A Balancing Act
By Karen Plawecki, M.S., R.D.
Minerals are amazing nutrients that provide critical functions to maintain health. Meeting mineral needs through foods and/or supplementation is a tricky balancing act in which many factors can help or hinder adequate mineral intake.
Mining for minerals
Minerals are noncarbon-based elements. They are virtually indestructible even under conditions such as light, alkalinity or intense, prolonged heating. Basically, calcium is always calcium. It can be used and excreted by the body, but it cannot be changed chemically. The only time when minerals (particularly sodium and potassium) are “lost” during cooking is when they are leached into the cooking water. If the cooking water is discarded, so are the minerals.
Certain food groups naturally offer a better source of key minerals than others. The following list outlines significant dietary mineral sources:
Grains: iron, magnesium (in unre- fined products), zinc, sodium
Vegetables: potassium, magnesium
Dairy: calcium, magnesium, phos- phorus, sodium
Meat and meat alternatives: iron, zinc, phosphorus, potassium, magnesium (legumes).
Minerals are absorbed via active (with carrier and energy) and passive transport. Active absorption is more efficient, but also has a certain capacity for each nutrient. Bioavailability refers to how well the body can absorb the nutrient. Simply because a mineral is present in a food does not mean that it is bioavailable to the consumer.
Most minerals are absorbed at a rate of 6% to 40%, except for sodium and potassium, which are absorbed at about 90%. However, absorption rate and bioavailability depend on several factors, including pH, mineral source and other food components, physiology and presence of other minerals.
pH: Minerals are charged elements, so they are better absorbed in an acidic environment.
Mineral source and other food components: The food source can either help or hinder mineral absorption. For example, dietary iron is found in two forms: nonheme (iron salts) in plants and dairy or heme (hemoglobin or myoglobin) in meats. Iron from heme is absorbed in quantities two to three times greater than nonheme iron.
Food components can also affect absorption. Plants contain compounds, including phytates, fiber and oxalates that lower mineral absorption, particularly calcium, iron, magnesium and zinc. Phytate (from legumes and unprocessed whole-grain products) binds the mineral and lowers absorption. Oxalates in spinach and rhubarb reduce calcium absorption to about 3% to 5%, compared to milk (approximately 32%) or calcium-fortified drinks (40% to 50%). Some compounds in foods promote absorption. Lactose and vitamins C and D improve nonheme iron absorption. One of the reasons that milk is considered a good calcium source is that it contains vitamin D and lactose, which increase calcium absorption; it also contains a balance of other bone-building minerals, such as magnesium and phosphorus.
Physiology: The body’s need for a mineral can affect its absorption. The absorption increases as the need increases. For example, women — who need to replenish iron lost in menstruation — absorb a greater percentage of iron as compared to men.
Presence of other minerals: In active absorption, only a limited number of spots exist to transport a mineral across the intestine. Minerals of similar charges actually compete with each other for absorption. Calcium competes with magnesium and iron; zinc competes with copper, etc. An excess of one mineral may lead to a deficiency of another, even if the diet is adequate. This situation can become an issue with improper supplementation and/or focusing only on a specific mineral in the diet.
Fortification and balance
According to law, iron is the only mineral that must be added back into grain products after processing, a process called enrichment. However, as long as a food is labeled accurately, food manufacturers can add different minerals through fortification. Many products on the market are fortified with one or more minerals, which helps to ensure adequate mineral intake.
Connie Weaver, Ph.D., Department of Foods and Nutrition, Purdue University, West Lafayette, IN, researches mineral bioavailability and metabolism, particularly calcium and iron. She considers milk as “a whole package.” However, for those looking at mineral fortification, “it depends on what other nutrients are in the food that you fortify — whether it’s juice or cereal or sugary beverage — that will affect the nutrient content,” she says. “A supplement may only provide calcium, but it’s better than not getting enough.”
To help ensure adequate nutrient intake, it is important to be aware that maintaining good health requires all minerals. The Dietary Reference Intakes addresses mineral bioavailability in determining nutrient requirements. One food cannot meet all the mineral requirements; a diet consisting of a variety of foods improves the chances of meeting mineral needs.
Karen Plawecki, a registered dietitian, earned her master’s degree in nutrition with an emphasis in food science from Purdue University. Currently, she teaches a nutrition and food course at the University of Illinois, Urbana-Champaign.
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