Major micronutrients

Microelements include iron, zinc, copper, selenium, iodine, fluorine, chromium, manganese, boron and vanadium.

• Iron

The total iron content of the body is about 5.0 and 3.8 mg kg body weight in men and women, respectively. Iron is involved in various processes associated with physical activity, such as the synthesis of hemoglobin and myoglobin, and is also a component of the cytochromes and tegemin iron compounds. Some iron-dependent enzymes (NADH2 and succinate dehydrogenase) are involved in oxidative metabolism.

Recommendations for physically active individuals. The incidence of anemia due to iron deficiency is approximately 5-6% 109, and may be higher in adults and women due to growth requirements and menstrual cycles. Iron deficiency, on the other hand, is quite common among athletes, ranging from 30 to 50%, especially in female athletes specializing in endurance sports.

Female athletes often have inadequate dietary iron intake (due to reduced calorie intake and/or reduced meat intake), and sweat iron losses, gastrointestinal bleeding, myoglobinuria, intravascular hemolysis-induced hemoglobinuria, and menstruation-induced hemoglobinuria may compromise their health and performance. The reduction in performance is not only due to anemia, decreased aerobic capacity, and decreased endurance. Anemia due to dietary iron deficiency has a negative impact on skeletal muscle ATP resynthesis and the ability to endure prolonged exercise. Recent studies have shown that iron-deficient women have reduced maximal oxygen consumption (V02max), which is a result of decreased iron stores rather than decreased oxygen transport. Changes in metabolic rate, thyroid hormone status, and thermoregulation have been noted in iron depletion and iron deficiency anemia. Mild iron deficiency anemia may also have a negative impact on psychomotor development and intellectual abilities.

Iron supplements. For people with iron deficiency anemia, dietary iron supplementation is most beneficial to increase iron stores and prevent harmful physiological effects. Ferrous sulfate is the cheapest and most readily available form of iron supplement. Adults with iron deficiency anemia are recommended to take at least 60 mg of iron daily between meals. There is a strong case for controlled iron supplementation by all athletes with low serum ferritin levels.

Factors Affecting Iron Absorption: As with calcium, some factors inhibit and others enhance iron absorption.

Factors that inhibit iron absorption:

• phytins;
• oxalates;
• polyphenols (tannins of tea and coffee);
• adequate iron stores in the body;
• excessive consumption of other macronutrients (zinc, calcium, manganese);
• decreased production of hydrochloric acid in gastric juice;
• some antacids.

Factors that enhance iron absorption:

• appropriate secretion of gastric acid;
• heme forms of iron;
• increased need for red blood cells (eg, blood loss, altitude, exercise, pregnancy);
• lack of iron in the body;
• meat protein factor;
• ascorbic acid.

Consuming vitamin C-containing foods or drinks with food, as well as tea or coffee 1 hour before or after meals, increases iron absorption. This regimen is highly recommended even for those diagnosed with iron deficiency anemia who take iron supplements.

• Zinc

Zinc is found in all organs, tissues, fluids and secretions. About 60% of all zinc in the body is in the muscles, 29% in the bones and 1% in the gastrointestinal tract, skin, kidneys, brain, lungs and sex glands. Zinc is involved in more than 30 metabolic reactions in the body. Alkaline phosphatase, Zn, Cu-superoxide dismutase are just some of the zinc metalloenzymes.

Recommendations for physically active individuals. Many people in the United States do not consume the required amount of zinc. Young women, for example, take only 9.7 mg of zinc per day, and about 50% of female runners also consume less than the recommended amount of zinc. More recent studies show that swimmers - both men and women - consume more than 70% of the recommended amount of zinc. Apparently, if zinc intake from food is sufficient, then physical activity does not negatively affect its status.

A transient effect of exercise on zinc status has been demonstrated. Zinc status has been shown to directly affect basal metabolic rate and thyroid hormone levels in men, which may have adverse effects on health outcomes. The effects of zinc supplementation on basal metabolic rate, thyroid hormone levels, and protein utilization were studied in six young men who participated in a 75-day metabolic study. They were initially given 16.5 mg zinc per day for 12 days and then 5.5 mg per day for 54 days, which resulted in zinc deficiency. Significant decreases in basal metabolic rate, thyroid hormone levels, and protein utilization were observed. This study provides insight into the relationships between zinc status, basal metabolic rate, and thyroid hormone levels. The long-term effects of daily exercise on zinc status require further study to develop more definitive recommendations for physically active individuals. Table 5.8 lists some food sources of zinc.

• Copper

The human body contains from 50 to 120 mg of copper. Copper is involved in enhancing iron absorption (with the help of the metalloenzyme ceruloplasmin), the formation of collagen and elastin, in the electron transport chain (cytochrome oxidase), and is also an antioxidant (Zn, Cu-superoxide dismutase).

Optimal intake. There are no RDIs for copper. Based on the 1989 recommendations, a safe and adequate daily dietary intake of copper has been developed, which recommends a daily intake of 1.5–3.0 mg for adults.

Sources. The copper content in food products is greatly influenced by the soil condition. Rich sources of copper are: liver, seafood (oysters), cocoa, mushrooms, various nuts, seeds (sunflower seeds), bread, cereals.

• Selenium

Selenium is well known as an antioxidant, as it is part of the enzyme glutathione peroxidase. It is involved in the metabolism of thyroid hormone. Recommendations for physically active individuals. Data on the selenium requirements of athletes and sedentary individuals are very limited.

It might be expected that because of increased oxidation during exercise, physically active individuals would require more selenium in their diet. However, a study in which 12 men consumed a diet containing 180 mcg of selenomethionine for 10 days and 12 men who received a placebo found that endurance training increased the antioxidant capacity of glutathione peroxidase, but selenium supplementation had no effect on the parameters. Because data on selenium intake are limited, physically active individuals should consume no more than the RDI.

Sources: Dietary sources, as for copper, vary widely and depend on the selenium content of the soil. Dietary sources of selenium include fish, shellfish, meat, eggs, and milk.

• Iodine

Thyroid hormones are synthesized from iodine and tyrosine, so iodine is necessary for normal metabolic rate.

Recommendations for physically active individuals. There are no data on iodine for physically active individuals, but inadequate iodine intake may affect thyroid hormone synthesis.

Sources: Iodine is found primarily in sea fish, molasses, iodized salt, and seafood.

• Fluorine

The main function of fluoride is to protect teeth and bones. It is known that adequate amounts of it in water help prevent dental caries. Fluoride stimulates bone growth (osteoblasts), increases the formation of trabecular bones and the mineral density of the bones of the spine. The appendix contains standards for fluoride.

Recommendations for physically active individuals. There is little research on the fluoride requirements of athletes. Most studies assess the effects of fluoride on bone mineral density and osteoporosis prevention.

Given the important role of fluoride in bone metabolism, further research on the role of fluoride in female athletes is needed.

Sources: Dietary sources of fluoride are limited to tea, seaweed, seafood, and fluoridated household water.

• Chromium

Chromium is a well-studied mineral. It enhances the action of insulin and thus affects the metabolism of carbohydrates, lipids and proteins. Chromium also has an anti-atherogenic effect, reducing serum cholesterol levels, but this data is not sufficiently substantiated. It has been proposed for weight gain as well as weight loss. However, the results of several studies have not confirmed this assumption.

Optimal intake. According to the calculation of safe and adequate daily intake, adults need 50-200 mcg of chromium per day. Anderson, Kozlovsky suggested an average value for chromium intake per day - 25 mcg for women and 33 mcg for men.

Recommendations for physically active individuals. Since chromium excretion in urine increases on training days, the athlete's diet requires increased chromium content. Since chromium can change the status of other minerals (e.g. iron), its consumption above the calculated level is not recommended until further studies are completed.

Sources: Food sources of chromium include whole grains, organ meats, beer, egg yolk, mushrooms, and nuts.

• Manganese

Manganese is involved in the antioxidant activity of the body, as it is part of superoxide dismutase. Manganese is also involved in carbohydrate and bone metabolism.

Optimal Intake: According to safe and adequate intake calculations, adults require 2-5 mg of manganese per day.

Recommendations for physically active individuals: There is no evidence that athletes need higher levels of manganese in their diet.

Sources: Food sources of manganese: whole grains, leafy vegetables, nuts, beans, tea.

• Molybdenum

Molybdenum interacts with copper and iron. Excess molybdenum can inhibit copper absorption. Molybdenum is involved in glucocorticoid metabolism.

Optimal Intake: The Recommended Dietary Allowance (RDA) for adults is 74–250 mcg of molybdenum per day.

Recommendations for physically active individuals: There are no data on the molybdenum requirements of physically active individuals.

Sources: Food sources of molybdenum include beans, nuts, whole grains, milk and dairy products.

• Boron

Boron is not currently considered an essential mineral for humans, but it may play a role in bone metabolism by interacting with calcitriol, estradiol, testosterone, magnesium, and calcium. Many athletes believe that boron increases body mass and bone mineral density. However, recent studies have not found these effects of boron.

Optimal Intake: There are no recommended dietary allowances for boron, but adults can consume 1-10 mg of boron per day.

Recommendations for physically active individuals. Much of the research has focused on the effects of boron on bone mineral density and body weight, but it is unclear whether athletes need more of this micronutrient in their diet.

Sources: Food sources of boron include fruits, vegetables, nuts, and legumes.

• Vanadium

Like chromium, vanadium has been shown to enhance the effects of insulin. Like chlorine, vanadium supplements (vanadyl sulfate) have been suggested for mass gain, but these anabolic effects have not been supported by research.

Optimal intake. There are no recommendations for dietary intake of vanadium. The requirements for adults are determined to be in the range of 10-100 mcg per day.

Recommendations for physically active individuals. There are no reports of increased requirements for vanadium or its ergogenic properties. Vanadium supplementation is not justified.

Sources: Food sources of vanadium include grains, mushrooms, and crustaceans.

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