Vitamin D

Vitamin D is a fat soluble vitamin and although there are several forms of the vitamin there are only two that are considered important for optimal health in humans, Vitamin D2 which is found in plant foods and Vitamin D3 which the body synthesises from a cholesterol molecule when the skin is exposed to ultra violet (UV) radiation from the sun.

Vitamin D is most commonly known for its important role in the absorption of calcium, however vitamin D also promotes absorption of phosphorus and prevents the kidneys from excreting protein in the urine. Vitamin D has similar characteristics to that of a hormone as it can be produced by the body, after which it is then carried from its site of production to act elsewhere. Because of its role in mineral absorption, vitamin D promotes the growth of strong bones and teeth.

Those who receive adequate exposure to sunlight generally do not need additional dietary intake of vitamin D. However the amount of vitamin D3 synthesised by the skin is largely determined by the intensity and amount the time exposed to sunlight, but also by the amount of melanin (pigment) in the skin. As a general rule it is recommended that to obtain adequate vitamin D, part of the skin (eg arms or legs) needs to be exposed to the sun for about 10 minutes a day. Please keep in mind that that prolonged exposure to sunlight increases the risk of skin cancer. Australia for example has the highest rate of skin cancer in the world with about 374,000 new cases diagnosed each year.

Deficiencies in vitamin D are rare, however infants living in cold climates are at greatest risk. Vitamin D deficiency in children can lead to rickets where the growth of bones is impaired as a result of the bones failing to calcify properly. The bones can become so weak that they bend just to support the child’s body weight. As an adult, a deficiency in vitamin D can lead to osteomalacia (adult form of rickets) and although this disease is rare it is commonly reported in the elderly, people in institutions (eg nursing homes) and those who wear clothes which limit sun exposure (eg Islamic women).

Vitamin D deficiency can also significantly increase the risk of osteoporosis another debilitating bone disease which is largely preventable. Several studies have shown that vitamin D when used in combination with calcium improves bone mineral status and reduces the risk of osteoporotic hip fractures. In particular 1 study showed that supplementation with 700IU of vitamin D reduced the rate of hip fractures by nearly 60%.

Good food sources of Vitamin D include fatty fish, margarine, eggs, and dairy products.

Additional Possible Benefits of Vitamin D¹

• May help to prevent osteoporosis.
• May reduce cancer risk and enhance immunity.

Recommended Dietary Intake for Vitamin D

A recent re-evaluation of dietary requirements for all vitamins and minerals was published by the National Health and Medical Research Council for Australia and New Zealand. The dietary recommendations for Vitamin D from this publication are summarised below.

Please refer to the following definitions when interpreting these recommendations:

RDI-Recommended Daily Intake
The average daily intake level that is sufficient to meet the nutritional requirements of nearly all (97-98%) health individuals in a particular life stage and gender group.

AI-Adequate Intake (used when an RDI cannot be determined)
The average daily nutrient intake level based on observed or experimentally-determined approximations or estimates of nutrient intake by a group (or groups) of apparently healthy people that are assumed to be adequate.

UL-Upper Limit of Intake
The highest average daily nutrient intake level likely to pose no adverse health effects to almost all individuals in the general population. As intake increases above the UL, the potential risk of adverse effects increases.

RECOMMENDATIONS BY LIFE STAGE AND GENDER – Vitamin D

Source: National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand. Department of Health and Ageing, Canberra 2006, copyright Commonwealth of Australia reproduced by permission.

Vitamin D back to Vitamins

References:
1. Wahlqvist, M.L., et al. Australia and New Zealand: Food and Nutrition. 2nd Ed. Allen and Unwin, Sydney; 2002 pp247-250.
2. Stanton R. Foods that harm, foods that heal: An A-Z guide to safe and healthy eating. Readers Digest; 2006, pp378-382.
3. Cancer in Australia: Facts and Figures http://www.cancer.org.au/content.cfm?randid=101127, (sighted 1.3.07)
4. MacWilliam, L.D. Comparative Guide to Nutritional Supplements. Northern Dimensions Publishing; 2005.
5. Murray M and Pizzorno J. Encyclopedia of Natural Medicine. Prima Health, Rocklin CA, 1998 pp 718-719.
6. Dawson-Hughes B et al. Rates of bone loss in post-menopausal women randomly assigned to one of two doses of vitamin D. Am J Clin Nutr 1995, 61: 1140-1145.
7. National Health and Medical Research Council. Nutrient Reference Values for Australia and New Zealand. Department of Health and Ageing, Canberra 2006, copyright Commonwealth of Australia reproduced by permission.