By Marie Spano, M.S., R.D., C.S.C.S., Contributing Editor
To fight the growing incidence of osteoporosis, calcium-fortified foods and beverages are essential. Yet calcium is complex, and many factors influence this vital nutrient’s absorption, including the type of calcium consumed, whether calcium is consumed in a fed state or on an empty stomach, the food it is consumed with, and the total calcium consumed at one time.
Importance of calcium
Calcium helps bones grow in length during adolescence, and in density up until approximately age 30, when peak bone-mineral density is achieved. After this age, bone-mineral density declines, but maintaining adequate calcium and vitamin D levels and participating in weight-bearing exercise can attenuate this decline. Maintaining bone-mineral density is crucial to preventing osteoporosis and helping bones withstand the force of impact without breaking.
Calcium plays other important roles in the body. It helps regulate muscular contractions and plays a role in normal nerve functioning, blood-vessel expansion and contraction, and hormone and enzyme secretion. When dietary calcium intake falls short, calcium is pulled from its storage site in bone to meet the demands of the body and keep calcium in blood, muscle and intercellular fluids within a constant concentration.
Calcium absorption
Several factors affect how much calcium is absorbed in the gut. These include age, the amount of calcium consumed in one sitting, vitamin D intake, other food components and the type of calcium consumed.
Infants and young children absorb significantly more calcium than adults, up to 60% of their calcium intake, because they need more to support bone growth. Calcium absorption decreases in adulthood and continues to decrease with age. Adults absorb just 15% to 20% of intake, according to the Food and Nutrition Board, Institute of Medicine.
Calcium absorption decreases as the calcium content of a meal increases. Therefore, it is generally recommended that people consume 500 mg or less calcium per serving. Serum vitamin D status also affects calcium absorption. Low serum vitamin D levels decrease intestinal calcium absorption (Journal of Human Nutrition and Dietetics, 2010; 23(1):54-60), and several studies indicate many people have insufficient or deficient levels of vitamin D (“What We Eat in America,” NHANES 2005-2006; Journal of Nutrition, 2007; 137:447-452; European Journal of Clinical Nutrition, 2009; 63:1,377-1,386). Phytic acid, found mainly in whole-grain products, beans, seeds, nuts, soybeans and soy protein, binds to calcium, inhibiting its absorption. Likewise, oxalic acid, which is mainly found in dark-green leafy vegetables, sweet potatoes and beans, also binds calcium, inhibiting its absorption. Though some types of dietary fiber also decrease calcium absorption, the prebiotic fiber inulin enhances calcium absorption (Journal of Nutrition, 2007; 137:2,527S-2,533S).
Formulating with calcium
Given the number of Americans consuming less than the Adequate Intake for calcium, calcium-fortified foods and beverages may help fill the gap, giving consumers more options for meeting their calcium needs. Fortification should be considered carefully and in conjunction with other bone-building nutrients, such as magnesium and vitamins D and K2.
