8.2: Antioxidant Nutrients

Learning Objectives

• List the antioxidant nutrients and describe their functions.
• Give examples of good food sources of antioxidant nutrients.

The market is flooded with advertisements for “super antioxidant” supplements that block free radical production, stimulate the immune system, prevent cancer, and reduce the signs of aging. Based on the antioxidant-supplement industry’s success, the general public appears to believe these health claims. However, these claims are not backed by scientific evidence; rather, there is some evidence suggesting supplements can actually cause harm. While scientists have found evidence supporting the consumption of antioxidant-rich foods as a method of reducing the risk of chronic disease, there is no “miracle cure;” no pill or supplement alone can provide the same benefits as a healthy diet. Remember, it is the combination of antioxidants and other nutrients in healthy foods that is beneficial. In this section, we will review how particular antioxidants function in the body, learn how they work together to protect the body against free radicals, and explore nutrient-rich dietary sources of antioxidants.

Vitamin E

Vitamin E is a fat-soluble vitamin. "Vitamin E" is the collective name for eight chemically similar forms of the nutrient; alpha-tocopherol appears to be the most potent form.

Functions of Vitamin E

The primary function of Vitamin E is as an antioxidant. Alpha-tocopherol and vitamin E’s other constituents are primarily responsible for protecting cell membranes against lipid destruction caused by free radicals. For example, free radical oxidation/damage to the lipids in low-density lipoproteins (LDLs) causes LDLs to stick to the walls of arteries. This contributes to the development of fatty streaks and, eventually, plaque, which hardens the arteries (also called atherosclerosis) and can lead to a heart attack. Alpha-tocopherol protects LDL from being oxidized, and therefore helps protect against heart disease. After alpha-tocopherol interacts with a free radical it is no longer capable of acting as an antioxidant unless it is enzymatically regenerated. Vitamin C helps to regenerate some of the alpha-tocopherol, but the remainder is eliminated from the body. Therefore, to maintain vitamin E levels, you ingest it as part of your diet.

In addition to its antioxidant functions, vitamin E plays a role in nerve and muscle development and function. It also helps increase vitamin A absorption if vitamin A intake from the diet is low.

Vitamin E Imbalances

Getting too much or too little vitamin E can cause issues. Vitamin E supplementation can increase the risk for prostate cancer and premature mortality. In addition, Vitamin E can interfere with anticoagulant medications (aspirin and Coumadin) and cause uncontrollable bleeding. Long term supplement use in some people can cause hemorrhagic stroke.

Vitamin E deficiencies are uncommon but can occur with diseases that cause fat malabsorption. Vitamin E deficiency can result in erythrocyte hemolysis (the rupture of red blood cells) and anemia.

Vitamin E Recommendations

The Recommended Dietary Allowances (RDAs) and Tolerable Upper Intake Levels (ULs) for different age groups for vitamin E are provided in Table \(\PageIndex{1}\). The RDAs are based on scientific evidence that these levels of vitamin E in the diet prevent conditions associated with vitamin E deficiency, which are rare but are primarily the result of nerve degeneration. People with malabsorption disorders, such as Crohn’s disease or cystic fibrosis, and babies born prematurely, are at higher risk for vitamin E deficiency.

Fat in the diet is required for vitamin E absorption as it is packaged into lipid-rich chylomicrons in intestinal cells and transported to the liver. The liver stores some of the vitamin E or packages it into lipoproteins, which deliver it to cells.

Table \(\PageIndex{1}\): Dietary Reference Intakes for Vitamin E¹

Age Group	RDA (mg/day)	UL (mg/day)
Infants (0–6 months)	4*	ND
Infants (7–12 months)	5*	ND
Children (1–3 years)	6	200
Children (4–8 years)	7	300
Children (9–13 years)	11	600
Adolescents (14–18 years)	15	800
Adults (> 19 years)	15	1,000
*denotes Adequate Intake; ND=not determined

Sources of Vitamin E

Vitamin E is found in many foods, especially those higher in fat, such as nuts and oils. See Table \(\PageIndex{2}\) for a list of foods and their vitamin E content.

Table \(\PageIndex{2}\): Vitamin E Content of Various Foods²

Food	Vitamin E (mg)	% Daily Value
Wheat-germ oil (1 Tbsp.)	20.3	135
Sunflower seeds (1 oz.)	7.4	49
Almonds (1 oz.)	6.8	45
Sunflower oil (1 Tbsp.)	5.6	37
Safflower oil (1 Tbsp.)	4.6	31
Hazelnuts (1 oz.)	4.3	29
Peanut butter (2 Tbsp.)	2.9	19
Peanuts (1 oz.)	2.2	15

Learn even more about this nutrient by reading the Vitamin E Fact Sheet from the National Institutes of Health, Office of Dietary Supplements.

Vitamin C

Vitamin C, also known as ascorbic acid, is a water-soluble micronutrient essential in the diet for humans, although most other mammals can readily synthesize it.

Functions of Vitamin C

Vitamin C’s ability to easily donate electrons makes it a highly effective antioxidant. It protects lipids both by disabling free radicals and by aiding in the regeneration of vitamin E. In addition to its role as an antioxidant, vitamin C is a required part of several enzymes involved in the synthesis of collagen, signaling molecules in the brain (neurotransmitters), some hormones, bile, and amino acids. Vitamin C also reduces formation of nitrosamines (cancer-causing agents in some cured and processed meats).

Vitamin C levels in the body are affected by the amount in the diet, which influences how much is absorbed and how much the kidney allows to be excreted, such that the higher the intake, the more vitamin C is excreted. Vitamin C is not stored in any significant amount in the body, but once it has reduced a free radical, it is very effectively regenerated and therefore it can exist in the body as a functioning antioxidant for many weeks.

Vitamin C Imbalances

High doses of vitamin C have been reported to cause numerous problems, but the only consistently shown side effects are gastrointestinal upset, abdominal cramps, and diarrhea. Very high doses of vitamin C from supplementation, in combination with iron, have sometimes been found to increase oxidative stress, reaffirming that getting your antioxidants from foods is better than getting them from supplements, as that helps regulate your intake levels. If a person has hemochromatosis (a disorder where extra iron builds up in the body) or preexisting kidney disease, excess Vitamin C can be harmful.

The classic condition associated with vitamin C deficiency is scurvy. The signs and symptoms of scurvy include skin disorders, bleeding gums, loose teeth, painful joints, weakness, depression, and increased susceptibility to infections.

Vitamin C Recommendations

The RDAs and ULs for different age groups for vitamin C are listed in Table \(\PageIndex{3}\). They are considered adequate to prevent scurvy. Vitamin C’s effectiveness as a free radical scavenger motivated the Institute of Medicine (IOM) to increase the RDA for smokers by 35 milligrams, as tobacco smoke is an environmental and behavioral contributor to free radicals in the body.

Table \(\PageIndex{3}\): Dietary Reference Intakes for Vitamin C¹

Age Group	RDA (mg/day)	UL (mg/day)
Infants (0–6 months)	40*	ND
Infants (7–12 months)	50*	ND
Children (1–3 years)	15	400
Children (4–8 years)	25	650
Children (9–13 years)	45	1200
Adolescents (14–18 years)	75 (males), 65 (females)	1800
Adults (> 19 years)	90 (males), 75 (females)	2000
*denotes Adequate Intake; ND=not determined

Sources of Vitamin C

Citrus fruits are great sources of vitamin C and so are many vegetables. In fact, British sailors in the past were often referred to as “limeys” as they carried sacks of limes onto ships to prevent scurvy. Vitamin C is not found in significant amounts in animal-based foods. For the vitamin C content of various foods, see Table \(\PageIndex{4}\). A couple of things to keep in mind about Vitamin C levels in food - cooking at high temperature for long periods of time and excessive exposure to oxygen (such as when there is only a little orange juice left in a large carton) destroys vitamin C.

Table \(\PageIndex{4}\): Vitamin C Content of Various Foods³

Food	Vitamin C (mg)	% Daily Value
Red pepper, raw (1/2 cup)	95	106
Orange juice (3/4 cup)	93	103
Grapefruit juice (3/4 cup)	70	78
Orange (1 medium)	70	78
Kiwifruit (1 medium)	64	71
Green pepper, raw (1/2 cup)	60	67
Broccoli, cooked (1/2 cup)	51	57
Strawberries, sliced (1/2 cup)	49	54
Grapefruit (1/2 medium)	39	43
Cantaloupe (1/2 cup)	29	32
Cassava, cooked (1 cup)	29	32
Potato, baked (1 medium)	17	19

Learn even more about this nutrient by reading the Vitamin C Fact Sheet from the National Institutes of Health, Office of Dietary Supplements.

Phytochemicals

Phytochemicals are chemicals in plants that may provide some health benefit. Carotenoids are one type of phytochemical that we will discuss in more detail below. Other phytochemicals include indoles, lignans, phytoestrogens, stanols, flavonoids, anthocyanidins, phenolic acids, and many more. They are found not only in fruits and vegetables, but also in grains, seeds, nuts, and legumes. Many phytochemicals act as antioxidants, but they have several other functions, such as mimicking hormones, altering absorption of cholesterol, inhibiting inflammatory responses, and blocking the actions of certain enzymes.

Phytochemicals are present in small amounts in the food supply, and although thousands have been and are currently being scientifically studied, their health benefits remain largely unknown. Also largely unknown is their potential for toxicity, which could be substantial if taken in large amounts in the form of supplements. Moreover, phytochemicals often act in conjunction with each other and with micronutrients. Thus, supplementing with only a few may impair the functions of other phytochemicals or micronutrients. As with the antioxidant vitamins, it is the mixture and variety of phytochemicals in foods that are linked to health benefits.

Functions of Carotenoids

Carotenoids are pigments synthesized by plants that give them their yellow, orange, red and deep green color. Over six hundred carotenoids have been identified; the vast majority are found in plant foods. Beta-carotene and some other carotenoids, such as lycopene and lutein, act as antioxidants. Through their antioxidant activities, carotenoids enhance immune function, protect cell membranes and LDL from oxidative damage, protect skin from sun damage, and protect our eyes from oxidative damage that can lead to vision impairment.

Carotenoid Imbalances

When considering if you can get too much beta-carotene, it appears that large quantities from food sources do not appear to be toxic. However, large quantities of beta-carotene from food can cause yellowing or even "oranging" of the skin. Beta-carotene supplements may be harmful in certain populations. On the other hand, there are no known deficiency symptoms from consuming too little beta-carotene.

Carotenoid Recommendations

Carotenoids are not considered essential nutrients, so no DRIs have been established. Eating five servings of fruits and vegetables daily provides 6-8 mg of beta-carotene.⁴ Beta-carotene is a provitamin (an inactive form of a vitamin that must be converted to the active form in the body). Beta-carotene is the precursor of retinol, the active form of Vitamin A.

Sources of Carotenoids

Carotenoids are not absorbed as well as vitamin A, but similar to vitamin A, they do require fat in the meal for absorption. In intestinal cells, carotenoids are packaged into the lipid-containing chylomicrons inside small intestine mucosal cells and then transported to the liver. In the liver, carotenoids are repackaged into lipoproteins, which transport them to cells.

In contrast to most micronutrients, carotenoid availability is actually increased by the cooking process because cooking releases carotenoids from the plant matrix. Thus, eating cooked tomato sauce provides more lycopene than eating a raw tomato. However, overcooking transforms some of the carotenoids into inactive products, and in general it is best to chop and lightly steam vegetables containing carotenoids to maximize their availability from foods. Cooking carotenoid-containing foods in oil also enhances the bioavailability of carotenoids. In the United States, the most consumed carotenoids are alpha-carotene, beta-carotene, beta-cryptoxanthin, lycopene, lutein, and zeaxanthin. See Table \(\PageIndex{5}\) for the beta-carotene content of various foods.

Table \(\PageIndex{5}\): Beta-Carotene Content of Various Foods⁵

Food	Serving Size	Beta-carotene (mg)
Sweet potato, baked	1 cup	23
Pumpkin, canned	1 cup	17
Collards (frozen), cooked	1 cup	11.6
Carrots, raw	1 cup	10.6
Butternut squash, baked	1 cup	9.4
Cantaloupe	1 cup	3.6

Selenium

Selenium is a trace mineral which means our body contains less than 5 grams of the nutrient and we need to consume it in small amounts (less than 100 mg per day).

Functions of Selenium

Selenium is a critical component of the glutathione peroxidase antioxidant enzyme system. As an integral functioning part of this enzyme system, selenium aids in the regeneration of glutathione and oxidized vitamin C. Selenium, as part of glutathione peroxidase, also protects lipids from free radicals, and, in doing so, spares vitamin E. This is just one example of how antioxidants work together to protect the body against free radical-induced damage. Other functions of selenium-containing proteins include protecting endothelial cells that line tissues, converting inactive thyroid hormone to the active form, and mediating inflammatory and immune system responses.

Selenium Imbalances

Selenium toxicity can result from supplements. Symptoms of selenium toxicity include brittle hair and nails, skin rashes, nausea, vomiting, and weakness. Selenium deficiency has been seen in areas of the world where the soil is very low in selenium. For example, Keshan disease (a heart disease) was discovered in the Keshan province of China where soil levels of selenium were low. Selenium deficiency can also lead to Kashin-Beck disease, a form of arthritis.

Selenium Recommendations

The Institute of Medicine set the RDA for selenium based on the amount required to maximize the activity of glutathione peroxidases found in blood plasma. The RDA for different age groups are listed in Table \(\PageIndex{6}\).

Table \(\PageIndex{6}\): Dietary Reference Intakes for Selenium¹

Age Group	RDA (mcg/day)	UL (mcg/day)
Infants (0–6 months)	15*	45
Infants (7–12 months)	20*	60
Children (1–3 years)	20	90
Children (4–8 years)	30	150
Children (9–13 years)	40	280
Adolescents (14–18 years)	55	400
Adults (> 19 years)	55	400
*denotes Adequate Intake

Sources of Selenium

Selenium is stored in the tissues of animals, so organ meats, muscle meats, and seafood tend to have high selenium content. Plants do not require selenium, so the selenium content in fruits and vegetables is usually low. Grains and some nuts contain selenium when grown in selenium-containing soils. See Table \(\PageIndex{7}\) for the selenium content of various foods.

Table \(\PageIndex{7}\): Selenium Content of Various Foods

Food	Selenium (mcg)	% Daily Value
Brazil nuts (1 ounce)	544	989
Yellowfin tuna, cooked (3 ounces)	92	167
Ham (3 ounces)	42	76
Turkey (3 ounces)	31	56
Cottage Cheese (1 cup)	20	36
Brown Rice, cooked (1 cup)	19	35
Whole Wheat Bread (1 slice)	13	24

Learn even more about this nutrient by reading the Selenium Fact Sheet from the National Institutes of Health, Office of Dietary Supplements.

Manganese, Copper, Zinc, and Iron

Manganese, copper, and zinc are part of the superoxide dismutase (SOD) antioxidant enzyme system which converts superoxide free radicals into hydrogen peroxide and oxygen. Iron is part of catalase which converts hydrogen peroxide to water and oxygen, thereby finishing the detoxification reaction started by SOD. In the proper doses they allow for optimal detoxification of free radicals. In excess and when not bound to proteins, manganese, iron, and copper actually accelerate the production of free radicals. This is an attribute of all antioxidants in general, although the effect is greater for certain antioxidants. Antioxidants can become pro-oxidants when the conditions are altered. Recall that oxidative stress results from an imbalance in free radicals with their detoxifying and repair systems. Another factor that can cause oxidative stress is a high level of antioxidants, as some will revert to acting as pro-oxidants.

Key Takeaways

• Consuming antioxidant-rich foods is scientifically supported to reduce the risk of chronic diseases.
• Antioxidants have specific functions and they can act in concert with each other to protect against free radicals.
• Some antioxidants are involved in the regeneration of other antioxidants.
• Many phytochemicals act as antioxidants, but they perform several other functions as well.
• Dietary antioxidants and phytochemicals in the proper doses are beneficial to health but can cause harm in excess.

References

1. Summary Report of the Dietary Reference Intakes. nationalacademies.org. https://www.nationalacademies.org/our-work/summary-report-of-the-dietary-reference-intakes. Accessed June 26, 2020.
2. Vitamin E - Health Professional Fact Sheet. ods.od.nih.gov. https://ods.od.nih.gov/factsheets/VitaminE-HealthProfessional/. Accessed June 26, 2020.
3. Vitamin C - Health Professional Fact Sheet. ods.od.nih.gov. https://ods.od.nih.gov/factsheets/VitaminC-HealthProfessional/. Accessed June 26, 2020.
4. Beta-Carotene: Medline Plus Supplements. mdlineplus.gov. https://medlineplus.gov/druginfo/natural/999.html. Accessed June 26, 2020.
5. USDA National Nutrient Database for Standard Reference, Release 28. ods.od.nih.gov. https://ods.od.nih.gov/pubs/usdandb/VitA-betaCarotene-Content.pdf. Accessed June 26, 2020.