Effects of sourdough- or regular-bread fermentation, and phytate reduction on iron bioavailability, absorption, and iron status in humans: a systematic review of intervention studies

This systematic review, conducted under the COST Action CA20218 “Promoting Innovation of fermented foods” (PIMENTO), aimed to evaluate whether sourdough- and regular-bread fermentation improve iron bioavailability, absorption, and status in humans. Screening of PubMed, Scopus, and Cochrane Library (January 1970–December 2024) identified 8 human intervention studies, in healthy or iron-deficient participants, that met inclusion criteria. EFSA’s scientific guidance for health claim applications, which integrates product characteristics and mechanisms of action to the human studies, was followed, and the extracted data were narratively presented. Results were inconclusive as acute postprandial studies increased non-haem iron bioavailability (especially in low-phytate breads); for example, low-phytate white bread produced a greater 2 h increase in serum iron than high-phytate wholemeal bread (59 vs. 30 μg Fe/100 mL), while exogenous phytase increased iron absorption by 50% for ferrous sulfate and 61% for iron bis-glycine chelate. However, long-term trials did not improve, and in one case even decreased, ferritin and total body iron; specifically, in the low-phytate sourdough rye bread group, ferritin declined from 32 ± 7 to 27 ± 6 μg/L and total body iron from 6.9 ± 1.4 to 5.4 ± 1.1 mg/kg over 12 weeks. On the other hand, phytate reduction combined with iron fortification showed positive effects on haemoglobin or prevented iron depletion; in anaemic children, fermented amaranth bread increased haemoglobin [adjusted β = 8.9 g/L (95% CI: 3.5–14.3)] and reduced anaemia prevalence (32% vs. 56%) compared to control bread. Despite convincing mechanistic evidence that the sourdough-fermentation process in bread fabrication improves iron bioavailability, through reduction of phytate, no human studies address this research question with the appropriate control and study quality. Systematic review registration – osf.io/gzt8m.