A little ray of sunshine

20-Jul-2004

Vitamin D is of particular concern for women, who live longer and need strong bones to prevent osteoporosis, writes Dr Terri Foran.

THE origin of modern man can be traced back to a tribe of hominid ancestors living in central Africa 120 million years ago. They had adapted to the intense sun exposure seen at these latitudes by developing a dark skin colour to protect them from skin cancer.

The success of this group as a species soon meant their numbers increased and they were forced to travel further and further afield to ensure the food supply necessary for their survival.

As they travelled north into what would later become Europe, the climate began to change and protection from the sun became less of an issue than protection from the cold winter temperatures. And interestingly, in an environment so different to that of Africa, those offspring born with paler skin seemed to do better in the lottery of survival that is natural selection.

The darkest skinned babies were physically deformed by the time they were a couple of years old. As they grew older they fractured easily and only ever achieved short stature. Even if they survived to maturity, most of these darker females died in labour because of obstructed delivery.

By contrast, their paler siblings thrived in the colder climate, eventually leaving a mark on the physical appearance of the entire population.

A second epidemic of this infant disease occurred at the time of the industrial revolution when factories produced so much environmental pollution that the skies darkened and the sun disappeared behind permanent smog. In this environment, even those with the palest skin colour failed to thrive and developed skeletal abnormalities.



THE ANSWER IS SKIN DEEP

The riddle as to why this occurred is contained in the skin itself and in the actions of an amazing substance called 7-dehydrocholesterol. Ultraviolet B light acts on this substance to make vitamin D3 or cholecalciferol — the major source of vitamin D in humans. This steroid is then hydroxylated in two steps.

The first step occurs in the liver, which provides the reserve compound that circulates throughout the body 25-hydroxyvitamin D (25O-HD).

The second step takes place in the kidney and requires the presence of parathyroid hormone. The kidney transforms the circulating 25OHD into the active hormone 1,25-dihydroxyvitaminD, which is sometimes called calcitriol.

But making the situation even more complicated is that the amount of sun exposure appears to be critical — too much and there is a further transition of vitamin D3 to inactive compounds that cannot be used by the body.

The compound found in modern vitamin supplements is vitamin D2, and is made through the effect of radiation on ergosterol, a substance found naturally in some plants.

It is also possible to ingest vitamin D in food products such as fish liver oil, although such sources have rarely played an important role in most human populations.

The tropical genesis of the human species also means that human breast milk contains little vitamin D. Mothers in more temperate climates soon realised the benefits of exposing their young children to the sun.

Even in sunny Australia this tradition, which had its origin in northern Europe, was continued until the latter half of last century with daily ‘sun kicks’ being advised to keep baby healthy.



HOW MUCH VITAMIN D DO WE NEED?

An overall daily intake of 400-1000IU/day of vitamin D is recommended. We know that exposure of the whole body to a dosage of UV light that just causes the faint redness of early sunburn is equivalent to a dosage of around 16,000IU.

From this we can estimate that exposure of the face/arms and hands to sunlight for about 5-20 minutes a day would produce about 800IU in the average person. Such exposure is best avoided between 11am and 3pm.

However, production of vitamin D is decreased in the elderly and in those with darker skin. These people may require more sun exposure to make an equivalent amount of vitamin D. Climate and culture may also play a role. Low levels of vitamin D have been recorded in children in the Tasmanian winter and in veiled women in western Sydney.



THE ROLE OF VITAMIN D

We are only just beginning to understand the critical role vitamin D plays in maintaining general health. We have known for many years that it increases the amount of calcium and phosphate absorbed from the gut, and that it promotes bone mineralisation. A lack of vitamin D in adults results in impaired bone metabolism and the bone softening known as osteomalacia.

When osteomalacia occurs in early childhood there are even more profound effects on the immature bones, resulting in the skull thickening, chest deformities and bowing and weakness of the long bones that are characteristic of rickets, the disease that affected the ancient European and industrial-age humans.

But it seems likely the effects of vitamin D go well beyond its effect on bone since there are receptors for this hormone in all body cells.

Another known target organ is muscle, and vitamin D deficiency can cause muscle weakness and pain. This can make it difficult to get up from a sitting position and may also result in excessive body sway and an increased tendency to fall. Other studies have suggested more subtle effects on balance associated with psychomotor and sensoriomotor co-ordination. Un-diagnosed vitamin D deficiency may then be an additional, hidden factor in the tendency to fall and fracture seen in elderly patients with known osteoporosis.

Interestingly, the relationship between vitamin D and skin cancer is more complex than it previously appeared. Since Australia has one of the highest rates of skin cancer in the world, the message of the ‘Slip, Slop, Slap’ campaign is an important one and may be even more so in the elderly patient where even small amounts of sun exposure can greatly increase their risk of skin damage.

In vitro studies suggest that treating skin cells with vitamin D reduces the risk of redness and peeling and may even reduce some of the oncogenic factors in the skin believed to be responsible for the development of skin cancer. There is also emerging evidence that vitamin D may influence the incidence and mortality of other cancers such as prostate, breast, colon and ovarian cancer.

A recent study has suggested that multiple sclerosis may also be associated with sun exposure and possibly vitamin D levels because there appears to be a significantly higher incidence of this condition in colder climates.



WHO IS AT RISK OF VITAMIN D DEFICIENCY?

Vitamin D deficiency is mainly seen in those who have little exposure to the sun, such as the elderly, the disabled and those with decreased mobility. It is also seen in those who have darker skin or whose cultural tradition demands that they cover up with clothing or veiling.

These groups, along with the mothers of infants with rickets, have been found to have an incidence of vitamin D deficiency of up to 80%.

It is also seen in those with liver or kidney disease because this will affect the production of the active steroid compound from its precursors. A low-calcium diet, sometimes seen in strict vegetarians, will reduce the amount of available vitamin D, as will malabsorption syndromes where there is poor absorption of calcium from the gut. Anticonvulsant medications, such as barbiturates and phenytoin, interfere with vitamin D metabolism and may be a contributing risk factor in institutionalised patients.



WHO DO WE TREAT?

We can’t measure vitamin D itself because its water solubility makes it extremely difficult to measure in vivo.

The available tests measure the liver metabolite 25OHD, which is known to have a linear relationship with the active hormone. This test is readily available through most laboratories and the newer automated tests are faster and more accurate than the assays used in the past.

Normal values for 25OHD are in the range 30-120nmol/L, but in recent times the concept of vitamin D insufficiency is gaining increased acceptance, with patients in the lower end of the normal range being considered for supplementation.

The general recommendation is that clinicians consider testing for vitamin D deficiency in all those with any of the risk factors identified above. Those with a 25OHD of less than 30nmol/L definitely require treatment while those with a result less than 50nmol/L will probably also benefit from supplementation.

Advice about increased sun exposure is an important part of therapy. While it is inappropriate that we try to persuade most grandmothers to wear tank tops or Islamic women to discard the veil, it may be worthwhile encouraging these groups to expose their skin to the sun in a private setting (such as a balcony or yard) for the short period each day recommended for adequate vitamin D synthesis.

Interestingly from a broader public health perspective, one study has indicated that even when lower risk groups, such as the community-dwelling elderly, were supplemented with additional vitamin D there was a 20% reduction in fracture rate in five years .



VITAMIN D SUPPLEMENTS

Unfortunately there is only one supplement available at this time in Australia —Ostelin, an oral preparation containing 1000IU of vitamin D. One gel cap a day is the usual recommended dose, although higher doses may be used initially in those with more severe deficiencies. Two months’ supply of Ostelin at the usual dose costs about $20. Some calcium supplements also contain vitamin D, but only in doses of 100-200IU. It is important to ensure that those on vitamin D supplements are getting adequate amounts of calcium as well, because low-calcium levels will mean any available vitamin D is poorly absorbed.

In many other parts of the world much higher doses of vitamin D are available cheaply in an injectable form. The preparation has a low incidence of toxicity and side effects, so doses of 100,000 to 300,000IU have been used at 6-12 month intervals. Such doses are particularly useful in those patients at high risk of vitamin D deficiency, such as those in nursing homes or institutional care.

It is hoped Australia will get access to such injectable forms of vitamin D soon because they not only have the potential to substantially reduce the morbidity and mortality in this vulnerable group, but will also make supplementation much easier.



WHY IS VITAMIN D A WOMEN’S HEALTH ISSUE?

Vitamin D deficiency can affect anyone — women and men, young and old. But in our society it is a particular issue for women because we live longer than our male counterparts and tend to spend a larger proportion of time at the end of our lives in institutional care.

An understanding of the role vitamin D plays in bone strength and mineralisation means it is becoming more important in any discussion about the management of osteoporosis risk. The suggestion that vitamin D may be partly responsible for maintenance of muscle strength and co-ordination makes it critical in reducing the risk of falls leading to hip fracture, which causes significant morbidity and mortality in older women.

In an increasingly multicultural Australian society, and one in which all women are alert to the potential damage done to their skin by excessive sun exposure, we may need to develop clearer messages for all women as to what constitutes a healthy, safe level of sun exposure.

And in the future, should the presently suggested link between vitamin D and the development of some female cancers and autoimmune diseases be established, we may find ourselves using vitamin D-like compounds in their prevention and treatment. We are only beginning to understand the impact vitamin D has, not just on the skeleton, but on multiple organ systems. It should be an interesting journey of discovery.

Dr Foran is a sexual health physician in private practice in Sydney, and conjoint lecturer at the University of NSW.


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