Read more: 鈥�Immune retune: Recharging your body鈥檚 natural defences鈥�
Having a healthy immune system isn鈥檛 all about cranking up the dial to maximum. There is a large class of conditions caused by immune cells attacking things they should leave in peace 鈥� namely the body itself. Known as autoimmune disorders, the list includes type 1 diabetes, inflammatory bowel disease, multiple sclerosis and rheumatoid arthritis.
These conditions have been linked to a lack of vitamin D 鈥� the so-called sunshine vitamin. Some foods such as dairy products and fish contain vitamin D but most is made in the skin when it is exposed to the sun鈥檚 ultraviolet rays.
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Vitamin D appears to play a key role in keeping the immune system in check so that it doesn鈥檛 react to things inappropriately. The first clue that this was the case was the higher rate of autoimmune disorders in parts of the world with less sunlight. Since then, researchers have found that vitamin D suppresses the immune system by inhibiting the proliferation of immune cells and the signalling factors that spur them into action. The compound is even being investigated as a way of stopping people rejecting organ transplants.
Sunshine鈥檚 effects stretch beyond those of vitamin D. Melatonin 鈥� a hormone secreted by a gland in the brain in response to changes in light 鈥� stimulates certain kinds of immune cells.
Vitamin D is also vital for calcium absorption and bone health. Unfortunately, growing awareness of the risks of skin cancer has led some people to shun the sun, hence a recent resurgence in childhood rickets.
Skin cancer aside, vitamin D appears to protect against many other common types of cancer, including those of the breast, prostate and colon. One research group has calculated that in the US, more people die from internal cancers caused by lack of sun exposure than from skin cancer itself 鈥� possibly four times as many (麻豆传媒, 9 August 2003, p 30).
鈥淢ore people may die from cancers caused by lack of sunshine than from skin cancer鈥�
So how much time should you spend in the sun? Michael Holick, a vitamin D researcher at Boston University in Massachusetts, reckons you should expose your hands, arms and face for a quarter of the time it would take to cause reddening two to three times a week.
Dermatologists tend to argue in favour of taking supplements instead, but multivitamin pills may not provide a high enough dose.