The human body’s cells produce free radicals during normal metabolic processes. However, cells also produce antioxidants that neutralize these free radicals. In general, the body is able to maintain a balance between antioxidants and free radicals. Oxidative stress can occur when there is an imbalance of free radicals and antioxidants in the body.

Several factors contribute to oxidative stress and excess free radical production. These factors can include: diet, lifestyle, certain health conditions, environmental factors such as pollution and radiation.
Athletes are disposed to strong oxidative stress when training and competing. Exercise is known to increase redox reactions. This isn’t inherently ‘bad’. In fact, redox is a necessary part of improving in our sport (i.e. physiological adaptation).
However, too much training, or a gross imbalance between the stimulus (training load) and recovery (rest and diet), can significantly upset redox balance and push the athlete into a fatigued, maladapted state with underperformance. That’s why athletes’ diet must be well balanced and rich with many nutrients in quantities which exceed the needs of a person not active in sports.
The human body works to maintain a state of redox balance (homeostasis), primarily through the activity of antioxidant enzymes in the body. However, food-derived antioxidants (what we eat, how much of it, how often and its metabolism by the gut) also play a prominent role.
As oxidative stress is present in higher lever with athletes, we recommend longer use of protective substances like polyphenols, as their body needs more of it to keep the homeostasis.
Healthy people, not that active in sports or not under high physical pressure, are disposed to different factors causing oxidative stress: environmental factors that increase redox reactions include pollution, lifestyle choices, dehydration, UVA, heat and especially stress. We all need protective substances, but sufficient intake is much lower.