Today we are introducing the fat soluble vitamins: A, D, E, and K.
They are so called because they are generally absorbed in fat globules. Once they are absorbed and stored in the tissue, fat soluble vitamins tend to stay there. This is why toxicity is seen more often with excessive consumption of fat soluble vitamins when compared to water soluble vitamins. This is also why it is important to eat these vitamins with a fat; sometimes diets that are too low in fat can limit absorption of these vitamins.
Toxicity typically only occurs with supplementation and not from over consumption foods containing these vitamins. However, this is not always the case. Furthermore, supplementation can sometimes be important if there is a known deficiency in the diet and in the body.
Each of these vitamins has a variety of functions; the body is pretty efficient at using things in multiple ways. Below are just a few of the ways that we use the fat soluble vitamins and where we can find them in our food.
Vitamin A, also known as retinol, is found in the highest amounts in liver and eggs. Provitamin A (i.e., carotenoids like beta carotene, which are found in fruit and leafy greens) is a precursor to vitamin A. The body can convert provitamin A with some efficiency into the active form; however, there is some evidence that even a diet very high in provitamin A (and little to no vitamin A) can lead to deficiencies in some people as they cannot convert provitamin A effectively enough.
Vitamin A is important for our vision, helping cells to differentiate, gene expression, growth, reproductive processes, and some immune function, among other things. The ancient Egyptians knew that liver could help with night blindness.
Vitamin D is known as the ‘sunshine vitamin’ because we can create it from sun exposure (UVB rays) and cholesterol. Dietary sources are mostly of animal origin and include liver, egg yolks, and fatty fish. Some foods are now fortified with vitamin D, including milk, cereals, and some orange juices.
Though we can produce vitamin D from the sun, it is important to remember that foods rich in vitamin D (or proper supplementation) becomes important for populations living in places with little sunlight or prolonged periods without sunlight. Those who live far enough from the equator may have trouble synthesizing enough vitamin D due to lower amounts of UVB rays during the autumn, winter, and spring seasons. In North America, this is anyone who lives north of the 37th parallel, or above the line drawn across the continent from the northern border of Arizona to the northern border of South Carolina.
Vitamin D is used to help maintain proper calcium metabolism and bone health, help with cell differentiation and growth, and aid the immune system. There are also studies showing that men who are deficient in vitamin D and have low testosterone can improve the status of both with vitamin D supplementation.
Vitamin E can be found in both animal and plant foods, though plant oils tend to be the richest source. The primary function of vitamin E is maintaining membrane health in the body’s cells. It does this by acting as an anti-oxidant of the unsaturated fatty acids that help to form the membranes.
It may also play a role in preventing the oxidation of LDL (bad cholesterol) and help maintain heart health. It may benefit those with type 2 diabetes by improving metabolic control, and it may act as an anti-inflammatory. Excessive vitamin E intake may inhibit both provitamin A and vitamin K absorption.
Vitamin K is the last of the fat soluble vitamins. It is found in the greatest amounts in leafy green vegetables and some other plants, although animal products can provide a small amount. Additionally, our gut bacteria can provide a good source of vitamin K when they are fed properly (think plants and fiber).
The two main functions of vitamin K are assisting with blood clotting and keeping calcium where it is supposed to be and out of the arteries, where calcium does not belong.
In the next post we will take a look at the water soluble vitamins: the B complex and vitamin C.