By James C. Qualified Nutritionist | 10th May 2016
Antioxidants are substances that protect cells against damage caused by a species of molecule known as free radicals. This article provides an overview of the protective mechanism of antioxidants, and examines five important antioxidants that are found in the diet:
A free radical is a highly reactive and unstable molecule that is usually characterised by having an unpaired electron. This spare electron accounts for the properties shared by the majority of free radicals in the body. Electrons usually exist in pairs, and free radicals steal an extra electron from certain components of the body’s cells, in a process called oxidation. When free radicals oxidise important parts of the cell, the cell becomes damaged, and this is why the damage caused by free radicals is often referred to as oxidative damage.
Free radicals are produced as a by-product of everyday human metabolism and are generated whenever food is converted into a usable form of energy, or when energy is expended during physical activity. There are a number of internal and external sources that expose the body to free radicals:
Oxidative stress results in damage to a variety of molecules in the body, including fats, proteins and the building blocks of living organisms known as nucleic acids. It is believed that oxidative stress is possibly involved in a wide range of diseases including:
Oxidation to some extent is inevitable, and normal physiological functioning is dependent on maintaining an appropriate balance between free radicals and antioxidants. After a free radical has stolen an electron from another molecule, the victim molecule is left with a spare electron and itself becomes a free radical with a need to steal an electron. This repetitive sequence is called a free radical chain reaction.
If free radicals are sufficiently abundant to outstrip the availability of the antioxidants needed for their regulation, the process of free radical generation continues unchecked. This unfavourably tips the balance between oxidative damage and the body’s ability to protect against it, resulting in a situation known as oxidative stress.
An antioxidant has the ability to satisfy a free radical’s need for an electron by donating one of its own, without sacrificing its own stability. This has a quenching effect on oxidation and explains why antioxidants are often referred to as free radical scavengers.
Vitamin A is an antioxidant vitamin found in the diet in both plant and animal foods. Several compounds are known to have vitamin A activity. The vitamin A found in animal sources is preformed vitamin A, known as retinol. The type of vitamin A found in plant foods is pro-vitamin A, of which the most common type is beta carotene. The body has the ability to convert beta carotene into retinol.
Vitamin C is an antioxidant vitamin that is found in a wide range of fruit and vegetables. Vitamin C, which has the chemical name of ascorbic acid, is a water soluble nutrient. This means that the human body has no capacity for its storage and any excess is excreted in the urine. Humans are one of the few species in the animal kingdom that lack the ability to synthesise vitamin C, and so an exogenous dietary supply is regularly required.
Vitamin E is an antioxidant vitamin that helps protects the body’s cells, tissue and organs against oxidative damage from free radicals. Vitamin E exists in nature in eight distinct forms and is found in a variety of foods. Since it is a fat-soluble vitamin, the body is able to store any vitamin E that is not used immediately. However, only one form of the nutrient, known as alpha-tocopherol, is believed to have activity in human metabolism, and the remaining forms are metabolised and excreted by the liver.
Selenium is a trace mineral, meaning that is only required by the body in relatively small amounts. A variety of rich dietary sources exist, and this important antioxidant is also added to certain foods and crops as a fortifying agent. Two forms of selenium exist, an inorganic form and an organic form. The type of selenium found in soil is inorganic, which plants accumulate and convert to the organic form. Humans are able to store selenium, and as much as 46% of the body’s selenium pool is found in the skeleton.
Vitamin A, beta carotene, vitamin C, vitamin E, and selenium are all dietary components that are nutritionally essential for humans. Although each of these nutrients has its own distinct set of metabolic roles in the body, they all share the ability to function as antioxidants and serve to combat the oxidative damage inflicted by free radicals.
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