We all enjoy the summer sun. low DR PAULINE
BALAC, senior lecturer in biology in the School of Applied
Sciences at Huddersfield University, tells us why it – not to
mention vitamin D – is good for us
HERE comes summer, we hope; but is the sun good or bad for us?
At last, after what has seemed like a never-ending winter, we
can finally hope for some sunshine.
So now is the time to top up with some vitamin D, the sunshine
vitamin. Most vitamins are essential compounds that we need to
obtain from our diet.
Vitamin D is unusual in that it can either be obtained from the
diet or by the action of sunlight on our skins. Vitamin D3 (cholecalciferol)
is formed in the skin from 7-dehydrocholesterol in a
photochemical reaction brought about by the ultra violet (UV)
part of sunlight.
Vitamin D3 is not itself biologically active but is converted by
enzymes in the liver and kidney to
1,25-dihydroxycholecalciferol. This is a hormone that regulates
calcium and phosphate levels in the bloodstream and in kidney
and bone.
By helping calcium absorption vitamin D helps to make strong
bones. Without vitamin D bones can become thin, brittle or
misshapen.
Rickets is a disease of defective bone formation caused by a
lack of vitamin D. Rickets was common in many industrialised
cold Northern towns, where smog, air pollution and heavy
clothing prevented any sunlight from getting to the skin.
In the 17th century rickets was so common it was called the
“children’s disease of the English”.
I’m sure you have all seen photographs of the characteristic bow
legs and knock knees of people suffering from rickets. Rickets
was first described in 1645, but it was not until the early part
of the 20th century that the cure was known.
By 1919 in England Sir Edward Mellanby had shown that dogs suffering from
rickets could be cured by giving them cod liver oil. That same year German
researcher K Huldschinsky found that exposing children to sun or UV light
could cure rickets.
The treatment for rickets is dietary supplementation of vitamin D. There are
only a few foods naturally rich in vitamin D. These include oily fish
(herrings, mackerel, sardines and pilchards), eggs, butter, cheese and
cream. Vitamin D is added by law to margarine and this is a more reliable
source than butter, which is lower in vitamin D in the winter.
In the United States milk has been fortified with vitamin D since the 1930s.
The additive to milk and margarine is actually vitamin D2 (ergocalciferol),
which is a commercial product similar in structure and activity to vitamin
D3.
It is now recognised that more than half the world’s population is at risk
of vitamin D deficiency. This may be because very few foods naturally
contain vitamin D or, if fortified, contain very low levels.
It may also be that people do not appreciate that you need a moderate amount
of sun exposure to make vitamin D.
In adults, vitamin D deficiency leads to softening and weakening of the
bones, which is called osteomalacia. This makes bones more prone to
fractures.
Muslim women wearing the full veil need to ensure that they eat plenty of
foods containing vitamin D as they will be unable to produce any vitamin D
from sunlight.
Osteomalacia is more common in Asian women and older white people who do not
go outside much. If the skin is exposed to sunlight during the summer there
is a good chance of making enough cholecalciferol to last through the
winter.
Vitamin D is well known to be associated with bone problems, but two studies
published at the beginning of this year have shown that lack of the sunshine
vitamin also increases the risk of heart disease and is linked to a poorer
outcome for some cancers.
The Framingham Heart Study was started in 1948, originally recruiting 5,209
men and women between 30 and 62 years of age who had no symptoms of
cardiovascular disease, from the town of Framingham, Massachusetts, USA.
Over the years, monitoring this population has led to the identification of
the major risk factors for heart disease, such as high blood pressure, high
cholesterol, obesity and diabetes.
A follow-up study over five years of 1,739 children of the original
participants in the Framingham Heart Study showed that those with low blood
levels of vitamin D had twice the risk of having a heart attack or stroke
than those with higher blood levels of vitamin D. The researchers are now
going on to see if giving vitamin D supplements could help to prevent
cardiovascular disease.
A second study in Norway showed that vitamin D production by sunlight is
nearly three-and-a-half times greater in countries at the Equator than in
the UK.
The number of skin cancers per year in fair-skinned people increases as you
go further south to the equator.
There are also more cases per year of major internal cancers as you go from
north to south.
However, the survival of people with these cancers seems to be significantly
improved in regions where there is more sunshine.
Warnings to avoid sunlight because of risk of skin cancer have to be
balanced against the health benefits of exposure to sunlight. Sun creams of
SPF 8 or greater will block the UV rays that produce vitamin D.
Remember the old Noel Coward song Mad Dogs and Englishmen Go Out in the
Midday Sun?
The Norwegian group also controversially suggest that the best time to get
most vitamin D but the least risk of skin cancer is noon. They say that
people are exposed to less of the harmful UVA radiation that causes skin
cancer and more of the vitamin D generating UVB radiation when the sun is
directly overhead.
The National Institute of Health in the USA suggest that an initial exposure
to sunlight of 10 to 15 minutes gives enough time for vitamin D synthesis.
This should then be followed by application of a sunscreen of SPF 15 or
above to protect the skin.
This routine twice a week should provide enough vitamin D for most people.
You should always wear sun protection of an appropriate factor for your skin
type and geographical place in the sun.
Remember the sun-safe rules; SLIP on a T-shirt and sunglasses, SLAP on a hat
and SLOP on some sun cream.
