Chronic Cardio = Excess Free Radicals

Free radicals are molecules with an unpaired electron that steal electrons from healthy atoms.  Not only causing damage to the cell in which it was taken, but causing a chain reaction of unpaired molecules.

free radical

Though our body has antioxidants to prevent these highly reactive molecules from causing damage (and can naturally increase our levels based on need) they can only handle so many before oxidative stress prevails.

The ‘free radical theory of aging‘ suggests that cells ‘age’ because of oxidative stress brought on by having more reactive oxygen species (free radicals) present in our body than antioxidants.


Other than rancid polyunsaturated fatty acids, and environmental pollutants, free radicals are produced during physical exercise through muscle contraction.

And sure, every workout type (aerobic or anaerobic, high-intensity or low-intensity, isometric or isokinetic) produces free radicals, but the amount generated, and whether there’s corresponding oxidative stress, varies greatly.  With lenghty bouts of aerobic activity being the most troublesome.


The biggest problem with aerobic or endurance exercise (consistent contraction for upwards of 60sec) is that oxygen is required to generate energy (ATP). Meaning free radicals are produced WITHIN the mitochondria of muscle cells.

And since mitochondria are the biggest producers of free radicals, skeletal muscle contains the most mitochondria, and muscle represents the largest organ in the body, this is a BIG problem.

Not only because of the excess free radical production in the mitochondria, and its potential damage to the rest of the body, but because damage to mitochondrial DNA appears to predict our lifespan.

An analysis of skeletal muscle from a 90-year-old man revealed that only 5% of his mitochondrial DNA was full length, while that of a 5-year-old boy was almost completely intact.

Some argue that more oxidative stress from exercise is beneficial because it increases the body’s internal production of antioxidants. As supposedly this promotes higher protective levels to deal with increased amounts of free radicals in the future (oxidative shielding).

However, this is only true to a certain extent.  Since research also suggests that free radicals outnumber antioxidants when exercise intensity or duration is excessive;

Which is outlined nicely in this 1992 paper by M.B. Reid, – showing that muscle fibers contain complex antioxidant defense systems to protect against oxidation (and muscle loss), but during strenuous exercise, free radicals are generated faster than any buffering agent can handle.

oxidative stress

In other words, our cells continue to increase their antioxidant status to protect against free radicals, but anything over the optimal threshold causes cell damage.

Point being, that those looking to “get fit” end up crossing that excessive line more often than not – because they think more exercise means more fat loss.  And so do those that treat running, cycling, and swimming as a sport – because continuous improvement requires higher speeds and longer distances.

When free radicals outnumber antioxidants we experience oxidative stress – leading to muscle dysfunction and loss, damage to protein, lipids and of perhaps highest concern, DNA!

Interestingly, it’s been suggested that cells overloaded with free radicals will often destroy themselves to protect the rest of the body (like Natural Selection). Which would explain a lot when looking at the stereotypical long-distance runner, cyclist, or swimmer, who chronically overloads the same areas and commonly ends up with cardiovascular (1, 2) and upper-respiratory damage (URTI’s).

Regardless, it’s clear that there’s a ‘safe’ level of exercise, and frequent cardio sessions for ridiculous durations are a problem. As we’re definitely capable of elevating antioxidants to support the additional stress on the body, but if we’re goofy enough to continue free radical production ‘everyday’ (or 4xWeek) and in large amounts, we’re on our own.


Stay Lean!
Coach Mike