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.
Trace minerals are the minerals that have a daily nutritional need of < 100 mg. It is generally difficult to produce a deficiency disease in a laboratory setting as they are required in such small amounts, so the actual requirement for some trace minerals is not clear. The trace minerals tend to have low bioavailability and have absorption rates of 1-6%, with plant sources, due to the potential inhibitors, tending to be less well absorbed compared to animal sources. Refining and processing of foods decreases trace mineral content compared to the natural form of the food.
The absorption of iron from food is determined by:
There are 2 forms of iron in food:
Heme iron is easier to absorb and the amount of heme in meat is higher than the amount of non-heme content of plant products. Factors that will influence non-heme absorption:
The absorption of iron is controlled via a mucosal block. This is necessary as once iron is absorbed, it can only be excreted with bloodletting. Women who have regular menstrual cycles will excrete iron. Postmenopausal women and men do not easily get rid of excess iron. The absorption of iron is controlled via the production in the small intestine cells of an iron bind protein called ferritin.
If iron stores are low, a small amount of ferritin is made. This decreases iron storage and increases absorption into the body.
If iron stores are adequate, a high amount of ferritin is made. The ferritin binds to the absorbed iron so iron will not enter the blood. The complex is sloughed off with intestinal cells.
Iron is transported in the blood by a protein called transferrin. Iron is stored in the liver as part of ferritin.
Functions of iron
1. Part of 2 proteins that get oxygen to cells:
2. electron transport chain – iron is part of the cytochromes (carries electrons to O2)
3. needed by the enzyme in the first step of the TCA cycle
4. cofactor for the enzyme that helps break down reactive O2 species, but iron can change valences so iron can catalyze formation of free radicals.
Deficiency: thought to be the most common micronutrient deficiency in the US. Risk of deficiency is higher in premenopausal women due to monthly loss and during periods of growth and in periods of rapid growth. Frequent blood donations can also lead to a deficiency.
Toxicity: excessive iron in the body can catalyze the formation of free radicals. A genetic disorder called hemochromatosis affects the mucosal block so iron is over absorbed. Iron accumulates in organs, like liver and heart, and damage the tissue.
Dietary sources of iron: heme is part of blood so it is found in the blood of meat. Non-heme is added to flour, but in a form that is not readily absorbed. Amount absorbed depends on form and total meal (presence of inhibitors, enhancers).
Absorption is influenced by:
Zinc absorption is not completely understood. When it is absorbed into the intestinal cell, zinc induces synthesis of metallothionein, a protein that binds zinc. If zinc is needed, it crosses to the blood and binds to blood proteins that transport it to the liver. Zinc can be released from liver proteins and carried in the blood by blood proteins. If zinc is not needed, it will be sloughed off with proteins and carried in blood by blood proteins. If zinc is not needed, it will be sloughed off with the intestinal cell and is excreted. A large dose of zinc can override the mucosal block.
Many enzymes require zinc, like enzymes for;
Deficiency: there is not a good test available for detecting zinc deficiency. Although blood levels are used, they are not a sensitive indicator. Zinc deficiency has been shown to produce growth retardation and inadequate sexual development.
Toxicity: mega doses of zinc can reduce HDL levels. Symptoms are diarrhea, cramps, nausea, vomiting, and a depressed immune system.
Dietary sources of zinc: animal flesh is the best source with highest amounts in beef and shellfish. Also found in nuts, beans, and whole grains.
Functions of copper:
Deficiency: a deficiency of copper produces an iron deficiency anemia because copper is needed to transport iron to hemoglobin. The most common cause of copper deficiency is using a mega does of zinc (supplement) as this will inhibit copper absorption.
Toxicity: Wilson’s disease is a genetic disorder where copper cannot be incorporated into ceruloplasmin and there is a decrease in the ability to excrete copper. Copper accumulates in the liver, brain, kidneys, and the eye. This lead to tissue damage if not treated. The typical eye finding is a ring in the iris that is called a “Kayser-Fleischer Ring”.
Dietary sources of copper: good sources of copper are liver, shellfish, nuts, seeds, dark chocolate, legumes, and whole grains.
Selenium has a higher bioavailability than iron or zinc. Unlike other minerals, the current need for selenium does not affect absorption and there is not mucosal block. Selenium is transported in the body with amino acids but the transport of selenium is not fully understood. Selenium is stored bound to the amino acid methionine.
Deficiency: a deficiency of selenium causes muscle pain and wasting. Selenium deficiency will also cause cardiomyopathy (heart muscle damage). There is a selenium deficiency disease called “Keshan Disease”. It results in various degrees of heart deterioration from inadequate selenium intake. Low blood selenium has been related to an increase in death from cardiovascular disease.
Toxicity: this would only occur with mega doses (supplements), results in hair loss, nausea, diarrhea, fatigue, and nail changes. It can also result in cirrhosis of the liver.
Dietary sources of selenium: selenium is found in animal products, whole grains, and nuts. The amount of selenium in a plant product depends on the amount in the soil when the food was being formed.
Iodide is efficiently absorbed and transported as both the free ion and bound to proteins. In humans, most (about 75%) of iodide is in the thyroid gland. A diet very low in iodide over time will result in an enlarged thyroid, or a goiter, as the gland compensates for the low iodide levels with induction of mechanisms to obtain more iodide from the blood.
Deficiency: insufficient thyroid hormone results from iodide deficiency. This leads to the adaptive response of enlargement of the thyroid gland, which is called a “goiter”. They symptoms include a fall in resting metabolic rate and an increase in blood LDL-c. A goiter can be painless or it can cause pressure on the trachea, which can lead to respiratory distress. Treatment with endogenous T4 (synthroid) can help, but surgery may be needed to remove the goiter. Iodide is added to most salt that can be purchased in the US, although iodide-free salt can be readily obtained.
Iodide deficiency during pregnancy is critical if it occurs during the last trimester. It can result in stillbirth, low birth weight, and impaired mental function and retarded development in the fetus. “Cretinism” is the result of severe iodide deficiency. Symptoms of cretinism include mental retardation and growth retardation.
Toxicity: high intake of iodide can inhibit the synthesis of T4. The toxicity can occur with excessive seaweed consumption.
Dietary sources of iodide: saltwater seafood, iodized salt, plant products grown near saltwater. Bioavailability is inhibited by goitrogens, which are found in raw turnip, cabbage, Brussel sprouts, cauliflower, and broccoli.
Fluoride is not really an essential nutrient because it is not needed for basic body function, but has a role in teeth and bone development/ strength.
Deficiency: none has been documented.
Toxicity: nausea, vomiting, diarrhea, pulmonary disturbances, cardiac insufficiency, convulsions, coma and, if severe, paralysis. If the toxicity is during development, it can weaken bones and teeth.
Dietary sources of fluoride: fluoridated water, tea, seaweed. Fluoride in toothpaste provides topical fluoride, which helps prevent caries.
Function: needed in glucose uptake in insulin-responsive cells. The mechanism is not known, but one hypothesis is that chromium can promote insulin receptor signaling through effects on enzymes (e.g., protein tyrosine phosphatases) that attenuate insulin signaling.
Deficiency: results in impaired glucose tolerance and elevated triglycerides.
Toxicity: can lead to production of harmful free radicals.
Dietary sources of chromium: while chromium is thought to be found in a variety of foods, it is not well studied and food tables typically do not include chromium.