eating chocolate or drinking water can blunt pain, reducing a rat's
response to a hot stimulus. This natural form of pain relief may help
animals in the wild avoid distraction while eating scarce food, but
in modern humans with readily available food, the effect may
contribute to overeating and obesity.
The study, published in the Journal of Neuroscience by authors Peggy
Mason, PhD, professor of neurobiology, and Hayley Foo, PhD, research
associate professor of neurobiology at the University of Chicago, is
the first to demonstrate that this powerful painkilling effect occurs
while the animals are ingesting food or liquid even in the absence of
"It's a strong, strong effect, but it's not about hunger or
appetite," Mason said. "If you have all this food in front of you
that's easily available to reach out and get, you're not going to
stop eating, for basically almost any reason."
In the experiments, rats were given either a chocolate chip to eat or
had sugar water or regular water infused directly into their mouth.
As the rat swallowed the chocolate or fluid, a light-bulb beneath the
cage was switched on, providing a heat stimulus that normally caused
the animal to lift its paw off the floor. Mason and Foo found that
rats were much slower to raise their paw while eating or drinking,
compared to tests conducted while they were awake, but not eating.
Surprisingly, the researchers found no difference in the delayed paw-
lift response between when the rat was eating chocolate and when it
was drinking water, despite previous research indicating that only
sugary substances were protective against pain.
"This really shows it has nothing to do with calories," Mason said.
"Water has no calories, saccharine has no sugar, but both have the
same effect as a chocolate chip. It's really shocking."
Mason and Foo then repeated the heat test as the rats were given
quinine, a bitter drink that causes rats to make an expression called
a gape that's akin to a child's expression of "yuck." During quinine
administration, the rats reacted to heat as quickly as when not
eating, suggesting that a non-pleasurable food or drink fails to
trigger pain relief.
The context of ingesting was also important to whether eating or
drinking blunted pain, the researchers found. When rats were made ill
by a drug treatment, eating chocolate no longer delayed their
response. However, drinking water still caused a reduced pain
response, indicating that drinking water was considered a positive
experience while ill.
By selectively inactivating a region in the brainstem called the
raphe mangus – an area previously shown to blunt pain during sleep
and urination – Mason and Foo were able to remove the effect of
drinking water on the rat's pain response. The brainstem controls
subconscious responses such as breathing and perspiration during
"You're essentially at the mercy of your brainstem, and the raphe
magnus is part of that," Mason said. "It tells you, 'you're going to
finish eating this, whether you like it or not,' just like you sweat
while running whether you like it or not."
In the wild, Mason said, rats and other animals would not want to be
distracted during the rare but important times that they were able to
eat or drink. Therefore, the activation of the raphe magnus during
eating or drinking would allow the rat to filter out distractions
until their meal was completed. For obvious reasons, this natural
pain relief would be activated when an animal is eating or drinking
something pleasurable, but not when it tastes something that could be
toxic or harmful.
Don Katz, an associate professor of psychology and neuroscience at
Brandeis University who studies taste, said that Mason and Foo's
paper brings together two systems – taste and pain – that are usually
"They're saying the purpose of the taste system is to give the animal
a cue that helps it decide what stimulus they should or shouldn't pay
attention to," Katz said. "This shows there is a whole region there
to enable the animal to keep eating."
Mason believes that this effect is also present in humans (studies by
other labs have observed similar pain reduction in infants receiving
sugar water during a booster shot), but that it has detrimental
effects in modern society given our ready access to large quantities
of pleasurable and fattening foods. Opening up a bag of chips could
activate the brainstem such that you don't stop eating until the bag
is empty, even while realizing that such behavior is bad for you.
"We've gotten a lot more overweight in last 100 to 150 years," Mason
said. "We're not more hungry; the fact of the matter is that we eat
more because food is readily available and we are biologically
destined to eat what's readily available."
But the painkilling effect can be turned to our advantage, Mason
said, perhaps as a replacement for the practice of using candy to
calm children – or even adults – in the doctor's office.
"Ingestion is a painkiller but we don't need the sugar," Mason said.
"So replace the doctor's lollipop with a drink of water."
The research was funded by a grant from the National Institute on
Drug Abuse and the Women's Council of the Brain Research Foundation.
The paper, "Analgesia accompanying food consumption requires
ingestion of hedonic foods," appears in the October 14th issue of the
Journal of Neuroscience.