Welcome to Part 2 of the Ultimate Vitamin Guide!

In the second part of my guide, I go into more detail about what fat-soluble vitamins are and their:
(1) food sources
(2) functions
(3) synthetic names to avoid

If you missed the first part where I covered: background information on vitamins, the different types of vitamin supplements, and what vitamins to take, you can read about it here.

Stay tuned for Part 3 to get all the important facts for water-soluble vitamins.

What are Fat-Soluble Vitamins?

The term “fat-soluble” refers to the ability of a substance to dissolve in fats (also known as lipids).

Thus, fat-soluble vitamins are found in the lipid layers of animals and vegetables, and are also stored in body tissues. The four fat-soluble vitamins are: A, D, E, and K.

Given a few exceptions, our bodies cannot produce vitamins – therefore we must obtain them from our diet. Being able to store these fat-soluble vitamins means that we can go on for a longer period of time without having to obtain them from food.

The caveat, however, is that there is a higher risk of toxicity that can occur if high dosages of these vitamins are taken on a regular basis. For this reason, caution must be taken if supplementing for fat-soluble vitamins.

Vitamin A

Vitamin A is available in foods in two different forms(1) as a precursor molecule, which then gets converted into vitamin A by the body; or (2) in a form that is usable right away.

The precursor molecule (also called provitamin A) can exist in a number of similar, but distinct forms belonging to the carotenoid family. Although there are various types of carotenoids, beta-carotene is the most important and the most studied.

In contrast, the second, more usable form of vitamin A belongs to the retinoid family. The three main chemicals of this group that are used by the body are retinol (the most usable), retinal, and retinoic acid. This group of compounds is also commonly called preformed vitamin A.


Vitamin A has a number of important functions in the body:

(1) Eyesight

Vitamin A (in the form of retinol) is needed for the formation of rhodopsin – a pigment in the eye that aids in nighttime vision. Another useful function related to the eye is this vitamin’s role in maintaining cornea health. The cornea is the protective layer over the eye. A deficiency in vitamin A can result in increased vulnerability to eye irritation and inflammation.

(2) Tissue growth and healing

When new tissue cells are made, it is important for them to attach together to form proper-functioning tissue. This process of attachment is mediated by special proteins called glycoproteins. Vitamin A is required to produce glycoproteins, thus it has an important role in ensuring proper tissue repair. For this reason, vitamin A is an important nutrient to have in the diet when recovering from an injury or surgery.

(3) Keeping skin healthy

The skin is the body’s largest organ, and is essential in protecting the body against infectious agents and environmental pollutants. Vitamin A helps the skin keep its structural integrity and ensures that the skin cells mature and grow normally. In turn, this helps the skin do its job in protecting the body. Vitamin A also plays this vital supportive role in the linings of the lungs, nose, throat, stomach, intestines, and eyes.

(4) Antioxidant properties

Free radicals are unstable molecules that are a product of a number of metabolic and biochemical reactions in the body. Because free radicals are damaging to cells and tissues, there are various mechanisms in place to protect the body against the damaging effects. Vitamin A and beta-carotene play critical roles in neutralizing free radicals to safeguard cell membranes and tissue linings.

(5) Supporting the immune system

The retinol form of vitamin A has two important roles related to immune system function: (1) optimizing the function of white blood cells, and (2) blocking the activity of certain viruses. There is also evidence that beta-carotene enhances immune function, though the exact mechanism is still unclear. These immune-supporting functions of vitamin A reduce the risk of cancer. Additionally, cancer risk is further reduced as a result of this vitamin’s support in structural cell integrity and in proper cell differentiation (mentioned above in #3).


The ready-to-use form of vitamin A is mainly found in animal sources such as fish oils, liver, egg yolks, milk, and other milk products.

In contrast, beta-carotene is found in leafy greens, as well as yellow- and orange-coloured fruits and vegetables such as carrots, squash, peppers, and cantaloupe.

Synthetic Names to Avoid

The palmitate and acetate forms of vitamin A should be avoided. If taken in high doses, these synthetic forms have a greater potential to produce toxic symptoms such as pressure headaches, irritability, dizziness, and hair loss.

The Bottom Line: Vitamin A

Vitamin A plays a number of important roles in the body:

  • maintains good eyesight
  • aids in tissue growth and healing
  • keeps the skin healthy and protective
  • neutralizes free radicals, preventing damage of the cell membrane and tissue lining supports the immune system

The two main sources of vitamin A are:
1) Animal products such as liver, egg yolks, and milk are rich in preformed vitamin A
2) Leafy greens, and yellow- and orange-coloured vegetables and fruit provide a good source of beta-carotene, a precursor molecule that the body can convert to vitamin A

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Vitamin D

Also known as the “sunshine” vitamin, vitamin D refers to a group of cholesterol-like substances that can be manufactured with sunlight exposure.

The two main types of vitamin D come from (1) plants and (2) animals.

The plant form of this vitamin – also known as D2 – is made from ergosterol.

The animal-derived vitamin D – called D3 – is made from cholesterol.

Ergosterol and cholesterol are similar in structure, however the cholesterol-based vitamin (D3) is the preferred form for optimal health in humans.


Vitamin D has two major functions:
1. regulation of calcium metabolism
2. ensuring normal calcification of the bone

It fulfills these functions by:
➙ increasing the absorption of calcium from the gut
➙ decreasing calcium’s excretion from the kidneys
➙ aiding in the proper deposit of calcium and phosphorus in teeth and bones, and also maintaining their levels in the blood


The main source of this fat-soluble vitamin is from the skin. When exposed to the ultraviolet rays from the sun, the body’s skin is able to manufacture and store vitamin D. Food sources of vitamin D include cod liver oil, egg yolks, butter, liver, salmon, mackerel, and sardines.

Synthetic Names to Avoid

Dietary supplements of vitamin D are available in both the D2 (plant-derived) and D3 (animal-derived) forms. Also known by the name ergocalciferol, vitamin D2 is best avoided due to its ineffectiveness in the body compared to vitamin D3.

Studies have shown that D2 metabolites only weakly bind vitamin-D-interacting proteins in plasma compared to D3.

The breakdown of vitamin D (which occurs in the liver) produces a compound named calcidiol. Measuring calcidiol levels in the blood is a standard clinical test that is acknowledged to be the best indicator of vitamin D status. It has been shown that D2 is less efficient at increasing calcidiol levels compared to D3. If supplementing for vitamin D, make sure that it is in the D3 form, also known as cholecalciferol.

The Bottom Line: Vitamin D

Vitamin D plays an important role in:

  • controlling calcium absorption and excretion
  • ensuring proper calcium and phosphorus levels in the blood and bones

These tasks are vital in maintaining strong and healthy bones and muscles (which also rely on calcium levels).

The best way of obtaining sufficient amounts of vitamin D is through natural sunlight exposure. For those living in colder climates, vitamin D can be maintained through regular consumption of animal-based sources, or with high-quality vitamin D3 supplements.

Vitamin E

Vitamin E refers to a group of fat-soluble compounds known as tocopherols and tocotrienols. Of these two groups, tocopherols are much more studied and better understood, with four identified as having vitamin activity. These four tocopherols are alpha, beta, gamma, and delta, with alpha-tocopherol being the most biologically active.


Perhaps the most well-known and important function of vitamin E is its role as an antioxidant. In other words, protecting cells and tissues from the damaging effects of free radicals.

More specifically, vitamin E protects the lipids in cell membranes against these highly reactive and detrimental molecules. This action of damage to cell membrane lipids mediated by free radicals is scientifically termed as lipid peroxidation. Together with selenium (an essential mineral), vitamin E has a synergistic effect in protecting biological cell membranes from peroxidation.

The alpha-tocopherol form of vitamin E also plays an important role in cell signaling. It modulates major signaling pathways that are involved with:
cell proliferation
platelet aggregation (i.e., formation of clots to stop bleeding)
NADPH oxidase activation

NADPH oxidase is an enzyme found in neutrophils (the most abundant type of white blood cell), and has an important role in killing bacteria and fungi.

Additionally, gamma-tocopherol is involved in protecting cells by trapping a highly reactive mutagen known as peroxynitrite.


The best sources of vitamin E are from nuts and seeds. Vitamin E can also be found in the protective layer or germ part of grains, however it is easily lost during the refinement of grains. Vegetables that include a decent amount of vitamin E are spinach, asparagus, kale, and cucumber.

Synthetic Names to Avoid

The synthetic version of vitamin E contains eight variations of the compound that differ by their physical structure. Only one of these eight variations is the natural form, and thus the most biologically active. Supplements that contain a mixture of the all the compound variations are labeled all-rac vitamin E, dl-tocopherol, or dl-tocopherol acetate, and should be avoided.

The Bottom Line – Vitamin E

Vitamin E has two main important functions:
1. as an antioxidant: protecting cell membranes from peroxidation
2. regulating cell signaling pathways: one of which involves supporting white blood cell antimicrobial function

The most biologically active form of vitamin E is alpha-tocopherol, though three other forms (beta, gamma, and delta) also have vitamin activity.

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Vitamin K

The last fat-soluble vitamin is vitamin K. This nutrient is found in plants and animal food sources, and is also produced by our intestinal bacteria. The vitamin found in plants is known as vitamin K1 or phylloquinone, whereas in animals it is called vitamin K2 or menaquinone. Both of these forms have the same function and can be used by the body.


The main function of vitamin K is to support blood clotting (also known as coagulation), which helps stop the bleeding from a wound. There are several proteins involved in this process, including prothrombin. Vitamin K is required to synthesize prothrombin, and it also helps convert prothrombin to thrombin, another protein involved in coagulation.

Vitamin K is also implicated in maintaining bone health, and has been shown to reduce the risk of osteoporotic bone fractures.


Vitamin K1 is mostly found in dark leafy greens such as spinach and kale, as well as in alfalfa, and kelp.

Vitamin K2 can be obtained from liver, milk, yogurt, egg yolks, and fish liver oils. The best source of K2 is from our own bodies. By supporting intestinal bacteria with probiotics (such as yogurt and kefir), vitamin K2 can be sustained at optimal levels. It is also important to support intestinal bacteria by minimizing the use of antibiotics and by avoiding the intake of excess sugar and processed foods.

Synthetic Names to Avoid

Menadione, also known as vitamin K3, is the synthetic form of vitamin K. A recent study showed that menadione induces the formation of reactive oxygen species, which are free radicals containing oxygen. As discussed previously, free radicals are detrimental to cells and tissues.

The Bottom Line – Vitamin K

Vitamin K plays an important role in:
(1) normal blood clotting
(2) supporting bone health

The K1 form is obtained from plant sources such as dark leafy greens, alfalfa, and kelp.

The K2 version of this vitamin is produced by the body’s friendly intestinal bacteria and can also be consumed from animal sources such as liver, milk products, and eggs.

Conclusion – Fat-Soluble Vitamins

Vitamins play an integral role in many diverse biological functions. It is important to strive to obtain these nutrients from the diet and avoid synthetic supplements, which simply do not function properly in the body.

Stay tuned for part 3 of this guide, where I will go into more detail on the water-soluble vitamins.

Thanks for reading!


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