Mary M. Fynn, Ph.D, RD, L.D.N
Minerals are divided into major and trace, dependent upon the requirement for the mineral. Major minerals are required in amounts > 100 mg/day and trace minerals are required in amounts < 100 mg/day.
Mineral bioavailability is defined as the degree to which the amount of the ingested mineral is absorbed and available to the body. This means that even if a food containing the mineral is present in the diet, the mineral may not get into the body.
Factors that will affect the bioavailability of a mineral:
While bioavailability makes the absorption of minerals difficult, most of the minerals can be quite toxic in the pill form. Some of the toxicities result from higher intake of the minerals being consumed, and others result from the disturbances that arise when the mega dose of the minerals causes a decrease in absorption of another mineral.
Minerals are found in both plant and animal foods. The animal sources tend to have greater bioavailability because there are fewer inhibitors present and the mineral content tends to be more concentrated. The one exception is magnesium, which is found only in plant products as magnesium is found in chlorophyll.
Sodium is the major extracellular ion. The functions of sodium include:
Deficiency: a deficiency of sodium can occur with a very low sodium diet, but it is more likely with persistent vomiting/ diarrhea, or excessive perspiration. Low serum levels of sodium, or “hyponatremia”, will result in muscle cramps, nausea, vomiting, and dizziness. Left untreated, hyponatremia can lead to a coma. Hyponatremia is a concern with fluid loss of more than 2-3% of the body weight (e.g., with prolonged, intense physical activity in high heat without sodium replacement with fluid).
Toxicity: most people can adapt to varying intakes of sodium. A high sodium intake will result in thirst and the fluid intake can balance the excessive sodium. About 10-15% of adults are sodium sensitive; in these people, excess sodium can lead to hypertension. High sodium intake will also increase calcium through the urine, which can contribute to osteopenia (loss of bone).
Dietary sources of sodium: there is very little sodium found naturally in food. Dietary sodium intake is primarily from salt, which is sodium (40%) and chloride (60%). About 33-50% of dietary sodium is from what is added to food in preparation. A major contributor to sodium intake is the food at Fast Food restaurants; table restaurants also contribute more than home intake. Another major contributor is processed foods; the addition of sodium in food processing is for both taste and food preservation. We can absorb most of the sodium that we consume.
Requirement for sodium: the recommended minimum sodium intake is 500 mg. Most Americans consume 3000 to 6000 mg.
Potassium is the major intracellular ion. The functions of potassium are:
Deficiency: low serum potassium, or hypokalemia, is life threatening. Severe potassium deficiency can cause an irregular heart beat and untreated can lead to death. Symptomatic potassium deficiency starts with muscle cramps, loss of appetite, and confusion. Potassium deficiency is not common, but can be found in poorly nourished elderly, person with eating disorders, and in athletes who exercise heavily without replacing electrolyte loss.
While the conventional dietary treatment of hypertension since the 1970’s has been to limit dietary sodium, more recent research has shown that potassium may have as much a role in treating hypertension as sodium does. The Dietary Approaches to Stop Hypertension (DASH) study tested the effect of increasing fruits and vegetable intake, initially without specifically lowering sodium. Participants who ate just under 9 servings (a serving = ½ cup) of fruits and vegetables a day had significant lowering of blood pressure. This is actually not a lot of fruits and vegetables. Most pieces of fruit are 2 servings and a half a cup of vegetables is quite small – about the amount eaten by someone who does not like vegetables. Nine servings could be 2 pieces of fruit and 2 ½ cups of vegetables, or a little more than a cup at both lunch and dinner. The mechanism for dietary potassium lowering blood pressure is likely related to fluid balance. Most Americans consume excessive sodium (mainly from Fast Foods and processed foods) while potassium intake tends to be low due to inadequate intake of fruits and vegetables. By increasing produce intake, you can increase the potassium intake to balance the sodium.
Toxicity: a toxicity of potassium is not likely if the kidneys are functioning properly. In diseased kidneys there may be an increase in blood potassium, or hyperkalemia. This can slow the heart beat and if left untreated, this can be fatal.
Dietary sources of potassium: all fruits and vegetables are rich sources of potassium. Bananas are commonly recommended as a source of potassium, however all produce has potassium and based upon serving size, the banana does not have more potassium than other choices. Other good dietary sources are milk, whole grains, beans, meat, and coffee.
Requirement for potassium: the minimum requirement for potassium is 2000 mg/day. This amount can be easily obtained in a varied diet that includes several servings of produce. Some diuretics, which can be used to treat hypertension, may deplete serum potassium. In this situation potassium supplements are typically prescribed as large doses will more quickly replenish the blood levels compared to trying to get sufficient amounts with diet.
Calcium is the most abundant mineral in the body. While > 99% of the total calcium is in bone and teeth, the remaining amount is in the blood and is needed by all cells for some vital functions. The amount of calcium in the body depends on how much is absorbed from the diet and how much is excreted. Absorption of dietary calcium is typically about 20 to 40% of dietary intake, but will increase during periods of increase need, such as growth and pregnancy, to 59 to 75%. Absorption of calcium is enhanced with a more acid environment and requires sufficient stores of calcitriol (active vitamin D). Absorption is inhibited by phytic acid, polyphenols (e.g., tannin – which is found in black tea and red wine) and decreases with age (some of which is related to the decrease in acid in the stomach that occurs with aging).
Regulation of blood calcium: every cell needs calcium and serum levels are not related to dietary intake, but with hormones. When blood calcium decreases, parathyroid hormone (PTH) is released. PTH works with calcitriol to increase kidney retention of calcium and increases calcium absorption in the intestines. PTH with calcitriol will also cause release of calcium from the bone to add to the blood calcium. When there is sufficient calcium absorbed, less calcium is taken from the bone.
Deficiency: a calcium deficiency primarily affects the bone. A low availability of dietary calcium which can result from not eating foods that contain calcium and /or inadequate absorption, and/ or excessive loss of calcium will contribute to osteoporosis. Osteoporosis is when osteopenia is 2 SD below the normal amount of bone. With osteoporosis, the bone composition is basically normal, but there is less bone. This leads to an increased risk of non-violent fracture or a fracture that occurs when the injury to the bone is not severe; for example, breaking a wrist while grabbing a hand rail. Osteoporosis is thought to affect about 25 million people. Loss of bone is a disease of aging and will affect anyone who lives into their 80’s. How severe the loss of bone will be depends on the amount of bone mineral density (BMD) one achieves in young adulthood. BMD is the total mineral content of the bone at a specific site. About 70% of BMD is genetically determined. Bone growth and calcification is most rapid pre-puberty, meaning this is probably the most important time to emphasize calcium intake. Additional mineral is laid down through adolescence but ends by about age 30 years. After the age of 30, bone is being lost. If one achieves a high bone mineral density, they can better sustain age related bone loss. Women make less bone than men and lose it at a faster rate due to loss of endogenous estrogen with menopause and women tend to live longer. For this reason, women are at a higher risk of osteoporosis than men.
The focus of American health professionals is to recommend an increase in calcium for all women over the age of about 50 years. However, while some studies show an increase in bone mineral density in adding calcium plus vitamin D to the diet of someone who had a low intake of calcium, there is little to no data to suggest than an increase in calcium alone will prevent bone fractures. The high rate of osteoporosis seen in the US may be due to other factors, particularly those that affect calcium loss. Both meat intake and high sodium intake will increase sodium loss in the urine.
The requirement for calcium is based upon 40% absorption. Person eating diets lower in meat and/ sodium have a lower requirement for calcium as they lose less calcium in the urine.
Toxicity: An intake of > 2000 mg/day may lead to high blood and urine calcium concentrations. Symptoms from calcium toxicity include headaches, soft tissue calcification, and kidney stones.
Dietary sources of calcium: Dairy products contain the most available dietary sources of calcium. Calcium is also found in leafy green vegetables; however, the oxalic acid in many leafy greens will bind the calcium and greatly decreases absorption.
Calcium supplements: it is best to take calcium supplements between meals to minimize the adverse effect on iron absorption.
Phosphorus is very efficiently absorbed (about 70% absorption). The absorption of phosphorus is enhanced by active vitamin D (1, 25 (OH) D3).
Deficiency: There is no deficiency disease for inadequate phosphorus status. However, chronically ill patients may become severely phosphorus deficient and this can lead to respiratory failure and death.
Toxicity: high intakes of phosphorus from cola beverages have been related to excessive bone fractures in adolescent females (1) and drinking as few as 3 cans of cola per week has been associated with lower hip density in older women (2). This is most likely aggravated when combined with a lower than required calcium intake.
Dietary sources of phosphorus: phosphorus is found in dairy products, meat, food additives, soft drinks, especially cola beverages (carbonation involves the use of phosphoric acid).
Deficiency: in humans, an irregular heartbeat, but magnesium is readily stored so it is hard to develop a deficiency. In animals, irritability and if untreated, can lead to death.
Toxicity: can be seen with kidney failure because the kidneys regulate the blood levels. Symptoms are weakness and nausea.
Dietary sources of magnesium: the best sources of magnesium are found in plant foods as magnesium is found in chlorophyll. Other good sources are whole grains, nuts, seeds, and broccoli. “Hard” tap water contains magnesium.