Vitamins, Explained.

Good morning!

Aside from the fact that they show up on food labels, supplement bottles, and skincare products, would you be able to tell someone what a vitamin is, what it does, and why they show up in both cereal boxes and in medication? That’s what this week’s newsletter is all about. Without further ado, let’s dive into the vitamin newsletter.

Key Takeaways

• Vitamins are essential organic compounds your body needs in small amounts, with 13 recognized vitamins that each perform specific biological roles.

• Vitamins are either fat-soluble (A, D, E, K) or water-soluble (C and the B vitamins), a distinction that affects how they’re absorbed, stored, excreted, and how excess intake can affect you.

• Vitamins can be added to foods through enrichment or fortification to help prevent deficiencies, and the same nutrients can be used in supplements or medications to address specific nutritional needs.

CORE

What a Vitamin Is

A vitamin is an organic compound your body needs in small amounts to keep normal biological processes running. The catch? Your body either can't make it at all or can't reliably make enough of it on its own. So, for the most part, you have to get it from food.

There are 13 recognized vitamins: A, C, D, E, K, and the eight B vitamins. Yes, eight. B vitamins apparently decided one letter wasn't enough and brought an entire committee. Each has specific biological roles that are well-established, and many vitamins participate in overlapping pathways rather than working in complete isolation.

Vitamins don't provide energy or serve as structural building blocks. Instead, they help run the machinery that keeps you alive, enabling the biochemical reactions involved in extracting energy from food, building and repairing tissue, supporting immune function, synthesizing hormones, and keeping your nervous system working properly. And while you don't need much of any given vitamin, "small amount" does not mean "unimportant." When a vitamin is missing, the specific processes that depend on it can slow down or stop, and another vitamin can't simply step in and do the same job.

How They’re Categorized

Every vitamin is classified as either fat-soluble or water-soluble. The difference affects how your body absorbs, stores, and gets rid of them, which in turn affects how quickly deficiencies can develop and how easily excess can build up.

Fat-soluble vitamins are A, D, E, and K. Because they dissolve in fat, your body needs dietary fat and bile to absorb them in the small intestine. From there, they're packaged up with dietary fat, transported through the lymphatic system, and eventually stored primarily in the liver and fatty tissues.

Think of these as the vitamins your body can put in the pantry for later. If your intake drops for a while, your body can dip into those reserves, so deficiencies generally take longer to develop.

But having a pantry comes with a downside. The body can't simply flush out large amounts of fat-soluble vitamins when you've had too much. They can accumulate over time, which is why consistently taking high doses can cause toxicity.

Water-soluble vitamins are vitamin C and all eight B vitamins. These dissolve in water and are absorbed into the bloodstream from the small intestine. Most aren't stored in large amounts, so your body generally has less of a reserve to fall back on. Excess is filtered by the kidneys and excreted in urine.

That makes deficiencies generally quicker to develop when intake is inadequate, and toxicity is less common than with fat-soluble vitamins. But "water-soluble" does not mean "you can take as much as you want because you'll just pee it out."

Vitamin B6 is a good example. Taking high supplemental doses for long enough can cause peripheral neuropathy, meaning nerve damage that can lead to symptoms like numbness and tingling. Vitamin C also has an upper intake limit, with excessive amounts potentially causing gastrointestinal problems and increasing kidney stone risk in susceptible people.

What Each Group Does

So what are all these vitamins actually doing?

Fat-soluble vitamins tend to have broader regulatory and protective roles.

Vitamin A helps your vision by making rhodopsin (a pigment in the retina that lets you see in the dark), protecting the cornea, and helping the body produce enough moisture and tears. It also supports healthy skin and the tissues lining your respiratory and digestive tracts. It also plays important roles in immune function and cell development.

Vitamin D helps your body absorb calcium and phosphate from the gut, making it essential for maintaining healthy bones. It also plays roles in muscle and immune function, although research into its other potential effects is still ongoing and the evidence isn't equally strong for every claim you might see online.

Vitamin E is best known as an antioxidant. It helps protect cell membranes, particularly the fatty components of those membranes, from oxidative damage.

Vitamin K is essential for blood clotting. Vitamin K2 has also attracted attention for its potential role in where calcium ends up in the body, including whether it accumulates in blood vessels. That's an interesting area of research, but we're not at the point where K2 can be called a proven treatment for arterial calcification or cardiovascular disease.

Then we have the water-soluble vitamins. Many of the B vitamins work as coenzymes, essentially helping enzymes carry out chemical reactions that keep your metabolism running.

B1, B2, B3, B5, B6, and B7 all participate in processes that help your body extract and use energy from carbohydrates, fats, and proteins. They're not giving you energy themselves, but they help your body turn the food you eat into something it can actually use.

B9 (folate) and B12 are especially important for DNA synthesis and cell division. Folate is particularly critical during early pregnancy, when adequate intake helps prevent neural tube defects. B12 also plays an important role in maintaining healthy red blood cells and the nervous system.

Vitamin C is essential for making collagen, the protein that gives structure to skin, tendons, blood vessels, and bone. It also helps your body absorb non-heme iron from plant foods and acts as an antioxidant in body fluids.

The important thing to remember is that vitamins aren't interchangeable. Your body doesn't have a backup vitamin waiting on the bench if one runs short. Each has its own jobs, and some of those jobs are surprisingly specific.

The Vitamins Added to Your Food

Vitamins appear on food labels in two ways that are worth distinguishing.

Enrichment means replacing nutrients lost during processing. Milling wheat into white flour removes the outer bran and germ layers along with most of the naturally occurring B vitamins and iron. Enrichment adds those back to approximately the levels present before processing. In the US, enriched flour is legally required to contain specified amounts of thiamine, riboflavin, niacin, folic acid, and iron. 

Fortification means adding nutrients that may be lacking in the diet, not necessarily because processing removed them. Vitamin D added to milk is the most familiar example. Vitamin D doesn't occur naturally in milk in meaningful amounts. Its addition, which became widespread in the 1930s, was specifically aimed at addressing rickets, a severe vitamin D deficiency disease that was widespread at the time. 

Today, food fortification operates at a significant scale. Wheat flour, maize flour, rice, cooking oil, salt, and dairy products are the most common vehicles globally. The vitamins most frequently added are A, D, B1, B2, B3, B9, and B12, targeting deficiencies most prevalent in each population. Some fortification is mandatory by law; some is voluntary. 

One technical challenge in fortification is stability. Fat-soluble vitamins in particular degrade during storage, cooking, and light exposure. Some manufacturers may add more than the target amount to account for losses between production and consumption. Water-soluble vitamins can leach into cooking water during boiling, which affects how much of the labeled amount actually reaches the person eating the food.

ENDNOTE

Final Thoughts

So why do vitamins show up in both cereal boxes and medication? Because they’re essential to keeping the body running, and they can be useful in very different contexts, from preventing deficiencies in everyday foods to treating specific deficiencies and medical conditions. We’ve only scratched the surface here. There’s a lot more to vitamins than what we could fit into one newsletter, from how they’re absorbed and activated to what happens when you get too little or too much.

Until next week!

Adrian Macdonald | Team Dietitian | The 8% Newsletter Author