Nutrition reviews
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Presented here is a system to standardize clinical studies of nutrient effects, using nutrient-specific physiological criteria. These guidelines are based mainly on analysis of the typical sigmoid curve of biological response to nutrients and are intended for design, interpretation, and pooling of studies of nutrient effects. Five rules have been articulated for individual studies of nutrients, and six for systematic reviews and/or meta-analyses.
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Adequate folate status in early pregnancy is critical to prevent congenital malformations, yet little is known about whether exposure to folate, specifically folic acid supplementation beyond the recommended dose, influences chronic health outcomes. The link between maternal folate levels and risk of childhood asthma and allergic disease has been investigated in 10 large prospective cohort studies that reported conflicting results. While the majority of studies reported no association, those supporting a positive relationship found a small increase in risk that was generally transient in nature, confined to early childhood, and associated with folic acid supplementation in late pregnancy. This systematic review presents background information on maternal folate exposure and childhood asthma, synthesizes the current epidemiologic evidence in the context of the methodological differences among studies and their potential limitations, and offers direction for future research.