What are free radicals and what do they do to your cells?

Free radicals are unstable molecules with an unpaired electron. They arise as a by-product of normal energy production in your cells, particularly in the mitochondria. Because they are so reactive, they attack DNA, proteins and fats in cell membranes. Each cell is estimated to sustain tens of thousands of such attacks on its DNA every single day. In the ageing brain, at least one in three proteins is consequently so damaged that it no longer functions properly.

Your body is not defenceless against this. It has multiple layers of defence: enzymes that neutralise radicals, and small protective molecules such as vitamin E and vitamin C. As long as those systems can keep pace with what is being produced, the damage remains limited. Only when production exceeds removal do we speak of oxidative stress, and it is then that damage accumulates in a noticeable way.

Mitochondria play a key role in ageing. Animals with a shorter maximum lifespan produce more oxygen radicals in their mitochondria, and this goes hand in hand with greater damage to the genetic material within those mitochondria themselves. In animal research, eating less (without malnutrition) has been shown to lower radical production in the mitochondria and to slow the ageing process. Whether this effect is equally large in humans has not yet been established.

Free radicals also play a role in specific disease processes. After a heart attack, when blood flow is restored, they can damage the fats in the cell membranes of heart muscle cells and disrupt the calcium balance. In Alzheimer's disease, certain protein fragments that enter the mitochondria produce extra radicals, causing further cell damage. In infections too, radicals can indirectly damage nerve cells through increased release of the signalling molecule glutamate, although that link has not yet been fully proven.

Finally, there is preliminary evidence that free radicals play a direct role in some forms of programmed cell death. This is not necessarily harmful: controlled cell death is part of normal cell renewal. When this process becomes dysregulated due to too many radicals, it can contribute to tissue damage.