You will be presented with the full text of a biomedical research paper. Please write an abstract for this paper. Your response should include the abstract and no additional text.

Paper text:
it promotes calcium and phosphate absorption in the intestine and maintains an adequate concentration of these minerals in circulation to allow for normal bone mineralization .
furthermore , vitamin d deficiency can lead to rickets in children and osteomalacia in adults .
recently , many nonskeletal actions of vitamin d on cellular proliferation and differentiation , muscle function , and immunity have also been suggested .
several clinical studies have shown that vitamin d deficiency or insufficiency is associated with an increased risk of nonskeletal health conditions , including cardiovascular diseases , diabetes mellitus , cancers , infections , and autoimmune diseases [ 2 - 15 ] .
although vitamin d can be obtained from dietary sources such as fatty fish and meat , the vitamin d in the human body is mainly acquired by cutaneous synthesis in response to sunlight .
once vitamin d is formed in the skin surface , it undergoes two sequential hydroxylations in the liver and the kidney in order to become biologically active . as people increasingly live in cities and
spend the majority of their time indoors , it has become more difficult to get sufficient sun exposure for adequate cutaneous production of vitamin d. furthermore , even when outdoors , many people use sunscreen and avoid direct exposure to sunlight due to concerns about skin cancer and aging .
therefore , there is a very high prevalence of vitamin d insufficiency worldwide . in particular , as korea has experienced rapid industrialization and economic development over the past few decades , vitamin d insufficiency has become a public health problem not only in elderly people , but also in younger generations due to changes in lifestyle and occupations .
it has been well established that vitamin d status is best assessed using the serum concentration of 25-hydroxyvitamin d [ 25(oh)d ] , which is produced in the liver and reflects vitamin d obtained from both cutaneous synthesis and dietary sources .
although previous studies have investigated what constitutes a sufficient level of 25(oh)d using several criteria , including serum parathyroid hormone suppression , calcium absorption , bone mineral density , and fall or fracture rates , there is no clear consensus on what level of serum 25(oh)d is sufficient . according to the world health organization , serum 25(oh)d levels below 25 nmol / l
however , there have been many proposals to increase the lower threshold of sufficient 25(oh)d level to upwards of 75 nmol / l .
suggested that the most advantageous serum 25(oh)d levels begin at 75 nmol / l , with the optimal level ranging from between 90 and 100 nmol / l .
most recently , the institute of medicine in the united states suggested that a serum 25(oh)d level of 50
nmol / l is sufficient to ensure bone health after an extensive literature review of vitamin d .
therefore , it is still controversial whether sufficient vitamin d status should be defined as a serum 25(oh)d level of above 50 or 75 nmol / l .
vitamin d status in the korean population has been previously investigated based on the korea national health and nutrition examination surveys iv conducted in 2008 ( table 1 ) . the mean serum 25(oh)d level measured by radioimmunoassay ( ria ) ( diasorin , stillwater , mn , usa ) was 52.9
the prevalence of vitamin d insufficiency , defined as a serum 25(oh)d level below 50 nmol / l , was 47.3% in males and 64.5% in females .
with a serum 25(oh)d level of 75 nmol / l as the threshold , the prevalence of vitamin d insufficiency rose to 86.8% in men and 93.3% in women .
interestingly , the study showed that serum 25(oh)d level increases with age from 20 - 29 to 60 - 69 years in both sexes .
furthermore , contrary to the common belief that vitamin d insufficiency is more prevalent in elderly people , vitamin d insufficiency was most prevalent in the 20 to 29 age group ( 65.0% in males and 79.9% in females , with 50 nmol / l as the threshold of vitamin d insufficiency ) . in the same study ,
vitamin d insufficiency was more prevalent among those who work indoor occupations such as sales , service , administration , clerical work , or specialists compared with those who have outdoor occupations such as agriculture , forestry , or fishing .
although vitamin d status has been studied in many countries worldwide , relatively few studies have assessed the vitamin d status of a population based on large representative samples . in the united states and canada
, the vitamin d status of the population has been investigated in nationwide surveys ( table 1 ) . in the united states ,
vitamin d status was investigated in the national health and nutrition examination survey ( nhanes ) ( table 1 ) .
serum 25(oh)d levels were measured using ria ( diasorin ) in the nhanes 1988 to 1994 and the nhanes 2001 to 2006 .
nmol / l in 1988 to 1994 and 55.2 nmol / l in 2001 to 2006 .
the prevalence of vitamin d insufficiency , defined as a serum 25(oh)d level of less than 50 nmol / l , was 29% in the nhanes 1988 to 1994 and 32% in the nhanes 2001 to 2006 . with a serum 25(oh)d level of 75 nmol /
l as the threshold , the prevalence of vitamin d insufficiency was 69% in the nhanes 1988 to 1994 and 76% in the nhanes 2001 to 2006 .
overall , there was a trend toward an increased prevalence of vitamin d insufficiency in 2001 to 2006 compared to 1988 to 1994 . in canada
, vitamin d status was investigated in the canadian health measures survey from 2007 to 2009 ( table 1 ) .
serum 25(oh)d levels were measured by a chemiluminescence assay using the liaison 25-hydroxyvitamin d total assay ( diasorin ) .
for both sexes , the serum level of 25(oh)d was highest among children and seniors , and lowest in those aged 20 to 39 years .
nmol / l was 67.0% in males and 62.2% in females and highest in those aged 20 to 39 years .
an individual 's vitamin d status is determined by several factors related to cutaneous synthesis or dietary intake of vitamin d. accordingly , traditional risk factors for poor vitamin d status include limited sun exposure , higher latitude , winter season , darker skin pigmentation , sunscreen use , skin - covering clothing , and a diet low in fish and dairy products .
older age is commonly suggested to be a risk factor for vitamin d insufficiency because the cutaneous synthesis of vitamin d declines with age . aging can decrease the capacity of the skin to produce previtamin d in response to ultraviolet radiation by greater than 2-fold .
however , in korea and canada , vitamin d insufficiency was more prevalent among young adults than in the older people .
although the cause of this finding is not clear , it may be due to behavioral factors such as an indoor lifestyle , sunscreen use , outdoor activity , or dietary habits .
previously , the national diet and nutrition survey 1992 to 2001 of the united kingdom also showed that vitamin d insufficiency was most prevalent among young adults aged 19 to 24 years as well as in elderly people over 85 years of age .
despite growing awareness of the multiple health benefits of vitamin d , vitamin d insufficiency remains a major health problem worldwide .
it is not only a health concern of elderly people , but has been shown to affect all age groups . in korea ,
compared with the united states and canada , korea has a lower mean 25(oh)d level and a higher prevalence of vitamin d insufficiency , although it is important to exercise caution when comparing results from surveys conducted in different laboratories or using different methods ( table 1 ) .
therefore , it should be a critical public health issue to improve the vitamin d status of the korean population . due to the heavily indoor lifestyle of modern society , it is difficult to get adequate sunlight exposure for sufficient vitamin d production .
furthermore , it is also challenging to change the dietary habits of a population in order to increase the intake of foods rich in vitamin d such as fish .
more realistic ways of improving vitamin d status include food fortification and supplementation with vitamin d. an aggressive national policy on food fortification for the general population should be adopted .
likewise , the promotion of vitamin d supplementation is necessary , especially for specific groups at risk for vitamin d insufficiency .
the majority of the vitamin d in our body is produced by cutaneous synthesis in response to sunlight . as more and more people live in cities and 
 spend the bulk of their time indoors , it can be difficult to get sufficient sun exposure for adequate cutaneous production of vitamin d. therefore , vitamin d insufficiency has become a very common health problem worldwide . 
 the korea national health and nutrition examination survey iv 2008 showed that the prevalence of vitamin d insufficiency , defined as a serum 25-hydroxyvitamin d [ 25(oh)d ] level below 50 nmol / l , was 47.3% in males and 64.5% in females . 
 only 13.2% of males and 6.7% of females had a serum 25(oh)d level of greater than 75 nmol / l . 
 in korea , vitamin d insufficiency was more prevalent in young adults than in elderly people , likely due to the indoor lifestyle of younger people . 
 compared with the united states and canada , korea has a lower mean 25(oh)d level and a higher prevalence of vitamin d insufficiency . to improve the vitamin d status of the korean population , 
 more aggressive policies on food fortification and vitamin d supplementation are needed .