Rights: University of Waikato Published 18 March 2011 Download

Plant & Food Research’s Dr David Stevenson explains what free radicals are and how they are produced. He outlines the role of cellular structures called mitochondria in the production of free radicals. He also describes the negative and positive effects of free radicals. The term ‘antioxidant’ is defined and common examples given.

Point of interest   
One of the problems that David faces in his research is that some phytochemicals show antioxidant activity in cells outside of the body but not necessarily in cells inside the body. A large number of health foods claim to have antioxidant activity, but is there evidence to show that the activity is in the living body (in vivo) or in a cell line outside the body (in vitro)?


Free radicals are essentially molecules with broken bonds, so they are highly reactive and will react with anything that they come into contact with. They are mainly produced in the body as a side-effect of generating energy. Our cells generate energy through little particles within them called mitochondria – they are sort of known as the powerhouses of the cell

Mitochondria carry out a controlled oxidation process of food – effectively the same as burning, but it’s much more controlled. In the process, they generate energy and they also reduce the oxygen to water, and the first stage of that is to convert the oxygen into a free radical called superoxide. Some of the superoxide doesn’t get further reduced, so it drifts away from the enzyme that produces it and can cause damage to proteins or DNA

An antioxidant – it’s more of a chemical definition than a biological one – it’s a compound which… you put it into a test where there is another compound that generates free radicals, it will neutralise those free radicals, and the less antioxidant you need to put into the test to get a response, the better it is.

The best known antioxidants that people would be familiar with would be vitamins C and E. They are made by plants. The type of antioxidant we would get a lot of in our diet will be the polyphenols, because when you put those into the chemical antioxidant assay, a lot of them work very well.

The term we use for excess free radical production is ‘oxidative stress’, so that’s when there’s so many free radicals produced that they leak out of the mitochondria and go around and cause a lot of damage. Now that might happen if, say, an unfit person takes up running and does far too much of it in one go.

We definitely don’t want to get rid of free radicals completely from the body because they are signalling molecules. The production of free radicals when we exercise sends a signal to various systems like the muscles, the mitochondria, the lungs, the arteries and everything to sort of up their game, and it tunes them up so that they will be able to function better next time we exercise.