Table 1.

Characteristics of studies included in the meta-analysis

First author, year and country	Study design	Setting, study population	Sample size (n)	Age, mean	Male (%)	BMI (kg/m2), mean	Selenium classification (range)	Definition of T2DM
Alehagen 2015, Sweden	Longitudinal study (data were extracted from baseline)	Community, rural municipality of inhabitants in the south-east of Sweden.	668	77.7	47.8	NR	Quartiles: <57.2, 57.2–67.1, 67.1–76.1, >76.1 μg/l	NR
Bleys 2007, USA	Cross-sectional study	Community, NHANES III participants	8876	45.2	49.1	26.9	Quintiles: <111.62, 111.62–120.17, 120.18–128.25, 128.26–137.65, >137.65 μg/l	Defined as a fasting plasma glucose ≥ 126 mg/dl, a self-report of a physician diagnosis of diabetes, or current use of insulin or oral hypoglycemic medication
Laclaustra 2009, USA	Cross-sectional study	Community, civilian non-institutionalized U.S. population from NHANES 2003–2004	890	54.4	47.0	28.8	Quartiles: <124, 124–133, 134–146, >146 μg/l	Defined as the presence of either a self-report of current use of hypoglycemic agents or insulin or fasting plasma glucose ≥ 126 mg/dL.
Stranges 2011, Italy	Longitudinal study (data were extracted from baseline)	Community, male workforce of the Olivetti factories in Pozzuoli (Naples) and Marcianise (Caserta), Southern Italy.	445	50.9	100	26.8	Tertiles: <69.7, 69.7–82.6, > 82.6 μg/l	Defined as a fasting blood glucose level ≥ 126 mg/dL or anti-diabetic therapy.
Gao 2014, Sweden	Longitudinal Study	Community, all 50-year-old men living in Uppsala County, Sweden.	1925	49.7	100	25.0	Tertiles: 61.4, 75.1, 91.1 μg/la	Defined by elevated fasting glucose levels and/or use of anti-diabetic medicine. Elevated glucose levels were assessed as fasting blood glucose ≥6.1 mmol/l at the age 60 years’ examination (equals ≥7.0 mmol/l of plasma glucose) and fasting plasma glucose ≥ 7.0 mmol/l at age 70 years and age 77 years

Abbreviations: NHANES National Health and Nutrition Examination Survey, NR not report

^a: number represents median level of each category