Most of us have heard about antioxidants in the news or at the supermarket. But what are they? And how are they related to damage causing particles called free radicals? In this post, recently completed PhD Shona Smith discusses her research on these important molecules, and explores their relationship to how fast organisms grow.
Growing quickly comes at a price
To produce the energy needed to survive, all animals burn food in a process called respiration. But unstable molecules called free radicals are produced as a by-product and these can attack the body’s tissues, damaging proteins, lipids and DNA. This oxidative damage can have serious consequences and has been implicated in numerous diseases—like cancer, cardiovascular disease, diabetes and Alzheimer’s—and might be the reason why we age. Although everybody produces free radicals, not everyone suffers from such diseases, and there are differences between individuals in how much damage they have. So, what makes certain individuals more vulnerable to this oxidative damage?
How fast an animal grows from conception to adulthood (and beyond in animals that continue to grow throughout adulthood, like many fishes and molluscs) is likely important. When an animal grows more quickly, their body needs to work harder, leading to increased respiration, and potentially more free radicals being produced. More free radicals means potentially more oxidative damage. It is well known that many animals don’t grow as fast as physiologically possible—perhaps this is why!
Then again, the level of oxidative damage we experience depends, in part, on our antioxidant defences. Our bodies makes these amazing molecules (they can also come from things we eat, like blueberries—a ‘superfood’) to fight off the free radicals before any harm is done. Therefore, a high level of antioxidants may prevent the buildup of tissue damage caused by free radicals, and reduce the risk of an early death. However, it must be noted that it is the balance between these molecules that is important—a high antioxidant capacity may not be much help if free radical production is even higher.
If this balance is in favour of free radicals, or antioxidant defences are low, oxidative damage will occur. Antioxidant levels might be reduced in the face of accelerated growth simply because the system is competing for limited resources, for instance there are certain amino acids that are essential for both growth and antioxidant defence. As such, there might be a resource trade-off between growth and self-maintenance processes, like antioxidant protection.
If the antioxidant defence system draws resources away from growth, could growth be constrained by low antioxidant levels, when free radical production rises and a greater antioxidant investment is needed? The repair and replacement of damaged cells and tissues might also draw resources away from growth. I tested this idea by analysing data from 61 published studies, spanning eight taxonomic classes, that measured growth rates after experimentally changing an organism’s antioxidant or damage levels. I found a consistent pattern: where antioxidants had been reduced, or damage had been increased, then the growth rate usually declined. This implies resources had been diverted from growth to either improve antioxidant defences when levels were low and free radical production rose, or to repairing damaged tissues.
What’s more, I also found that animals experimentally induced to grow more quickly suffered greater oxidative damage. However, remarkably, there was no overall change in antioxidant levels in response to growth. If faster growth was causing resources to be diverted away from antioxidant defences, then we would expect these to decline in fast-growing animals; as this did not happen, I suggest that faster growing individuals are working harder and so generating more free radicals, leading to greater damage.
So, growing quickly comes at a price, but is this just the tip of the iceberg? Could those faster growing individuals, with greater oxidative damage levels, be at a disadvantage later in life, having a lower reproductive success or survival rate? Both fast growth and raised oxidative damage levels have been associated with various diseases, and perhaps this is why most animals don’t grow as fast as they can. So, can we prevent such diseases, and live longer, by improving our antioxidant status with supplements? This is another broad area of research and the answer is not black and white. Furthermore, ‘super’ species (e.g. naked mole rat) can tolerate relatively high levels of oxidative damage, whilst living for a very long time, suggesting there’s more to it than simply increasing antioxidant levels. We need more studies that follow the same individuals over key life events, such as growth, reproduction and ageing, to see if high oxidative damage from fast growth really matters.
Some of the animals included in Shona’s analysis:
Feature image: Shona Smith conducting fieldwork. Image supplied.