Obesity and dopamine dysfunction
Obesity is a major problem worldwide. In the United Sates, more than 66% of adults are overweight, with 1 out of every 3 being considered obese (National Institutes of Health). Obesity is determined based on a person’s Body Mass Index or BMI. BMI is calculated by dividing your weight in kilograms by your height squared in meters. A healthy BMI ranges from 18.5 to 24.9, a BMI of 25 to 29.9 is considered overweight, a BMI of 30+ is considered obese, and a BMI of 40+ is considered extreme obesity. To make sure your BMI is in a normal range, you can calculate it here:
Weight gain leading to situations of obesity result from an energy imbalance. When we consume (through food and drink) more calories than we burn, the body stores these calories as fat, and we gain weight. When this happens consistently over a period of time, this weight gain may lead to obesity. Obesity is associated with and may lead to many other health issues including high blood pressure and cholesterol, heart disease, type 2 diabetes, osteoarthritis, cancer, or stroke.
Obesity is also associated with decreased levels of physical activity. Along with maintaining a healthy diet, increasing levels of physical activity help combat obesity. However, recent research is showing that obese individuals may show an actual deficit in the motivation to engage in physical activity. In a recent review, Drs. Alexxai Kravitz, Timothy O’Neal, and Danielle Friend of the National Institute of Health hypothesized that impairments in the dopamine system of our brain may contribute to the sedentary behaviors seen in obese individuals – similar to what is happening in the brains of patients with Parkinson’s disease (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5063846/).
Dopamine is a neurotransmitter that is made in two regions of the brain, the ventral tegmental area and the substantia nigra pars compacta. These two areas are involved in aspects of reward, motivation, and the ability to move. The neurological disorder, Parkinson’s’ disease, highlights the importance of dopamine for proper movement. Parkinson’s disease is a movement disorder characterized by tremor, slowness of movement, stiffness, and postural instability, and is caused by the loss of dopamine neurons in the substantia nigra.
Interestingly, obesity has also been shown to be associated with impairments in the dopamine system. Evidence for this has been seen in both rodent and human studies. For example, rats that were fed high fat diets for long periods of time (comparable to years of life in humans) showed decreased levels of dopamine in areas of the reward circuitry (https://pubmed.ncbi.nlm.nih.gov/23512420/). Importantly, deficits in the dopamine system could be reversed with removal of the high-fat diet. Certain individuals, however, did not show brain changes after the removal of a high-fat diet. The authors suggest that these lasting brain changes may contribute to difficulty in losing and/or maintaining weight loss after a diet program has begun. In humans, genes linked to dopamine have been implicated in obesity. Monoamine oxidase is an enzyme (or protein) that breaks down neurotransmitters in the brain including dopamine. Monoamine oxidase dysfunction (either too much or too little activity) is thought to underlie a variety of psychiatric and neurological disorders – obesity may be one of these. Humans that have a genetic polymorphism or a change in their genetic code that leads to decreased activity of monoamine oxidase have higher rates of obesity (https://pubmed.ncbi.nlm.nih.gov/16674552/). In addition, using positron emission tomography, researchers examined dopamine functioning in the brains of humans (https://pubmed.ncbi.nlm.nih.gov/11210998/). Dopamine binds to receptors, which are proteins that are located on the cell membranes of neurons; some of these are known as D2 dopamine receptors. Compared to normal weight controls, they found that obese individuals showed lower levels of the D2 receptors in the striatum (part of the basal ganglia and a region of the brain that helps regulate motivation and movement). Moreover, individuals with the largest BMIs showed the lowest levels of D2 receptors. As we all know, eating can be a pleasurable experience; this is because food consumption activates the reward centers of our brains. These studies suggest that obese individuals may be overeating to stimulate the dysfunctional or depressed reward centers of the brain.
In conclusion, obesity may be caused by changes in the brain’s dopamine system. The lack of physical activity common in people with obesity may be due to these central nervous system changes. Future studies will be needed to fully understand the relationship between obesity, physical inactivity, and changes in the dopamine system. These type of studies will help support new methods to help increase physical activity levels in obese individuals and help them to regain healthy weights.
References:
Carlin, J., Hill‐Smith, T. E., Lucki, I., & Reyes, T. M. (2013). Reversal of dopamine system dysfunction in response to high‐fat diet. Obesity, 21(12), 2513-2521.
Kravitz, A. V., O'Neal, T. J., & Friend, D. M. (2016). Do dopaminergic impairments underlie physical inactivity in people with obesity?. Frontiers in human neuroscience, 10.
National Institutes of Health: https://www.niddk.nih.gov/health-information/health-statistics/overweight-obesity.
Need, A. C., Ahmadi, K. R., Spector, T. D., & Goldstein, D. B. (2006). Obesity is associated with genetic variants that alter dopamine availability. Annals of Human Genetics, 70(3), 293-303.
Wang, G. J., Volkow, N. D., Logan, J., Pappas, N. R., Wong, C. T., Zhu, W., ... & Fowler, J. S. (2001). Brain dopamine and obesity. The Lancet, 357(9253), 354-357.
