VITAMIN D- JUST ANOTHER VITAMIN?
We all know that calcium is good for bones, and much has been written about this, but calcium goes hand in hand with vitamin D and we will attempt to highlight the importance of vitamin D in ensuring bone health. Vitamin D is essential for the development and maintenance of bone, both for its role in assisting calcium absorption from food in the intestine, and for ensuring the renewal and mineralization of bone tissue. Although classified as a nutrient, vitamin D is actually a pro-hormone that is produced chemically in the skin from 7-dehydrocholesterol. Due to the fact that sun-exposure has been found to be a major culprit in causing skin cancer, the use of sun screens has increased and an increasing body of evidence suggests that on a global level, vitamin D deficiency is widespread, even in sunny countries like South Africa.
Vitamin D is the generic term for a number of components of which vitamin D2 or ergocalciferol
(found mainly in diet) and vitamin D3 or cholecalciferol (made in the skin) are the most important. Both are found in the body and their amounts are directly linked to the amount of sun-exposure and dietary intake of vitamin D. The vitamin D compound that is measured in blood to test vitamin D status is called 25-hydroxyvitamin D. The kidney converts this form into1,25-dihydroxyvitamin D, which is the active form that promotes intestinal calcium absorption.
We have already mentioned that vitamin D aids in calcium absorption from the gut and renews and mineralizes bone tissue. Bone is a living tissue and old bone constantly gets “eroded” and new bone formed in the “erosion” zones. In childhood, a vitamin D deficiency prevents mineralization (hardening) of the new bone leading to growth retardation and bone deformities known as rickets. This was first described in the 17th Century, but it was only in the late 19th to early 20th Centuries that german doctors noticed that cod liver oil reversed this condition. In adults, this same deficiency causes softening of the bones and muscle weakness known as osteomalacia. A longterm insufficiency of vitamin D can lead to osteoporosis because the calcium metabolism is disturbed. Poor vitamin D status in the elderly also leads to an increase in falling (and therefore fractures) because vitamin D is also important for the correct functioning of the muscles and nervous system. Sufficient vitamin D levels in the elderly therefore increases their reaction time and is very important in the prevention of falls.
Factors influencing vitamin D3 formation
A number of factors can influence the amount of vitamin D formed in the skin. The amount of ultraviolet-B-radiation that reaches the earth is dependent on the angle of the sun, amount of pollution and degree of cloud cover. The further one moves from the equator, the less UV radiation reaches the earth. Vitamin D formation is therefore reduced in high lattitude countries and also during winter months. This has been well shown in South Africa where in
Johannesburg vitamin D formation occurs throughout the year, but in Cape Town there is limited vitamin D formation during the months of April to September.
Ozone pollution have a significant impact on the amount of UV-B that reaches the earth as it effectively absorbs UV radiation.

Melanin pigmentation in the epidermis of the skin also absorbs UV-B radiation and the darker the skin the greater the amount of sun exposure required to produce a specific amount of vitamin D.
Another important factor in determining the production of vitamin D in the skin, is the amount of skin exposed to sunlight. The bigger the surface, the more vitamin D will be made and vice versa. In countries where especially women are covered from head to toe, this plays a significant role in vitamin D insufficiency.
As the dangers of sun-exposure and the link to skin cancer are very real, more people wear sunscreen and stay out of the sun. Any factor more than F8 effectively blocks out UV-B formation and therefore skin synthesis of vitamin D does not occur.
Sources of vitamin D2
Very few food sources containing vitamin D exist and it is basically found in oily fish such as salmon and mackerel and also in cod liver oil. Dietary supplements containing D2 and D3 are available, but it is thought that vitamin D3 is more effective to elevate 25- hydroxyvitamin D as it maintains blood concentrations of this for longer. Very few foods in South Africa are fortified with vitamin D and supplementation becomes more important during the winter months.
APPROXIMATE VITAMIN D LEVELS IN FOOD
Cod liver oil *
Salmon, grilled
AMOUNT
1 tbsp
100 g
I/U per
SERVING
924
284
% RDA
231
71
Mackerel, grilled
Tuna in brine
Sardines in brine
Liver, fried
100 g
100 g
100 g
100 g
352
144
184
36
88
36
46
9
Egg (medium) 50 g
* Also high in vitamin A – toxic when consumed in excess
36
How much is enough?
9
The current recommendations for vitamin D have been derived from calculations of the intakes required to achieve an optimal blood level of 25-hydroxyvitamin D that would be optimal for fracture prevention (70-80 nmol/L). This equates to an intake of 800-1000 IU of vitamin D per day in older men and women, which is 2x higher than the current recommended intakes in most countries.

Groups at risk for vitamin D deficiency
 Although breastmilk is still the best nutrition for infants, vitamin D intake in breastmilk is low and dependent on the mother’s vitamin D levels. Babies are also not exposed to sufficient sunlight.
 Adults older than 50 and especially the elderly, institutionalized individuals with limited sun exposure are at risk and need supplementation with vitamin D.
 The degree of melanin pigmentation has already been discussed and it is important to remember that dark skins need longer exposure to sunlight and are also prone to vitamin
D insufficiency.
 Vitamin D is fat-soluble and requires some dietary fat in the gut for absorption and if there is a history of fat-malabsorption (e.g. pancreatic enzyme deficiency, Chrohn’s disease, cystic fibrosis, celiac disease, gastric bypass operations), the chances of vitamin
D insufficiency increase.
Effects on bone density and fracture risk
Most trials done to measure the effects of vitamin D on bone health also included calcium supplementation. Several studies where vitamin D only was supplemented, as well as those with calcium supplementation demonstrated a reduced risk of falling in older men and women, thus reducing the number of fractures.
Health risks from excessive vitamin D
Vitamin D toxicity is very rare and one needs to maintain an intake of 2000 IU per day for this to occur. Nausea, vomiting, constipation, anorexia, weakness and weight loss are symptoms of this. It can also increase blood levels of calcium to such an extent that confusion and arrythmias ensue. Special precautions need to be taken when there is a history of kidney stones or high levels of calcium in the urine.
Special considerations in the elderly
 With ageing there is a decrease in dietary calcium intake due to poorer appetite, chronic illness and social and economic factors.
 There is also a decrease in the intestinal absorption of calcium which is exacerbated in the presence of a vitamin D insufficiency.
 Elderly, housebound or institutionalized individuals are less frequently exposed to sunlight and are therefore prone to vitamin D insufficiency.
 The skin’s capacity to synthesize vitamin D also decreases.
 The kidneys lose their ability to retain calcium and there is an increased calcium loss in the urine.
 There is also a decrease in the capacity of the kidneys to convert vitamin D into the most active form, 1,25-dihydroxyvitamin D.

In summary:
Calcium and vitamin D alone are not sufficient to prevent rapid post-menopausal bone-loss or to reduce fracture risk optimally, but they are essential components in the treatment and the prevention of osteoporosis.
References
1.
Dawson-Hughes B, Heany RP, Holick MF, et al. (2005) Estimates of optimal vitmain D status. Osteoporos Int 16: 713-716
2.
Pfeifer M, Begerow B, Minne HW, et al. (2000) Effects of a short-term calcium and vitamin D supplementation on body sway and secondary hyperparathyroidism in elderly women. J Bone Miner Res 15: 1113-18
3.
Harvey NC, Martin R, Javaid MK, et al. (2006) Maternal 25(OH)-vitamin-D status in late pregnancy and MRNA expression of placental calcium transporter predict intrauterine bone mineral accrual in the offspring. Osteoporos Int 17(Suppl.2): S9 (OC9)
4.
Dawson-Hughes B, Harris SS, Krall EA, et al. (1997) Effect of calcium and vitamin D supplementation on bone density in men and women 65 years of age or older. N Engl J
Med 337:670-76
5.
Chapuy MC, Arlot ME, Duboeuf F, et al (1992) Vitamin D3 and calcium to prevent hip fractures in elderly women. N Engl J Med 327:1637-42
6.
Jackson RD, LaCroix AZ, Gass M, et al. (2006) Calcium plus vitamin D supplementation and the risk of fractures. N Engl J Med 354: 669-83
7.
Bischoff-Ferrari HA, Willet WC, Wong JB, et al. (2005) Fracture prevention with vitamin
D supplementation: a meta-analysis of randomized controlled trials. JAMA 293:2257-64
8.
Pettifor JM, Moodley GP, Hough FS, et al. (1996) The effect of season and lattitude on in
vitro vitamin D formation by sunlight in South Africa. S Afr Med J 86: 1270-1272
9.
Holick MF (1995) Environmental factors that influence the cutaneous production of vitamin D. Am J Clin Nutr 61: 638S-645S
10.
Norman AW (1998) Sunlight, season, skin pigmentation, vitamin D, and 25hydroxyvitamin D: intergral components of the vitamin D endocrine system. Am J Clin
Nutr 67:1108-1110