Micronutrients and Diabetes
Micronutrients are vitamins and minerals that are required in small quantities for specific physiological functions. Micronutrients often function as coenzymes or cofactors essential for metabolic processes (glycolysis, lipid metabolism, amino acid metabolism, etc.) and are thus essential to sustaining life (Shils 2005). Vitamins and minerals have been studied in the prevention and treatment of both type 1 and type 2 diabetes, as well as for the treatment of diabetes complications (Mooradian 1994). “Natural” medicines, including micronutrients, are widely used by people with diabetes. Some surveys indicate that as many as 60% of people with diabetes use some form of alternative medicine (Yeh 2002). Accordingly, it is not uncommon for consumers with diabetes to ask a variety of questions about the utility of micronutrients and supplements in the management of their diabetes.
The objective of this chapter is to assist health care professionals in answering some of the questions posed by people with diabetes concerning micronutrients. This chapter, which focuses on micronutrients for the treatment of diabetes, begins with a summary of dietary intake requirements for micronutrients for people with diabetes from the book American Diabetes Association Guide to Medical Nutrition Therapy of Diabetes (Franz 1999). The chapter then reviews and updates currently available data for select micronutrients and antioxidants on carbohydrate/glucose metabolism and/or insulin activity and reviews the U.S. Food and Drug Administration (FDA) regulation processes for supplement products.
REQUIREMENTS FOR MICRONUTRIENTS: DIETARY REFERENCE INTAKES
Before addressing the role of vitamins and minerals in diabetes, it is helpful to review requirements for micronutrients and how they are determined. Vitamins and minerals are substances required in very small amounts to promote essential biochemical reactions in cells. Together, vitamins and minerals are called micronutrients. At low nutrient levels (deficiency), dependent biological functions are impaired. In contrast, high intakes can result in toxicity and decreased absorption of other micronutrients because of competitive inhibition.
Micronutrients are specific in their functions, and most cannot be made by the body or be replaced by chemically similar elements. They must come from food or supplements. Small amounts of micronutrients are needed for optimal performance, yet lack of a micronutrient for a prolonged period can result in disease seemingly disproportionate to the amount missing. For example, although only small amounts of vitamins are needed, lack of vitamin C results in scurvy, lack of adequate thiamin in beriberi, and lack of adequate niacin in pellagra.
Several factors make determining exact individual requirements for micronutrients difficult. First, metabolism and use of micronutrients are homeostatically regulated, making requirements and the effect of supplementation dependent on an individual’s nutritional status. For example, if intake of a particular micronutrient is low, absorption may be increased, and when intake is adequate, excess nutrient may be excreted in the feces and in small amounts in the urine.
Assessment of micronutrient status is difficult. It is assumed that levels of micronutrients in body fluids (plasma) reflect tissue and intracellular status and, therefore, that decreased serum levels indicate suboptimal status. However, plasma levels generally do not reflect intracellular status. Correlations between plasma levels and tissue status, especially in marginal deficiencies, are not always apparent.
Furthermore, metabolism and use of nutrients in general is highly integrated with other nutrients, hormones, and physiological factors. With excessive (or deficient) intakes of a particular micronutrient, the balance of this highly orchestrated scheme is disrupted, which leads to a cascade of effects. For example, calcium use is affected by a high protein intake, phosphorus and vitamin D intakes, and parathyroid hormone. Changes in any of these factors may affect dietary calcium requirements.
Requirements for micronutrients have been historically based on the 10th edition of Recommended Dietary Allowances (RDAs) (Food and Nutrition Board 1989). However, the RDAs, published since 1941 by the Food and Nutrition Board of the Institute of Medicine, National Academy of Sciences, are being replaced by a new approach called Dietary Reference Intakes (DRIs) (Yates 1998). The DRIs are developed by the Food and Nutrition Board in partnership with Health Canada and Canadian scientists. DRIs are reference values that are quantitative estimates of nutrient intakes to be used for planning and assessing diets for healthy people. The standards are for apparently healthy people and are not meant to be applied to those with acute or chronic disease or for the replacement of nutrient levels in previously deficient individuals. For individuals with specific needs, adjustments in the values may need to be made. They consist of four reference intakes: RDA, Adequate Intake (AI), Tolerable Upper Intake Level (UL), and Estimated Average Requirement (EAR).
The four primary uses of the DRIs are for assessing intakes of individuals, assessing intakes of population groups, planning diets for individuals, and planning diets for groups. RDAs and AIs both may be used as goals for individual intakes, whereas EARs may be used to examine the possibility of inadequacy and ULs the possibility of overconsumption for individuals. EARs are also used as guides to limit individual intake and to set goals for the mean intake of groups or of a specific population, as well as for the assessment of inadequate intakes within a group. Table 3.1 summarizes the definitions for the various reference values.
Table 3.1 Terms for Nutrient Requirements
• Adequate Intake (AI): An AI is provided instead of an RDA when sufficient scientific evidence is not available to calculate an EAR. The AI is a recommended daily intake level based on observed or experimentally determined approximations of nutrient intake by a group of healthy people that are assumed to be adequate (Yates 1998). The primary use of the AI is as a goal for the nutrient intake of individuals.
• Daily Value (DV): This term is used for nutrient levels on food and supplement labels. DVs are derived from RDAs (or AIs) to represent both sexes and most age-groups.
• Dietary Reference Intakes (DRIs): This is an umbrella term for a set of four reference values: EAR, RDA, AI, and UL.
• Estimated Average Requirement (EAR): The process for setting the RDA depends on being able to set the EAR. The EAR is the amount of nutrient that is estimated to meet the nutrient requirements of half the healthy individuals in a life stage and gender group (Yates 1998). When selecting the EAR, reduction of disease risk is considered, along with many other health parameters. No RDA is proposed if it is determined that an EAR cannot be set. The EAR is used to assess adequacy of intakes of population groups.
• Recommended Dietary Allowance (RDA): The RDA is the average daily dietary intake level of a nutrient that is sufficient to meet the nutrient requirement of nearly all (97–98%) healthy individuals in a particular life stage (life stage considers age and, when applicable, pregnancy or lactation) and gender group (Yates 1998). The RDA includes a generous safety factor related to a bell-shaped curve. The majority of the population actually requires only approximately two-thirds of the RDA. This is in contrast to energy requirements, which are based on average needs.
• Tolerable Upper Intake Level (UL): The UL is the highest level of nutrient intake that is likely to pose no risks or adverse health effects to almost all individuals in the general population. As intake increases above the UL, the risk of adverse effects increases. The UL is not intended to be a recommended level of intake. There is no established benefit for healthy individuals if they consume nutrient intakes above the RDA or AI (Yates 1998). The UL applies to chronic daily use. It is useful because of the increased interest in availability of fortified foods and the increased use of dietary supplements.
Adapted from Yates 1998.
REQUIREMENTS FOR MICRONUTRIENTS IN DIABETES
The literature on the micronutrient status of people with diabetes contains conflicting reports depending on the population studied and because of the uncertainties in methodologies (Mooradian 1994; Mooradian 1987). Adequately controlled studies that establish the role of trace elements in the pathogenesis of carbohydrate intolerance are not available. Although