Overnutrition is a risk factor for iron deficiency in children and young people: a systematic review and meta-analysis of micronutrient deficiencies and the double burden of malnutrition

Introduction Traditionally associated with undernutrition, increasing evidence suggests micronutrient deficiencies can co-exist with overnutrition. Therefore, this work aimed to systematically review the associations between iron, zinc and vitamin A status and weight status (both under- and overweight) in children and young people. Methods Ovid Medline, Ovid Embase, Scopus, and Cochrane databases were systematically searched for observational studies assessing micronutrient status (blood, serum, or plasma levels of iron, zinc, or vitamin A biomarkers) and weight status (body mass index or other anthropometric measurement) in humans under 25 years of any ethnicity and gender. Risk of bias assessment was conducted using the American Dietetic Association Quality Criteria Checklist. Where possible, random effects restricted maximum likelihood (REML) meta-analyses were performed. PROSPERO (CRD42020221523). Results After screening, 83 observational studies involving 190,443 participants from 44 countries were identified, with many studies having reported on more than one micronutrient and/or weight status indicator. Iron was the most investigated micronutrient, with 46, 28, and 27 studies reporting data for iron, zinc, and vitamin A status, respectively. Synthesizing 16 records of odds ratio (OR) from 7 eligible studies, overnutrition (overweight and obesity) increased odds of iron deficiency (OR [95%CI]: 1.51 [1.20, 1.82], p<0.0001, I2=40.7%). Odds appeared to be higher for children living with obesity (1.88 [1.33, 2.43], p<0.0001 I2=20.6%) in comparison to those with overweight (1.31 [0.98, 1.64], p<0.0001 I2=40.5%), although between group differences were not significant (p=0.08). Conclusions Overnutrition is associated with increased risk of iron deficiency, but not zinc or vitamin A deficiencies, with an inverted U-shaped relationship observed between iron status and bodyweight. Our results highlight significant heterogeneity in the reporting of micronutrient biomarkers and how deficiencies were defined. Inflammation status was rarely adequately accounted for, and the burden of iron deficiency may well be under-recognised, particularly in children and young people living with overnutrition.