Supplemental iron intake and the risk of glucose intolerance in pregnancy: re-analysis of a randomised controlled trial in Finland.

Observational studies suggest that high iron intake during pregnancy is associated with the risk of gestational diabetes. As such studies are prone to bias, we re-analysed data from a randomised controlled trial of iron supplementation to see whether it supports the risk found in observational studies. The trial was conducted in primary health care setting in five municipalities in Finland in 1985-1986. The participants were 2944 women (95% of pregnant women in the area) who were randomly allocated either to (1) the selective iron group (elemental iron 50 mg twice a day only if diagnosed as anaemic, continuing until their haemoglobin increased to 110 g L(-1)) or (2) the routine iron group (elemental iron 100 mg day(-1) throughout the pregnancy regardless of haemoglobin level). The numbers of women in the analyses were 1358 and 1336, respectively. The main outcome measure was a composite variable including any glucose intolerance-related outcome (e.g. glucosuria, gestational diabetes, large-for-gestational-age child) in mothers' or children's patient records during pregnancy and post-partum. There were no statistically significant differences in the incidence of the primary outcome between the selective iron and the routine iron groups (13.0 vs. 11.0%, P = 0.12). The most common outcome was large-for-gestational-age calculated from children's hospital data (8.3 vs. 8.2%, P = 0.95). The results were mainly similar when stratified by the mothers' baseline haemoglobin level, body mass index or gestational weight gain. Routine iron supplementation throughout pregnancy did not increase the risk of glucose intolerance during pregnancy. The results need to be confirmed in future trials.

Iron intake, haemoglobin and risk of gestational diabetes: a prospective cohort study.

OBJECTIVE: To investigate the possible association between total daily iron intake during pregnancy, haemoglobin in early pregnancy and the risk of gestational diabetes mellitus (GDM) in women at increased risk of GDM. DESIGN: A prospective cohort study (based on a cluster-randomised controlled trial, where the intervention and the usual care groups were combined). SETTING: Primary healthcare maternity clinics in 14 municipalities in south-western Finland. PARTICIPANTS: 399 Pregnant women who were at increased risk of GDM participated in a GDM prevention trial and were followed throughout pregnancy. MAIN OUTCOME MEASUREMENTS: The main outcome was GDM diagnosed with oral glucose tolerance test at 26-28 weeks' gestation or based on a diagnosis recorded in the Finnish Medical Birth registry. Data on iron intake was collected using a 181-item food frequency questionnaire and separate questions for supplement use at 26-28 weeks' gestation. RESULTS: GDM was diagnosed in 72 women (18.1%) in the study population. The OR for total iron intake as a continuous variable was 1.006 (95% CI 1.000 to 1.011; p=0.038) after adjustment for body mass index, age, diabetes in first-degree or second-degree relatives, GDM or macrosomia in earlier pregnancy, total energy intake, dietary fibre, saturated fatty acids and total gestational weight gain. Women in the highest fifth of total daily iron intake had an adjusted OR of 1.66 (95% CI 0.84 to 3.30; p=0.15) for GDM. After excluding participants with low haemoglobin levels (≤120 g/l) already in early pregnancy the adjusted OR was 2.35 (95% CI 1.13 to 4.92; p=0.023). CONCLUSIONS: Our results suggest that high iron intake during pregnancy increases the risk of GDM especially in women who are not anaemic in early pregnancy and who are at increased risk of GDM. These findings suggest that routine iron supplementation should be reconsidered in this risk group of women.