User talk:Ghadeermm

Free Radicals

Antioxidants and Free radicals

Antioxidants are intimately involved in the prevention of cellular damage -- the common pathway for cancer, aging, and a variety of diseases. The scientific community has begun to unveil some of the mysteries surrounding this topic, and the media has begun whetting our thirst for knowledge. Athletes have a keen interest because of health concerns and the prospect of enhanced performance and/or recovery from exercise. The purpose of this article is to serve as a beginners guide to what antioxidants are and to briefly review their role in exercise and general health. What follows is only the tip of the iceberg in this dynamic and interesting subject.

It's the radicals, man

Free radicals are atoms or groups of atoms with an odd (unpaired) number of electrons and can be formed when oxygen interacts with certain molecules. Once formed these highly reactive radicals can start a chain reaction, like dominoes. Their chief danger comes from the damage they can do when they react with important cellular components such as DNA, or the cell membrane. Cells may function poorly or die if this occurs. To prevent free radical damage the body has a defense system of antioxidants.

Antioxidants are molecules which can safely interact with free radicals and terminate the chain reaction before vital molecules are damaged. Although there are several enzyme systems within the body that scavenge free radicals, the principle micronutrient (vitamin) antioxidants are vitamin E, beta-carotene, and vitamin C. Additionally, selenium, a trace metal that is required for proper function of one of the body's antioxidant enzyme systems, is sometimes included in this category. The body cannot manufacture these micronutrients so they must be supplied in the diet.

Vitamin E : d-alpha tocopherol. A fat soluble vitamin present in nuts, seeds, vegetable and fish oils, whole grains (esp. wheat germ), fortified cereals, and apricots. Current recommended daily allowance (RDA) is 15 IU per day for men and 12 IU per day for women.

Vitamin C : Ascorbic acid is a water soluble vitamin present in citrus fruits and juices, green peppers, cabbage, spinach, broccoli, kale, cantaloupe, kiwi, and strawberries. The RDA is 60 mg per day. Intake above 2000 mg may be associated with adverse side effects in some individuals.

Beta-carotene is a precursor to vitamin A (retinol) and is present in liver, egg yolk, milk, butter, spinach, carrots, squash, broccoli, yams, tomato, cantaloupe, peaches, and grains. Because beta-carotene is converted to vitamin A by the body there is no set requirement. Instead the RDA is expressed as retinol equivalents (RE), to clarify the relationship. (NOTE: Vitamin A has no antioxidant properties and can be quite toxic when taken in excess.)

Preventing cancer and heart disease -- do antioxidants help?

Epidemiologic observations show lower cancer rates in people whose diets are rich in fruits and vegetables. This has lead to the theory that these diets contain substances, possibly antioxidants, which protect against the development of cancer. There is currently intense scientific investigation into this topic. Thus far, none of the large, well designed studies have shown that dietary supplementation with extra antioxidants reduces the risk of developing cancer. In fact one study demonstrated an increased risk of lung cancer in male smokers who took antioxidants vs. male smoker who did not supplement. Whether this effect was from the antioxidants is unknown but it does raise the issue that antioxidants may be harmful under certain conditions.

Antioxidants are also thought to have a role in slowing the aging process and preventing heart disease and strokes, but the data is still inconclusive. Therefore from a public health perspective it is premature to make recommendations regarding antioxidant supplements and disease prevention. New data from ongoing studies will be available in the next few years and will shed more light on this constantly evolving area. Perhaps the best advice, which comes from several authorities in cancer prevention, is to eat 5 servings of fruit or vegetables per day.

Exercise and oxidative damage

Endurance exercise can increase oxygen utilization from 10 to 20 times over the resting state. This greatly increases the generation of free radicals, prompting concern about enhanced damage to muscles and other tissues. The question that arises is, how effectively can athletes defend against the increased free radicals resulting from exercise? Do athletes need to take extra antioxidants?

Because it is not possible to directly measure free radicals in the body, scientists have approached this question by measuring the by-products that result from free radical reactions. If the generation of free radicals exceeds the antioxidant defenses then one would expect to see more of these by-products. These measurements have been performed in athletes under a variety of conditions.

Several interesting concepts have emerged from these types of experimental studies. Regular physical exercise enhances the antioxidant defense system and protects against exercise induced free radical damage. This is an important finding because it shows how smart the body is about adapting to the demands of exercise. These changes occur slowly over time and appear to parallel other adaptations to exercise.

On the other hand, intense exercise in untrained individuals overwhelms defenses resulting in increased free radical damage. Thus, the "weekend warrior" who is predominantly sedentary during the week but engages in vigorous bouts of exercise during the weekend may be doing more harm than good. To this end there are many factors which may determine whether exercise induced free radical damage occurs, including degree of conditioning of the athlete, intensity of exercise, and diet.

Can antioxidant supplements prevent exercise induced damage or enhance recovery from exercise?

Although it is well known that vitamin deficiencies can create difficulties in training and recovery, the role of antioxidant supplementation in a well nourished athlete is controversial. The experimental studies are often conflicting and conclusions are difficult to reach. Nevertheless, most of the data suggest that increased intake of vitamin E is protective against exercise induced oxidative damage. It is hypothesized that vitamin E is also involved in the recovery process following exercise. Currently, the amount of vitamin E needed to produce these effects is unknown. The diet may supply enough vitamin E in most athletes, but some may require supplementation. There is no firm data to support the use of increased amounts of the other antioxidants.

Performance

In general, antioxidant supplements have not been shown to be useful as performance enhancers. The one exception to this is vitamin E which has been shown to be useful in athletes exercising at high altitudes. A placebo controlled study done on mountaineers demonstrated less free radical damage and decline in anaerobic threshold in those athletes supplemented with vitamin E. Although difficult to generalize, this finding suggests that supplementation with vitamin E might be beneficial in those triathletes who are adapting to higher elevations.

How much is enough?

Although there is little doubt that antioxidants are a necessary component for good health, no one knows if supplements should be taken and, if so, how much. Antioxidants supplements were once thought to be harmless but increasingly we are becoming aware of interactions and potential toxicity. It is interesting to note that, in the normal concentrations found in the body, vitamin C and beta-carotene are antioxidants; but at higher concentrations they are pro-oxidants and, thus, harmful. Also, very little is known about the long term consequences of megadoses of antioxidants. The body's finely tuned mechanisms are carefully balanced to withstand a variety of insults. Taking chemicals without a complete understanding of all of their effects may disrupt this balance.

اعداد الطالبة : غدير الموسى ٢ع/٧