Commercial production of vitamin C for fortification and dietary supplements

In most mammals, vitamin C is made in the liver from blood sugar using a four-step enzymatic process. Humans and about 20 other mammalian species lack one of these enzymes, requiring us to both obtain vitamin C from our diet and to sustain internal levels of vitamin C using a variety of other food antioxidants.

But rather than extracting vitamin C from foods, the modern way to manufacture it is to start with dextrose, a sugar identical to blood glucose, and manipulate it into ascorbic acid, which is vitamin C; mimicking the biological production technique to some extent. The result is a nature-identical vitamin indistinguishable from the form in foods.

The primary method of biosynthesizing vitamin C starts with the conversion of glucose into sorbitol. Dextrose is converted to sorbitol by adding hydrogen to the molecule; that is typically done with a catalyst (defined as an easily recoverable material that facilitates a chemical reaction but is not used up in the reaction nor added to the molecule). The only added element is hydrogen, which is applied in a pressure tank.

Converting the sorbitol into sorbose and then into an intermediate known as 2-KLG is done using a two-step fermentation process

Then a final lactonization step (reacting 2-KLG with acid and/or dehydration is used to produce the lactone known as l-ascorbic acid/vitamin C; chemical form C₆H₈O₆).

Natural versus Synthetic: Not Always Relevant

Synthetic processes are often irrelevant; the resulting form is the most important consideration. For example, most mammals synthesize ascorbic acid in their livers from blood sugar, so synthetic processes are essential in biological ascorbic acid production and all of their internally produced vitamin C is quite literally synthetically derived. This process is mimicked in commercial production.

This means that synthetic is not a synonym for unnatural; it depends on the process and the end result. If the compound is a synthetic form, such as dl-ascorbic acid, that is not so good. But if it results in a natural form, such as l-carnitine rather than d-carnitine, and mimics the synthesis of that substance in the body, our biology can’t distinguish between the internally and externally produced sources.

"Synthetic" loses its automatically negative connotation when considered in conjunction with these factors because we internally synthesize thousands of compounds every day and only the unnatural forms are presumed to be inherently negative when ingested.

How are vitamins C, D and K in supplements made?

Almost all D’s on the market are nature-identical synthesized forms. D3 and D2 are both naturally occurring in foods, but in supplements the same forms are usually from synthesized sources. So D3 synthesized from sheep lanolin is no more natural than D2 from plants or fungi. In fact, the D3 made in the skin-liver-kidneys from cholesterol and sunlight is also literally synthesized. Either plant sterols or animal sterols (cholesterol in humans, lanolin from sheep) are irradiated with UV-B light to make D2 or D3, respectively. It is a synthetic process, either internally or to produce the supplemental form; in much the same way that vitamin C can be synthesized by most mammals (not humans) from blood glucose in a process that mirrors the commercially synthesized nature-identical form. By the way, Vitamin K is also synthesized as the exact same form found in green foods (K1). Fermented foods like cheese can also contain K2 (MK-4), which is synthesized by microbes like bacteria.