By the completion of this module, the nurse should be able to:
- Examine the history of dietary supplements.
- Consider the regulatory process of dietary supplements.
- Discuss current recommendations for dietary intake of various nutrients.
- Explore the three most common categories of dietary supplements.
- Discuss the difference between fat-soluble and water-soluble vitamins.
- Describe the benefits of amino-based dietary supplements.
- Consider the implications of the use of herbal supplements.
- Explore the therapeutic use of probiotic supplements.
- Discuss the potential benefits of fish oil supplements.
Amino acids are the building blocks of life, a class of organic compounds that combine to form proteins (US National Library of Medicine [NLM], 2020a).
Antioxidants are man-made or natural substances that can prevent or delay various types of cellular damage. They are found in fruits and vegetables or as dietary supplements. Examples are beta-carotene, lutein, lycopene, selenium, and vitamins A, D, and E (NLM, 2020f).
Dietary supplements are vitamins, minerals, herbs, and other products that can play an important role in health by replacing dietary intake through supplements; they can be in the form of powder, pills, capsules, drinks, or energy bars (NLM, 2018).
Fat-soluble vitamins are like oil and do not dissolve in water. They are most abundant in high-fat foods and are better absorbed into circulation when eaten with fat. Fat-soluble vitamins are stored in body tissues and remain there, leading to overdose if a person takes too many over time. The four fat-soluble vitamins in the human diet are vitamins A, D, E, and K (Arnarson, 2017; Youdim, 2019).
Macronutrients are essential nutrients that have a large minimum daily requirement, including proteins, carbohydrates, fats, and water (Youdim, 2019).
Micronutrients are essential nutrients that are needed in minute amounts, such as vitamins and minerals (Youdim, 2019).
Water-soluble vitamins dissolve in water and are not stored by the body. Most of these vitamins must be replenished via the diet or dietary supplements on an ongoing basis since they are not stored. Any excess vitamin in the body is excreted. Vitamin B12 is an exception and can be stored in the liver for an extended period. Water-soluble vitamins are vitamin C, niacin, pantothenic acid, riboflavin, thiamin, vitamin B6, and vitamin B12 (Youdim, 2019).
Our current knowledge of dietary supplements has evolved over the past hundred years significantly; however, since 400 B.C., the link between food and health has been known. See Table 1 for a brief overview of the history of dietary supplements.
Over the past century, there have been 13 essential vitamins identified. These are given the term essential since the body must ingest them to maintain health and function. In their absence, the human body will not function properly, and nutritional diseases will develop. Beyond the 13 essential vitamins, dietary supplements are not well defined. Manufacturers take great liberty in developing a broad range of dietary supplements, and there is little scientific proof of necessity or outcome with their intake. Today, over half of US adults take at least one dietary supplement per day, and there are as many as 80,000 dietary supplement products on the market with an estimated $40 billion in annual sales. With this number of products in use, consistency, quality, and safety are of grave concern to most (Kantor et al., 2016; Mozaffarian et al., 2018).
Dietary supplement regulation did not start until early in the 20th century, partly because a structured federal regulatory agency did not exist until 1906; also, there was little to no knowledge of the elements of food that were responsible for various aspects of health and nutrition until the early 1900s-1920s. In 1994, the US Congress passed the Dietary Supplement Health and Education Act (DSHEA), defining a dietary supplement as a product that supplements the diet under the category of food (rather than a medication). The DSHEA further notes that dietary supplements must contain one or more ingredients or their constituents (component, part, or ingredient of a larger whole, for example, valerenic acid is a constituent of the dietary supplement valerian). According to the DSHEA, supplements are to be taken orally (by mouth) as a capsule, pill, tablet, or liquid and should be labeled on the front panel as a dietary supplement. Products can range from children's vitamins to sports nutrition or weight-loss products for adults. The DSHEA provides the US Food and Drug Administration (FDA) with the regulatory authority and the ability to enforce processes that ensure consumer access to dietary supplements. While the FDA regulates dietary supplements, the process is much less stringent than with other food and drug products. Manufacturers do not have to prove that dietary supplements are safe and effective through testing or clinical trials prior to marketing as other drugs require. The DSHEA also mandates that dietary supplements should be produced in compliance with current Good Manufacturing Practices, and the FDA has the authority to remove potentially unsafe products from the market (NIH, 2020f).
When a manufacturer develops a new product, they do not have to notify the FDA unless the product contains a new dietary ingredient (NDI) that includes any vitamin, mineral, or herb that was not on the market prior to October 15, 1994. Of interest, there is no definitive list of dietary ingredients contained in dietary supplements prior to this date, but manufacturers and distributors are expected to determine all ingredients in their supplements and ensure compliance. The FDA notes that a dietary ingredient is "a vitamin; a mineral; an herb or other botanical; an amino acid; a dietary substance for use by man to supplement the diet by increasing total dietary intake; or a concentrate, metabolite, constituent, extract, or combination of any of the above dietary ingredients." In the event of an NDI, the manufacturer is simply required to notify the FDA and provide rationale as to why they feel it is appropriate to add the ingredient, a description of the ingredient, and any identifying published materials that are available (FDA, 2019).
Dietary supplements that have false or misleading labeling are considered misbranded foods. Misbranding provisions are applied if the label does any of the following:
- Fails to list each ingredient’s name and quantity;
- Fails to identify the product as a dietary supplement; or
- Fails to state from which plant each ingredient is derived (FDA, 2019; NIH, 2020f).
In 2015, the US Department of Health and Human Services (HHS) and the USDA jointly formed the Dietary Guidelines Advisory Committee (DGAC). This committee's focus is to evaluate dietary guidelines and identify common characteristics of healthy diets, identify critical new research, and develop food-based recommendations critical to good health. Of interest, the agencies in tandem with the DGAC indicate that 50% of adults in the US suffer from preventable, chronic disease, and two-thirds of adults (and nearly one-third of children) in the US are either overweight or obese with poor dietary habits partially responsible for these disparities. The report notes that positive changes to dietary habits have the potential to modify individual patient outcomes related to the impact of preventable disease progression. While this report primarily focuses on good nutrition through a healthy diet, the value and limitations of dietary supplements are also discussed. Dietary supplements are typically utilized for wellness rather than disease treatment; however, there is recognized value in some disease processes. It is advised that healthcare providers discuss the use of dietary supplements with their patients to determine the best supplemental regime for each individual. Currently up to 50% of adults in the US take at least one dietary supplement each day (USDA, 2015).
Dietary supplements may have claims of health, nutrient, structure, or function. Each type of claim has implications based on FDA expectations. Health claims may describe a relationship between a food or dietary supplement ingredient and a reduced risk of disease or health-related condition. A nutrient content claim describes the amount of a nutrient or dietary substance in a product. A structure or function claim describes how the product may affect a body system or organ, i.e., the cardiovascular system or the liver. If a structure or function claim is made, the manufacturer must notify the FDA with the text of the claim at least 30 days before the product is put on the market for sale. There must be a disclaimer added to any structure or function claim that states, "This statement has not been evaluated by the FDA. This product is not intended to diagnose, treat, cure, or prevent any disease" (NIH, 2020e).
The Food and Nutrition Board (FNB) is the component of the Institute of Medicine that manages food safety and quality issues; establishes guidelines for adequate dietary intake by individuals; and gives authoritative judgments on the relationship between food intake, nutrition, and overall health. This board is part of the National Academies of Sciences, Engineering, and Medicine (NASEM); part of their task is to provide policy guidance to improve the health of Americans using food and nutrition science. The board was established in 1940, evaluating issues related to the safety and adequacy of the US food supply and continuing with this monitoring today. Part of their study of nutrition is focused on the relationship between food intake, nutrition, health maintenance, and prevention of disease (NASEM, 2018).
The dietary reference intake (DRI) is a general term for a set of reference values the FNB uses to plan and assess nutrient intakes of healthy individuals (USDA, 2015). Also, DRIs can be used to develop optimal diet plans and dietary supplement options. The values often differ based on age and sex, but include:
- Recommended daily allowance (RDA) is the average daily level intake that is enough to meet the nutritional requirements of most healthy individuals.
- Adequate intake (AI) is the level assumed to ensure adequate nutrition when evidence is insufficient to determine the RDA.
- Tolerable upper intake level (UL) is the maximum daily intake that is unlikely to cause harmful health effects (NIH, n.d.b).
Crucial to remember for nurses and patients is that good nutrition is foundational to good health, and nutritional assessments and education are vital to assisting patients in optimal health and wellness. Dietary supplements are widely marketed to the American consumer and have a variety of ingredients intended to provide multiple benefits for health and wellness (NIH, n.d.b).
Common Categories of Dietary Supplements
For the purposes of this module, dietary supplements discussed are limited to those characterized by the NIH Office of Dietary Health, including a broad overview of vitamins, minerals, herbs/botanicals, amino acid, probiotics, and fish-oil containing products. The NIH Office of Dietary Health offers more in-depth individual educational resources and fact sheets on their website (NIH, n.d.a).
As previously noted, there are 13 vitamins needed by the body for normal growth, health, and wellness. These vitamins are vitamin A, B vitamins (thiamine, niacin, riboflavin, biotin, pantothenic acid, vitamin B6, vitamin B12, and folate), vitamin C, vitamin D, vitamin E, and vitamin K. Most of these vitamins are obtained from dietary intake. Each vitamin has unique jobs within the body; low levels can lead to health problems and the proper amount can help prevent problems. An example is a lack of vitamin A, which can lead to night blindness. Conversely, high doses of many vitamins can also cause health problems (NLM, 2019c). See Table 2 for the 13 essential vitamins, their potential benefits, and nursing implications; Table 3 contains the RDAs of these vitamins.
Complete nutrition is dependent on both vitamins and minerals. The body uses minerals to keep the bones, muscles, brain, and heart properly functioning. Minerals are also necessary for making hormones and enzymes. There are two types of minerals, macrominerals and trace minerals. The body needs a larger amount of macrominerals than trace minerals. These macrominerals include calcium, potassium, sodium, phosphorus, magnesium, chloride, and sulfur. Trace minerals are only needed in small amounts and include iron, copper, iodine, manganese, zinc, fluoride, cobalt, and selenium. Generally, minerals are obtained through dietary intake, but for those who do not eat a wide variety of foods, a mineral supplement may be recommended. Individuals with certain health conditions, such as chronic kidney disease, need to limit certain minerals such as potassium (NLM, 2020e). See Table 4 for minerals and their implications and Table 5 for RDAs of Minerals.
A botanical is a plant or a plant portion that is used for its therapeutic or medicinal properties, flavors, or scents. Herbs are a type of botanical. Products that are made from botanicals to maintain or improve health are called botanical products, herbal products, or phytomedicines. Botanicals may be sold as fresh or dried products, liquid or solid extracts, tablets, capsules, powders, or teas. Typically, dietary supplements are in tablet or capsule forms. Botanicals may have specific chemicals known as markers that are used to manufacture a consistent or standardized product; this should be the portion of the botanical that provides the therapeutic effect. It is difficult to determine the quality of a botanical dietary supplement from its label as the quality is related to the manufacturer, supplier, and production process. Many people take botanicals and herbs for various conditions because they are natural substances, but the validity of their usefulness is primarily unsubstantiated by research. Many of these dietary supplements can be harmful, causing serious adverse effects. Those at highest risk for these effects are pregnant women and those who take anticoagulation therapy or have bleeding problems. Specific herbs will be discussed below with individual concerns (NIH, 2011). See Table 6 for common botanicals and herbs.
Amino acids are naturally occurring in foods but can also be taken as dietary supplements to enhance mood or athletic performance; they are categorized as essential, nonessential, or conditionally essential (Kubala, 2018). The body needs 20 specific amino acids for proper growth and development, but nine of these are considered essential (histidine, leucine, methionine, threonine, valine, L-tryptophan, isoleucine, lysine, and phenylalanine). These are considered essential because the body is unable to produce them endogenously and it depends entirely on dietary intake. Optimal sources for amino acids are animal proteins, including meat, eggs, and poultry. Proteins are broken down into amino acids for use by the body. Nonessential amino acids include those produced endogenously by the body (alanine, arginine, asparagine, aspartic acid, cysteine, glutamine, glutamic acid, proline, glycine, serine, and tyrosine). Conditionally essential amino acids are those that become essential under special circumstances such as stress or illness (i.e., cancer, wound healing, liver disease, premature infants). The conditionally essential amino acids are glutamine, cysteine, tyrosine, and ß-hydroxy ß-methylbutyrate (HMB). While all amino acids will not be discussed in this module, several of the more common ones used by athletes or for specific conditions are listed below. For further information on various amino acids in dietary supplements, a complete listing can be found in the US National Library of Medicine-Herbs and Supplements (NLM, 2020a). See Table 7 for a listing of amino acids and their nursing implications.
Branched-Chain Amino Acids (BCAAs)
Three of the essential amino acids, leucine, isoleucine, and valine, are BCAAs. BCAAs are primarily found in eggs, meat, and dairy products. There are five proven benefits of BCAA supplements: increased muscle growth, decreased muscle soreness, reduction in exercise fatigue, prevention of muscle wasting, and improvement in liver disease such as cirrhosis. Research has provided evidence of these benefits, such as a study that followed people drinking 5.6 g of BCAAs after their resistance workout which lead to a 22% increase in muscle protein synthesis in comparison to the placebo group. A similar study assessed people that drank a BCAA supplement before a squat exercise who report reduced muscle fatigue and delayed onset muscle soreness (DOMS) in comparison to the placebo group. Other studies have shown that mental focus during exercise improves when taking BCAAs in comparison to a placebo, and muscle protein breakdown is inhibited with the use of BCAA supplements among the elderly and those with cancer at high risk for muscle wasting (Van de Walle, 2018a).
Probiotics are living microorganisms that have health benefits when consumed or applied to the human body. Sources include yogurt, dietary supplements, or beauty products. Most people consider bacteria or microorganisms as potentially harmful, yet many bacteria are beneficial in food digestion, produce vitamins, or help destroy disease-causing cells. The microorganisms in probiotic dietary supplements are like the microorganisms found on humans naturally. Probiotics contain several different microorganisms, but the most commonly used supplements belong to the groups Lactobacillus or Bifidobacterium. The yeast used as a probiotic is Saccharomyces boulardi. Approximately four million adults in the US used probiotics within a 30-day period in a 2012 survey according to the NCCIH (2019). They are the third most common dietary supplement behind vitamins and minerals. Probiotics work by helping the body maintain a healthy community of cooperative microorganisms or return to a healthy condition after an illness or the use of certain medications such as antibiotics. They influence the body's immune response and produce substances that support optimal GI health. While many studies related to probiotics are inconclusive, some areas of study have shown promise. Research has been favorable regarding the use of probiotics in the prevention of antibiotic-associated diarrhea, the prevention of necrotizing enterocolitis and sepsis in infants, the treatment of infant colic, or the treatment of periodontal disease. Other claims by manufacturers of probiotics are related to a decrease or prevention of allergies, skin conditions such as acne, dental caries, UTIs, and upper respiratory infections. Probiotics appear to be safe, with few, if any, adverse effects in healthy individuals. The risk of side effects goes up with immunocompromised patients or those with severe illnesses. Some probiotic supplements have been reported to contain microorganisms other than the ones listed on the label, and those contaminants could pose health risks (NCCIH, 2019). See Table 8 for probiotics.
Fish oil is a dietary source of omega-3 fatty acids, which are polyunsaturated fatty acids needed for numerous body functions ranging from muscle activity to cellular growth. Specific functions include blood clotting, cell division and growth, and fertility. Fish oil supplements typically contain two omega-3 fatty acids: docosahexaenoic acid (DHA) and eicosapentaenoic acid (EPA). DHA is needed for brain development and function. Omega-3 fatty acids are found in many foods. The primary dietary sources for DHA and EPA are cold-water fatty fish such as salmon, mackerel, trout, mussels, oysters, shellfish, and crabs. Other foods such as eggs, yogurt, infant formula, and milk may be fortified with DHA and other omega-3s. Nuts, seeds, and vegetable oils (including flaxseed, soybean, and canola oils) also contain a third omega-3 fatty acid called alpha-linolenic acid (ALA). ALA is considered essential and must be obtained from food sources since the body cannot make it. Similarly, EPA and DHA are mostly obtained from food sources as the body is only able to convert ALA into EPA and then DHA in very limited amounts (roughly 15%). In addition to dietary consumption, EPA and DHA can be supplemented via fish oil. Research supports the use of fish oil supplements to reduce the risk of CVD, hypertension, elevated triglycerides or cholesterol, preterm delivery, and RA (Mayo Clinic, 2017; NIH, 2019). See Table 9 for more information regarding fish oils.
The NCCIH has information throughout its website that dietary supplements have many ingredients, and all may not show up on their labels. While there have been many studies on dietary supplements with evidence of value, many do not have solid clinical research that confirms their value or efficacy of use for various conditions. Safe use of supplements requires that the user read the label carefully and follow the directions for safe use. Also, the user should remember that natural sources do not always translate to safety, particularly where herbs or botanicals are concerned. Often unknown or multiple ingredients are inside the products. There is always the possibility of food or drug interactions with dietary supplements, and some of the interactions pose a serious risk. While the major interactions have been shared in this module, there could be other significant risks. Most supplements are not approved for children or pregnant/breastfeeding women. Finally, while the FDA oversees dietary supplements, their regulation is much less rigorous or controlled than prescription drugs or over-the-counter medications (Kantor et al., 2016).
Arnarson, A. (2017). The fat-soluble vitamins: A, D, E, and K. Healthline. https://www.healthline.com/nutrition/fat-soluble-vitamins
Healthline. (2018). 11 signs and symptoms of thiamine (vitamin B1) deficiency. https://www.healthline.com/nutrition/thiamine-deficiency-symptoms
Kantor, E. D., Rehm, C. D., Du, M., White, E., & Givannucci, E. L. (2016). Trends in dietary supplement use among US adults from 1999-2012. JAMA, 316(14) 1464-1474. https://doi.org/ 10.1001/jama.2016.14403.
Kubala, J. (2018). Essential amino acids: Definition, benefits, and food sources. Healthline. https://www.healthline.com/nutrition/essential-amino-acids
Mayo Clinic. (2017). Fish oil. https://www.mayoclinic.org/drugs-supplements-fish-oil/art-20364810
Menayang, A. (2018). Leucine intake from a supplement may be better for muscle protein synthesis than leucine from a meal. https://www.nutraingredients-usa.com/Article/2018/10/19/Leucine-supplement-fared-better-than-leucine-from-meal-for-muscle-building
Mozaffarian, D., Rosenberg, I., & Uauy, R. (2018). History of modern nutrition science-implications for current research, dietary guidelines, and food policy. BMJ, 361, k2392. https://doi.org/10.1136/bmj.k2392.
National Academies of Sciences, Engineering and Medicine. (2018). Food and nutrition board. http://nationalacademies.org/hmd/about-hmd/leadership-staff/hmd-staff-leadership-boards/food-and-nutrition-board.aspx
The National Center for Complimentary and Integrative Health. (2016a). Echinacea. https://www.nccih.nih.gov/health/echinacea
The National Center for Complimentary and Integrative Health. (2016b). Garlic. https://www.nccih.nih.gov/health/garlic
The National Center for Complimentary and Integrative Health. (2016c). Ginkgo. https://www.nccih.nih.gov/health/ginkgo
The National Center for Complimentary and Integrative Health. (2016d). Saw palmetto. https://www.nccih.nih.gov/health/saw-palmetto
The National Center for Complimentary and Integrative Health. (2016e). St. john’s wort. https://www.nccih.nih.gov/health/st-johns-wort
The National Center for Complimentary and Integrative Health. (2019). Probiotics: What you need to know. https://www.nccih.nih.gov/health/probiotics-what-you-need-to-know
The National Institutes of Health. (n.d.a). Dietary supplement fact sheets. https://ods.od.nih.gov/factsheets/list-all/
The National Institutes of Health. (n.d.b). Nutrient recommendations: Dietary reference intakes (DRI). https://ods.od.nih.gov/Health_Information/Dietary_Reference_Intakes.aspx
The National Institutes of Health. (n.d.c). Promoting vitamin C. https://profiles.nlm.nih.gov/spotlight/mm/feature/medicine
The National Institutes of Health. (2011). Botanical dietary supplements. https://ods.od.nih.gov/factsheets/BotanicalBackground-HealthProfessional/
The National Institutes of Health. (2018). Black cohosh. https://ods.od.nih.gov/factsheets/BlackCohosh-HealthProfessional/
The National Institutes of Health. (2019). Omega-3 fatty acids. https://ods.od.nih.gov/factsheets/Omega3FattyAcids-HealthProfessional/
The National Institutes of Health. (2020a). Biotin. https://ods.od.nih.gov/factsheets/Biotin-HealthProfessional/
The National Institutes of Health. (2020b). Calcium. https://ods.od.nih.gov/factsheets/Calcium-HealthProfessional/
The National Institutes of Health. (2020c). Chromium. https://ods.od.nih.gov/factsheets/Chromium-HealthProfessional/
The National Institutes of Health. (2020d). Copper. https://ods.od.nih.gov/factsheets/Copper-HealthProfessional/
The National Institutes of Health. (2020e). Dietary and herbal supplements. https://www.nccih.nih.gov/health/dietary-and-herbal-supplements
The National Institutes of Health. (2020f). Dietary supplements. https://ods.od.nih.gov/factsheets/DietarySupplements-HealthProfessional/#h1
The National Institutes of Health. (2020g). Folate. https://ods.od.nih.gov/factsheets/Folate-HealthProfessional/
The National Institutes of Health. (2020h). Iodine. https://ods.od.nih.gov/factsheets/Iodine-HealthProfessional/
The National Institutes of Health. (2020i). Iron. https://ods.od.nih.gov/factsheets/Iron-HealthProfessional/
The National Institutes of Health. (2020j). Magnesium. https://ods.od.nih.gov/factsheets/Magnesium-HealthProfessional/
The National Institutes of Health. (2020k). Manganese. https://ods.od.nih.gov/factsheets/Manganese-HealthProfessional/
The National Institutes of Health. (2020l). Molybdenum. https://ods.od.nih.gov/factsheets/Molybdenum-HealthProfessional/
The National Institutes of Health. (2020m). Pantothenic acid. https://ods.od.nih.gov/factsheets/PantothenicAcid-HealthProfessional/
The National Institutes of Health. (2020n). Phosphorus. https://ods.od.nih.gov/factsheets/Phosphorus-HealthProfessional/
The National Institutes of Health. (2020o). Potassium. https://ods.od.nih.gov/factsheets/Potassium-HealthProfessional/
The National Institutes of Health. (2020p). Riboflavin. https://ods.od.nih.gov/factsheets/Riboflavin-HealthProfessional/
The National Institutes of Health. (2020q). Selenium. https://ods.od.nih.gov/factsheets/Selenium-HealthProfessional/
The National Institutes of Health. (2020r) Vitamin A. https://ods.od.nih.gov/factsheets/VitaminA-HealthProfessional/
The National Institutes of Health. (2020s). Vitamin B6. https://ods.od.nih.gov/factsheets/VitaminB6-HealthProfessional/
The National Institutes of Health. (2020t). Vitamin B12. https://ods.od.nih.gov/factsheets/VitaminB12-HealthProfessional/
The National Institutes of Health. (2020u). Vitamin C. https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/
The National Institutes of Health. (2020v). Vitamin D. https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/
The National Institutes of Health. (2020w). Vitamin E. https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/
The National Institutes of Health. (2020x). Vitamin K. https://ods.od.nih.gov/factsheets/vitaminK-HealthProfessional/
The National Institutes of Health. (2020y). Zinc. https://ods.od.nih.gov/factsheets/Zinc-HealthProfessional/
Patel, K. (2019). Valine. https://examine.com/supplements/valine/
Robertson, R. (2017). 9 ways lactobacillus acidophilus can benefit your health. https://www.healthline.com/nutrition/lactobacillus-acidophilus
RxList. (n.d.). Aspartates. Retrieved April 25, 2020, from https://www.rxlist.com/aspartates/supplements.htm
Smith, R. (2004). Let food be thy medicine…. BMJ, 328(7433). https://www.ncbi.nlm.nih.gov/pmc/articles/PMC318470/
Sullivan, D. (2017). What’s to know about vitamin B3 deficiency? https://www.medicalnewstoday.com/articles/319429
Swan, J.P. (2015). The history of efforts to regulate dietary supplements in the USA. Drug Testing and Analysis, 8(3-4), 271-282. https://doi.org/10.1002/dta.1919.
Tinsely, G. (2018). Glutamine: Benefits, uses, and side-effects. Healthline. https://www.healthline.com/nutrition/glutamine
University of Florida. (2016). Facts about vitamin A. https://edis.ifas.ufl.edu/pdffiles/fy/fy20600.pdf
US Department of Agriculture. (2015). Scientific report of the 2015 dietary guidelines advisory committee. https://health.gov/sites/default/files/2019-09/Scientific-Report-of-the-2015-Dietary-Guidelines-Advisory-Committee.pdf
US Food & Drug Administration. (2019). New dietary ingredients in dietary supplements: Background for industry. https://www.fda.gov/food/new-dietary-ingredients-ndi-notification-process/new-dietary-ingredients-dietary-supplements-background-industry#what_is
US National Library of Medicine. (n.d.) Phenylalanine. Retrieved on April 26, 2020 from https://pubchem.ncbi.nlm.nih.gov/compound/L-phenylalanine
US National Library of Medicine. (2018). Dietary supplements. https://medlineplus.gov/dietarysupplements.html
US National Library of Medicine. (2019a). Fluoride in diet. https://medlineplus.gov/ency/article/002420.htm
US National Library of Medicine. (2019b). Sodium. https://medlineplus.gov/sodium.html
US National Library of Medicine. (2019c). Vitamins. https://medlineplus.gov/vitamins.html
US National Library of Medicine. (2020a). Amino acids. https://medlineplus.gov/ency/article/002222.htm
US National Library of Medicine. (2020b). Chloride. https://medlineplus.gov/ency/article/002417.htm
US National Library of Medicine. (2020c). Herbs and supplements. https://medlineplus.gov/druginfo/herb_All.html
US National Library of Medicine. (2020d). Low blood sodium. https://medlineplus.gov/ency/article/000394.htm
US National Library of Medicine. (2020e). Minerals. https://medlineplus.gov/minerals.html
US National Library of Medicine. (2020f). Vitamin A. https://medlineplus.gov/vitamina.html
Van de Walle, G. (2018a). 5 proven benefits of BCAAs (branch-chain amino acids). Healthline. https://www.healthline.com/nutrition/benefits-of-bcaa
Van de Walle, G. (2018b). Tyrosine: Benefits, side effects and dosage. Healthline. https://www.healthline.com/nutrition/tyrosine
Venkatraman, S. & Dandekar, S. (2015). Nutrition and biochemistry for nurses (2nd ed.) Elsevier.
WebMD. (n.d.a). Arginine. Retrieved on April 25, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-875/l-arginine
WebMD. (n.d.b). Histidine. Retrieved on April 24, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-467/histidine
WebMD. (n.d.c). L-tryptophan. Retrieved on April 24, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-326/l-tryptophan
WebMD. (n.d.d) Methionine. Retrieved on April 24, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-42/methionine
WebMD. (n.d.e). Nickel. Retrieved on April 22, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-1223/nickel
WebMD. (n.d.f). Ornithine. Retrieved on April 25, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-200/ornithine
WebMD. (n.d.g). Taurine. Retrieved on April 24 from https://www.webmd.com/vitamins/ai/ingredientmono-1024/taurine
WebMD. (n.d.h). Threonine. Retrieved on April 24, 2020 from https://www.webmd.com/vitamins/ai/ingredientmono-1083/threonine
The World Health Organization. (2020). WHO guidance helps detect iron deficiency and protect brain development. https://www.who.int/news-room/detail/20-04-2020-who-guidance-helps-detect-iron-deficiency-and-protect-brain-development
Youdim, A. (2019). Overview of nutrition. Merck Manual. https://www.merckmanuals.com/professional/nutritional-disorders/nutrition-general-considerations/overview-of-nutrition?qt=&sc=&alt=