Mineral Nutrients: From Macro-Level to Ultra Trace

Abstract

Minerals are nutrients are essential for human function, much like vitamins. The minerals represent about 5% of body weight, most of which comes from calcium and phosphorus. The minerals are subdivided into: (1) macrominerals that are present in the body in an amount >0.01% of weight and where >100 mg/day is needed from the diet and (2) microminerals or trace elements which are present in the body in an amount of ≤0.01% of weight and required dietary intake is <15 mg/day. Studies of total parenteral nutrition (TPN) have determined the essentiality of ultra-trace minerals, where daily dietary requirements are in micrograms. Exact requirements have not been established for all of the minerals.

All minerals, except heme iron , are absorbed in the ionic state. Therefore, minerals that remain bound to organic molecules (chelated) or remain as inorganic complexes after digestion usually are not biologically available. Some minerals may be absorbed better in a chelated form when they are bound to an amino acid (e.g., selenomethionine ). Most minerals, especially cations, rely on active transport mechanisms to be absorbed. Unabsorbed minerals remain in the intestinal cells; when the intestinal cells die and slough off, the minerals they contained are excreted. This may be a protective mechanism to prevent toxicity from excessive absorption.

Bioavailability refers to the proportion of a mineral that can be absorbed after its digestion and before its use in tissues and cells. Factors that can reduce bioavailability include the formation of soaps (e.g., calcium and magnesium binding to free fatty acids in the intestinal lumen due to fat malabsorption) and from precipitation when one of a pair of ions (e.g., calcium, which combines with phosphates) is present in the lumen in high concentrations. Mineral–mineral interactions, such as excess zinc intake that reduces copper absorption, can also reduce bioavailability.

Many organic molecules in foods can either inhibit or enhance absorption. Examples of inhibitors include the binding of calcium and other divalent cations by phytates and oxalates. Enhancers include ascorbate for nonheme iron or the hemoglobin protein for iron. Vegetarians tend to consume foods with higher quantities of inhibiting factors, but they also typically ingest more ascorbic acid, which is an enhancer. Other factors that affect mineral bioavailability include gastric acidity, homeostatic adaptations, and stress, which can alter gastrointestinal function. Certain minerals have low bioavailability from foods (e.g., iron, chromium), whereas others have high bioavailability (e.g., sodium, potassium, chloride, iodide, fluoride). Table 26.1 provides an overview of the minerals according to their functions, food sources, recommended levels of intake to support health, deficiency and toxicity symptoms, as well as population groups that may be at risk of deficiency.