12.11: EDIT Vitamins As Antioxidants - An Introduction
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The Vitamins
Vitamins are organic compounds that are traditionally assigned to two groups: fat-soluble(hydrophobic) or water-soluble (hydrophilic).
- The fat-soluble vitamins include vitamins A, D, E, K.
- The water-soluble vitamins include the B vitamins and vitamin C.
- This classification determines where they act in the body. The fat-soluble vitamins are largely responsible for protecting cell membranes from free radical damage. The water-soluble vitamins act in the cytosol of cells or in extracellular fluids such as blood; The body can synthesize some vitamins, but others must be obtained from the diet.
- Another important difference between fat-soluble vitamins and water-soluble vitamins is the way they are absorbed in the body. Vitamins are absorbed primarily in the small intestine and their bio-availability is dependent on the food composition. Including a small amount of fat or oil in your meal can enhance fat-soluble vitamin absorption.
- Once fat-soluble vitamins have been absorbed in the small intestine, they are packaged and incorporated into chylomicrons along with other fatty acids
and transported in the lymphatic system to the liver. - Water-soluble vitamins, on the other hand, are absorbed in the small intestine but are transported to the liver through blood vessels.
- Vitamins are obtained from the different types of foods that we consume. If a diet is lacking a certain type of nutrient, a vitamin deficiency may occur.
The Antioxidant Vitamin A
Vitamin A is a generic term for a group of similar compounds called retinoids. Every vitamin has a biologically active form that plays a very important role in Nutrition. Retinol is the form of vitamin A found in animal-derived foods and is converted in the body to the biologically active forms of vitamin A: retinal and retinoic acid (thus retinol is sometimes referred to as “preformed vitamin A”). About 10 percent of plant-derived carotenoids, including beta-carotene, can be converted in the body to retinoids and are another source of functional vitamin A. Carotenoids are pigments synthesized by plants that give them their yellow, orange, and red color. Over six hundred carotenoids have been identified and, with just a few exceptions, all are found in the plant kingdom. There are two classes of carotenoids—the xanthophylls, which contain oxygen, and the carotenes, which do not.
In plants, carotenoids absorb light for use in photosynthesis and act as antioxidants. Beta-carotene, alpha-carotene, and beta-cryptoxanthin are converted to some extent to retinol in the body. The other carotenoids, such as lycopene, are not. Many biological actions of carotenoids are attributed to their antioxidant activity, but they likely act by other mechanisms, too.
Vitamin A is fat-soluble and is packaged into chylomicrons in the small intestine, and transported to the liver. The liver stores and exports vitamin A as needed; it is released into the blood and is transported to cells. 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 and then transported to the liver. In the liver, carotenoids are repackaged into lipoproteins, which transport them to cells.
The retinoids are aptly named as their most notable function is in the retina of the eye where they aid in vision, particularly in seeing under low-light conditions. This is why night blindness is the most definitive sign of vitamin A deficiency. Vitamin A has several important functions in the body, including maintaining vision and a healthy immune system. Many of vitamin A’s functions in the body are similar to the functions of hormones (for example, vitamin A can interact with DNA, causing a change in protein function). Vitamin A assists in maintaining healthy skin and the linings and coverings of tissues; it also regulates growth and development. As an antioxidant, vitamin A protects cellular membranes, helps in maintaining glutathione levels, and influences the amount and activity of enzymes that detoxify free radicals.
There is more than one source of vitamin A in the diet. There is preformed vitamin A, which is abundant in many animal-derived foods, and there are carotenoids, which are found in high concentrations in vibrantly colored fruits and vegetables and some oils.
Some carotenoids are converted to retinol in the body by intestinal cells and liver cells. However, only minuscule amounts of certain carotenoids are converted to retinol, meaning fruits and vegetables are not necessarily good sources of vitamin A.
The Dietary Sources and Measurements
The R.D.A. for vitamin A includes all sources of vitamin A. The R.D.A. for vitamin A is given in mcg of retinol activity requirements (R.A.E.) to take into account the many different forms it is available in. The human body converts all dietary sources of vitamin A into retinol. Therefore, 1 mcg of retinol is equivalent to 12 mcg of beta-carotene, and 24 mcg of alpha-carotene or beta-cryptoxanthin. For example, 12 micrograms of fruit- or vegetable-based beta-carotene will yield 1 microgram of retinol. Currently, vitamin A listed in food and on supplement labels use international units (I.U.s). The following conversions are listed below3:
- 1 I.U. retinol = 0.3 mcg R.A.E.
- 1 I.U. beta-carotene from dietary supplements = 0.15 mcg R.A.E.
- 1 I.U. beta-carotene from food = 0.05 mcg R.A.E.
- 1 I.U. alpha-carotene or beta-cryptoxanthin = 0.025 mcg R.A.E.
The R.D.A. for vitamin A is considered sufficient to support growth and development, reproduction, vision, and immune system function while maintaining adequate stores (good for four months) in the liver.
Sources of Beta-Carotene
In the United States, the most consumed carotenoids are alpha-carotene, beta-carotene, beta-cryptoxanthin, lycopene, lutein, and zeaxanthin.
| Food | Serving | Vitamin A (I.U.) | Percent Daily Value |
|---|---|---|---|
| Pumpkin, canned | 1c. | 17.00 | 11.70 |
| Carrot juice | 1c. | 22.00 | 10.20 |
| Carrots, cooked | 1c. | 13.00 | 5.90 |
| Carrots, raw | 1 medium | 5.10 | 2.10 |
| Winter squash, baked | 1c. | 5.70 | 1.40 |
| Collards, cooked | 1c. | 11.60 | 0.20 |
| Tomato | 1 medium | 0.55 | 0.10 |
| Tangerine | 1 medium | 0.13 | 0.09 |
| Peas, cooked | 1c. | 1.20 | 0.09 |
The Antioxidant Vitamin E
Vitamin E occurs in eight chemical forms, of which alpha-tocopherol appears to be the only form that is recognized to meet human requirements. Alpha-tocopherol and vitamin E’s other constituents are fat-soluble and primarily responsible for protecting cell membranes against lipid destruction caused by free radicals, therefore making it an antioxidant. When 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 must ingest it as part of your diet.
Insufficient levels are rare (signs and symptoms of such conditions are not always evident) 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.
Vitamin E has many other important roles and functions in the body such as boosting the immune system by helping to fight off bacteria and viruses. It also enhances the dilation of blood vessels and inhibits the formation of blood clotting. Despite vitamin E’s numerous beneficial functions when taken in recommended amounts, large studies do not support the idea that taking higher doses of this vitamin will increase its power to prevent or reduce disease risk.
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.
Dietary Reference Intakes for Vitamin E
| Age Group | R.D.A. Males and Females (m.g. Per Day) | U.L. |
|---|---|---|
| Infants (0–6 months) | 4* | None |
| Infants (7–12 months) | 5* | None |
| 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 |
Vitamin E is found in many foods, especially those higher in fat, such as nuts and oils. Some spices, such as paprika and red chili pepper, and herbs, such as oregano, basil, cumin, and thyme, also contain vitamin E. (Keep in mind spices and herbs are commonly used in small amounts in cooking and therefore are a lesser source of dietary vitamin E.)
Dietary Sources of Vitamin E
| Food | Serving Size | Vitamin E (M G) | Percent Daily Value |
|---|---|---|---|
| Sunflower seeds | 1 oz. | 7.4 | 37 |
| Almonds | 1 oz. | 6.8 | 34 |
| Sunflower oil | 1 Tbsp | 5.6 | 28 |
| Hazelnuts 1 oz. | 1 oz. | 4.3 | 22 |
| Peanut butter | 2 Tbsp. | 2.9 | 15 |
| Peanuts 1 oz. | 1 oz. | 2.2 | 11 |
| Corn oil 1 Tbsp. | 1 Tbsp. | 1.9 | 10 |
| Kiwi | 1 medium | 1.1 | 6 |
| Tomato | 1 medium | 0.7 | 4 |
| Spinach | 1 c. raw | 0.6 | 3 |
The Antioxidant Vitamin C
Vitamin C, also commonly called ascorbic acid, is a water-soluble micronutrient essential in the diet for humans, although most other mammals can readily synthesize it. Vitamin C’s ability to easily donate electrons makes it a highly effective antioxidant. It is effective in scavenging free radicals. 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 like signaling molecules in the brain, some hormones, and amino acids. Vitamin C is also essential for the synthesis and maintenance of collagen. Collagen is the most abundant protein in the body and used for different functions such as the structure for ligaments, tendons, and blood vessels and also scars that bind wounds together. Vitamin C acts as the glue that holds the collagen fibers together and without sufficient levels in the body, collagen strands are weak and abnormal.
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.
The classic condition associated with vitamin C deficiency is scurvy. The signs and symptoms of scurvy include skin disorders, bleeding gums, painful joints, weakness, depression, and increased susceptibility to infections. Scurvy is prevented by having an adequate intake of fruits and vegetables rich in vitamin C.
Vitamin C’s ability to prevent disease has been debated for many years. Overall, higher dietary intakes of vitamin C (via food intake, not supplements), are linked to decreased disease risk. A review of multiple studies published in the January issue of Nutrition Research concludes there is a limited role for vitamin C supplementation in certain population groups (older people, individuals with lower Vitamin C status, those at higher risk of cardiovascular disease) to improve cardiovascular health. Vitamin C levels in the body have been shown to correlate well with fruit and vegetable intake, and higher plasma vitamin C levels are linked to a reduced risk of some chronic diseases.
There is some evidence that a higher vitamin C intake is linked to a reduced risk of cancers of the mouth, throat, esophagus, stomach, colon, and lung, but not all studies confirm this is true. As with the studies on cardiovascular disease, the reduced risk of cancer is the result of eating foods rich in vitamin C, such as fruits and vegetables, not from taking vitamin C supplements. In these studies, the specific protective effects of vitamin C cannot be separated from the many other beneficial chemicals in fruits and vegetables.
Vitamin C does have several roles in the immune system, and many people increase vitamin C intake either from diet or supplements when they have a cold. Many others take vitamin C supplements routinely to prevent colds. Contrary to this popular practice, however, there is no good evidence that vitamin C prevents a cold. A review of more than fifty years of studies published in 2004 in the Cochrane Database of Systematic Reviews concluded that taking vitamin C routinely does not prevent colds in most people, but it does slightly reduce cold severity and duration. Moreover, taking mega doses (up to 4 grams per day) at the onset of a cold provides no benefits.2
Gout is a disease caused by elevated circulating levels of uric acid and is characterized by recurrent attacks of tender, hot, and painful joints. There is some evidence that a higher intake of vitamin C reduces the risk of gout.
Dietary Reference Intakes for Vitamin C
| Age Group | R.D.A. Males and Females (Milligrams Per Day) | U.L. |
|---|---|---|
| Infants (0–6 months) | 40* | None |
| Infants (7–12 months) | 50* | None |
| 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) | 2,000 |
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. Because vitamin C is water-soluble, it leaches away from foods considerably during cooking, freezing, thawing, and canning. Up to 50 percent of vitamin C can be boiled away. Therefore, to maximize vitamin C intake from foods, you should eat fruits and vegetables raw or lightly steamed.
Vitamin C Content of Various Foods
| Food | Serving | Vitamin C (milligrams) | Percent Daily Value |
|---|---|---|---|
| Orange juice | 6 oz. | 93 | 155 |
| Grapefruit juice | 6 oz. | 70 | 117 |
| Orange | 1 medium | 70 | 117 |
| Strawberries | 1 c. | 85 | 164 |
| Tomato | 1 medium | 17 | 28 |
| Sweet red pepper | ½ c. raw | 95 | 158 |
| Broccoli | ½ c. cooked | 51 | 65 |
| Romaine lettuce | 2 c. | 28 | 47 |
| Cauliflower | 1 c. boiled | 55 | 86 |
| Potato | 1 medium, baked | 17 | 28 |
Source: Dietary Supplement Fact Sheet: Vitamin C. National Institutes of Health, Office of Dietary Supplements.
References
1.Institute of Medicine (IOM). Food and Nutrition Board. 2000. Dietary Reference Intakes for Vitamin C, Vitamin E, Selenium and Carotenoids. Washington, DC: National Academy Press.
2. Bjelakovic, G., D. Nikolova, and C. Gluud. 2013. Meta-regression analyses, meta-analyses, and trial sequential analyses of the effects of supplementation with beta-carotene, vitamin A, and vitamin E singly or in different combinations on all-cause mortality: Do we have evidence for lack of harm? PLoS ONE 8(9):e74558. doi: 10.1371/journal.pone.0074558
3. McCollum, E. V. 1957. A History of Nutrition. Boston: Houghton Mifflin Co.
4. Albanes, D., O. P. Heinonen, J. K. Huttunen, et al. 1995. Effects of alpha-tocopherol and beta-carotene supplements on cancer incidence in the Alpha-Tocopherol Beta-Carotene Cancer Prevention Study. The American Journal of Clinical Nutrition 62(suppl.):1427S–1430S.
5. Joshipura, K. J., F. B. Hu, J. E. Manson, et al. 2001. The effect of fruit and vegetable intake on risk for coronary heart disease. Annals of Internal Medicine 134:1106–1114.