of a varied diet, is necessary for good health. This is both due directly to your own body’s functioning, as well as to the symbiotic (and commensalistic) bacteria living in your gut.