Calcium, Magnesium and Phosphorus

1. What are the best methods of assessment for calcium, magnesium and phosphorus?

• Phosphorus can be assessed by monitoring serum concentration and urinary excretion; when dietary intake of phosphorus increases the serum level increases as well; also, when urinary excretion increases this can indicate high dietary intake, high PTH and acidosis. We also have to keep in mind that when urinary excretion decreases this means that there is an effect on calcitriol, alkalosis, estrogen, thyroid and growth hormones.

• For Magnesium, serum levels are tightly controlled, and due to this, it is not a good marker for the status of magnesium in the body. Therefore, the best way to assess magnesium is through urine excretion before and after magnesium is administered into the body intravenously. When the body is deficient in magnesium, it will hold onto it opposed to excreting it through the urine. When an individual is deficient in magnesium less than 80% of the magnesium is secreted.

• Like magnesium, serum calcium is tightly regulated making this a poor marker for the status of calcium. Instead serum calcium reflects alterations of calcium metabolism, in this we need to keep in mind that albumin levels must be considered, as calcium binds to albumin to be transported through the blood. The best way to assess calcium status is to observe bone mass. DEXA is used to measure bone mineral contents this is the best method because it is accurate and repeatable.

6. What are the best foods for calcium, magnesium and phosphorus – and which foods/nutrients inhibit absorption?

• Milk and dairy are the best sources of calcium. Other sources include tofu and sardines with the bone. However, orange juice fortified with calcium is also a good source because the acidity in the orange juice will help the body absorb more of the calcium. Leafy greens, including spinach, turnip greens and collards are also good non-dairy sources of calcium. Some nutrients that help to enhance calcium absorption include: vitamin D, sugars and sugar alcohol, and protein. On the other hand, nutrients/substances that inhibit calcium absorption include: fiber, phytic acid, oxalic acid, unabsorbed fatty acid and excessive zinc and magnesium.

• Magnesium is in a wide variety of foods, food processing and preparation may reduce the magnesium content of the food. High sources of magnesium are sesame seeds, halibut, almonds and cashews, other sources that are not as high include: spinach, black beans, and brown rice. Nutrients/substances that help enhance the absorption of magnesium include: vitamin D, protein, carbohydrates and fructose. There are some interactions with magnesium and they include calcium, phosphorus, and potassium. Nutrients that inhibit the absorption of magnesium are phytic acid, fiber and excess unabsorbed fatty acids.

• Phosphorus is the second most abundant mineral, and like magnesium, it is in a wide variety of foods. Some high sourced foods include: beef liver, and milk, other sources include: chicken, oysters and peanut butter. It has been studied and determined that animal products are a better source of phosphorus than plant sources. One nutrient that can help with the absorption of phosphorus is vitamin D, and nutrients that can inhibit the absorption are phytic acid, and excessive intakes of magnesium, calcium and aluminum [1].
        The NIH suggested that the two forms of calcium in supplements and carbonate and citrate. Calcium carbonate is more commonly available and is both inexpensive and convenient due to its dependence on stomach acid for absorption, it explains that calcium carbonate is absorbed most efficiently when taken with food, whereas calcium citrate is absorbed equally well when taken with or without food. This will explained comments on orange juice and other fortified juices with high acidity and their function on absorption of calcium in the body [2].
        Some absorbed calcium is eliminated from the body in urine, feces and sweet. Per the NIH protein intake with high sodium increases urinary calcium excretion; recent research suggested that high protein intake also increases intestinal calcium absorption therefore the conclusion is that protein with help with calcium absorption with moderate sodium intake. Caffeine intake is another way of losing calcium from the body for example one cup of regular brewed coffee causes a loss of 2-3 mg of calcium. Phosphorus intake, will have some effect on calcium excretion however, is minimal. Several observational studies suggest that consumption of carbonated soft drinks with high levels of phosphate is associated with reduced bone mass and increased fracture risk. However, the effect is probably due to replacing milk with soda rather than the phosphorus itself [2].

1. Gropper S, Smith J, Carr T., Advanced Nutrition and Human Metabolism. 7th ed. Boston, MA: Cengage Learning; 2018

2. NIH. Calcium. Website. Last Updated November 17, 2016. Accessed on November 5, 2017.


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