Over the course of the next few weeks we will be taking a deep dive into all of the essential vitamins and minerals. The series will focus on the role that each plays in the body and the foods that provide them. The first up is Vitamin A.
Vitamin A is involved in immune function, vision, reproduction and cellular communication. It plays a critical part in vision as it makes up a part of the protein rhodopsin which absorbs light in the retinal receptors. It also supports the normal differentiation and functioning of conjunctival membranes and the cornea. Another important function of Vitamin A is the role it plays in cell growth and differentiation, playing an integral role in the formation and maintenance of the heart, lungs, kidneys and other organs.
There are two forms of Vitamin A that are available in the diet, preformed Vitamin A and pro-Vitamin A carotenoids. Preformed Vitamin A is found in animal sources including dairy, eggs, fish and meat(especially liver). Pro-Vitamin A carotenoids are alpha-carotene, beta-carotene and beta-crytoxanthin with beta-carotene being the most important by far. The body converts these plant pigments into Vitamin A. Other carotenoids that are found in food such as lycopene, lutein and zeaxanthin are beneficial but are not converted into Vitamin A.
Preformed Vitamin A concentrations are highest in liver and fish oil; eggs and dairy products are good sources as well. Dietary pro-Vitamin A comes from leafy greens, orange and yellow vegetables, tomato products, fruits, and some types of vegetable oil. The top food sources for providing Vitamin A in the typical American diet are dairy products, liver, fish and fortified cereals. The most common sources of pro-Vitamin A in the American diet are carrots, broccoli, cantaloupe and squash.
Vitamin A has been shown to be beneficial in many different circumstances. Observational studies have shown that higher intake of carotenoids in the form of fruits and vegetables are associated with a lower risk of lung cancer. This link is only with carotenoids that are obtained from the diet, there are no signs that supplemental Vitamin A provides a similar effect. Carotenoids also have anti-oxidant properties that can aid in the alleviation of the cumulative effects of oxidative stress that can lead to age-related macular degeneration. Finally, Vitamin A deficiency is a known risk factor for severe measles. The World Health Organization recommends administering a large oral dose of Vitamin A to children over the age of one when they contract measles in an area where Vitamin A deficiency is prevalent. Studies have shown that these high doses of Vitamin A lead to a reduced mortality rate in children who have measles, as well as for those with pneumonia.