New research finds a link between brain circuits associated with reward and motivation, eating behavior, and chronic pain.

Researchers investigated how lower back pain interacts with eating behaviour via behavioural and neuroimaging experiments.
They found that pain alters food satiety in those who have chronic lower back pain and those who have recovered from subacute back pain.
Further studies are necessary to understand the mechanisms behind their results.
Studies show that people often experience chronic pain and obesity together and that both pain and eating interact with motivation and decision-making. However, how this happens remains unknown.

One theory, known as the “fear-avoidance model,” suggests that people with pain avoid physical activity to avoid movement-associated pain until they become averse to physical activity and therefore put on weight.

However, some studies have found that patients with chronic back pain have similar levels of physical activity as those who do not experience pain. Therefore, the fear-avoidance model may not explain the link between chronic pain and obesity.

Other research suggests that the brain’s reward system may be an important factor in linking pain and obesity. Neuroimaging studies have shown that it is possible to predict the transition to chronic pain from the structural and functional properties of the limbic system.

Knowing more about the relationship between chronic pain and eating behaviour could help researchers and clinicians manage or reverse the negative emotional and motivational aspects of chronic pain better.

In a recent study, researchers led by Paul Geha, M.D., an assistant professor at the University of Rochester, NY, investigated eating behaviours of those with lower back pain before and after it transitioned to chronic pain or subsided. They also assessed changes in the nucleus accumbens, a part of the brain responsible for decision-making and reward.

They found that disrupted eating behaviour sets in after pain becomes chronic and is accompanied by structural changes in the nucleus accumbens.

Mac and cheese and Jell-O
For the study, the researchers recruited:

43 people with chronic lower back pain, which is back pain lasting 12 weeks or more
51 people with subacute back pain, which is back pain lasting 6–12 weeks
36 pain-free healthy individuals as controls
The participants underwent two experiments on different days no more than a week apart.

In the first experiment, the researchers asked participants to arrive at the lab feeling neither hungry nor full and to rate their hunger on a visual analog scale of 0 — “I am not hungry at all” — to 100, i.e., “I have never been more hungry.” Those who reported feeling more than 30 out of 100 on the scale were offered a small snack and asked to wait 30 minutes, after which the participants repeated their hunger ratings.

The team then invited the participants to taste three blocks of Jell-O and three blocks of pudding and to record their sense perceptions and feelings on different scales to indicate aspects, including:

sweetness
overall intensity
liking or disliking
hunger
fullness
thirst
oiliness
fattiness
creaminess
wanting more of the food
In the second experiment, the researchers invited the participants to the lab at around lunchtime and asked them to come feeling hungry. Upon arriving, the team assessed their body fat percentage and then offered them a portion of mac and cheese to mimic the effects of a meal.

Next, they offered each participant the pudding they rated the highest during the previous experiment and asked them to eat as much as they liked and provide the same ratings as those in the previous experiment.

The participants also filled in various questionnaires relating to:

pain
anxiety
impulsivity
eating style
reward sensitivity
hours of physical activity per week
specific food addiction
They also received an MRI scan so that researchers could visualize their nucleus accumbens.

The participants underwent the above procedures twice, approximately 1 year apart.

At the second visit, the researchers noted that 20 patients in the subacute back pain group recovered — defined as pain perception dropping by more than 30% — while 16 experienced persistent pain.

The team also noted that there were no significant differences in age, sex, body mass index, years in education, and anxiety between the groups. While those with chronic pain reported higher depression scores than the other groups, none of the groups exhibited clinical depression.

They found that subacute back pain patients who recovered at the follow-up and patients with chronic lower back pain had disrupted eating behaviour, likely from disrupted satiety signalling.

In contrast, those with subacute back pain whose symptoms persisted at the follow-up had intact eating behaviour similar to the control subjects.

MRI scans from those with subacute back pain at the follow-up and those with chronic pain demonstrated a strong correlation between liking fat-rich food and nucleus accumbens volume.

However, the same was not true for those with subacute back pain who had recovered at the follow-up.

Unclear mechanisms
To explain their findings, the researchers said that changes in the nucleus accumbens in those with subacute back pain at follow-up may protect them from hedonic eating disruption during the early phases of the condition.

“Pain is an emotional and sensory experience, which requires decision-making to plan future steps. (‘Should I sustain the pain to keep running, or should I stop?),“ Dr. Geha told MNT. “Pain is, therefore, part of motivated behaviour — evidence from brain imaging established now that chronic pain is associated with significant change in the motivational circuitry in the brain where the nucleus accumbens is a hot spot.”

“Consistent with the anhedonia and loss of motivation we see in the patients [living with] chronic pain, brain imaging has shown that these patients have altered structure, function, and dopaminergic transmission (dopamine is a major motivation related neurochemical). When we ask chronic pain patients to rate their pain intensity in the scanner, we consistently see that accumbens activity tracks these ratings,” he added.

“The accumbens is also very important in food ingestion, which is a primary motivated behaviour. The accumbens is important in learning to associate behaviour (‘pressing a lever’) with outcome (‘obtaining a food pellet’). There is also evidence that the accumbens is important in attributing hedonic value on food, especially highly palatable food. This summary explains the background of the results we saw,” he explained.

Ivan E. de Araujo, D.Phil., professor of neuroscience at Icahn School of Medicine at Mount Sinai, NY, another of the study’s authors, told MNT that while it seems the nucleus accumbens has links with appetite in chronic pain, how so remains unknown: “The mechanism linking nucleus accumbens alterations to altered appetite in chronic pain remains unclear,” he told MNT.

“A plausible idea is that pain-induced peripheral signals, such as circulating inflammatory or immune factors, reach accumbens neurons via circulation. Equally possible [are] altered sensory pathways linking the body to the brain. Those are [a] critical focus for future research,” he added.

“The relationship of the subjects with subacute back pain, recovered, and chronic low back pain (CLBP) is confusing at best,” Khalid Afzal, M.D., DFAACAP, associate professor of psychiatry and behavioural neuroscience at the University of Chicago, not involved in the study, told MNT.

“The logic suggests that CLBP patients should be more consistent with subacute pain patients, a persistent group. However, that was not the case. [We] need to do similar studies in a wider population to confirm the findings.”

The researchers concluded that while their findings demonstrate a link between eating behaviour, chronic pain, and the nucleus accumbens, further research is necessary to understand the long-term impact of altered eating behaviour in patients with chronic pain.

Study limitations
They also note several limitations to their findings, including small sample size, the need to reproduce their results on a larger scale, and that they only tested one type of pain.

“There are several limitations in this study,” added Dr. Afzal, “The authors focused on only neuropathic pain, while chronic low back pain can occur for other reasons such as structural or bone related. Although the number of subjects [was] decent, there was no mention of the cultural, racial and ethnic variability in the perception of pain and eating behaviours.”

“Further, the study subjects’ average age was 30. It would be great to do similar studies in later age groups. The study focused on low back pain, and I wonder if a similar link exists with other chronic pain syndromes such as fibromyalgia, complex regional pain syndrome, or other neuropathic pains,” he added.

Chronic pain management
When asked how these findings may affect care strategies for those with chronic pain, Dr. de Araujo said, “Some of the patients suffering from a recent onset of pain are at a greater risk of developing an altered appetite. Appropriate attention to changes in eating patterns is thus required from health professionals.”

“The findings shift the focus from the control of sensory aspects of pain, like decreasing the intensity, to controlling the negative motivational aspects of pain. In addition to medications and surgeries that aim to decrease the intensity of pain, we need interventions that help us cope or reverse the negative emotional and motivational aspects of chronic pain,” added Dr. Geha.

Meanwhile, Dr. Afzal said that it is too early to see how these findings may affect care strategies for chronic pain: “It is too early to see how these findings will impact strategies for chronic pain, although, I think we are moving in the right direction toward the beginning of understanding the complex neurobehavioral links between chronic pain and disordered eating.”

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