Micronutrient: a chemical element or substance required in trace amounts for the normal growth and development of living organisms.
The Periodic Table and the Required Trace and Major Minerals Six major minerals and at least eight trace minerals are vital for health and are therefore essential nutrients.
Influence of Micronutrients on Bone and Muscle
Mirconutrients, vitamins and minerals, are essential for wide variety of functions within the human body. In fact, athletes actually need more micronutrients because the demands placed on the body through intensified physical activity. A higher percentage of micronutrients are needed because of increased loss from sweating, decreased nutrient absorption, and increased tissue repair/maintenance (Lorincz, Manske, & Zernicke, 2009). Most notably, calcium and vitamin D are crucial in promoting healthy bone. Other important micronutrients that influence bone health include vitamin B6, vitamin B12, vitamin C, folate, fluoride, iron, magnesium, sodium and zinc. Micronutrients that influence muscle are calcium, choline, B vitamins, vitamin C, vitamin E, iron, magnesium, phosphorus and zinc.
Biotin
|
Food |
Serving Size |
Amount (mg) |
DRI/DV (%) |
Peanuts |
0.25 cup |
26.3 |
87.6 |
Tomatoes |
1 cup |
7.2 |
24 |
Almonds |
0.25 cup |
14.7 |
49.1 |
Eggs |
1 each |
8.0 |
26.7 |
Onions |
1 cup |
8.0 |
26.6 |
Carrots |
1 cup |
6.1 |
20.3 |
Romaine Lettuce |
2 cups |
1.8 |
6.0 |
Cauliflower |
1 cup |
1.6 |
5.4 |
Sweet Potato |
1 medium |
8.6 |
28.7 |
Oats |
0.25 cup |
7.8 |
26.0 |
Calcium
|
Calcium is required for a number of physiologic processes including muscle contraction, nerve impulse transmission, and for maintaining hydroxyapatite, the inorganic portion of bone mainly composed of calcium and phosphate. The optimal calcium intake for young adults is 1200 mg/day, for women between the ages of 25-50 years is 1000 mg/day, and 1500 mg/day for postmenopausal women (not of hormone replacement therapy) or women who are at risk of developing amenorrhea. Athletes have a higher risk for calcium deficiency, a status which can negatively affect bone health and physical performance (Lorincz, Manske, & Zernicke, 2009). Good sources of calcium include tofu, sardines, sesame seeds, dairy products, collard greens and spinach (Mateljan, 2001).
Food |
Serving Size |
Amount (mg) |
DRI/DV (%) |
Tofu |
4 oz |
774.5 |
77.5 |
Collard Greens |
1 cup |
267.9 |
26.8 |
Spinach |
1 cup |
244.8 |
24.5 |
Turnip Greens |
1 cup |
197.3 |
19.7 |
Mustard Greens |
1 cup |
165.2 |
16.5 |
Beet Greens |
1 cup |
164.2 |
16.4 |
Bok Choy |
1 cup |
158.1 |
15.8 |
Yogurt |
1 cup |
296.5 |
30 |
Swiss Chard |
1 cup |
101.5 |
10 |
Kale |
1 cup |
93.6 |
9 |
Choline
|
Choline is an essential micronutrient required for the structure of cell membrane and is required to form actlycholine (Ach), a neurotransmitter released in skeletal muscle. Ach begins a series of chemical events that cause the muscle to contract (Birch, MacLaren, & George, 2004). A reduction of Ach is theorized to contribute to fatigue, thus suggesting that supplementing choline may reduce fatigue. However, research suggests choline does not prolong aerobic training (Williams, 2004). Good sources of choline include shrimp, eggs, scallops, chicken, turkey, tuna, cod and salmon (Mateljan, 2001)
Vitamin D
|
Vitamin D is an essential chemical messenger that controls calcium levels within the blood. When calcium levels within the blood are low, vitamin D can enhance calcium absorption within the intestines. In addition, Vitamin D is able to send messages to breakdown existing bone and release calcium into the blood stream. When calcium levels within the blood are too high, vitamin D can assist in removing calcium from the blood by stimulating a bone forming process. Vitamin D3, the preferred form of Vitamin D, can be created within the skin via the interaction of sunlight, or ultraviolet radiation, and a couple Vitamin D precursors. Vitamin D3 can also be obtained through diet, in an inactive form, which is eventually converted in the liver to become active. The current daily adequate intake for vitamin C is 200 IU/day for adults up to 50 years of age, 400 IU/day for adults between 51-70 years of age, and 600 IU/day for adults over 71 years of age. However, researchers suggest at least 1000-4000 IU/day to ensure adequate protective health effects. Sources of vitamin D include regular sun exposure (about 2 times/week with the arms and legs exposed 5-30 minutes depending on season, latitude and skin pigmentation) or vitamin D3 supplementation (Lorincz, Manske, & Zernicke, 2009). Other sources of vitamin D include salmon, sardines and tuna (Mateljan, 2001).
B Vitamins
|
The B vitamins, including vitamin B6, vitamin B12 and folate, play an important role in growth and bone health. B vitamins, B1 (thiamin), B6 (pyridoxine), B12 (methylcobalamin) and folate, play a direct role in metabolizing macronutrients. The B vitamins aid in the metabolism of an amino acid called homocysteine. Although the mechanism is not exactly clear, researchers have determined that high concentrations of homocysteine in conjunction with low levels of vitamin B12 and folate are associated with low BMD, and a higher risk of fractures in the geriatric population. B vitamins can be obtained through both plant (besides vitamin B12) and animal sources. While deficiency of B vitamins impairs physical performance, supplementing with B-vitamins does not seem to enhance performance (Williams, 2004). The recommended dietary allowance (RDA) for vitamin B6 is 1.3 mg/day for both men and women between the ages of 19-50. Good sources of vitamin B6 include fish, chicken, nuts, legumes, potatoes and bananas. The RDA for thiamin is 1.1 mg/day for females and 1.2 mg/day for males. Good sources of thiamin include pork, whole grains, legumes, tuna, soy milk. The RDA for folic acid (folate) is 400 μg/day for men and women of all ages. Good sources of folate include green leafy vegetables, citrus fruits and legumes. The RDA for vitamin B12 is 2.4 μg/day for men and women of all ages. Vitamin B12 can only be obtained through animal sources. Good sources of vitamin B12 include clams, mussels, mackerel, beef, salmon and eggs (Fratoni & Brandi, 2015).
Vitamin C
|
Vitamin C is an antioxidant required for the synthesis of collagen, the largest component in the organic portion of bone. Collagen also serves to function as the connective tissue forming layers within the muscle, which is divided into three categories: epimysium, perimysium, and endomysium (Birch, MacLaren, & George, 2004). Research conducted on animals demonstrates that vitamin C deficiency is correlation with impaired bone bass, cartilage and connective tissue. In addition, research as concluded that there is a positive correlation between vitamin C intakes and BMD at multiple bone sites. Researchers suggest that vitamin C may also decrease oxidative stress, which may help prevent osteoporosis (Sahni et al., 2008). As an antioxidant, vitamin C helps reduce oxidative damage to cells and enhances the immune system. Thus, vitamin C has been shown to improve performance in deficient subjects, but this effect is not seen in well-nourished athletes (Kreider et al., 2010), (Williams, 2004). While research in inconsistent, vitamin C may potential treat or reduce delayed onset muscle soreness (DOMS), which is characterized as soreness, stiffness, inflammation, and/or pain that may occur after an acute resistance training session (Birch, MacLaren, & George, 2004), (Maes & Kravitz, 2003). The current RDA for vitamin C is 90 mg/day and 75 mg/day, respectively for adult men and women. Since smoking increases oxidative stress and the metabolism of vitamin C, smokers are advised to increase vitamin C intake by 35 mg/day (Krinsky et al., 2000). Good sources of vitamin C include papaya, bell peppers, broccoli, Brussels sprouts, strawberries, pineapple, oranges and kiwi (Mateljan, 2001).
Vitamin E
|
Vitamin E is another antioxidant, that is known to reduce free radicals, which are highly reactive molecules that causes oxidative damage to cells. Inconsistent research supports whether or not vitamin E may also reduce DOMS (Maes & Kravitz, 2003). However, other research has supported vitamin E enhancing the utilization of oxygen at high altitudes, by preventing the destruction of red blood cells (Williams, 2004). The current RDA for vitamin E is 15 mg/day for both adult men and women (Ross, Taylor, Yaktine, & Del Valle, 2011). Good sources of vitamin E include sunflower seeds, almonds, spinach, swiss chard and avocado (Mateljan, 2001).
Fluoride |
Some research on fluoride has been shown to stimulate bone formation and increase bone density (Dure-Smith et al., 1991). However, it should be noted that fluoride should be consumed via food sources rather than via drinking water, as more recent research conducted on animals suggests that fluoride may decrease bone strength by altering the microarchitechture, thus increasing the risk of fracture (Doull et al., 2006). Fluoride has been shown to accumulate in tissues within the human body, including bone (Doull et al., 2006) and the pineal gland (Luke, 1997). The current adequate intake (AI) for fluoride is 3 mg/day for adult females and 4 mg/day for adult males (Ross, Taylor, Yaktine, & Del Valle, 2011). Most foods have concentrations of fluoride well below 0.05 mg/100 g, except fluoridated water (Young et al., 1997). Keep in mind, the chemicals used to fluoridate water (fluorosilic acid, sodium flurosilicate, sodium fluoride) are not naturally occurring (“NSF Fact Sheet on Fluoridation,” 2013).
Iron
|
Iron, serving as a component of hemoglobin, binds and carries oxygen within red blood cells. Adequate consumption of iron is suggested, as iron deficiency can lead to anemia, a condition with a decreased ability of the blood to carry oxygen. Research suggests that supplementation of iron does not enhance performance in those who meet the recommended intake (Kreider et al., 2010). The current RDI for iron is 8 mg/day for men and 18 mg/day for women (Ross, Taylor, Yaktine, & Del Valle, 2011). Good sources of iron include soybeans, lentils, spinach and sesame seeds (Mateljan, 2001).
Magnesium |
Magnesium, an essential micronutrient for all living cells, plays many roles in the body as it required for more than 300 enzymes (including the ATP reaction) and is required for anaerobic/aerobic energy production. Magnesium is crucial for optimal bone and muscle health, as about 60% of magnesium is stored in bone, and about 30% of the magnesium within the body is found in skeletal muscle. Low levels of magnesium promotes osteoporosis. Magnesium deficiency is associated with fragile and brittle bones, microfractures, impaired mechanical properties and low grade inflammation. Research suggests magnesium deficiency impairs protein synthesis, cell reproduction, as well as many hormones and neurotransmitters. Magnesium depletion may also result in muscle cramps, hypertension and is linked to a variety of diseases within the cardiovascular and neuromuscular systems, as well as diabetes. Dietary recommendations for athletes is suggest to be at or above the current dietary reference intake (DRI) (Young et al., 1997). Most research suggest that supplementation of magnesium does not enhance performance unless a deficiency is present (Kreider et al., 2010). Diets rich in processed foods and poor in micronutrients result in inadequate dietary magnesium intake (Castiglioni, Cazzaniga, Albisetti, & Maier, 2013). The current dietary reference intake (DRI) for magnesium is 400 mg/day and 310 mg/day respectively for males and females between the ages of 19-30 (Ross, Taylor, Yaktine, & Del Valle, 2011). Good sources of magnesium include pumpkin seeds, spinach, soybeans, sesame seeds, quinoa, black beans, cashews and sunflower seeds (Mateljan, 2001).
Phosphorus
|
Phosphorus is essential in component of animal tissues, and is most commonly found in the form of phosphate. Phosphorus is a structural component of cell membranes and nucleic acids. Roughly 15 percent of the total phosphorus in the body is distributed in soft tissues, the remain 85 percent is within bone. Most foods, besides plant seeds, display an adequate phosphorus bioavailability. Phosphorus sources are ubiquitous and include nearly all foods that are naturally occurring (Young et al., 1997). The current RDI for phosphorus is 700 mg/day for both men and women (Ross, Taylor, Yaktine, & Del Valle, 2011).
Sodium |
Sodium plays a very important role in body, however the concentration of sodium must be optimal as too much or too little can result in adverse effects. Consumption of too much sodium promotes urinary calcium excretion, which in turn may lead to increased bone loss (Heaney, 2006). The current AI for sodium is 2.3 g/day for adults (Ross, Taylor, Yaktine, & Del Valle, 2011).
Zinc |
Zinc serves many functions within a variety of enzymes, such as supporting the structure of proteins, regulates gene expression, and assists growth and development (Russell et al., 2001). Regarding bone, zinc stimulates bone formation and collagen synthesis in osteocytes (Seo, Cho, Kim, Shin, & Kwun, 2010). Deficiency of zinc leads to impairment of growth and development (Russel et al., 2001). Supplementing of zinc appears to enhance immune function (Kreider et al., 2010). The current DRI for zinc is 8 mg/day for adult females and 11 mg/day for adult males (Ross, Taylor, Yaktine, & Del Valle, 2011). Good sources of zinc include beef, sesame seeds, pumpkin seeds, lentil, garbanzo beans and cashews (Mateljan, 2001)
References
Birch, K., MacLaren, D., & George, K. (2004). Instant notes in sport and exercise physiology. London: BIOS Scientific Publishers.
Castiglioni, S., Cazzaniga, A., Albisetti, W., & Maier, J. A. M. (2013). Magnesium and osteoporosis: Current state of knowledge and future research directions. , 5(8), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775240/
Doull, J., Boekelheide, K., Farishian, B., Isaacson, R., Klotz, J., Kumar, J., & Limeback, H. (2006). Fluoride in Drinking Water: A Scientific Review of EPA’s Standards. Retrieved December 3, 2016, from The National Academies Press, https://www.nap.edu/read/11571/chapter/7#142
Dure-Smith, B., Kraenzlin, M., Farley, S., Libanati, C., Schulz, E., & Baylink, D. (1991). Fluoride therapy for osteoporosis: A review of dose response, duration of treatment, and skeletal sites of action. Calcified tissue international., 49, . Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/1933602
Fratoni, V., & Brandi, M. L. (2015). B vitamins, Homocysteine and bone health. , 7(4), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4425139/
Heaney, R. (2006). Role of dietary sodium in osteoporosis. Journal of the American College of Nutrition., 25, . Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/16772639
Krinsky, N., Beecher, G., Burk, R., Chan, A., Erdman, J., Jacob, R., … Prentice, R. (2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Retrieved December 3, 2016, from The National Academies Press, https://www.nap.edu/read/9810/chapter/7
Lorincz, C., Manske, S. L., & Zernicke, R. (2009). Bone health. , 1(3), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3445243/
Luke, J. (1997). The effect of Fluoride on the Physiology of the Pineal Gland. Retrieved December 3, 2016, from http://epubs.surrey.ac.uk/895/1/fulltext.pdf
Maes, J., & Kravitz, L. (2003, November 16). Treating and Prenting DOMS. Retrieved December 6, 2016, from University of New Mexico, http://www.unm.edu/~lkravitz/Article%20folder/domos.html
Mateljan, G. (2001). Calcium. Retrieved December 1, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=45
Mateljan, G. (2001). Choline. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=50
Mateljan, G. (2001). Iron. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=70
Mateljan, G. (2001). Magnesium. Retrieved December 3, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=75
Mateljan, G. (2001). Vitamin C. Retrieved December 3, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=109
Mateljan, G. (2001). Vitamin D. Retrieved December 1, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=110
Mateljan, G. (2001). Vitamin E. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=111
Mateljan, G. (2001). Zinc. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=115
NSF Fact Sheet on Fluoridation. (2013, February 13). Retrieved December 3, 2016, from National Sanitation Foundation, https://www.nsf.org/newsroom_pdf/NSF_Fact_Sheet_on_Fluoridation.pdf
Ross, C. A., Taylor, C. L., Yaktine, A. L., & Del Valle, H. B. (2011). - dietary reference Intakes for calcium and vitamin D - NCBI bookshelf. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK56068/table/summarytables.t3/?report=objectonly
Russell, R., Beard, J., Cousins, R., Dunn, J., Ferland, G., Hambidge, M., … Ross, C. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Retrieved December 4, 2016, from The National Academies Press, https://www.nap.edu/read/10026/chapter/14
Sahni, S., Hannan, M. T., Gagnon, D., Blumberg, J., Cupples, A. L., Kiel, D. P., & Tucker, K. L. (2008). High vitamin C intake is associated with lower 4-Year bone loss in elderly men. The Journal of Nutrition, 138(10), 1931–1938. Retrieved from http://jn.nutrition.org/content/138/10/1931.full
Seo, H.-J., Cho, Y.-E., Kim, T., Shin, H.-I., & Kwun, I.-S. (2010). Zinc may increase bone formation through stimulating cell proliferation, alkaline phosphatase activity and collagen synthesis in osteoblastic MC3T3-E1 cells. , 4(5), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2981717/
Williams, M. H. (2004). Dietary supplements and sports performance: Introduction and vitamins. Journal of the International Society of Sports Nutrition, 1(2), 1. doi:10.1186/1550-2783-1-2-1
Young, V., Erdman, J., King, J., Allen, L., Atkinson, S., & Dwyer, J. (1997). Read “dietary reference Intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride” at NAP.edu. Retrieved from https://www.nap.edu/read/5776/chapter/10#292
Castiglioni, S., Cazzaniga, A., Albisetti, W., & Maier, J. A. M. (2013). Magnesium and osteoporosis: Current state of knowledge and future research directions. , 5(8), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3775240/
Doull, J., Boekelheide, K., Farishian, B., Isaacson, R., Klotz, J., Kumar, J., & Limeback, H. (2006). Fluoride in Drinking Water: A Scientific Review of EPA’s Standards. Retrieved December 3, 2016, from The National Academies Press, https://www.nap.edu/read/11571/chapter/7#142
Dure-Smith, B., Kraenzlin, M., Farley, S., Libanati, C., Schulz, E., & Baylink, D. (1991). Fluoride therapy for osteoporosis: A review of dose response, duration of treatment, and skeletal sites of action. Calcified tissue international., 49, . Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/1933602
Fratoni, V., & Brandi, M. L. (2015). B vitamins, Homocysteine and bone health. , 7(4), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4425139/
Heaney, R. (2006). Role of dietary sodium in osteoporosis. Journal of the American College of Nutrition., 25, . Retrieved from https://www.ncbi.nlm.nih.gov/pubmed/16772639
Krinsky, N., Beecher, G., Burk, R., Chan, A., Erdman, J., Jacob, R., … Prentice, R. (2000). Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium, and Carotenoids. Retrieved December 3, 2016, from The National Academies Press, https://www.nap.edu/read/9810/chapter/7
Lorincz, C., Manske, S. L., & Zernicke, R. (2009). Bone health. , 1(3), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3445243/
Luke, J. (1997). The effect of Fluoride on the Physiology of the Pineal Gland. Retrieved December 3, 2016, from http://epubs.surrey.ac.uk/895/1/fulltext.pdf
Maes, J., & Kravitz, L. (2003, November 16). Treating and Prenting DOMS. Retrieved December 6, 2016, from University of New Mexico, http://www.unm.edu/~lkravitz/Article%20folder/domos.html
Mateljan, G. (2001). Calcium. Retrieved December 1, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=45
Mateljan, G. (2001). Choline. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=50
Mateljan, G. (2001). Iron. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=70
Mateljan, G. (2001). Magnesium. Retrieved December 3, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=75
Mateljan, G. (2001). Vitamin C. Retrieved December 3, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=109
Mateljan, G. (2001). Vitamin D. Retrieved December 1, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=110
Mateljan, G. (2001). Vitamin E. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=111
Mateljan, G. (2001). Zinc. Retrieved December 4, 2016, from The World’s Healthiest Foods, http://whfoods.com/genpage.php?tname=nutrient&dbid=115
NSF Fact Sheet on Fluoridation. (2013, February 13). Retrieved December 3, 2016, from National Sanitation Foundation, https://www.nsf.org/newsroom_pdf/NSF_Fact_Sheet_on_Fluoridation.pdf
Ross, C. A., Taylor, C. L., Yaktine, A. L., & Del Valle, H. B. (2011). - dietary reference Intakes for calcium and vitamin D - NCBI bookshelf. Retrieved from https://www.ncbi.nlm.nih.gov/books/NBK56068/table/summarytables.t3/?report=objectonly
Russell, R., Beard, J., Cousins, R., Dunn, J., Ferland, G., Hambidge, M., … Ross, C. (2001). Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc. Retrieved December 4, 2016, from The National Academies Press, https://www.nap.edu/read/10026/chapter/14
Sahni, S., Hannan, M. T., Gagnon, D., Blumberg, J., Cupples, A. L., Kiel, D. P., & Tucker, K. L. (2008). High vitamin C intake is associated with lower 4-Year bone loss in elderly men. The Journal of Nutrition, 138(10), 1931–1938. Retrieved from http://jn.nutrition.org/content/138/10/1931.full
Seo, H.-J., Cho, Y.-E., Kim, T., Shin, H.-I., & Kwun, I.-S. (2010). Zinc may increase bone formation through stimulating cell proliferation, alkaline phosphatase activity and collagen synthesis in osteoblastic MC3T3-E1 cells. , 4(5), . Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2981717/
Williams, M. H. (2004). Dietary supplements and sports performance: Introduction and vitamins. Journal of the International Society of Sports Nutrition, 1(2), 1. doi:10.1186/1550-2783-1-2-1
Young, V., Erdman, J., King, J., Allen, L., Atkinson, S., & Dwyer, J. (1997). Read “dietary reference Intakes for calcium, phosphorus, magnesium, vitamin D, and fluoride” at NAP.edu. Retrieved from https://www.nap.edu/read/5776/chapter/10#292