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Wildland Firefighter Health and Safety
Recommendations of the April 1999 Conference

Oxidative Stress and Antioxidants: Fighting the Fire Within

Wayne Askew, Ph.D.
Professor and Director
Division of Foods and Nutrition
University of Utah


Introduction

There is a fire smoldering within the cells of your body that could burst into flame at any moment. Scientists who study aging, chronic disease, and preventive medicine call the “smoldering fire” free radical production and the “flame” oxidative stress. As long as the fire just smolders, it is under control by endogenous antioxidant defenses. However, if it is fanned into “flame” by certain events such as exercise, inhalation of pollutants, excessive exposure to sunlight or poor dietary intakes of antioxidant nutrients, oxidative stress can rage out of control, inflicting cumulative damage to important tissues within the body. The damage is not always noticeable, but may exact a serious toll over time.

Consider this scenario: Jack Jones, wildland firefighter, is aboard an airplane on an unplanned flight. He left his home—now 200 miles behind—in haste. No time to eat or even tell the wife and kids goodbye. Five thousand acres of pristine wilderness forest is burning and his crew is assembling to help bring the fire under control. In 10 minutes he will be on the ground racing toward the blazing inferno. Still no time to eat. There may be time for that later. Now, more important tasks face the Forest Service team assembled at the site. Smoke fills the air. The sun beats down, causing rivulets of sweat to run down his face, washing off the sunscreen he hastily applied before leaving the plane. As he nears the fire, smoke swirls down and engulfs him, causing him to cough and burning his eyes. Maybe the wind will blow the smoke in the other direction soon. The team swings into action, cutting break lines in the underbrush. Heavy chain saws whine, emitting dust and exhaust fumes. Pulaskis are wielded, giving off a staccato uneven beat as the crew warms to the physically demanding task. The heat is intolerable. The sun beats down unmercifully, its burning ultraviolet rays reflecting off rocks and onto the unfortunate workers. As the day winds into night it is time to change shifts. The workers on this team have expended a gargantuan amount of energy, approximately 6,000 kcal on this one day alone, almost twice their normal energy expenditure. Sore muscles tighten up and inflammation grips backs, knees, shoulders and wrists. Jack is hungry, but too exhausted to eat much of the food provided. He grabs a hamburger and a piece of pie and washes them down with some lemonade. He is asleep before his head hits the pillow. This scenario will be repeated for days until his team is relieved to recover and return again if the blaze still rages. Jack is experiencing repetitive periods of oxidative stress.

Now fast-forward 25 years into the future. Jack is now nearing 60 years of age. Things are not as tough physically now, he has desk job, but he feels increasingly sore and stiff in his joints. Jack’s opthamologist is concerned about something he called the ‘macula’ of his eye, his vision is spotty and detail is hard to make out, even with new glasses. He looks into the mirror and wonders at the number of age spots that seem to have taken residence on his face and the backs of his hands. He thinks, “Maybe I should get some of these checked out the next time I have a physical.” He can remember how worried his mother was when several spots on his Dad’s face were diagnosed as cancerous. He sits back in his chair and sighs as he massages his arthritic knees. “I guess I am just getting old,” he thinks. I always knew it had to happen, but I didn’t think it would be this soon!

Jack is correct about one thing: He is undergoing accelerated aging and is suffering from what we term (for lack of a better word) chronic disease. Chronic diseases are those maladies we often associate with the aging process, such as arthritis, macular degeneration and even some forms of cancer. Can the onset of these symptoms be delayed or even prevented? Scientists who study chronic diseases believe that many are preventable. The key seems to involve diet (Garewal 1997, Papas 1999). Epidemiologists who study the relationship between diet and disease have built a convincing case for the importance of certain nutrients contained in fruits and vegetables (McLarty 1997, Mayne and Ziegler 1999). These important nutrients are called antioxidants. Fewer than three in five Americans consume enough of them on a daily basis (Krebs-Smith et al. 1995). These antioxidant nutrients from fruits and vegetables help us control oxidative processes within our bodies that can damage important cellular components. High rates of energy expenditure, inflammation, excessive exposure to sunlight, and poor dietary habits all promote oxidative stress (Askew 1995). Severe oxidative stress can injure or kill cells (Gutteridge and Halliwell 1994). Scientists believe that excessive oxidative stress in our tissues may be at the root of many chronic diseases.

Oxidative stress is the term used to describe the damaging oxidation of biological tissues by free radicals (Gutteridge and Halliwell, 1994). Free radicals are unstable, short-lived molecules that are especially reactive chemical species because they have an unpaired electron in their outer orbit. These molecules seek to stabilize their structure (pair the electrons in their outer orbit) by “stealing an electron” from an unsuspecting neighbor (Figure 1). When this happens, the free radical is once again stable, but the molecule that lost an electron may become nonfunctional or even become a free radical itself.

Image showing free radical damage.
Figure 1—What is oxidative stress?

Free radicals are unstable chemical molecules that are continually being formed and destroyed in your body. They are most often oxygen-, carbon-, or nitrogen-based molecules. Oxygen molecules with unpaired electrons in their outer orbits are an especially damaging species. They are called “reactive oxygen species.” Reactive oxygen species are unstable molecules containing oxygen with unpaired electrons in their outer orbit (Figure 2). Examples of reactive oxygen species are the superoxide molecule, the hydroxyl anion, and singlet molecular oxygen (Gutteridge and Halliwell 1994). Normally, oxygen molecules within the body are essential for life and are not damaging. However, a small percentage of these oxygen molecules can become potentially damaging reactive oxygen species or free radicals (Gutteridge and Halliwell 1994) . Since humans must exist in an atmosphere of oxygen and consume large quantities to survive, they have developed very effective antioxidant defense systems to neutralize these reactive oxygen species. A certain amount of free radical formation is a normal part of metabolism. Our antioxidant defense systems usually prevent free radicals from causing excessive damage.

Figure showing what a free radical is.
Figure 2—What are free radicals?

Certain situations or conditions are believed to lead to the formation of free radicals and increased levels of oxidative stress (Table 1, Askew 1995). High rates of energy expenditure and their relationship to oxidative stress can be better understood by considering the analogy of “water pressure and the leaky hose.” Just as more water pressure applied to a hose with a pinpoint leak causes the leak to become more pronounced, so does “metabolic pressure” (high rates of energy expenditure) cause more electrons to “leak” out of the normally tightly coupled electron transport chain into the cytoplasm of the cell where these electrons can reduce molecular oxygen to the free radical, superoxide.

Risk factors for oxidative stress:

  1. High rates of energy expenditure
  2. Hypoxia/reoxygenation
  3. Sunlight (UV) exposure
  4. Inhaling atmospheric pollutants
  5. Poor dietary intake of antioxidant nutrients

Table 1—Risk factors for oxidative stress.

Another theory suggests tissue that is “starved for oxygen” can suffer free radical damage when it is reoxygenated with blood that has regained its normal oxygen content. This theory is called “fanning the glowing embers,” and can be likened to a smoldering coal bursting into flames when air is supplied to it. This situation might occur following exhaustive exercise at altitudes where the oxygen tension is not adequate to support the energy expenditure required.

High-energy rays found in the ultraviolet spectrum of sunlight can actually “penetrate” the dermis of skin and impart enough energy to oxygen molecules in the dermis to form a special type of “reactive oxygen species,” singlet molecular oxygen. This molecule has become “excited” by the ultraviolet energy and one of its electrons has absorbed enough energy to cause it to move up into a higher orbital by itself, hence forming the potentially damaging unpaired electron configuration. Singlet molecular oxygen can “strip” electrons from cell membranes, causing internal cellular damage. Skin cancer caused by excessive sunlight exposure may get its start in this manner.

Another contributor to oxidative stress involves the biochemical processes associated with inflammation. Phagocytotic cells rushing to the site of damaged or injured tissue can release substantial amounts of free radicals in the process of fighting infection and inflammation.

Inhalation of toxic compounds in the air we breathe may also give rise to free radical production (Table 2). Smoke is particularly irritating to the lungs and may be damaging due to increased free radical formation (Gutteridge and Halliwell 1994). Most of the evidence for the damaging effects of acute and chronic smoke inhalation come from studies of tobacco smoking, but inhaling smoke from a forest fire might be expected to elicit acute responses similar to (but less chronic than) cigarette smoking. Little data exists to confirm or deny smoke inhalation as a high risk item for wildland firefighters. Sharkey (1997a) has recommended a prudent approach that firefighters avoid smoke exposure and eat a diet including antioxidant foods to maintain a healthy immune system.

Smoke

  • Smoke contains free radicals (oxides of nitrogen NO & NO2) in both the gas and tar phases which irritate lung membranes
  • Chronic smoke inhalation causes tissue inflammation which in turn leads to macrophage and neutrophil infiltration of the lungs
  • Acute smoke inhalation and bacteria in the inflamed tissue stimulates neutrophils to produce superoxide (O2-)
  • O2- and NO2 can lead to lung tissue damage

Table 2—Smoke and free radicals.

The good news is that the human body is remarkably well equipped to minimize oxidative stress caused by free radical damage (Table 3), provided we do our part. We must provide the our body with the “biological flame retardants” or antioxidant nutrients that it needs to neutralize free radicals. Antioxidant nutrients can be minerals, vitamins, or plant phytochemicals. The minerals zinc, selenium, magnesium, and manganese function as important nutrient cofactors or “helpers” for enzymes such as superoxide dismutase and glutathione reductase. These minerals and enzymes work in a concerted manner to neutralize free radicals such as superoxide. Left unchecked, these bad characters can strip electrons from unsuspecting cellular component neighbors, leading to leaky cell membranes, nonfunctional enzyme proteins, and even coding errors in DNA molecules.

Antioxidant Defense Systems:

The body's "firefighters"

  • Antioxidant enzymes
  • Antioxidant vitamins
  • Phytochemicals from fruits and vegetables

Table 3—Antioxidant defense systems.

Antioxidant enzymes are fast acting and very effective as long as the level of invading free radicals is not excessive. When severe oxidative stress is present and the antioxidant enzymes are overwhelmed by the invading free radicals, the antioxidant vitamins and antioxidant phytochemicals become the last line of defense between our cells and free radical damage. The large, complex molecules of antioxidant vitamins and phytochemicals can directly interact with a free radical and “neutralize” it by absorbing the full force of its attack, much as a sumo wrestler would envelop a smaller foe. The antioxidants neutralize free radicals by donating an electron to stabilize the free radical. Unlike smaller molecules that may be attacked by a free radical, these large organic, antioxidant molecules “internalize” the loss of an electron through resonance between carbon bonds. They do not themselves become electron-seeking free radicals. Some antioxidant vitamins such as vitamin E (alpha-tocopherol) and vitamin C (ascorbate) act as “tag team” partners to intercept and neutralize free radicals. Vitamin E is lipid soluble and can position itself in the membrane of cells and lipoproteins where it can “intercept” free radicals that attack lipid-containing cell membranes. Once vitamin E has intercepted a free radical, it can pass the “hit” to water-soluble vitamin C. In this manner, vitamin C regenerates the immobile vitamin E in the membrane and can, in turn, be regenerated by other antioxidant phytochemicals in the cell or pass out of the cell, into the blood and ultimately be excreted in the urine.

Phytochemicals are molecules of plant origin (such as carotenoids, flavenoids, phytosterols, chlorophylls, terpenoids, indoles and allylic compounds) consumed in the diet that are not true vitamins, but can be very potent antioxidant nutrients (Papas 1999). Plants have evolved very sophisticated mechanisms to protect themselves from the large amount of oxygen generated by photosynthesis. The conversion of even small quantities of that oxygen to free radicals would potentially be lethal to the plant (Halliwell and Gutteridge 1993). We gain the protective effect of these powerful antioxidant phytochemicals when we consume fruits and vegetables. Five servings of fruits and vegetables per day supply us with enough of these antioxidants to prevent or combat many chronic diseases such as cancer and macular degeneration (Block et al. 1992).

However, active people working in remote outdoor environments usually do not have access to ample quantities of fresh fruits and vegetables (Askew 1995). The National Cancer Institute and other agencies concerned with disease prevention recommend eating five servings of fruits and vegetables per day. In fact, many persons do not even consume five servings of fruits and vegetables per day when they are eating at home, close to the local supermarket (Krebs-Smith et al. 1995). This information does not diminish the importance of diet, but strengthens the case for supplementing the diet with antioxidant nutrients for people who might be subject to increased oxidative stress.

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