Vitamin D- just another vitamin?

d2

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. 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. 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 (found mainly in diet) and vitamin D3 or (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.

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. 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. Ozone pollution have a significant impact on the amount of UV-B that reaches the earth as it effectively absorbs UV radiation.

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. 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.

Dietary 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.

APPROXIMATE VITAMIN D LEVELS IN FOOD

  AMOUNTI/U per   SERVING%  RDA
Cod liver oil1 tbsp  924231
Salmon, grilled100 g284 71
Mackerel, grilled100 g35288
Tuna in brine100 g14436
Sardines in brine100 g 18446
Liver, fried100 g 36 9
Egg (medium)  50 g 36 9

* Also high in vitamin A – toxic when consumed in excess

How much is enough?

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.

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:

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.