Eating is essential for life. Animals must eat to live, grow, and reproduce. But like most things, eating comes with tradeoffs. Dr. Zach Stahlschmidt of the University of the Pacific and his colleagues have found that along with the benefits of eating, there's a price to pay.
This price is oxidative damage -- damage caused by an increase in chemically reactive molecules containing oxygen that result in harm to cells and DNA. This harm is severe enough that it's believed that its accumulation over time contributes to aging. Surprisingly, says Stahlschmidt, this cost of eating has been gravely overlooked.
"It seemed like this is a hidden piece of the puzzle that no one had investigated that might be really important, for lots of reasons" Stahlschmidt says.
But in order to look for oxidative damage during eating, Stahlschmidt and his collaborators had to compare the amount of reactive oxygen molecules in an organism during digestion and well after the animal was done absorbing a meal. Otherwise, it would be difficult to tell what the 'normal' amount of circulating reactive oxygen molecules in the animal's body was. So they had to work with an organism that didn't feed continuously. Snakes, which eat several times a week or up to months apart, turned out to be ideal.
The team worked with the corn snake Pantherophis guttatus, a commonly studied snake that can be fed one mouse every two weeks. Because the corn snake's digestion and metabolism has been well-studied, the team knew exactly when to draw blood at peak digestion and post-absorption times. This enabled them to discover how the amount of oxidative damage was changing over time.
What Stahlschmidt and his collaborators found was unexpected. In the corn snakes, oxidative damage increased by almost 180% during digestion. However, antioxidant capacity, the ability of the body to fight the damaging effects of the oxidizing molecules, only increased by 6%. So every time these animals ate, they were accruing damage. What was surprising was that even though feeding was something the snakes were doing regularly, their bodies were not balancing the oxidative damage with an equal amount of protective support.
"The levels of damage we saw were really similar to or exceeded -- by quite a bit -- things as stressful as flying 200 kilometers in a bird, or mounting an immune response. Both of these things seem really stressful and may induce oxidative damage, and they do, but much less than actually eating a meal," Stahlschmidt says.
Stahlschmidt and his team think that what might be causing the damage is the immune system. The immune system may kick in when animals eat, releasing reactive oxygen molecules to kill microbes on food, helping to protect from disease. But the molecules also affect the cells of the animal ingesting the food, by damaging the DNA, proteins, and other critical parts of the cells in their body.
Stahlschmidt says the increased immune response during feeding makes sense, "You're ingesting something with microbes all over it and inside it."
So the immune system, which is normally considered to be working to protect us, is causing both help and harm, a tradeoff that could be affecting more animals than just snakes. All animals eat, and oxidative damage during eating could play a larger role in evolution than previously thought.
Stahlschmidt believes that this larger role may be in shaping the life history evolution of species. Life history traits are the things in an animal's life that affect life expectancy -- critical issues like reproduction, growth, and survival.
Stahlschmidt says that many life history traits are associated with oxidative damage. And life history evolution involves strategic tradeoffs in terms of how an animal is using resources across these traits. If an animal uses most of its resources for reproduction, it can't use them to grow. If oxidative damage is more or less of a cost of any or all of these traits, that may affect the life history evolution of the animal.
"Lots of these major tradeoffs or shifts that we are seeing between traits are underlied by oxidative stress or antioxidant capacity -- some kind balancing act," Stahlschmidt says.
Stahlschmidt and his collaborators suggest that their results from one species of snake could have ramifications for different animals and other types of studies examining the role of oxidative damage during other activities, such as flying and immune response. Unless the digestive status of the animal in the study is known, it's possible that oxidative damage from digestion could provide misleading data.
The question brought up by this research is how prevalent this trend is across other types of animals -- whether all species deal with such high levels of oxidative damage when they eat is an open question. Eating is necessary for all animals, but perhaps there is cost to every benefit.
Stahlschmidt presented his team's research results at the 2016 annual conference of the Society of Integrative and Comparative Biology in Portland, Oregon.
