New York: Don't forget to use sunscreen to avoid getting a tan -- for the darkening of the skin to protect itself against harmful ultraviolet (UV) radiation blocks vitamin D synthesis and limits the skin's ability to produce more of the vital vitamin, finds a new study.

Researchers found that even people exposed to high levels of sunlight may be deficient in serum vitamin D because it is mainly induced by UV irradiation and synthesised in the skin.

"Our research showed that in a large sample of individuals living in a tropical region located eight degrees south of the equator with very high rates of sun exposure and extremely high UV irradiation, most people had serum vitamin D below 30 ng/ml (nanograms per ml), the cutoff for normal," said lead study author Francisco Bandeira from the University of Pernambuco Medical School in Recife, Brazil.

"Our findings suggest that skin tanning, which is a natural protection against the harmful effects of UV irradiation, limits the progressive rise in serum vitamin D towards optimal concentrations," Bandeira added.

The team evaluated 986 participants between 13 and 82 years of age and all participants had high rate of daily sun exposure and did not regularly use sunscreen or take vitamin D supplements.

The researchers evaluated each participant's Fitzpatrick skin phototype scale to estimate the response of different skin types to UV light.

In general, higher Fitzpatrick scale scores indicate deeper colour and tendency to tan rather than burn in the sun.

They also calculated each participant's sun index, the number of hours of sun exposure per week multiplied by the fraction of body surface area exposed.

They measured everyone's serum vitamin D levels and compared them with their skin phototype and sun index scores.

The participants with deficient serum vitamin D tended to be older and have lower sun index values than those with normal levels.

The findings suggest that 72 percent of participants had vitamin D deficiency. 

Although the individuals with greater sun exposure had skin that was more tanned and less vitamin D deficiency than other participants, most of those with very high daily exposure had serum vitamin D levels below the normal cutoff.