Objective: Ensuring adequate exposure to elements during infancy is vital for lifelong health. This study examines dietary factors influencing essential and non-essential element exposure at 3 months and the impact of solid food introduction at 18 months.

Methods: We analyzed data from the population based NELA birth cohort study in a south-eastern Mediterranean region of Spain, including sociodemographic, dietary, and urine metal concentration information for 490 participants at 3 months and 290 participants at 18 months. Among them, 179 infants provided paired urine samples at both time points. Urinary concentrations of essential elements (Co, Cu, Mo, I, Fe, Mn, Se, Zn, and Ni) and non-essential elements (Al, V, As, Cd, Sb, Tl, and Pb) were measured using inductively coupled plasma-mass spectrometry.

Results: At 3 months, formula-fed infants (9.2% of the total population) had higher urinary concentrations of Al, V, Fe, Co, Se, Mo, Sb, and Tl compared to infants exclusively breastfed or exclusively formula fed or mixed diet. By 18 months, statistically significant increases in concentrations were observed compared to those at 3 months. The medians at 18 months were 18.60, 0.24, 4.41, 50.0, 0.69, 23.1, 0.11, and 0.11 µg/L for As, Co, Cu, Mo, Pb, Se, Tl, and V, respectively.

Conclusion: Formula-fed infants showed higher non-essential element exposure compared to mixed or exclusively breastfed infants, with average Al, V, and Tl exposure being 1.2, 3, and 30 times higher, respectively. Introduction of solid foods at 18 months significantly increased exposure to non-essential elements, e.g., As and Pb exposure rose by 25 and 5 times, respectively. Conversely, the essential element Mo experienced a 25-fold increase with solid food introduction. These findings emphasize the importance of minimizing non-essential element exposure while ensuring adequate intake of essential elements for long-term infant health.