Vitamins are classified as either water soluble or fat soluble. The water-soluble vitamins include all of the B vitamins and vitamin C. These vitamins dissolve in water and are not stored by the body—any excess is flushed out. Fat-soluble vitamins—A, D, E, and K— must be dissolved in fat before they enter the bloodstream and any excess is stored by the body in the liver and fat tissue; they are released as needed. Because the body has the ability to store the fat-soluble vitamins, it is erroneously assumed that we get plenty from our diet, thus there is no need to supplement and that supplementation could be toxic. However, for most people, this is simply not true. Case in point: vitamin D. Even though it is a fat-soluble vitamin and can be stored by the body, (and can even be made by the body) most people are lacking optimal levels and many are downright deficient.

The best sources of the fat-soluble vitamins (with the exception of vitamin E) are animal foods, especially fatty fish (and fish liver oils), organ meats, eggs from pastured hens, and full-fat dairy from grass fed cows. For most Americans, these foods rarely, if ever, make an appearance at mealtimes. This is especially true if you follow a low-fat, vegetarian, or vegan diet.

Why is it important to maintain optimal levels of the fat-soluble vitamins? Unlike the water-soluble vitamins, which act as "helper" molecules, or cofactors, to the proteins responsible for biochemical activity in the body, the fat-soluble vitamins act in a more fundamental way, actually influencing the production and activation of these proteins. Vitamins A and D specifically play important roles in gene expression, promoting the synthesis of certain proteins, while vitamin K’s role is to activate specific proteins.^{1 2} In other words, other vitamins and minerals have nothing to work with if the fat-soluble vitamins are lacking.

Chances are you've heard of at least some of the many health benefits of vitamin D. There are vitamin D receptors in almost every tissue in the human body, affecting the health of every body system. Vitamin D has been shown to play a role in bone and muscle health; cardiovascular health; brain health; and in a healthy immune response. High levels of vitamin D are also associated with a lower risk of a number of cancers, including breast, ovarian, prostate, colon, and lung. And low levels have been associated with everything from autoimmune diseases to dementia. Vitamin D has proven to be invaluable to good health.^{3 4 5}

Vitamin A is an umbrella term for a family of related compounds, the most common including retinol, retinal, and retinoic acid. There is a long-held misconception that beta carotene, found in vegetables, is the same as vitamin A. It is not. Beta carotene is the precursor to vitamin A. In a perfectly healthy body, some beta carotene can be converted to vitamin A, but very little, and the conversion is highly variable from person to person, with factors such as gut integrity, nutritional status, fat intake, disease states, and genetic variances influencing the conversion.^{6 7 8}

A sort of unsung hero in the nutrient world, vitamin A is responsible for so much (like vitamin D, there are vitamin A receptors in almost all tissues in the body), yet often left out of a good supplement routine. It is essential for fetal development and reproductive health; it is necessary to maintain the health of the skin and the mucous membranes that line the airways, the urinary tract, and the digestive tract; it is crucial for immune function; it is vital for eye health, including the proper development of the retina; it plays an important role in bone health; and it is important in healthy cell differentiation, the process in which stem cells mature into specific types of cells/tissue.^{9 10 11 12 13} Note: Because of the risk of birth defects at high doses, women of childbearing age should consume no more than 5,000 IU of vitamin A daily.

Like vitamin A, vitamin K is not a single vitamin, but a group of structurally similar, fat soluble vitamins that are necessary to activate proteins in the body that are essential for blood coagulation and calcium metabolism (directing calcium to the bones and teeth and away from soft tissues). For this reason, vitamin K plays a tremendous role in cardiovascular and bone health; it is also important in reproductive health; dental health; cancer prevention; and insulin regulation.^{14 15 16}

Like its cousins A and K, vitamin E is actually a group of fat-soluble nutrients including tocopherols and tocotrienols. But unlike the other fat-soluble vitamins, vitamin E’s primary role is that of an antioxidant. Considered to be the most potent fat-soluble antioxidant, vitamin E maintains the health of the cells by protecting polyunsaturated fatty acids in the cell membranes from oxidative damage. Vitamin E also plays a crucial role in reproductive health, influencing the release of the reproductive hormones from the pituitary gland, including follicle-stimulating hormone (FSH) and luteinizing hormone (LH), which control the menstrual cycle and ovulation in women and sperm production in men.¹⁷ There is preliminary evidence that suggests vitamin E also affects gene expression, including genes involved in immunity.^{18 19}

The fat-soluble vitamins A, D, E, and K have a uniquely synergistic relationship, balancing and enhancing each other, and as a group, they have a profound influence on human health. When you take more of one fat-soluble vitamin you create a greater need for the others. While there are no established guidelines for an optimal ratio of these important nutrients, the best approach is to take them in a healthy balance, through both a nutrient-dense traditional foods diet and smart supplementation.

References available upon request