What Are Fat Soluble Vitamins?
Vitamins can be divided into two types – fat-soluble and water-soluble. Fat-soluble vitamins, vitamins A (see ‘Vitamin A’), D (see ‘Vitamin D’), E (see ‘Vitamin E’), and K (see ‘Other Vitamins’), are found in meat (especially liver), dairy products, eggs, oily fish, nuts, seeds, grains and leafy green vegetables (vitamin K is also made by some bacteria in the gut). Vitamin D is formed in the skin following exposure to sunlight. Water-soluble vitamins are the B vitamins and vitamin C
Fat-soluble vitamins are not destroyed during cooking, and do not leach into the water (see ‘Cooking to Preserve Vitamins’), unlike water-soluble vitamins. Fat-soluble vitamins are absorbed through the walls of the small intestine in small droplets of fats, and enter the bloodstream.
What Causes Deficiencies in Fat-Soluble Vitamins?If someone does not eat enough food containing fat-soluble vitamins, they will develop what is called a ‘primary deficiency’. If, however, they eat enough of the vitamins but cannot absorb or use them, whether caused by disease or drugs, they have a ‘secondary deficiency’. People with secondary deficiencies need to take additional supplements of fat-soluble vitamins.
Very low fat diets can cause deficiencies in fat-soluble vitamins – this is a primary deficiency.
Any disorders that affect how the body absorbs fats will affect the absorption of fat-soluble vitamins – these include the bowel disorders Crohn’s disease and coeliac disease, and cystic fibrosis, an inherited lung and bowel disorder, as well as people with chronic diarrhoea.
Drugs that reduce the absorption of fat from the diet will reduce the absorption of fat-soluble vitamins. These include cholestyramine (Questran), which is used to treat high cholesterol, and Xenical (orlistat), which is used to help people lose weight (see ‘How Drugs and Foods Interact With Vitamins’). As some vitamin K is produced by bacteria in the gut, long-term antibiotics may cause a vitamin K deficiency. These are secondary deficiencies.
Fat-soluble vitamins are stored in the liver and fatty tissues, so diets have to be low in fat-soluble vitamins for a long period before the symptoms of deficiency show.
Overdoses of Fat-Soluble VitaminsBecause fat-soluble vitamins are stored in the liver and fatty tissues, once the levels get too high, they can remain too high for a long period, leading to something called ‘hypervitaminosis’. This is unlikely to be caused by vitamins occurring naturally in the diet, but may be caused by taking high levels of supplements.
Too much vitamin A can cause dry skin, itching, headaches, jaundice, feelings of sickness and dizziness, vomiting, diarrhoea, blurred vision, muscle pain, tiredness, fever, insomnia, anaemia, and increased risk of bone fractures, including hip fractures. Too much vitamin A in children can slow growth, and too much in pregnancy can cause birth defects.
Too much vitamin D can cause raised levels of calcium in the blood, high blood pressure, nausea, sickness, increased urination, increased thirst, itching and kidney failure. Too much vitamin D in children can slow mental and physical growth.
Too much vitamin K can damage the liver and blood cells.