Vegetarian Diets Across the Lifecycle

Abstract

Optimal zinc status is an important consideration when evaluating the nutritional adequacy of vegetarian diets. In the absence of animal tissue sources of zinc and with increased intake of inhibitors of zinc absorption, phytic acid in particular, the bioavailability of zinc is thought to be lower from vegetarian as compared to omnivorous diets. The aim of this chapter is to review the research that examines the effects of vegetarian compared to omnivorous diets on zinc intake and zinc status in the elderly, adults, children, pregnancy, and lactation. A narrative review of the published literature was undertaken, focusing on observational studies in humans that reported zinc intake and biomarkers of zinc status at various stages of the life cycle. Compared to their respective nonvegetarian control groups, adult male and female vegetarians have lower dietary zinc intakes and serum zinc concentrations. However in the elderly, children, and in women during pregnancy and lactation, there is insufficient evidence to determine whether zinc intake and status are lower in vegetarians compared to omnivores. Inconsistencies in study findings reflect variations inherent in the definition of vegetarian diets, and in many instances compromised statistical power due to a small sample size. Improved methods for the assessment of zinc status are required to determine whether homeostatic responses are sufficient to maintain an adequate zinc status in vegetarians, particularly during times of increased requirement. Appropriate dietary advice to increase the zinc content and bioavailability of vegetarian diets throughout the life cycle is prudent.

Introduction

A considerable body of scientific information reports on the health implications of observing a vegetarian diet. The American Dietetic Association and Dietitians of Canada (2003) have concluded that “appropriately planned” vegetarian diets are healthful and may provide benefits in the prevention and treatment of certain diseases. Plant-based diets are reported to contain less saturated fatty acids and cholesterol, and more folate, fiber, and phytochemicals than omnivorous diets (Bingham, 1999, Phillips, 2005). Vegetarian diets have been associated with a reduction in several of the established risk factors for cardiovascular disease, including more favorable blood lipid profiles, lower body mass index, and lower systolic and diastolic blood pressures (Phillips, 2005), which is consistent with the lower mortality rate from coronary heart disease reported for vegetarians compared with meat eaters (Key et al., 1998). There are several nutrients that require particular consideration in the planning of a nutritionally adequate vegetarian diet including vitamin B₁₂, iron, and zinc: the latter have poorer bioavailability when obtained from plant-derived compared to animal food sources.

Zinc is an essential trace element and is involved in many biological processes that include enzyme action, stabilization of cell membranes, regulation of gene expression, and cell signaling (Foster and Samman, 2010, Samman, 2012); hence, the effects of zinc deficiency have the potential to be wide-ranging. Deficiencies associated with low intakes of absorbable zinc may be exacerbated during times of increased requirement, including growth, pregnancy and lactation, and physiologic changes associated with aging. The aim of this chapter is to review the observational studies that compare the effects of vegetarian and omnivorous diets on zinc intake and serum/plasma zinc concentrations at various stages of the life cycle.