Vitamin D
General Description
Cholecalciferol, commonly referred to as Vitamin D, supports the body’s absorption of calcium and phosphorus, which are vital for regulating bone growth. It is commonly found in animals and humans and is produced on our skin when exposed to sunlight. It indirectly facilitates bone growth and mineralisation by enhancing plasma calcium and phosphorus levels. It is synthesised or obtained from 7-dehydrocholesterol, a derivative of cholesterol.
Vitamin D is not only a nutrient. While we say it is a fat-soluble vitamin, it is also a hormone with many different functions. One of its primary functions is maintaining acceptable calcium levels in the blood, playing a fundamental role in bone mineralisation. Another function is its role in neuromuscular activity, signal transduction, defence against infections, and more.
vitamin’s synthesis and metabolisation process
This vitamin’s synthesis and metabolisation process started at the beginning of the 20th century. The process begins with the transformation of 7-dehydrocholesterol to provitamin D and then to vitamin D, requiring two hydroxylations. The first occurs in the liver, and the second in the kidney, although it can also be performed to a lesser extent in other tissues such as the breast, prostate, or colon, to be biologically active.
Functions of Vitamin D in the Body
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Vitamin D helps maintain adequate phosphate concentrations to enable normal bone mineralisation.
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Vitamin D helps prevent hypocalcemia tetany (involuntary contraction of muscles, leading to spasms and cramps).
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Vitamin D is also needed for bone growth and remodelling by osteoblasts and osteoclasts. Without sufficient vitamin D, bones can become thin, brittle, and misshapen.
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Vitamin D sufficiency helps prevent rickets in children and osteomalacia in adults.
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Vitamin D also helps in the protection against osteoporosis in older adults.
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Vitamin D has various roles, including reducing inflammation and modulating processes like cell growth, immune function, neuromuscular activity, and glucose metabolism.
Sources of Vitamin D
There are three sources of vitamin D. The main one for Australians is exposure to sunlight. It’s essential to realise that although we live in a generally sunny country, many factors affect UV radiation exposure and vitamin D synthesis in the body. These factors include season, time of day, length of the day, cloud cover, smog, skin melanin content, and sunscreen. Also, UVB radiation does not penetrate through glass, so exposure to sunlight through a window does not produce vitamin D.
The factors affecting vitamin D levels make it challenging to provide general guidelines on adequate sun exposure. Several vitamin D researchers have suggested that about five to thirty minutes of sun exposure a week without sunscreen between 10 AM and 3 PM typically leads to sufficient vitamin D synthesis on the face, arms, legs, or back. It has also been found that moderate use of commercial tanning beds emitting UVB radiation at 2% to 6% intensity can also be effective. Individuals with limited sun exposure should include a good source of vitamin D in their diet or take supplements.
The best food sources of vitamin D
Vitamin D is also available through a small number of foods. The best food sources of vitamin D are margarine, fatty fish (sardines, herring, mackerel, tuna, and salmon), milk (especially fortified kinds of milk), vitamin D-fortified soy drinks, dairy blend spread, red meat, and eggs containing tiny amounts of vitamin D. However, dietary sources rarely provide enough vitamin D to meet daily requirements.
The third source of vitamin D is a vitamin supplement. The strength will vary, but vitamin D is commonly available in a 1000 IU dose. Vitamin D tablets, capsules, and liquids are available to ensure sufficient intake. Supplements come in various forms, including tablets, liquids, and pills.
Food Sources
Vitamin D can be found in supplements and fortified foods, such as:
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Cod liver oil
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Salmon
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Swordfish
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Tuna fish
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Orange juice
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Plant milks
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Sardines
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Beef liver
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Egg yolk
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Fortified cereals
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Soft margarine
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Fatty fish
Deficiency Symptoms
Vitamin D deficiency is often asymptomatic. However, severe or prolonged deficiency may cause the following symptoms:
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Bone discomfort or pain in the lower back, pelvis, or lower extremities
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Falls and impaired physical function
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Muscle aches
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Proximal muscle weakness
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Symmetric low back pain (in women)
Vitamin D is a fat-soluble vitamin that relies on the gut’s ability to absorb dietary fat. Deficiency may occur from poor absorption, lack of diet, or a metabolic need for higher amounts. Risk factors for deficiency include insufficient vitamin D intake, lack of ultraviolet sun exposure, and conditions like lactose intolerance or vegan diets.
High-risk groups include:
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People with inflammatory bowel disease (Crohn’s disease, ulcerative colitis) or other conditions that affect normal digestion of fat
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Obese individuals, as vitamin D accumulates in extra fat tissue but is not readily available for use
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Those who have undergone gastric bypass surgery
Conditions Resulting from Prolonged Deficiency
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Rickets: A condition in children and infants involving skeletal deformities and soft bones caused by the failure of bone tissue to harden.
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Osteomalacia: A condition in which bones become weak and soft but can be reversed with supplementation. Unlike osteoporosis, it can be treated.
Toxicity
Vitamin D toxicity most frequently occurs from taking supplements. The low amounts of vitamins in food are unlikely to reach a toxic level. Excessive sun exposure does not lead to toxicity because excess heat on the skin prevents D3 from forming. A vitamin D supplement containing more than 4,000 IU daily should only be taken with a doctor’s supervision.
Symptoms of Toxicity:
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Weight loss
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Anorexia
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Irregular heartbeat
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Hardening of tissues and blood vessels due to increased calcium levels in the blood, potentially damaging the heart and kidneys
Conclusion
Whether vitamin D is a risk marker or has some beneficial effects on non-skeletal diseases remains to be elucidated. It is widely accepted that vitamin D is essential for maintaining skeletal health. Evidence suggests that vitamin D insufficiency is an independent risk factor for various extra-skeletal diseases, including cancer.
Current recommendations regarding preventive medicine should focus on the skeletal effects of vitamin D. While RDAs have been published, there is still a significant gap between these recommendations and actual intake levels in the general population. Public health authorities and professionals must emphasise combating vitamin D deficiency through food fortification, supplementation for at-risk groups, outdoor physical activity, careful sunlight exposure, and a healthy diet.
Additionally, further studies are needed to determine the most optimal means of achieving healthy vitamin D status.