The Fat-Soluble Vitamins: A, D, E and K
Fat-soluble vitamins are essential for human life. They include vitamin A (retinol), vitamin D3 (cholecalciferol) and vitamin E (tocopherol). Vitamin C plays an important role in cell metabolism. These three nutrients play a major role in maintaining healthy skin, hair and nails.
Vitamin A, which is known as retinal, helps with vision. It protects against eye diseases such as macular degeneration and cataracts. Vitamin D3 helps regulate calcium levels in the body. It also plays a vital role in bone growth and development. Vitamin E has antioxidant properties that protect cells from damage caused by free radicals or reactive oxygen species (ROS).
These ROS can cause cancer, heart disease, diabetes mellitus and other illnesses.
In addition to these three vitamins, there are many other substances found in food that have beneficial effects on health. For example, certain amino acids found in meat and fish may prevent cancer. Certain minerals like zinc and selenium may reduce the risk of developing certain cancers. Some dietary supplements contain compounds that act similarly to vitamin A, but without any side effects.
Vitamin A, also called retinol, is a fat-soluble vitamin required by all higher life forms. It is used by the body mainly to keep the skin, hair, mucus membranes, and eyes healthy and functional. It also plays a role in bone growth, cell reproduction, and cell differentiation.
Vitamin A is found in many fruits, vegetables and animal products like milk. Carrots are an especially rich source of vitamin A. Carotene, the pigment that gives carrots their color, is a precursor to vitamin A. Many other vegetables like broccoli, cabbage and leafy greens also contain provitamin A carotenoids. Dairy, specifically milk and cheese, contains vitamin A in the form of retinol.
Animal sources like fish oils, eggs and liver contain retinol as well.
However, vitamin A is also available in supplement form. Some people choose to take these supplements instead because they find it easier to meet their daily requirements in this way or because they are unwilling or unable to consume foods like liver or whole milk which contain high levels of this nutrient.
The recommended dietary allowance (RDA) for vitamin A is 900 micrograms (μg) per day for adult males and 700 μg per day for adult females. Pregnant women should increase their intake by 770 μg and breastfeeding women by 1300 μg.
Vitamin A overdose is possible with supplements but extremely unlikely with food. In fact, eating the skin of carrots or other vegetables and fruits high in carotene can even be beneficial due to the provitamin A properties. However, taking in too much vitamin A through supplementation can cause negative health effects. Taking in more than 10,000 μg (10 milligrams) per day can lead to nausea, vomiting, liver damage and hair loss.
Vitamin D3 (cholecalciferol), popularly known as sunshine vitamin or the bone vitamin, is a steroid hormone that can be synthesized in the skin by the action of UVB rays in the range of 240 to 300 nanometers (nm) upon a cholesterol base. Sunlight is the natural source of this vitamin.
Upon exposure to UVB rays, 7-dehydrocholesterol in the skin is converted to vitamin D3 which is later converted by the liver to calcidiol (also known as calcitriol or Vitamin D3). Calcidiol enters the bloodstream and transported to different body organs like the bone, intestine, kidney etc. where it is converted to calcitriol.
Vitamin D3 can also be ingested by food and supplements. It is found in oily fish like sardines, mackerel and salmon, egg yolk, shiitake and portabella mushrooms, cheese and pork among others. In India, amrat anna or bahadur ras (which contains small amounts of naturally occurring vitamin D3) is used to prevent and treat rickets.
People at risk of vitamin D3 deficiency are people who live in areas far away from the equator, people who cover most of their skin while outdoors, people whose jobs require them to stay indoors like IT professionals, people with fat malabsorption conditions like cystic fibrosis and obesity and people who have malnutrition.
Though no specific symptoms exist for vitamin D3 deficiency, some of the common symptoms include bone pain, muscle weakness and stiffness, bone fractures, hypertension etc.
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