The Connection Between Obesity and the Reward System

Obesity may alter the brain's reward system and affect the enjoyment of activities.

Posted November 26, 2024 | Reviewed by Abigail Fagan

Since the 1970s, the global trend of obesity has been steadily increasing, earning it the designation of a 21st-century epidemic. Obesity is the main risk factor for various serious health conditions, including cardiovascular diseases, diabetes, and certain types of cancer. However, recent studies suggest that obesity also affects brain functions, highlighting the need to understand this condition beyond its physical effects.

Obesity, particularly abdominal obesity, has significant neurological and psychological health consequences, including mild cognitive decline , mood disorders, sleep disturbances, and risk of neurodegenerative diseases. Obesity also causes an imbalance between sympathetic and parasympathetic activity, resulting in metabolic dysregulation in metabolic tissues and hypertension. While inflammation and its subsequent extension into the brain is the main mediator of obesity-associated neurological issues, other mechanisms, such as insulin resistance, oxidative stress , and dysfunction of cellular organelles, may also play a role [1].

Overeating and obesity change the brain’s reward system

A recent study by Amsterdam UMC and Yale University, published in the journal Nature Metabolism, revealed that the brain's response to the intragastric administration of certain nutrients is disturbed in obese people. Obese individuals also exhibited decreased dopamine release in regions of the brain that regulate the motivational aspects of food consumption. The trial involved injecting glucose and lipids into the stomachs of 30 healthy-weight and 30 obese participants, while simultaneously measuring their brain activity using MRI and dopamine release using SPECT scans. Interestingly, a 10% reduction in body weight (following a 12-week diet ) was not sufficient to restore these brain responses in obese subjects, suggesting that long-term brain adaptations occur in obesity and persist even after weight loss [2].

Previous studies, particularly those involving animal models, have shown similar results. These findings confirm that dopamine levels and the content of dopamine receptors (D2) decline in this condition and ultimately the brain’s dopaminergic reward system becomes less sensitive in obese people [ 3 ].

How does obesity impair the brain functions?

Excess body fat can lead to mild to moderate inflammation throughout the body. This inflammation arises from various factors, including stress on cells and damage to cellular structures. Enlarged fat cells and infiltrated immune cells release inflammatory substances that can disrupt normal bodily functions. In physiological conditions, the resolution of inflammation typically occurs in the later stages of the inflammatory response, following the elimination of the initial cause and the effective repair of tissue. However, factors such as aging and metabolic disorders can hinder this resolution, resulting in sustained inflammation.

Chronic inflammation can develop in the brain through several pathways. It can disrupt the blood-brain barrier, allowing harmful substances and immune cells to enter the brain. Additionally, inflammatory signals can activate brain cells known as microglia, which further contribute to inflammation.

The vagus nerve also plays a role in this process, linking peripheral inflammation to brain function. As a result, chronic inflammation in the body can lead to neuroinflammation in the brain. Neuroinflammation affects key areas of the brain involved in regulating metabolism, worsening the metabolic issues associated with obesity. Understanding these connections is crucial for addressing both the physical and mental health challenges posed by obesity [4].

Several factors activate the dopamine reward system, which may be affected by desensitization due to obesity

Several behavioral and psychological factors activate the brain's reward system. Activities such as eating delicious food, exercising, engaging in social interactions, and using certain drugs like methamphetamine, cocaine, and alcohol , as well as pursuing success in work or studies and experiencing sexual drive, stimulate the reward system and increase dopamine levels.

In addition to some brain disorders, desensitization of the reward system, due to reduced levels of dopamine or its receptors, is often caused by chronic exposure to addictive substances or behaviors. This condition causes several psychological and mental consequences including lack of pleasure in previously enjoyable activities ( anhedonia ), reduced motivation , increased apathy, impulsive behaviors, reduced interest in social interactions, and depression . One of the main consequences of low dopaminergic system function is seeking increasingly intense stimuli to experience the same level of pleasure, leading to risky and compulsive behaviors [5]. Despite the finding of some link between obesity and addictive behavior, the results of epidemiological studies are controversial and are not convincing.

These new findings indicate that we need further clinical studies to investigate the role of obesity-associated desensitization of the reward system and its role in risky and addictive behaviors.

O'Brien PD, Hinder LM, Callaghan BC, Feldman EL. Neurological consequences of obesity. Lancet Neurol. 2017 Jun;16(6):465-477.
van Galen KA, Schrantee A, Ter Horst KW, la Fleur SE, Booij J, Constable RT, Schwartz GJ, DiLeone RJ, Serlie MJ. Brain responses to nutrients are severely impaired and not reversed by weight loss in humans with obesity: a randomized crossover study. Nat Metab. 2023 Jun;5(6):1059-1072.
Volkow ND, Wang GJ, Baler RD. Reward, dopamine and the control of food intake: implications for obesity. Trends Cogn Sci. 2011 Jan;15(1):37-46.
1. Schmitt LO, Gaspar JM. Obesity-Induced Brain Neuroinflammatory and Mitochondrial Changes. Metabolites. 2023 Jan 5;13(1):86.
Barry D, Clarke M, Petry NM. Obesity and its relationship to addictions: is overeating a form of addictive behavior? Am J Addict. 2009 Nov-Dec;18(6):439-51.

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Hamid Zand, Ph.D., is a professor of Biochemistry at the Department of Cellular and Molecular Nutrition at Shahid Beheshti University of Medical Sciences in Tehran, Iran.

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This article is part of the Bringwise Psychology Journal — daily insights on human behavior, mental health, and personal growth.