Why and how a UV lamp can help boost vitamin D (fundamental theory)
Ultraviolet (UV) rays are electromagnetic waves with wavelength of between 400nm and 10nm. UV can be divided into three components according to wavelength: UVA (320-400 nm), UVB (290-320 nm), and UVC (100-290 nm). They have different skin penetration, and generate different biological effect.
UVB plays a key role in vitamin D formation. It is absorbed by the epidermal layer, where the highest concentration of 7-dehydrocholesterol exist. Th e absorption of UVB in the epidermal layer , as is shown in figure.1, is marked as the first step of vitamin D formation process. UV lamp irradiates UVB thus has the same effect of vitamin D production as sunshine (see figure.3). One advantage of UVB exposure in formation of vitamin D is that UVB exposure does not result in excessive production of vitamin D, which causes risks of intoxication. This can be explained that the previtamin D3 that is formed and the thermal isomerization product vitamin D3 that does not go into the circulation , absorb UVB radiation and isomerize to several photoproducts which have little activity on calcium metabolism .
The efficiency of UVB in boosting Vitamin D
Several studies have proved the effect of UV lamp in boosting vitamin D content. For example one study on elderly living in residential nursing home showed that daily exposure to UVB radiation from UV lamp can help improve vitamin D status . Another 12-week investigation conducted in nursing home demonstrated that a few minutes per day of UVB irradiation generated the same effect of oral vitamin D3 intake . For people who can not absorb vitamin D supplement efficiently, UV lamp has been proved as a good alternative to keep (maintain) vitamin D status. A recent study using UV lamp in treatment of vitamin D deficiency , resulted from cystic fibrosis (CF) showed that serum 25(OH)D level increased from 21 ng/ml to 27 ng/ml over 8 weeks (figure.4) .
Choice of wavelength
It has been found that optimum wavelength for the production of vitamin D was between 295 and 300 nm. This narrow range is sometimes referred to as D-UV . One concern associated with this waveband is the erythema effect, which is result of cell irritation and destruction caused by ultraviolet radiation. In UVB region, the wavelength between 290 and 297 nm has the greatest erythema effect on human body with a steep decrease above 297 nm . Therefore UVB tube with spectrum feature of minimal irradiance from 290 to 297 nm should always be chosen in treating vitamin D deficiency.
Generally, there are two types of UVB tube for medical purpose s : UVB-broadband and UVB-narrowband. Compared with UVB-broadband, narrowband is a relatively new development. Nonetheless it is now used widely in UK , continental Europe, and increasingly in the USA [49,50]. The great advantage of narrowband over broadband is that it covers less erythema area than the latter. Thus in theory narrowband is safer to human body than broadband.
Vitamin D production
The production of vitamin D depends on three factors: UV lamp power output, exposure duration, and body exposure area (see figure.5). In theory, the higher the power output, the longer the exposure duration, the larger the body exposure area, the higher vitamin D production.
With power output and exposure duration, UV radiation can be calculated. Limitations should be put on this parameter for safety considerations . Minimal erythema dose (MED) is defined as the minimum amount of UVB radiation that produces redness 24 hours after exposure. It is used when using UVB to treat psoriasis in order to minimise the potential for developing erythema. Thus UVB radiation should not exceed one MED, in other words, exposure (treatment) duration should be well controlled.