Bariatric surgery alters brain’s response to hunger, appetite and food intake

Three types of brain responses which help to prevent weight regain following bariatric surgery have been identified by researchers from Imperial College London and Imperial College Healthcare NHS Trust. In the study, ‘Weight Loss by Low-Calorie Diet Versus Gastric Bypass Surgery in People With Diabetes Results in Divergent Brain Activation Patterns: A Functional MRI Study’, published in Diabetes Care, the researchers looked at the brain activity of sixteen people with T2DM or pre-diabetes who underwent bariatric surgery. The brain activity was measured using magnetic resonance imaging (MRI) scans that detected the blood flow through different areas of the brain.

They found that there were three distinct types of responses in areas of the brain that control hunger, appetite and food intake which were different from a separate group of people who had used a very low calorie diet to lose weight. The researchers believe that these changes in brain activity after weight loss surgery can help explain why people who undergo this type of treatment have successful and long term weight loss, in comparison with those who have followed a very low calorie diet, where very often there is a regain of weight.

"This study is unique because we pulled together three interconnecting theories about the brain changes that explain why patients who have had weight loss surgery find it so much easier to maintain their weight loss compared with those who have been on a strict calorie-controlled diet.”

The researchers suggest that any weight loss programme aimed at helping people with Type 2 diabetes could be assessed using similar scans to understand whether any weight loss is sustainable in the long-term.

"Tackling obesity and diabetes is a priority for many healthcare systems around the world. There are many different weight loss programs and methods, with varying results on long term weight loss,” said Professor Tricia Tan, principal investigator of the research and Professor of Practice, Metabolic Medicine & Endocrinology, at Imperial College London. “Although very low calorie diets can help people to lose a great deal of weight in the short term, in the long term they often find it difficult to keep the weight off.”

Weight loss achieved with very low calorie diets can produce remission of type 2 diabetes lasting up to two years in some people. However, severe calorie restriction induces hormonal and metabolic responses that encourage high rates of weight regain. Only weight loss or bariatric surgery has been shown to produce the enduring weight loss that translates into long-lasting remission of diabetes and a reduced risk of other diseases such as cardiovascular disease and cancer. However, the mechanisms by which this surgery produces its benefits are not fully understood.

A number of studies have reported changes in brain activation patterns before and after weight loss surgery but few have focused on participants with diabetes and pre-diabetes. None have investigated the evolution of these patterns in participants with obesity and diabetes that lose weight through diet versus surgery.

"Our study is the first to show how three types of brain response are activated in people who have had weight loss surgery. These responses are related to how the mind takes pleasure in food and eating; limiting the impulse to eat; and how the brain and body control body weight,” added Tan. “We found that these brain responses were not activated in our participants who undertook the very low calorie diet. This may help to explain why those who have weight loss surgery experience continued weight loss benefits and those who embark on low calorie diets find continued weight loss difficult to sustain.”

The researchers wanted to investigate changes in brain activity and weight responses following surgery or a very low calorie diet (VLCD) in people with obesity and type 2 diabetes or pre-diabetes. Sixteen participants were recruited to the study at Hammersmith Hospital, part of Imperial College Healthcare NHS Trust, between 2016-2018. The team also recruited 19 matched participants who undertook a very low calorie (meal replacement) diet for four weeks.

The participants in the surgery group underwent an MRI scan where they were shown images of low and high calorie food, as well as other non-food objects to measure the brain's response to these images. They then underwent gastric bypass surgery, followed by a further MRI scan four weeks after the procedure.

Participants in the VLCD group underwent the same MRI scans before and after starting their 800-calorie daily diet for four weeks. They were then followed up a year after the trial to monitor their progress and compare results between the groups.

Following the trial, the team found that participants in the surgery group lost 12.3kg on average whereas participants in the VLCD group lost 8.4kg on average at four weeks. At 12 months, the VLCD group regained all the weight they had previously lost, while those in the surgery group continued to lose weight.

After analyzing the MRI scans before and after the interventions, the team found that people in the surgery group experienced three different types of brain responses that are linked to long-term weight loss.

Firstly, when surgical participants looked at images of food compared to non-food objects after surgery, there was a significant reduction in areas of the brain associated with the 'reward' response to food. In contrast, in those who had had the VLCD, there was an increase in activation of these same areas in response to the food pictures.

Secondly, in the VLCD group, after viewing pictures of food, there was an increased activation of brain areas associated with restraint against over eating. This was not found in the surgical group. The researchers believe that following VLCD participants may need to engage a greater cognitive effort to suppress food craving, compared with those who had bariatric surgery.

Lastly, the team found that the hypothalamus—an area of the brain which subconsciously controls appetite and weight—is more strongly linked to higher brain centers, which are involved in conscious thought, after surgery in comparison to the VLCD group. The researchers suggest that this stronger link, by providing subconscious feedback from the hypothalamus to the conscious mind, may help to dampen hunger cravings and any compulsions to eat high calorie foods.

The team believe that the three types of brain responses interlink with each other and may explain why, after weight loss surgery, people have a long lasting loss of weight, but, after VLCD, people tend to have a regain of weight.

"These findings can be used to further develop weight loss programmes and treatments which are not surgical. By using magnetic brain scans to examine how the brain responds to a treatment, we might be able to improve the performance of these programs,” she added. “This was a small study so we need to do more work to see if this result is replicated in a larger group of people who undergo weight loss surgery."

Population-based weight loss strategies are being heavily invested in by health systems around the world to reduce these health problems, but there is ongoing debate about the most effective way to achieve long-term improvements in health. With the discovery of these brain responses, it may be possible to assess the durability of weight loss after different types of weight loss treatments such as new dietary regimens and medications using MRI scanning.

"This study is unique because we pulled together three interconnecting theories about the brain changes that explain why patients who have had weight loss surgery find it so much easier to maintain their weight loss compared with those who have been on a strict calorie-controlled diet,” said Lead study author, Dr Victoria Salem, Clinical Senior Lecturer in Endocrinology, Department of Bioengineering, Imperial College London.