Table 2.

Summary of studies on nutrition associations with gestational diabetes among Iranian women

Author (ref); year	City	Study design	Sample size; NO. of GDM	Mean maternal age ± SD	Exposure/intervention	Time of exposure/ intervention †	Significant association adjusted for confounders
Vitamin D
Maghbooli [44]; 2008	Tehran	Cross-sectional	579; 52	GDM:30.2±5.7 Non-GDM: 25.1 ± 4.4	Serum vitamin D	First half of the pregnancy	• Lower serum vitamin D in GDM compared to non-GDM (16.5 ± 10.4 vs. 22.9 ± 18.3 nmol/l; p=0.001). • Serum D < 25 nmol/l was 86.5% in the GDM and 71% in non-GDM • Serum D <12.5 nmol/l was 44.2% in GDM and 23.5% in non-GDM
Soheilykhah [22]; 2010	Yazd	Case-control	165; 54	27.39 ± 5.08	Serum vitamin D	24-28	• Lower serum vitamin D in GDM compared to non-GDM (median (IQR): 9.62 (8.26) vs. 12.9 (14.3); p=0.03). • Prevalence of vitamin D deficiency (<20 ng/ml) was higher in GDM than non-GDM women (83.3% vs. 71.2%; p=0.03). • Odds serum vitamin D<15 ng/ml was 2.66 (1.26, 5.6) higher in GDM compared to non-GDM women
Akhlaghi [18]; 2014	Mashhad	Case-control	130; 61	GDM: 27.5 ± 5.54 control: 28.6±6.19	Serum vitamin D	24-28	• Lower serum vitamin D in GDM compared to non-GDM ( 8.06 ± 3.85 ng/ml vs 17.75 ± 5.38 ; p <0.001)
Sobhani [19]; 2016	Rasht	Case-control	154; 77	GDM: 27.1±5.7 Healthy 26.8 ± 5.0	Serum vitamin D	First half of the pregnancy	• Lower serum vitamin D in GDM compared to non-GDM (13.4 ±6.8 vs. 17.8 ±11.1 nmol/l; p=0.003). • Inverse association between serum vitamin D and odds of GDM (OR=0.95 (95%CIs=0.91-0.99; p=0.021).
Haidari [26]; 2016	Ahvaz	Case-control	90; 45	GDM: 29.33 ± 4.31 NGT: 27.51 ± 4.87	Serum Vitamin D	20-30	• Serum vitamin D was significantly lower in GDM than non-GDM women (13.46 ± 5.18 vs. 16.97 ± 5.56 ng/ml; p-adjusted: 0.034). • Vitamin D status was not significantly different between the two groups.
Shahgheibi [17]; 2016	Sanandaj	Randomized clinical trial	I:46 P: 44	I: 31.3 ± 6.4 P: 29.0 ±6.2	5000 unit/week vitamin D	From first trimester till gestational week 26 th	• Lower incidence of diabetes in the intervention group compared to the placebo (11.4 vs. 34.8%; p=0.009).
Rostami [54]; 2018	Shushtar and masjed soleiman	Field trial	900 control and 900 intervention	18-40	Vitamin D supplementation with different doses based on the status of vitamin D	initiated 4 to 8 days after the first prenatal visit	• Lower odds of GDM in the vitamin D supplemented group vs. non-supplemented women (OR=0.5; 95%CI= 0.34-0.88).
Mostali [20]; 2018	Tehran	Case-control	96; 48	30.1± 5.67	Serum vitamin D	24-28	• No significant difference in serum vitamin D between GDM and non-GDM women ( 21.4±41.5 vs. 18.2±21.2 nmol/l; p=0.21)
Hosseini [21]; 2018	Tehran	Case-control	164; 82	GDM: 29.4±4.8 Non-GDM: 29.6±4.8	Serum vitamin D	24-28	• Lower serum vitamin D in GDM women than non-GDM (24.2 ± 13.3 vs. 29.4 ± 19 nmol/l; p=0.042)
Iron
Afkhami-Ardekani [14]; 2009	Yazd	Case-control	68; 34	NA	ferritin, serum iron, TIBC, Hb, MCV	24-28	• Higher serum iron (100 ±22.1 vs. 56.9±23.0 μg/dl), Hb (13.4 ±1.1 vs. 11.8 ± 1.43 g/dl), ferritin (73.3 ±31.7 vs. 41.6 ± 28.3 ng/ml), transferrin saturation (26.5 ± 5.94 vs. 12.8 ± 5.67%), and MCV (85.3±10.8 vs. 77.7 ± 6.46 fl) in GDM than non-GDM women. • Lower TIBC GDM than non-GDM women (383±30.6 vs. 458±58.2 μg/dl).
Ouladsahebmadarek [47]; 2011	Tehran	Randomized clinical trial	I: 410 P:372 Non-iron deficient , non-anemic women	I:26.3±5.25 P:25.5±4.96	30 mg elemental iron	from 13 weeks of pregnancy	• No significant difference in frequency of GDM in iron-supplemented women compared to the placebo (0.5 vs. 0.8%; p=0.67).
Behboudi-Gandevani [37]; 2013	Tehran	prospective	1,033, 72	27.6±4.84	Serum /dietary Iron	14-20	• Higher serum iron in GDM vs. non-GDM women (143.8 ±48.7 vs. 112.5 ±69.4 μg/dl; p<0.001) • Higher odds of GDM (adjusted OR=1.006, 95 % CI 1.002 to 1.009; P=0.001) for serum iron levels in early pregnancy • No significant difference in dietary intakes of iron between the two groups (101.1 ±74 in GDM vs. 120.1 ±101 mg/day in non-GDM; p>0.05)
Nasiri Amiri [16]; 2013	Babol	Case-control	200; 100	Case: 25.7 ±5.33 Control: 25.0±5.33	Ferritin, serum iron	24-28	• No significant difference in serum iron in GDM than non-GDM women (95.8 ± 41.1 vs. 91.5 ± 31.8 μg/dl; p=0.06) • Higher ferritin in GDM than non-GDM women (52.1 ± 47.2 vs. 30.4 ± 23.3 ng/ml; p=0.001) • No significant differences in TIBC between the two groups (355±40.5 vs. 353±51.1; p=0.1)
Javadian [23] ; 2014	Tehran	Case-control	102; 52	Case: 31.2±6.71 Control: 28.9 ± 6.58	Serum Ferritin and Hb	34	• Higher serum ferritin (31.22 ± 15.4 vs. 24.8 ± 63.2 ng/ml; p=0.012) and Hb (12.9 ± 0.72 vs. 12.2 ± 0.71; p=0.005) in GDM than non-GDM women
Didedar [31]; 2018	Zabol	Case-control	120; 60	Case: 34.9 ±8.10 Control: 33.9 ±7.96	Serum chromium, iron, and copper	20-40	• No significant differences in serum chromium (6.52±2.74 vs. 7.76 ±2.83 ng/dl; p=0.30), iron (0.55±0.14 vs. 0.60±0.10 μg/dl; p=0.14) and copper (0.83±0.22 vs. 0.97 ±0.22 μg/dl; p=0.50) in GDM and Non-GDM women
Taghavi [39]; 2018	Bandar Abbas	Prospective	120; 9	GDM: 24.6±5.92 Non-GDM: 24.3±7.27	Hb and dietary intakes iron	Hb: first trimester and 24-28 Dietary intakes:6-10 and 16-20	• Higher Hb in GDM than non-GDM at the first trimester (11.1±3.63 vs. 10.1±5.71; p=0.05). • Higher Hb in GDM than non-GDM at gestational age of 24-28 (13.3±7.29 vs. 10.9±5.18 g/dl; p=0.001). • Higher dietary intake of iron in the GDM than non-GDM women at gestational age of 6-10 weeks (61.8±0.33 vs. 55 ± 0.32 mg/d; p=0.05) and 16-20 weeks (68.9±0.42 vs. 53.9±0.19; p=0.05) • Higher intake of iron from supplement in GDM than non-GDM women ( 61.8±0.33 vs.55±0.33 mg/d; p=0.05)
Asadi [57]; 2019	Shiraz	non-randomized clinical trial	90; 3 groups with 30	Group 1: 29.6 ± 16.1 Group 2: 29.6 ± 12.3 Group 3: 27.5 ± 8.3	A.Non-anemic women with prophylactic iron B. non-anemic women not receiving the supplementation C. anemic women with standard iron supplementation	The first trimester	• Higher frequency of GDM in the group A compared to the other groups (16.7% vs. 0% in the two other groups).
Hajianfar [43]; 2020	Isfahan	Prospective	812	29.40 ± 4.84	Heme and non-heme dietary iron intakes	8–16 (first trimester)	• Higher odds of high FPG with higher intakes of heme iron (ORT2 = 052 (95%CIs= 0.32, 0.86), ORT3 = 0.72 (0.42, 1.2); p-trend=0.04)
Zinc
Rahimi Sharbaf [62]; 2008	Tehran	Case-control	70/35 (50%)	Case: 28 ±12.5 Control: 28.2 ± 9.5	Serum zinc	24-28	• Lower serum zinc in GDM than non-GDM women (94.8±13.7 vs. 103±7.87mg/dl; p<0.001).
Behboudi-Gandevani [37]; 2013	Tehran	prospective	1,033, 72	27.6±4.84	Serum /dietary zinc	14-20	• No significant difference in serum zinc in GDM than non-GDM (84.8 ±44 vs. 83.5 ±44.4 μg/dl) • No significant difference in dietary intake of Zinc between the two groups • No significant association between serum zinc and odds of GDM
Parast [27]; 2017	Isfahan	Case/control	80; 40	Case: 29.4 ± 4.9 Non-control: 28.9 ± 5.2	Dietary zinc	24-48	• Lower dietary intake of zinc in GDM than non-GDM (7.4 ± 1.9 vs. 9.1 ± 1.7 mg/d; p<0.001)
Magnesium
Mostafavi[45]; 2015	Tehran	Cross-sectional	40	28.4 ±5.37	Serum magnesium	24-48	• Lower serum magnesium in GDM than non-GDM (0.75 ± 0.04 vs. 0.79 ± 0.08 mmol/l; p=0.05) • Obesity (relative risk = 20.6, p-value = 0.002), low-normal magnesium level (relative risk = 4.2, p-value = 0.009), and their interaction (p-value<0.001) were significant predictors of GDM.
Zarean [52]; 2017	Isfahan	randomized controlled trial	180; three groups of 60	group A: 29.8±5.05 group B: 29.7±6.21 group C: 29.4±5.68	A.Normomagnesemia with one tablet multimineral till end of the pregnancy B.hypomagnesaemia receiving one tablet multimineral till end of the pregnancy C.hypomagnesaemia receiving one tablet multimineral till end of the pregnancy plus 200 mg effervescent magnesium tablet for one month	12–14 till end of pregnancy	• Lower frequency of GDM in group C compared to group B; 8.3 % vs. 21.7%.
Musavi [46]; 2019	Babol	cross-sectional	GDM:96 Control: 122	GDM: 26.6±4.9 Control:26.4±6.0	Serum and RBC magnesium	24-32	• Serum magnesium was not significantly different in GDM vs. control (0.71 ± 0.05 vs. 0.71 ± 0.04 mmol/l; p=0.9). • RBC-mg was significantly lower in GDM vs. control (1.93 ± 0.1 vs. 2.10± 0.07 mmol/l; p=0.001)
Antioxidant
Hekmat [24]; 2014	Ahvaz	Case-control	82 ; 41	Case: 29.4±4.68 Control:28.3±6.42	Serum retinol and α-tocopherol	≥32	• Lower serum retinol in the GDM women than non-GDM (0.46 ± 0.19 vs. 0.59 ± 0.25 μg/dl; p<0.01). • No significant difference in α-tocopherol GDM women than non-GDM (6.21 ± 2.69 vs. 6.92 ± 2.43mg/dl; P=0.45)
Parast [27]; 2017	Isfahan	Case-control	80; 40	Case: 29.4 ± 4.9 Non-control: 28.9 ± 5.2	Serum TAC, dietary intakes of antioxidants including vitamin E, C, b-carotene, and Se	24-28	• Lower serum TAC in GDM than non-GDM women ( 2.3 ± 0.7 vs. 3.7 ± 0.1 μmol/l; p<0.001) • Lower dietary intake of selenium GDM than non-GDM women (81 ± 26 vs. 95 ± 36 μg/d; p=0.037). • Lower dietary intake of vitamin E in GDM than non-GDM women (11.8 ± 3.1 vs. 16.2 ± 3.1 mg/d; p<0.001). • No significant difference in dietary intake of vitamin C between the two groups (223 ± 132 vs. 235 ± 122 mg/d; p=0.675) • No significant difference in dietary intake of β-carotene between the two groups (803 ± 512 vs. 1009 ± 635μg/d; p=0.114)
Protein, cholesterol, trans-fatty acid
Hezaveh [34]; 2019	Tehran	Case-control	320; 152	Case: 30.8±5.28 Control: 28.8±5.42	Total protein, vegetable protein, animal protein	24-40	• No significant association between intakes of total protein, vegetable protein, protein intakes from red and process meat, poultry, seafood, and dairy, and odds of GDM. • Lower odds of GDM in women in the highest quartile of protein intake from egg than those in the lowest quartile (OR= 0.43 (95%CIs= 0.28, 0.89).
Milajerdi [32]; 2018	Isfahan	Case-control	463; 200	22-44	Cholesterol	5-28	• No significant association between cholesterol intakes and odds of GDM
Alamolhoda [56]; 2019	Tehran	Randomized controlled trial	800	Intervention: 24.4±2.9 Control: 24.6 ±2.7	Intervention: Daily intake of trans-fatty acid content was less than1%	≥7	• No significant effect on the development of GDM by Reducing the dietary intakes of trans-fatty acids • Frequency of GDM was 5% in the intervention group and 8% in the control group (p=0.08).
Food groups
Lamyian [38]; 2017	Tehran	prospective	1026; 71	26.7 ± 4.3	total fast foods, French fries, hamburger, bologna, sausages, Pizza	≤ 6	• Higher odds of GDM with higher consumption of fast food (p-trend: 0.03); odds of GDM in women in the highest vs. lowest quartile of fast food was 2.12 (95%CI: 1.12–5.43) • The odds of GDM in women in the highest quartile vs. lowest quartile of French fries was 2.18 (95%CI: 1.05, 4.70). • No significant association between odds of GDM and intakes of other fast food subtypes
Goshtasebi [40]; 2018	Tehran	Prospective	1,026; 71	26.7 ± 4.3	legumes, potatoes, and other starchy vegetables (corn, squash, green pea, and green lima beans)	≤ 6	• No significant association between total starchy vegetable and other starchy vegetable and GDM • Inverse association between potato intakes and risk of GDM that became non-significant in the fully adjusted model • Higher legumes consumption associated with lower odds GDM; OR for those who consumed ≥ 3.3 servings/week (T3) was 0.33 (0.16 – 0.66); p-trend:0.002
Milajerdi [32]; 2018	Isfahan	Case-control	463; 200	22-44	egg	5-28	• The risk of GDM in the highest vs. lowest tertile of egg intakes were 41 lower (95% CIs: 0.35, 0.99; p=0.01).
Mirmiran [42]; 2019	Tehran	Prospective	1026; 71	26.7 ± 4.3	Fruit and vegetables (combined and per se)/ dairy	≤ 6	• Higher intakes of fruits and vegetables were associated with lower odds of GDM; (OR=0.85; 95%CI: 0.74-0.98) • Higher intakes of fruits were associated with lower odds of GDM (OR=0.82; 95%CI: 0.73-0.96). • Women in the highest quartiles of vegetable intakes had lower odds of GDM compared to those in the lowest quartiles (OR=0.46; 95%CI: 0.22-0.99). • Vegetable intakes as a continuous variable was not associated with GDM. • No significant association between dairy intake s and odds of GDM.
Periori dietary pattern
Izadi [25]; 2016	Isfahan	Case-control	460; 200	22-44	Mediterranean & DASH diets	5-28	• Odds of GDM in the tertile 3 of the MED diet was 0.20 (95% CIs= 0.50–0.70), compared to the lowest tertile. • Odds of GDM in the tertile 3 of the DASH diet was 0.29 (95%CIs: 0.17–0.48), compared to the lowest tertile.
Saraf-Bank [29]; 2017	Isfahan	Case-control	460; 200	22-44	Dietary acid load (DAL): protein to potassium ratio (Pr/K) and potential renal acid load (PRAL)	5-28	• Higher odds of GDM with higher score for pr/k; OR (95%CI) in tertile 3 vs.1 was 7.60 (3.43-16.84); p-trend<0.001. • Higher odds of GDM was higher with higher score for PRAL; OR (95%CI) in tertile 3 vs. 1 was 9.27 (4.00-21.46); p-trend<0.001.
Zamani [35]; 2019	Isfahan	Case–control	460; 200	22-44	overall plant-based dietary index (PDI), healthy plant-based diet (hPDI), and unhealthy plant-based diet index (uPDI)	25-28	• The odds of GDM according to the tertiles of PDI was 0.79 (0.50, 1.25) and 0.47 (0.28, 0.78) in tertile 2 and 3 vs. 1 (p-trend: 0.004) in the fully adjusted model. • No significant associations between hPDI and uPDI and odds of GDM.
Shivappa [36]; 2019	Tehran	Case-control	388; 122	GDM: 29.76± 4.26 Control: 29.64± 4.52	Dietary inflammatory index	24-28	• Odds of GDM was significantly higher in the tertile 3 vs. 1 of DII (OR: 2.10 (95%CIs: 1.02, 4.34) after adjusting for all potential variables. • No significant association was observed regarding DII as a continuous variable.
Posteriori dietary pattern
Sedaghat [28]; 2017	Tehran	Case-control	388; 122	Case: 29.76 ± 4.26 Control: 29.64 ± 4.52	Dietary pattern using PCA	24-28	• The odds of GDM was higher in women with dietary scores for western diet above the median compare to below the median (OR (95%CI): 1.68 (1.04–2.72)). • No significant association between prudent diet and odds of GDM.
Zareei [30]; 2018	Fasa	Case/control	204; 104	GDM: 30.9 ±5.59 Control: 27.9±4.93	Dietary pattern using PCA	24-28	• Odds of GDM was 2.84 (1.04, 7.75; p=0.042) in the quartile 4 vs. 1 of the unhealthy dietary pattern. • Odds of GDM was 0.28 (0.096, 0.84; 0.023) in the quartile 4 vs. 1 of healthy dietary pattern.
Hajianfar [41]; 2018	Isfahan	Prospective	812	Aged 2-40 years	Dietary pattern using PCA	8–16	• Neither dietary patterns of healthy, western, and traditional were associated with odds of GDM.
Asadi [33]; 2019	Yazd	Case-control	278; 130	GDM: 29.0 ± 5.17 Control: 27.50 ± 4.92	Dietary pattern using PCA		• No significant association between western diet and GDM. • The odds of GDM was significantly lower in tertiles 2 (OR: 0.38; 95%CI: 0.18-0.79) and 3 (OR: 0.88; 95%CIs: 0.44-0.99) compared to the tertile 1 in the prudent dietary pattern.
Probiotic
Asgharian [60]; 2020	Tabriz	Randomized controlled trial	64 in each group	Probiotic: 29.5 ± 6.2 Conventional : 29.4 ± 5.5	Daily intake of 100 g probiotic yoghurt (intervention) and conventional yoghurt (control)	from 24 weeks of gestation until delivery (4-weeks for FPG and GDM)	• After 4- weeks, FPG (mean difference (95%CIs): -4.0 (-6.9, -1.1 mg/dl) and 2-h OGTT (-13.5 (-22.8, -5.0) were significantly lower in the probiotic vs. conventional yoghurt. • GDM was diagnosed in 9% of participants in the probiotic and 17% in the conventional yoghurt (p=0.184). • Intake of probiotic compared to conventional youghurt had no effects on odds of GDM (OR (95%CI): 0.5 (0.2, 1.5).

^†According to gestational weeks.

CIs, confidence intervals; FPG, fasting plasma glucose; GDM, gestational diabetes mellitus; Hb, hemoglobin; 2-h OGTT, 2-hour oral glucose tolerance test; IQR, interquartile range; MCV, mean corpuscular volume; NA, not available; OR, odds ratio; PCA, principal component analysis; RBC, red blood cell; T, tertile; TIBC, total iron binding capacity.