REVIEW URRENT C OPINION
Dietary approaches to assessing iron-related nutrition Kathryn L. Beck a and Anne-Louise M. Heath b
Purpose of review Both dietary and nondietary factors contribute to iron deficiency, the most common nutritional deficiency worldwide. Identifying dietary factors associated with iron deficiency is challenging due to the number of components in food affecting iron absorption. This review describes recent advances in dietary approaches to assessing iron-related nutrition. Recent findings Most research investigating the relationship between dietary intake and iron deficiency has focussed on individual foods and nutrients, despite several components in foods influencing iron absorption. More recently, studies have considered the overall diet and combinations of foods eaten, through the analysis of dietary patterns and practices. This includes the development and validation of dietary assessment tools to assess iron-related dietary patterns. Summary Dietary pattern analysis which considers the whole diet and combinations of foods eaten may enhance our understanding of how diet impacts on iron deficiency. The analysis of dietary patterns offers an alternative and complementary approach to the traditional focus on individual foods and nutrients when investigating dietary factors associated with iron deficiency. Keywords dietary assessment, dietary patterns, food frequency questionnaire, iron deficiency
INTRODUCTION Iron deficiency is the most common nutritional deficiency worldwide and is associated with a number of negative health consequences [1]. Both dietary and nondietary factors impact on an individual’s iron status, but the assessment of dietary factors is challenging due to the number of components in food which affect iron absorption. Earlier studies investigating associations between dietary intake and iron status were limited by their focus on individual foods and nutrients. New approaches to assessing iron-related nutrition include dietary pattern analysis which considers the whole diet and combinations of foods eaten [2]. This review describes recent advances in dietary approaches to assessing iron-related nutrition.
[3]), genetics [4,5], ethnicity (Beck K, unpublished data) and possible inflammation associated with obesity [6,7]. Iron homeostasis is maintained primarily at the absorptive level [8], as there is no physiological mechanism for iron excretion [9]. Iron absorption is determined by an individual’s iron status (increases during iron deficiency), unidentified physiological factors influenced largely by genetics, and a range of dietary factors [10]. Absorption is primarily regulated by hepcidin [11], a hepatic peptide which responds to physiological stimuli [12 ] such as iron availability and erythropoietic demand for iron [13]. &
a Institute of Food Nutrition and Human Health, Massey University, North Shore City and bDepartment of Human Nutrition, University of Otago, Dunedin, New Zealand
IRON METABOLISM, INCLUDING DETERMINANTS OF IRON STATUS AND ABSORPTION
Correspondence to Kathryn Beck, Institute of Food Nutrition and Human Health, Massey University Albany, Private Bag 102 904, North Shore City 0745, New Zealand. Tel: +649 443 9649; e-mail: K.L.Beck@ massey.ac.nz
Nondietary factors affecting iron status in healthy populations include blood loss (e.g. blood donation
Curr Opin Clin Nutr Metab Care 2013, 16:712–718
www.co-clinicalnutrition.com
DOI:10.1097/MCO.0b013e328364f382 Volume 16 Number 6 November 2013
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Dietary approaches to assessing iron-related nutrition Beck and Heath
KEY POINTS The assessment of the relationship between diet and iron status is challenging due to the number of food components affecting iron absorption. Until recently, most studies investigating associations between dietary intake and iron status have focussed on individual foods and nutrients, even though iron absorption is affected by food components from multiple foods eaten as part of a whole diet. The use of dietary patterns (derived theoretically or empirically) which consider the whole diet and how combinations of foods are eaten, offers an alternative and complementary approach to studying iron-related nutrition. The validity and reproducibility of dietary assessment tools used to derive dietary patterns must be determined prior to use.
DIETARY FACTORS AFFECTING IRON ABSORPTION Dietary iron exists in two forms – haem iron (derived primarily from haemoglobin and myoglobin and found in meat, poultry, fish and seafood) and nonhaem iron (found in animal tissue and in cereals, eggs, beans, peas, lentils, fruit, vegetables and fortified foods). Although haem iron contributes only a small proportion of total dietary iron intake, it is relatively well absorbed (15–35%) [9,14] and largely unaffected by other food components, however, there is some suggestion that its absorption may be inhibited by calcium [15]. In contrast, nonhaem iron is poorly absorbed (2–20%) [16]. Several dietary components affect nonhaem iron absorption and both vitamin C and an unidentified factor in animal tissue are well known enhancers of nonhaem iron absorption [9,17,18]. Phytic acid, polyphenols, calcium and some proteins (e.g. milk, egg and soy) inhibit nonhaem iron absorption [9,19–24]. Although several dietary components impact on iron absorption, this does not always translate to an effect on iron status. For example, although numerous studies have shown that vitamin C enhances iron absorption, the majority of intervention studies have demonstrated no effect on iron status [25,26]. This lack of impact of isolated food components on iron status is presumably due, at least in part, to the complexity of whole diets. In the free-living human, iron is consumed in a complex matrix of enhancers and inhibitors from multiple foods. It is important, therefore, to investigate not just iron intake, but also foods and the overall diet.
ASSESSING IRON-RELATED NUTRITION – NUTRIENTS, FOODS OR DIETARY PATTERNS To date, most studies investigating associations between dietary intake and iron status have focussed on individual nutrients (e.g. vitamin C) or foods (e.g. meat). Although this approach has been very useful for identifying associations between isolated components of the diet and iron status, it has a number of limitations: (1) Individuals do not consume foods and nutrients in isolation, but as meals containing a variety of foods and nutrients which interact [2,27]. (2) An individual’s health is more likely to be associated with the sum of the total diet, rather than the independent effects of isolated foods and nutrients [28]. (3) The analysis of multiple individual foods or nutrients and their associations with iron status may produce statistically significant results by chance [2,27]. (4) Conversely, associations between single foods or nutrients and iron status may not be large enough to be identified when investigated alone, however the additive effect of several nutrients or foods may be detectable [2,27]. (5) A number of nutrients are highly correlated (e.g. calcium and phosphorus), making it difficult to investigate their independent associations with iron status [2,27]. (6) It is not clear whether associations observed between dietary intake and iron status are the independent effect of a particular food or nutrient, or whether the food or nutrient represents an overall lifestyle or dietary pattern [29]. As a number of foods and nutrients affect iron absorption, and given that people consume foods in combination as part of a whole diet, it makes sense to consider associations between the whole diet and iron status, rather than focusing solely on individual foods and nutrients. This view is supported by the Academy of Nutrition and Dietetics that states ‘the total diet or overall pattern of food eaten is the most important focus of healthy eating’ (page 307) [30 ]. Dietary pattern analysis is one approach that considers the whole diet and the combinations of foods eaten, and offers an alternative and complementary approach to studying individual foods and nutrients and their associations with iron status. In recent years, associations between dietary patterns and iron status have been explored (Table 1) [31 ,32,33]. Most recently, Beck et al. [31 ] found
1363-1950 ß 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
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&
&
www.co-clinicalnutrition.com
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Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Micronutrients Table 1. Studies investigating associations between dietary patterns and iron status Author
Population, country
Dietary patterns
Dietary patterns associated with iron status
Beck et al. & [31 ]
375 women, 18–44 years, New Zealand
a
‘Meat and vegetable’ reduced odds of suboptimal iron status (serum ferritin
Dietary approaches to assessing iron-related nutrition Kathryn L. Beck a and Anne-Louise M. Heath b
Purpose of review Both dietary and nondietary factors contribute to iron deficiency, the most common nutritional deficiency worldwide. Identifying dietary factors associated with iron deficiency is challenging due to the number of components in food affecting iron absorption. This review describes recent advances in dietary approaches to assessing iron-related nutrition. Recent findings Most research investigating the relationship between dietary intake and iron deficiency has focussed on individual foods and nutrients, despite several components in foods influencing iron absorption. More recently, studies have considered the overall diet and combinations of foods eaten, through the analysis of dietary patterns and practices. This includes the development and validation of dietary assessment tools to assess iron-related dietary patterns. Summary Dietary pattern analysis which considers the whole diet and combinations of foods eaten may enhance our understanding of how diet impacts on iron deficiency. The analysis of dietary patterns offers an alternative and complementary approach to the traditional focus on individual foods and nutrients when investigating dietary factors associated with iron deficiency. Keywords dietary assessment, dietary patterns, food frequency questionnaire, iron deficiency
INTRODUCTION Iron deficiency is the most common nutritional deficiency worldwide and is associated with a number of negative health consequences [1]. Both dietary and nondietary factors impact on an individual’s iron status, but the assessment of dietary factors is challenging due to the number of components in food which affect iron absorption. Earlier studies investigating associations between dietary intake and iron status were limited by their focus on individual foods and nutrients. New approaches to assessing iron-related nutrition include dietary pattern analysis which considers the whole diet and combinations of foods eaten [2]. This review describes recent advances in dietary approaches to assessing iron-related nutrition.
[3]), genetics [4,5], ethnicity (Beck K, unpublished data) and possible inflammation associated with obesity [6,7]. Iron homeostasis is maintained primarily at the absorptive level [8], as there is no physiological mechanism for iron excretion [9]. Iron absorption is determined by an individual’s iron status (increases during iron deficiency), unidentified physiological factors influenced largely by genetics, and a range of dietary factors [10]. Absorption is primarily regulated by hepcidin [11], a hepatic peptide which responds to physiological stimuli [12 ] such as iron availability and erythropoietic demand for iron [13]. &
a Institute of Food Nutrition and Human Health, Massey University, North Shore City and bDepartment of Human Nutrition, University of Otago, Dunedin, New Zealand
IRON METABOLISM, INCLUDING DETERMINANTS OF IRON STATUS AND ABSORPTION
Correspondence to Kathryn Beck, Institute of Food Nutrition and Human Health, Massey University Albany, Private Bag 102 904, North Shore City 0745, New Zealand. Tel: +649 443 9649; e-mail: K.L.Beck@ massey.ac.nz
Nondietary factors affecting iron status in healthy populations include blood loss (e.g. blood donation
Curr Opin Clin Nutr Metab Care 2013, 16:712–718
www.co-clinicalnutrition.com
DOI:10.1097/MCO.0b013e328364f382 Volume 16 Number 6 November 2013
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Dietary approaches to assessing iron-related nutrition Beck and Heath
KEY POINTS The assessment of the relationship between diet and iron status is challenging due to the number of food components affecting iron absorption. Until recently, most studies investigating associations between dietary intake and iron status have focussed on individual foods and nutrients, even though iron absorption is affected by food components from multiple foods eaten as part of a whole diet. The use of dietary patterns (derived theoretically or empirically) which consider the whole diet and how combinations of foods are eaten, offers an alternative and complementary approach to studying iron-related nutrition. The validity and reproducibility of dietary assessment tools used to derive dietary patterns must be determined prior to use.
DIETARY FACTORS AFFECTING IRON ABSORPTION Dietary iron exists in two forms – haem iron (derived primarily from haemoglobin and myoglobin and found in meat, poultry, fish and seafood) and nonhaem iron (found in animal tissue and in cereals, eggs, beans, peas, lentils, fruit, vegetables and fortified foods). Although haem iron contributes only a small proportion of total dietary iron intake, it is relatively well absorbed (15–35%) [9,14] and largely unaffected by other food components, however, there is some suggestion that its absorption may be inhibited by calcium [15]. In contrast, nonhaem iron is poorly absorbed (2–20%) [16]. Several dietary components affect nonhaem iron absorption and both vitamin C and an unidentified factor in animal tissue are well known enhancers of nonhaem iron absorption [9,17,18]. Phytic acid, polyphenols, calcium and some proteins (e.g. milk, egg and soy) inhibit nonhaem iron absorption [9,19–24]. Although several dietary components impact on iron absorption, this does not always translate to an effect on iron status. For example, although numerous studies have shown that vitamin C enhances iron absorption, the majority of intervention studies have demonstrated no effect on iron status [25,26]. This lack of impact of isolated food components on iron status is presumably due, at least in part, to the complexity of whole diets. In the free-living human, iron is consumed in a complex matrix of enhancers and inhibitors from multiple foods. It is important, therefore, to investigate not just iron intake, but also foods and the overall diet.
ASSESSING IRON-RELATED NUTRITION – NUTRIENTS, FOODS OR DIETARY PATTERNS To date, most studies investigating associations between dietary intake and iron status have focussed on individual nutrients (e.g. vitamin C) or foods (e.g. meat). Although this approach has been very useful for identifying associations between isolated components of the diet and iron status, it has a number of limitations: (1) Individuals do not consume foods and nutrients in isolation, but as meals containing a variety of foods and nutrients which interact [2,27]. (2) An individual’s health is more likely to be associated with the sum of the total diet, rather than the independent effects of isolated foods and nutrients [28]. (3) The analysis of multiple individual foods or nutrients and their associations with iron status may produce statistically significant results by chance [2,27]. (4) Conversely, associations between single foods or nutrients and iron status may not be large enough to be identified when investigated alone, however the additive effect of several nutrients or foods may be detectable [2,27]. (5) A number of nutrients are highly correlated (e.g. calcium and phosphorus), making it difficult to investigate their independent associations with iron status [2,27]. (6) It is not clear whether associations observed between dietary intake and iron status are the independent effect of a particular food or nutrient, or whether the food or nutrient represents an overall lifestyle or dietary pattern [29]. As a number of foods and nutrients affect iron absorption, and given that people consume foods in combination as part of a whole diet, it makes sense to consider associations between the whole diet and iron status, rather than focusing solely on individual foods and nutrients. This view is supported by the Academy of Nutrition and Dietetics that states ‘the total diet or overall pattern of food eaten is the most important focus of healthy eating’ (page 307) [30 ]. Dietary pattern analysis is one approach that considers the whole diet and the combinations of foods eaten, and offers an alternative and complementary approach to studying individual foods and nutrients and their associations with iron status. In recent years, associations between dietary patterns and iron status have been explored (Table 1) [31 ,32,33]. Most recently, Beck et al. [31 ] found
1363-1950 ß 2013 Wolters Kluwer Health | Lippincott Williams & Wilkins
&
&
&
www.co-clinicalnutrition.com
713
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Micronutrients Table 1. Studies investigating associations between dietary patterns and iron status Author
Population, country
Dietary patterns
Dietary patterns associated with iron status
Beck et al. & [31 ]
375 women, 18–44 years, New Zealand
a
‘Meat and vegetable’ reduced odds of suboptimal iron status (serum ferritin
