Fat-Soluble Vitamins

Vitamins A, D, E and K are referred to as fat-soluble vitamins because they are found in animal fats and plant oils. They are absorbed into the body in the small intestine along with dietary fats. People consuming very low-fat diets (less than 10% of calories from fat), individuals taking certain medications including Xenical or Alli for weight management, and those with medical conditions that limit their ability to absorb dietary fats can develop deficiencies of fat-soluble vitamins.

Fat-soluble vitamins are stored in the liver and in the adipose (fatty) tissue. With the exception of vitamin K, fat-soluble vitamins are excreted less efficiently and quickly than water-soluble ones, and an imbalance of fat-soluble vitamins can lead to toxicity. This is particularly true for vitamins A and D, both of which can accumulate in excessive amounts in the human body and cause toxic side effects.

Vitamin A

In 1913, vitamin A became the first fat-soluble vitamin identified by scientists. It is found in two forms in the diet, as active retinols and in a preliminary form known as caratinoids (yellow-orange pigments found in fruits and vegetables). The best-known caratinoid is the beta-carotene found in carrots. Caratinoids are absorbed from the intestine and converted to the active retinol form of vitamin A in the liver.

The retinoid form of Vitamin A is vulnerable to destruction by excessive heat and prolonged exposure to light and the oxygen in air. The caratinoid form is more stable.

Vitamin A is found naturally in liver and fish oils. The preliminary caratinoid form can be found in dark, leafy green vegetables, carrots, winter squash, sweet potatoes, and other orange, red, or yellow fruits and vegetables.

Recommended Intake

Adult men and women need 700-900 mg of vitamin A or the equivalent amount of caratinoids daily.

Deficiency

Vitamin A is used to make a visual pigment that allows people to see in dim light and also supplies the pigments needed for color vision. Vitamin A is also used in cell growth and differentiation, including bone cells. Night blindness, dry skin, poor growth, and impaired immune function are signs of vitamin A deficiency. A condition known as Xeropthalmia – in which the surface of the eye becomes dry, opaque, and cracked occurs in cases of severe vitamin A deficiency.

Thanks to the fortification of milk and other foods, deficiencies of vitamin A are rare in the US and other industrialized countries, but vitamin A deficiency remains a leading cause of blindness in children in developing countries.

Toxicity

Toxicity symptoms include headache, vomiting, double vision, very dry and painful skin and mucus membranes, joint pain, fractures, hair loss, and severe birth defects.

Vitamin D

Vitamin D, originally called the anti-rickettic factor because of its ability to prevent the bone development disease rickets, is unique among vitamins because it is actually a hormone-like substance that can be made by the body if skin is exposed to adequate amounts of sunlight. Vitamin D helps the body build strong bones by keeping the appropriate balance of the minerals calcium and phosphorus in the bloodstream. Vitamin D also aids in those minerals’ absorption from the GI tract.

Fortification of foods, particularly milk, with vitamin D has greatly reduced the incidence of rickets in the US, but this disease can still be seen in children with malabsorption syndromes and in developing nations. If developed during adulthood, a vitamins D deficiency results in a condition called osteomalacia, or “soft bones,” which increases the risks of fractures and osteoporosis (porous bones). As people age, their bodies’ ability to synthesize vitamin D decreases and they tend to spend less time outdoors, so dietary vitamin D becomes increasing important to help reduce the incidence of osteoporosis and bone factures in senior citizens.

Recommended Intake

The daily recommended intake of vitamin D is based on the assumption that an individual receives no sun exposure. Until they reach 50 years of age, people consume receive 5 mg a day; from age 51-70, they should consume 10 mg, and anyone over 71 should consume 15 mg.

Deficiency

Living in a northern climate, dark pigmented skin, lack of time outdoors, advanced age, use of sunscreen with an SPF factor of 8 or greater, and lack of supplementation are all risk factors for vitamin D deficiency. People can get a natural dose of vitamin D by exposing their skin to summer sunlight for 15 minutes before putting on sunscreen. To ensure that adults get enough vitamin D, both calcium supplements and calcium-fortified orange juice are now available fortified with vitamin D.

Toxicity

When consumed in excessive amounts (greater than 50 mg per day), Vitamin D has the potential to be a very toxic vitamin. Vitamin D toxicity cause high levels of calcium in the bloodstream, which, in turn, causes kidney stones and calcium deposits in soft tissues. The body regulates its production of vitamin D to avoid toxic levels. In other words, you can’t get vitamin D toxicity by being out in the sun too long only from consuming too much of the vitamin.

Vitamin E

Vitamin E is one of the best-known vitamins. Thanks to aggressive advertising by beauty product manufacturers, people know it is an antioxidant and associate it with anti-aging products. Vitamin E is found naturally in plant oils, nuts and seeds, and in foods made from these ingredients, so dietary deficiencies are rare in healthy people. Because it is a powerful antioxidant, it is vulnerable to destruction by free radical oxygen (single molecule oxygen; the oxygen we breathe travels in pairs). Consequently, long-term storage of foods can lead to loss of vitamin E via oxidation.

Recommended Intake

Healthy adult men and women need 15 mg of vitamin E daily. No more than 1000 mg should be consumed to avoid toxicity symptoms. This vitamin’s antioxidant properties are enhanced by the presence of other antioxidants like vitamin C and the trace mineral selenium. Although scientists have speculated that oxidative damage may promote arthritis, cancer, heart disease and even aging, the role of vitamin E in quelling these conditions has yet to be proven.

Deficiency

Vitamin E deficiencies have been seen in premature infants became they are born with low reserves of this vitamin and in people with malabsorption syndromes. Smokers and people eating very lowfat diets are also at risk for vitamin E deficiency.

Deficiency symptoms include dry, scaly skin; hemorrhagic anemia (destruction of red blood cell membranes by oxygen in the absence of vitamin E), and loss of sensory nerve cells.

Toxicity

Toxicity of vitamin E is rare but can occur in people who take excessive amounts of vitamin E supplements. High levels interfere with vitamin K metabolism, including vitamin K’s ability to promote the formation of blood clotting factors.

Vitamin K

Vitamin K’s role as a cofactor for the formation of seven essential blood clotting proteins has been recognized since the late 1920s. These proteins prevent excessive blood loss from occurring after injury. In fact, vitamin K is so named because coagulation is spelled with a “K” in Danish, the native language of its discoverer. More recently, scientists have discovered that vitamin K plays an important role in bone health. Specifically, it turns on proteins in bone tissue, allowing them to hold on to more calcium, thus strengthening bone structure. In light of this discovery, many calcium supplements now include vitamin K.

Phylloquinone, the most active form of vitamin K, is found in green vegetables and liver. Bacteria living in the colons of humans and animals synthesize a form of vitamin K known as menaquinones, which can be absorbed into the bloodstream. However, menaquinones make up only about 10% of the total vitamin K intake.

Recommended intake

The recommended intake of vitamin K is 90 mg per day for adult men and 120 mgs per day for adult women.

Deficiency

Developing a vitamin K deficiency from too little vegetable intake is rare. However, people with fat malabsorption syndromes can develop deficiencies of this vitamin. Long-term use of antibiotics can kill off enough gut bacteria to reduce vitamin K production and cause low grade deficiencies. Taking large doses of vitamins A and E, both of which can interfere with vitamin K absorption, can also cause low grade deficiencies.

Toxicity

No real instances of vitamin K toxicity have been recorded. However, high levels of vitamin K intake can interfere with coumadin, a medication used to control blood clot formation. People taking this and similar medications need to maintain a constant level of vitamin K intake.