We read with interest the work by Jaeggi et al
1 and Paganinni et al
2 and commend their efforts. Despite differences in iron concentration, infants’ age
and sequencing techniques, both studies demonstrate unfavourable iron effects on gut
microbiota with decreased abundance of bifidobacteria and lactobacillus, and increased
abundance of pathogenic bacteria in iron-deficient/anaemic Kenyan infants.
We have investigated changes in gut microbial composition due to iron fortification
or supplementation in healthy, Swedish infants. Iron-sufficient infants at 6 months
of age were randomly allocated to receive low-iron-fortified formula (1.2 mg Fe/day;
n=24), high-iron-fortified formula (6.6 mg Fe/day; n=24) or no-added-iron formula
with liquid ferrous sulfate supplementation (iron drops; 6.6 mg Fe/day; n=24) for
45 days. All participants gave their informed consent before inclusion through parents
or legal guardians. Total iron intake was 1.2, 6.4 and 5.7 mg/day (all differences
p<0.01) in the low-iron, high-iron and iron-drops group, respectively. Stool samples
were collected before and after the intervention. We applied 16S rRNA gene amplicon
sequencing of the V3–V4 region to profile the gut microbiome using Illumina MiSeq.
We used QIIME3 to assess composition and diversity of gut microbiota and the DESeq2
package4 to investigate differences in relative abundance of gut bacteria among the
groups. PICRUSt was used to predict metagenome functional content.5
Vaginally delivered infants (n=53) with paired stool samples were included in the
analyses. There were no significant differences in anthropometrics or iron-related
biomarkers among the randomisation groups; no adverse effects were reported (diarrhoea,
increased rates of infections, other illnesses, etc), and growth was not affected
(table 1).6
Table 1
Baseline characteristics of the study participants and anthropometric and biochemical
values at the 45-day follow-up.
Low-iron formula
High-iron formula
Fe drops
Participants (n)
18
18
17
Girls (n)
7
9
11
Birth weight (g)*
3717±560
3548±425
3800±436
Birth length (cm)*
51.1±2.2
50.2±1.6
51.7±1.7
Age at inclusion (months)*
6.1±0.3
6.1±0.2
6.1±0.3
Baseline
Follow-up
P values†
Baseline
Follow-up
P values†
Baseline
Follow-up
P values†
P values‡
Weight (kg)*
8.3±1.0
9.1±1.1
<0.001
8.0±1.2
8.8±1.1
<0.001
8.4±0.9
9.2±0.9
<0.001
0.49
Length (cm)*
68.4±2.4
71.3±2.7
<0.001
67.4±2.8
69.9±2.6
<0.001
68.2±2.3
71.7±3.9
<0.001
0.26
Hb (g/L)*
111.6±6.0
110.2±9.0
0.71
112.2±7.0
112.9±5.9
0.62
118.0±11.5
112.2±5.8
0.06
0.51
S-Fe (µmol/L)*
9.5±4.2
9.5±4.3
0.66
9.7±3.8
8.7±3.6
0.42
8.8±4.5
9.6±3.6
0.64
0.78
S-transferrin (g/L)*
2.2±0.3
2.4±0.4
0.07
2.2±0.3
2.2±0.3
0.66
2.3±0.4
2.2±0.2
0.70
0.32
S-ferritin (µg/L)§
89.4±44.7
61.2±32.5
<0.001
72.3±40.7
70.5±47.0
0.81
109.3±85.8
92.2±62.9
0.14
0.17
F-calprotectin (µg/g)¶
132 (71, 241)
121 (55, 211)
NS**
120 (59, 238)
105 (62, 421)
NS**
263 (104, 345)
151 (109, 492)
NS**
NS††**
Data are mean/geometric mean±SD or median (25th, 75th percentile).
*Mean ±SD.
†P values for within-group differences, paired-samples t-test.
‡P values for between-group differences, ANOVA.
§Geometric mean ±SD.
¶Median (25th, 75th percentile).
**P values for within-group differences, related-samples Wilcoxon signed-rank test.
††P values for between-group difference, independent-samples Kruskal-Wallis test.
F, faecal; Hb, haemoglobin; NS, not significant at p=0.05; S, serum.
In this study, we confirm findings that consumption of high-iron formula is associated
with decreased relative abundance of Bifidobacterium (p<0.001, 60% vs 78%) after only
45 days of intervention, but we did not detect enhanced growth of pathogenic bacteria.
However, we were able to partly confirm previous findings regarding abundance of lactobacilli
due to iron consumption. We found lower relative abundance of Lactobacillus sp (p<0.007,
8% vs 42%) in infants who received iron drops versus high-iron-formula group. Unexpectedly,
we also found higher relative abundance of Lactobacillus sp (p<0.0002, 42% vs 32%)
in high-iron compared with low-iron formula group; this result challenges the hypothesis
that the mode of iron administration has a direct effect on lactobacilli colonisation
in the gut. Furthermore, the iron-drops group had lower abundance of Streptococcus
(p<0.0003, 0.2% vs 0.9%) but higher abundance of Clostridium (p<0.05, 25% vs 9%) and
Bacteroides (p<0.02, 1.2% vs 0.9%) compared with the high-iron formula group (figure
1). In the present study, all groups received formula with added galacto-oligosaccharides
(GOS) at 3.3 g/L. This prebiotic may mitigate adverse effects of iron fortification
on gut microbiota,2 but in the iron-drops group, iron was administered apart from
the formula meals. Thus, we cannot exclude a possible protective effect of GOS on
the gut microbiota of infants in our study.
Figure 1
Differences in gut bacterial composition depend on the concentration and administration
mode of the consumed iron. In the cladogram, showing the results of the microbiome
analysis over time, taxa are grouped on the basis of synapomorphy. The outermost small,
white circles represent the 561 OTUs (operational taxonomic units). Differences in
gut microbial composition between the high-Fe-formula group versus the low-Fe-formula
group over time are presented in the yellow component around the cladogram, where
blue bars represent lower relative abundance of bacteria in the high-Fe-formula group
compared with the low-Fe-formula group and the red bars represent higher relative
abundance in the high-Fe-formula group compared with the low-Fe-formula group, respectively.
Differences in gut microbial composition between the high-Fe-formula group versus
the Fe-drops group over time are presented in the red component around the cladogram,
where the blue bars represent lower relative abundance of bacteria in the high-Fe-formula
group and the red bars represent higher relative abundance in the high-Fe-formula
group compared with the Fe-drops group, respectively. OTU, operational taxonomic unit.
As in the study by Paganinni et al,2 faecal calprotectin did not differ between the
groups (table 1), but in our study, it correlated positively with Clostridium difficile
in high-iron-formula (rSpearman=0.4, p<0.01) and iron-drops intervention groups (rSpearman=0.48,
p<0.004). The bacterial function pathway related to Staphylococcus aureus infection
(KEGG module 05150)5 was significantly lower in the iron-drops group compared with
the low-iron-formula group (p=0.027). This is a novel finding which suggests that
changes in bacterial composition due to administration of iron drops may reduce the
protective response of the gut microbiota to bacterial infections. Nevertheless, no
effects on the health of the participants were seen due to this.
To summarise, in healthy, non-anaemic Swedish infants, consumption of high-iron formula
is associated with significantly lower abundance of bifidobacteria compared with low-iron
formula, and administration of iron as drops, even in a dose comparable with the daily
iron requirement and for a short time, leads to decreased relative abundance of lactobacilli
and potentially increases susceptibility to bacterial infection.