Body cells can suffer damage due to oxidation. This happens when atoms in the body loses an electron from its outer shell. This makes the atom highly unstable. In this unstable form (free radical), they try to regain stability by robbing an electron from nearby molecules thus causing them to become unstable. This can set off a chain reaction resulting in much cellular damage including damage to cell membrane, lipoproteins, and DNA. As a result, they promote inflammation in the body and are associated with diseases including cardiovascular disease, type 2 diabetes, arthritis, cataracts, Alzheimer’s disease and Parkinson’s disease. Free radicals are naturally produced during cellular respiration and during times when the immune system is fighting infection. However, they also occur due to exposure to harmful chemicals, radiation and smoking.
Luckily, certain nutrients (vitamins and minerals) and phytochemicals that we eat can control free-radical formation by acting as antioxidants. They do this in the following ways:
- Vitamins and phytochemicals donating their electrons to free radicals
- Certain minerals (e.g. selenium, iron, zinc and manganese) acting as cofactors in antioxidant-enzyme systems that convert free radicals to less harmful substances. Examples of antioxidant-enzyme systems are (1) superoxide dismutase which is responsible for breaking down superoxide free radicals to hydrogen peroxide, (2) catalase that breaks down hydrogen peroxide to oxygen and water, and (3) glutathione peroxidase that also breaks down hydrogen peroxide.
Antioxidant Activity of Vitamin E
Vitamin E is fat-soluble and is therefore absorbed wherever there is fat in our body such as adipose tissue and cell membrane. They protect against cellular damage including erythrocyte lysis (red blood cell damage) by donating an electron to free radicals. The RDA for vitamin E for the day is based on the amount that is needed to prevent erythrocyte lysis, which is 15 mg of vitamin E in its alpha-tocopherol form. Vitamin E is often added to food oils to prevent them from oxidizing and turning rancid. Foods rich in vitamin E include vegetable oils (e.g. sunflower, safflower and canola), breakfast cereals, almond, peanut butter, and avocado. Consuming too much vitamin E can cause:
- Increased risk of prostate cancer
- Premature mortality in men and women
- Hemorrhaging, due to its anticoagulant activity
Consuming too little vitamin E can cause erythrocyte lysis leading to anemia, and loss of muscle coordination, speech and vision problems. Deficiencies are usually rare however, since our body does a good job of storing vitamin E in our fat cells.
Antioxidant Activity of Vitamin C
Vitamin C is also known as ascorbic acid. It is known to prevent scurvy due to the role it plays in building the connective tissue, collagen. Collagen is the most abundant protein in the body providing structure to bone, teeth, skin, tendons, blood vessels, tissues and organs. Vitamin C contributes to collagen-building by acting as a coenzyme for the enzymes involved in its production. In addition to collagen production and many other functions (including its ability to improve iron absorption), vitamin C is also an antioxidant, protecting against cellular and tissue damage. They protect white cells from damage during an inflammatory response which is why you might have been told to drink orange juice when you have the flu. Unfortunately, there is no evidence that this helps the flu while you are having it. It appears that you need to make sure you are consistently getting enough vitamin C to lower the risk of a flu in the first place, rather than curing it when it happens. Vitamin C supports vitamin E activity by donating an electron to replace the one vitamin E gave away to a free radical. In that way vitamin E activity is regenerated. When ascorbic acid gives away its electron it becomes dehydroascorbic acid. Dehydroascorbic acid can be regenerated to ascorbic acid by accepting an electron from glutathione (GSH). Fruits and vegetables are the best source of vitamin C, so be sure to get lots of those in your diet. Because vitamin C is easily excreted from the body, toxicity is rare unless you are taking mega-doses of supplements for a prolonged time. In that case you may experience nausea, diarrhea, nose bleeds, abdominal cramps and kidney stone formation.
So far, we have talked about antioxidant properties of nutrients. However non-nutritive compounds in food can also offer antioxidant protection. These compounds are called phytochemicals. Phyto means plant, so phytochemicals are only found in plant foods. One such phytochemical that we are studying this week is β-carotene. β-carotene is a provitamin classified in the group called carotenoids. Other phytochemicals in that group includes lycopene (the pigment that gives tomato its red color) and zeaxanthin (the pigment in bell pepper and corn). A provitamin is an inactive form of a vitamin which can be activated to the active form. Beta-carotene can be converted by β-carotene-15,15′-dioxygenase in the wall of the small intestine to form vitamin A (retinol form). β-chemicals are quantified in foods based on their retinol activity equivalent (RAEs). This value tells you how much vitamin A will be produced when it is converted.
Apart from its antioxidant properties, β-carotene provides other amazing benefits such as building the immune system and protecting eye health. However, there is no RDA for β-carotene. This is because there are no deficiencies associated with not eating enough of it. The only concern in eating too much is your skin turning yellow which has been found to be reversable and harmless. However, if you take excess in supplement form, this has been found to increase the risk of premature deaths from lung cancer and heart disease. So, you are probably much better off avoiding β-carotene supplements.
The three active forms of vitamin A are retinol, retinal and retinoic acid. When β-carotene is cleaved, it is converted in the small intestine to retinal and then to retinol. Retinal is also converted to retinoic acid, but this takes place in the liver. Since retinol is fat soluble, it absorbs into the lymphatic system and goes to the liver where up to 90% of it is stored. From the liver, retinol-binding proteins carries retinol to where it is needed in the body.
You might have been told to eat up your carrots for good eyesight. This is because carrots contain β-carotene which can be converted to vitamin A, and vitamin A helps in improving eyesight. The eye contains the retinal form of vitamin A located in the retina (the back of the eye where light falls allowing us to see even in dim light).
Retinal is located in cells called rods which are sensitive to black and white images. Retinal combines with a certain protein called opsin which together is called rhodopsin. In dim light environments, light hitting the retina causes retinal to change from its cis to its trans configuration. This sets off a cascade of biochemical reactions that results in the transmission of vision messages to the brain via the optic nerve. Retinal in the rhodopsin is regenerated from retinal in the blood.
Deficiency symptoms of vitamin A gives us evidence of their importance for example:
- Blindness or night blindness: As mentioned, vitamin A is an important component of retina in the eye
- Dry skin and dry eyes: Vitamin A is involved in cell differentiation of epithelial cells responsible for lubricating the skin and eliminating irritants and microbes
- Delayed growth: Vitamin A assists in bone development
- Poor wound healing and infections: Vitamin A is involved in differentiation of specialized immune cells called T-Cells
- Difficulty conceiving: Vitamin A is necessary for sperm production and reproduction
Sources of Vitamin A
By far, liver contains the highest amount of vitamin A. Only 3 ounces which is the amount that I suppose most of us would normally eat in a serving, contains more than 6 times the RDA. Vitamin A is fat-soluble, so it is easy to accumulate in the body and cause toxicity if taken in high amounts. Try pumpkin, carrot and sweet potato as healthier options to liver and definitely avoid mega-dose supplementation. Toxicity can cause permanent damage to your liver and your vision.
Reference: Thompson,& J., Manore, M., Vaughan, L. (2020). The science of nutrition (5th ed.). New York. Pearson