Table 1 Characteristics of included studies

Outdoor Workers

Haddad and Chyu (1971) [102]

Lifeguards

8

St. Louis, Missouri, USA (38°63′ N)

Descriptive

160.7 ± 21.7

Devgun (1981) [85]

Gardeners

18

Dundee/Scotland (56°30′N)

Descriptive

59.4 ± 19.7

Devgun (1981) [25]

Male outdoor workers

20

Scotland (56°30′N)

Descriptive

62.1 ± 18.8

Devgun (1983) [103]

1.Outdoor workers

9

Scotland (56°30′N)

Descriptive

73 (1977),

54 (1979)

Average reported as median

2. Outdoor workers

9

55 (1976),

84 (1977)

Excluded from meta-analysis

Azizi (2009) [104]

Agriculture, physical education, construction

122

Beer Sheva, Israel (31°25′ N)

Case-control

67.6 ± 21.3

Norsang (2009) [105]

1. Farmers

20

Shigatse, China (29°19′ N)

Descriptive

81.0 ± 18.0

<75 nmol/L: 40%

<50 nmol/L: 0%

2. Farmers

6

Tingri, China (28°34′N)

67.0 ± 27.0

<75 nmol/L: 67%

<50 nmol/L: 17%

3. Farmers

6

Chonggye, China (29°02′ N)

46.0 ± 27.0

<75 nmol/L: 83%

<50 nmol/L: 17%

Azizi (2012) [91]

Israel National Water Company

1. Group 1

34

Israel (31°05 N)

Interventional

74.4 ± 15.2

2. Group 2

67

98.3 ± 17.2

80.4 ± 27.7

3. Group 3

53

Choi (2011) [106]

Agriculture, Forestry, Fishery

644

South Korea (33° - 38°N)

Cross-sectional

61.3 ± 18.5

<75 nmol/L: 76.2%

<50 nmol/L: 27.8%

Roomi (2015) [55]

Outdoor workers

15

Lahore, Pakistan (31°55′N)

Cross-sectional

31.4 ± 3.8

Oh (2015) [107]

1. Craft, equipment, machine operating, and assembling workers

2812

South Korea (33° - 38°N)

Cross-sectional

<50 nmol/L: 61.6%

2. Skilled agricultural, forestry, and fishery workers

2572

<50 nmol/L: 38.5%

Bacchel (2015) [27]

Farmers

6

North-West Punjab, India (31°15′N)

Cross-sectional

<75 nmol/L: 50%

<25 nmol/L: 50%

Indoor Workers

Devgun (1981) [85]

Laboratory staff

8

Dundee, Scotland (56°30′N)

Descriptive

43.9 ± 17.0

Devgun (1981) [25]

Indoor workers

9

Scotland (56°30′N)

Descriptive

42.9 ± 15.3

Maeda (2007) [108]

Plant and office workers

28

Sao Paulo, Brazil (23° 34′S)

Descriptive

94.0 ± 32.6

Gulvady (2007) [109]

Senior executives

75

Mumbai, India (19°08′ N)

Descriptive

<50 nmol/L: 83%

Islam (2008) [110]

Garment factory workers

200

Dhaka, Bangledesh (23°81′N)

Descriptive

36.7 ± 11.2

<50 nmol/L: 86.5%

Azizi (2009) [104]

Industry, civil service, etc.

104

Israel, Beer Sheva (31°25′ N)

Interventional

57.5 ± 20.8

Itoh (2011) [28]

Indoor daytime workers

4

Kawasaki City, Japan (35°53′N)

Interrupted time series

49.7 ± 7.9

Vu (2011) [111]

Office workers

213

Brisbane, Australia (27°S)

Descriptive

62.2 ± 22.5

<75 nmol/L: 43.3%

<50 nmol/L: 42.5%

Choi (2011) [106]

1. Administration, clerical work

1047

Cross-sectional

45.8 ± 16.5

<75 nmol/L: 93.4%

<50 nmol/L: 87.1%

53.8 ± 18.5

<75 nmol/L: 87.1%

<50 nmol/L: 50%

2. Engineering, assembling, and technical work

572

South Korea (33° - 38°N)

Xiang (2013) [56]

Pregnant indoor workers

311

Guiyang, China (26°65′ N)

Descriptive

36.7 ± 17.0

<75 nmol/L: 12.5%

<50 nmol/L: 83.6%

Cinar (2014) [112]

Premenopausal female and male office workers.

118

Ankara, Turkey (39°93′N)

Prospective observational

52.8 ± 28.4

<75 nmol/L: 24.2%

<50 nmol/L: 54.3%

Jeong (2014) [23]

Managers, experts, specialists, etc.

2659

South Korea (35°91′N)

Descriptive

40.6 ± 18.0

<50 nmol/L: 80.4%

Sharma (2015) [113]

Office working women

50

Udaipur, Rajasthan, India (24°59′N)

Descriptive

46.7 ± 17.4

Yosephin (2015) [114]

Garment factory workers

39

Indonesia (3°35′N)

Randomized control-trial

39.5 ± 12.0

<75 nmol/L: 82%

Roomi (2015) [55]

Indoor workers

73

Lahore, Pakistan (31°55′N)

Cross-sectional

19.0 ± 1.1

Kwon (2015) [54]

Manufacturing workers

1054

Busan, Gyeongnam/South Korea (35°18′N)

Cross-sectional

22.7 ± 8.1

<50 nmol/L: 97.2%

Deficiency

<25 nmol/L:68.4%

Insufficiency:

<50 nmol/L: 97.2%

Oh (2015) [107]

1. Clerks

2357

South Korea (33° - 38°N)

Cross-sectional

<50 nmol/L: 74.7%

2. Managers, professionals and related work

3597

<50 nmol/L: 74.8%

Bacchel (2015) [27]

1. Public servants

69

North-West Punjab, India (31°15′N)

Cross-sectional

<75 nmol/L: 100%

<25 nmol/L: 56.5%

2. Professionals (businessmen) working indoors

13

<75 nmol/L: 100%

<25 nmol/L: 16.7%

Shift Workers

Ward (2011) [41]

1. Shiftworkers with constant regular work hours

4496

United Kingdom (55°39′N)

Cohort

53.0

<50 nmol/L: 81%

Average reported as geometric mean. Excluded from meta-analysis

2. Shiftworkers with varying number of hours worked per week

6136

52.3

<50 nmol/L: 79.4%

Itoh (2011) [28]

1. Rotating shift workers without night shift

4

Osaka Prefecture, Japan (34.5°N)

Cross-sectional

63.1 ± 6.3

2. Rotating shift workers with night shift

4

64.4 ± 8.1

Kim (2013) [115]

Shift workers without day shift

627

South Korea (35°91′N)

Descriptive

40.0 ± 14.7

Jeong (2014) [23]

Shiftworkers

969

South Korea (35°91′N)

Descriptive

40.0 ± 17.7

<50 nmol/L: 80.1%

Kwon (2015) [54]

Manufacturing workers

872

Busan, Gyeongnam/South Korea (35°18′N)

Cross-sectional

22.2 ± 8.1

<25 nmol/L: 71%

≥25 nmol/L: 29%

Romano (2015) [45]

Shiftworkers

96

Northern Italy, Milan Province, Lomabardy (45° 30′N)

Cross-sectional

33.4 ± 13.2

<75 nmol/L: 24%

<50 nmol/L: 66.8%

Lead/Smelter

Greenberg (1986) [79]

Lead and cadmium exposed workers

37

Pittsburgh & Cleveland, United States (40°44′N & 41°50′N)

Descriptive

62.5 ± 24.5

5.4%

Mean 1α, 25-(OH)₂D: 122.7 ± 36.5 pmol/L. Vitamin D deficiency was not defined.

Mason (1990) [78]

Lead exposed workers

63

United Kingdom (55°39′N)

Cohort

Mean 1α, 25-(OH)₂D: 90.5 ± 29.5 pmol/L

Chalkley (1998) [39]

Smelter workers

19

England (52°36′N)

Descriptive

Mean: 71.4 Median: 71.0

1α, 25-(OH)₂D3 Mean: 77.3 pmol/L; Median: 84.0 pmol/L.

Excluded from meta-analysis

Kristal-Boneh (1998) [77]

1. Lead-exposed factory workers (battery and recycling)

56

Israel (31°25′ N)

Cross-sectional

86.0 ± 25.2

Mean 1α, 25-(OH)₂D: 198 ± 64.8 pmol/L

2. Non-lead exposed workers

90

79.0 ± 20.5

Mean 1α, 25-(OH)₂D: 165 ± 42.3 pmol/L

Potula (2005) [116]

Smelter workers

73

Bunker Hill, Idaho, USA (43°91′N)

Descriptive

Mean 1α, 25-(OH)₂D: 115.9 ± 38.0 pmol/L

Coal-miners

Shuster (1981) [82]

1. Underground miners

101

Newcastle Upon Tyne (54°98′N)

Cross-sectional

73.8 ± 73.4

2. Surface workers

19

82.3 ± 67.6

3. Miners not at work

6

83.5 ± 67.4

Shuster (1982) [81]

1. Underground miners

60

United Kingdom (55°39′N)

Cross-sectional

58.5 ± 24.3

2. Surface workers

28

62.6 ± 21.7

Sarikaya (2006) [83]

1. Underground miners

50

Zonguldak, Turkey (41°45′N)

Cross-sectional

24.5 ± 28.2

2. Surface workers

50

35.3 ± 29.3

Healthcare Students

Maeda (2007) [108]

Medical students

44

Sao Paulo (23° 34′S), Brazil

Descriptive

81.5 ± 35.8

Gonzalez-Padilla (2011) [117]

Medical students

103

Gran Canaria, Canary Islands (27°92′N)

Descriptive

69.6 ± 31.0

<75 nmol/L: 28.6%

<50 nmol/L: 32.6%

Paper reported unit as ng/dL but ng/ml was used for calculation

Kaehler (2012) [118]

Female healthcare professional students

215

Innsbruck, Austria (47°27′N)

Cross-sectional

50.3 ± 16.6

<75 nmol/L: 33.5%

<50 nmol/L: 55.8%

Al-Elq (2012) [119]

Medical students

198

Dammam, Saudi Arabia (26°39′N)

Cross-sectional

21.2 ± 11.9

<75 nmol/L: 4%

<50 nmol/L: 96%

Manickam (2012) [120]

Medical students and residents

104

Chicago, IL, USA (42°N)

Descriptive

54.0 ± 28.0

<75 nmol/L: 77%

Zabihiyeganeh (2014) [121]

Medical students

100

Tehran, Iran (35° 69′N)

Cross-sectional

42.0 ± 11.7

<75 nmol/L: 15%

<50 nmol/L: 84%

Milovanovic (2015) [122]

Medical, pharmacy and dental students

86

Kragujevac, Serbia (44°N)

Descriptive

33.1 ± 12.1

<50 nmol/L: 88.4%

Medical Residents

Haney (2004) [61]

Medical residents

34

Portland, OR, USA (45°52′ N)

Interrupted series

56.4 ± 20.1

<50 nmol/L: 38%

Paper reported unit as ng/dL but ng/ml was used for calculation

Maeda (2007) [108]

Medical residents

49

Sao Paulo, Brazil (23° 34′S)

Descriptive

67.1 ± 27.0

Orlandin Premaor (2014) [123]

Medical residents

73

Porto Alegre, Brazil (30°S)

Cross-sectional

44.7 ± 20.0

<50 nmol/L: 57.4%

Multani (2010) [76]

Medical residents

214

Mumbai, India (19°08′ N)

Cross-sectional

31.1 ± 18.6

<50 nmol/L: 87.2%

Singh (2011) [124]

Medical residents

80

Varanasi, India (25°N)

Cross-sectional

22.8 ± 18.2

<75 nmol/L: 11%

<50 nmol/L: 89%

Growdon (2012) [125]

Trainee doctors (Residents)

102

Boston, MA, USA (42°36′ N)

Descriptive

67.0 ± 26.0

<75 nmol/L: 44%

<50 nmol/L: 25%

Mendoza (2013) [126]

Medical residents

20

Mexico City, Mexico (19°43′N)

Cross-sectional

42.4 ± 13.0

<50 nmol/L: 75%

Ramirez-Vick (2015) [127]

Medical residents

51

San Juan, Puerto Rico (18°47′ N)

Descriptive

54.3 ± 19.4

<75 nmol/L: 45%

<50 nmol/L: 43%

Practising Physicians

Gann (1996) [128]

Male physicians

414

United States (37°09′N)

Case-control

Median: 71.1

<50 nmol/L: 6.5%

Goswami (2000) [129]

Physicians and nurses

19

Delhi, India (28°61′N)

Descriptive

13.0 ± 7.9

Mean 1α, 25-(OH)₂D: 93.6 ± 29.0 pmol/L

Kramm (2010) [130]

Physicians

28

Madison, Wisconsin (43°07′N)

Descriptive

80.0 ± 25.0

<50 nmol/L: 21%

25-(OH)D deficiency was defined as <62.5 nmol/L

Mahdy (2010) [75]

Physicians and nurses

340

Doha, Qatar (25°29′N)

Observational

29.3

<75 nmol/L: 9.5%

<50 nmol/L: 87%

SD was not provided. Excluded from meta-analysis

Lee (2011) [131]

Male physicians

389

United States (37°09′N)

Case-control

64.0

Only control values were used for analysis

Haliloglu (2016) [132]

Medical doctors

Unknown

Istanbul/Turkey (41°N)

Prospective observational

Winter: 42.8 ± 22.5 Summer: 58.1 ± 24.3

Total number of healthcare workers were 190. Actual number of medical doctors was not provided

Munter (2015) [133]

Hospital- and community-based physicians

81

Jerusalem (31.4°N), Israel

Descriptive

56.2 ± 16.3

<75 nmol/L: 24.6%

<50 nmol/L: 67.6%

Nurses

Platz (2000) [134]

Nurses

326

United States (37°09′N)

Case-control

67.0 ± 25.5

Mean 1α, 25-(OH)₂D: Controls: 77.3 ± 20.6 pmol/L

Eliassen (2011) [135]

Nurses

1218

United States (37°09′ N)

Case-control

62.4 ± 24.0

Only control values used

Hattapornsawan (2012) [136]

Nurses

217

Nonthaburi, Thailand (13°86′N)

Cross-sectional

<75 nmol/L: 45.6%

<50 nmol/L: 49.8%

Wallingford (2014) [137]

Premenopausal nurses

83

Kingston, ON, Canada (44°23′N)

Cross-sectional

83.5 ± 36.2

<50 nmol/L: 11.2%

Wang (2014) [138]

Nurses

584

United States (37°09′N)

Case-control

61.1 ± 22.8

Only control values used

Haliloglu (2016) [132]

Nurses

Unknown

Istanbul/Turkey (41°N)

Prospective observational

41.8 ± 16.8

Authors did not report number of subjects

Bertrand (2016) [139]

Nurses

835

Boston/USA (42°36′N)

Case-control

68.0 ± 25.8

Only control values used

Madani (2015) [140]

Nurses

200

Kashan/Iran (33°98′N)

Cross-sectional

42.4 ± 52.8

<75 nmol/L: 178 (89%)

≤25 nmol/L: 91 (45.5%)

<25–75 nmol/L: 87 (43.5%)

25-(OH)D 25–75 nmol/L (deficiency);

25-(OH)D ≤ 25 nmol/L (severe deficiency)

Other Healthcare Employees

Platz (2000) [59]

Health professionals

150

United States (37°09′N)

Cohort

45.5 ± 15.0

Mean 1α, 25-(OH)₂D: 79.6 ± 15.7 pmol/L

Nakamura (2001) [141]

Nursing home employees

77

Niigata, Japan (37° 48′ to 59′N)

Cross-sectional

42.1 ± 15.1

<50 nmol/L: 26%

Mean 1α, 25-(OH)₂D: 111.1 ± 33.6 pmol/L

Platz (2004) [142]

Health professionals

460

United States (37°09′N)

Case-control

59.7 ± 20.5

Mean 1α, 25-(OH)₂D: 83.8 ± 17.0 pmol/L.

Arya (2004) [143]

Hospital staff

92

Lucknow, 26.55°N, 80.59°E

Descriptive

30.7 ± 27.2

<50 nmol/L: 78.3%

Mean 1α, 25-(OH)₂D: 97.4 ± 48.2 pmol/L

Hanwell (2010) [144]

1. Hospital workers (in winter)

47

South Italy (latitude 40°N)

Descriptive

38.8 ± 29.0

<75 nmol/L: 87%

<30 nmol/L: 53%

2. Hospital workers (in summer)

23

58.6 ± 16.5

<75 nmol/L: 78%

<30 nmol/L: 4%

Beloyartseva [29]

Healthcare professionals

2119

India, various cities (20°59′N)

Descriptive

35.8 ± 26.5

<75 nmol/L: 15%

<50 nmol/L: 79%

Plotnikoff (2012) [145]

Health care system employees

10,646

Minnesota, United States (46°73′ N)

Prospective observational

70.1 ± 34.0

<75 nmol/L: 31.9%

<50 nmol/L: 28.9%

Porojnicu[146]

Hospital employees

105

Bucharest, Romania (45° N)

Descriptive

<80 nmol/L: 17%

<50 nmol/L: 80%

Gannage-Yared (2014) [147]

Hospital employees

392

Beirut, Lebanon (33°89′N)

Descriptive

39.0 ± 19.7

< 75 nmol/L: 23.5%

< 50 nmol/L: 71.4%

Skarphedinsdottir (2014) [148]

1. Anaesthesia health care staff

106

Reykjavik, Iceland (64°08′N)

Madison, WI, USA (43°07′N)

Descriptive

70.5 ± 30.9

<75 nmol/L: 56.6%

<50 nmol/L: 39.6%

2. Anaesthesia health care staff

124

70.0 ± 30.0

<75 nmol/L: 61.3%

<50 nmol/L: 29%