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.

What is already known on this topic -summarise the state of scientific knowledge on this subject before you did your study and why this study needed to be done

Low-and middle-income countries are increasingly facing a double burden of malnutrition; that is, the coexistence of undernutrition (stunting, wasting, underweight) with overnutrition (overweight and obesity).

While the relationship between undernutrition and critical micronutrients for childhood growth and development (e.g., iron, zinc, and vitamin A) is well established, less is known about the risk of micronutrient deficiencies in children and adolescents with overweight or obese, a hidden form of malnutrition.

There are limited data summarising associations between biomarkers of the most commonly limiting micronutrients and body weight status, particularly in children and young people.

What this study adds -summarise what we now know as a result of this study that we did not know before

Overnutrition increases the risk of iron deficiency, but not zinc or vitamin A deficiencies.

There is an inverted U-shaped relationship observed between iron status and bodyweight in children and young people, with iron deficiencies observed more frequent in both under- and overnutrition.

Studies done to date have been heterogeneous in terms of populations studied, diagnostic criteria, and approaches to data analysis; few followed current guidelines for measuring inflammation and defining micronutrient deficiencies.

How this study might affect research, practice or policy -summarise the implications of this study

Increased recognition by healthcare practitioners that children and young people living with overweight, or obesity are likely to be iron deficient; thereby improving clinical practice and care.

The gaps in evidence highlighted are addressed, with more research from currently underrepresented counties examining micronutrient deficiencies and the double burden of malnutrition.

The weaknesses in study design and reporting highlighted are addressed, with improved quality and reporting of micronutrient assessment in children and young people.

The authors have declared no competing interest.

https://www.crd.york.ac.uk/PROSPERO/display_record.php?RecordID=221523

This research was funded by the project titled Addressing micronutrient deficiencies associated with the double burden of childhood malnutrition in China, a combined food system framework; BB/T008989/1 from UK Biotechnology and Biological Sciences Research Council (BBSRC).

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