Low levels of vitamin D linked to respiratory-tract infectionsby Jennifer Bowden
We synthesise it when we are exposed to sunlight; we ingest it when we eat oily fish and eggs. But new research suggests that vitamin D supplements may have health benefits for some of us.
Acute respiratory-tract infections (ARTI) are a major cause of illness and death. Most adults will experience at least one a year and those in contact with younger children will probably have more since kids typically have between 6 and 10: the common cold, bronchitis, middle-ear infection or worse. Starship Children’s Health paediatrician Dr Cameron Grant, says upper respiratory-tract infections are the commonest reason that children are taken to the doctor and the most frequent cause for antibiotics to be prescribed.
New Zealand children under 2 are more than twice as likely to be hospitalised with bronchiolitis as those in England or the US; and our hospitalisation rates for pneumonia in children under 2 are double those of the US, says Grant, co-author of a recent major vitamin D study published in the British Medical Journal.
Observational studies have found a consistent link between low circulating levels of 25-hydroxyvitamin D (the major form in which vitamin D circulates in our blood) and susceptibility to ARTIs. Notably, vitamin D is involved in the modulation of our immune system – including our innate immune system, which produces molecules capable of killing viruses, bacteria and other organisms.
There are three main sources of vitamin D for humans: exposure to ultraviolet B (UVB) from sunlight; foods, such as fatty fish and eggs, which provide small quantities; and dietary supplements. Sunlight exposure is the main source for most people, but there’s a well-known risk of skin cancer. In any case, many variables affect the amount we get from sunlight exposure: latitude (distance from the equator), sunscreen use, clothing, skin pigmentation and how much time we spend outside.
Seasonal variations in UV levels here make it almost impossible for some New Zealanders to maintain a healthy vitamin D status. In Auckland, for example, levels can vary by a factor of 10 between summer and winter.
About one in 20 New Zealand adults aged 15 and over are vitamin D deficient (defined as having 25-hydroxyvitamin D levels lower than 25nmol/L), according to the Ministry of Health’s 2012 report Vitamin D Status of New Zealand Adults. Other studies found that a quarter of breastfed New Zealand infants in the first six months of life and 13% of those aged 6 to 23 months in Auckland had very low vitamin D levels (less than 27.5nmol/L).
It may be even worse: Grant points out that the United States Institute of Medicine now sets the baseline at 50nmol/L, which means almost half of New Zealand infants, schoolchildren and women of childbearing age are deficient.
Vitamin D supplements could significantly improve the health of these people, according to the BMJ study, which used data from 25 clinical trials in 14 countries (including two from New Zealand) and discovered that, when they took supplements, people with particularly low vitamin D levels reduced by 70% their risk of having at least one ARTI a year, and those with higher levels reduced their risk by 25%. What’s more, taking daily or weekly supplements produced greater effects than taking them less frequently.
“These findings have important implications for New Zealand, where both vitamin D deficiency and acute respiratory infections are common,” says Grant.
People with naturally very dark skin and those who are housebound or avoid sun for medical reasons are at high risk of a vitamin D deficiency. For them and for people living in our cooler southern regions or anyone who spends little time outdoors, now is a good time to consider vitamin D supplementation: it may ward off those winter ills.
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