TABLE 1.
Summary of published studies examining the risk of GDM associated with dietary or supplemental iron intake during or before pregnancy1
| Study, year | Comparison groups | Crude effect size RR/OR (95% CI) | Adjusted effect size RR/OR (95% CI) | Covariates |
| Dietary heme iron intake | ||||
| Bowers et al. (27), 2011 | Highest vs. lowest quintile (median 1.60 vs. 0.66 mg/d) | 2.13 (1.70, 2.67) | 1.55 (1.13, 2.13) | Age, parity, BMI, physical activity, glycemic load, polyunsaturated fat intake, cereal fiber, smoking, alcohol, total calories, saturated fat, dietary cholesterol, and family history of diabetes |
| Qiu et al. (28), 2011 | Highest vs. lowest quartile (median 1.43 vs. 0.30 mg/d) | 2.12 (1.31, 3.43) | 2.15 (1.09, 4.27) | Age, daily energy intake, race/ethnicity, parity, physical activity, prepregnancy BMI, dietary fiber, vitamin C, saturated fat, cholesterol, and red and processed meat intake |
| Darling et al. (29), 2016 | Highest vs. lowest quintile (mean 50 vs. 0.28 mg/d) | 2.53 (1.70, 3.78) | 1.55 (0.98, 2.46) | Total calories, nonheme iron intake, iron supplements use, BMI, and dietary cholesterol |
| Dietary nonheme iron intake | ||||
| Bowers et al. (27), 2011 | Highest vs. lowest quintile (median 45.33 vs. 7.58 mg/d) | 0.79 (0.65, 0.96) | 0.97 (0.78, 1.20) | Age, parity, BMI, physical activity, glycemic load, polyunsaturated fat intake, cereal fiber, smoking, alcohol, total calories, and family history of diabetes |
| Qiu et al. (28), 2011 | Highest vs. lowest quartile (median 19.60 vs. 7.16 mg/d) | 0.54 (0.29, 0.99) | 0.61 (0.31, 1.18) | Age, daily energy intake, race/ethnicity, parity, physical activity, prepregnancy BMI, dietary fiber, vitamin C, saturated fat, cholesterol, and red and processed meat intake |
| Darling et al. (29), 2016 | Highest vs. lowest quintile (mean 13.39 vs. 9.85 mg/d) | 0.53 (0.34, 0.83) | 0.48 (0.28, 0.81) | Total calories, heme iron intake, iron supplements use, age, glycemic index, and dietary fiber |
| Supplemental iron | ||||
| Palma et al. (32), 2008 | Iron supplementation users vs. nonusers | Not provided | Not provided (null association) | Not applicable |
| Bo et al. (33), 2009 | Iron supplementation users vs. nonusers | 3.03 (2.18, 4.20) | 3.36 (1.50, 7.53) | Age, family history of diabetes, prepregnancy BMI, education, smoking, parity, duration of iron supplementation, and employment |
| Darling et al. (29), 2016 | Iron supplementation users vs. nonusers | Not provided | 0.78 (0.60, 1.02) | Total calories, dietary iron intake, and BMI |
| Bowers et al. (27), 2011 | Highest vs. lowest quintile (median 60 vs. 0 mg/d) | 0.99 (0.81, 1.20) | 1.04 (0.84, 1.28) | Age, parity, BMI, physical activity, glycemic load, polyunsaturated fat intake, cereal fiber, smoking, alcohol, total calories, and family history of diabetes |
| Chan et al. (34), 2009 | Supplementation with 300 mg ferrous sulfate tablet vs. placebo | 1.04 (0.70, 1.53) | Not applicable | Not applicable |
| Ouladsahebmadarek et al. (35), 2011 | Supplementation with 30 mg elemental iron vs. placebo | Not provided | Not provided (null association) | Not applicable |
| Kinnunen et al. (36), 2016 | Selective iron supplementation vs. routine iron supplementation | Not provided | Not provided (null association) | Not applicable |
| Dietary total iron | ||||
| Bowers et al. (27), 2011 | Highest vs. lowest quintile (median 18.90 vs. 10.30 mg/d) | 0.83 (0.68, 1.02) | 1.12 (0.87, 1.45) | Age, parity, BMI, physical activity, glycemic load, polyunsaturated fat intake, cereal fiber, smoking, alcohol, total calories, and family history of diabetes |
| Behboudi-Gandevani et al. (31), 2013 | Deficient iron intakes (<50% RDA recommendation) vs. sufficient iron intake | Not provided | 0.70 (0.40, 1.13) | Age, BMI, education, parity, passive smoking, history of GDM, family history of diabetes, deficient zinc intake, and serum zinc, iron and hemoglobin concentrations |
| Total iron intake (including supplements) | ||||
| Bowers et al. (27), 2011 | Highest vs. lowest quintile (median 49.80 vs. 10.70 mg/d) | 0.78 (0.64, 0.96) | 0.90 (0.72, 1.12) | Age, parity, BMI, physical activity, glycemic load, polyunsaturated fat intake, cereal fiber, smoking, alcohol, total calories, and family history of diabetes |
| Helin et al. (30), 2012 | Highest 20% vs. lowest 80% (≥110 vs. <14.2 mg/d) | 1.31 (0.70, 2.44) | 1.66 (0.84, 3.30) | Age, BMI, family history of diabetes, prior GDM or macrosomia, total energy intake, gestational weight gain, and dietary fiber and SFAs |
1
GDM, gestational diabetes mellitus; RDA, Recommended Dietary Allowance.