Dieting and the Brain

Another famous New Year's resolution is to loose weight by dieting. This is also the single most common New Year's resolution to fail. For our September 2015 issue Bettina Schmerl asked "Why is it so hard to lose weight?"

source: pixabay

In industrialized countries, obesity is a socioeconomic burden and among the most rapidly increasing diseases. Simultaneously, popular role models make a very skinny physique desirable. Irrespective of whether for aesthetic or health reasons, many people go on a diet to lose weight at some point in their lives.
There are innumerable dieting strategies, but what counts in the end is less calorie intake in relation to the daily need. Despite this rather simple truth, people usually have a very hard time when they try to shrink their bellies. Why do people on a diet suddenly start craving food? Does dieting suddenly change our food preferences? Our personality? Our brains?
Let’s start out by explaining why it is so hard to lose weight: Evolution has not made creatures to be size zero models, but to survive. It is only nowadays that high-caloric food is readily available while physical activity is minimal. A few generations ago, dying from starvation was much more likely than suffering from obesity. Therefore, several mechanisms developed to gain and maintain body weight rather than providing a convenient way to lose weight.

How the Brain Changes With Dieting
Three neural pathways are involved in regulating food intake: the homeostatic system (hypothalamus), the reward-related system (mesolimbic pathways), and the inhibitory system (dorsolateral prefrontal cortex). These pathways interact and modulate food intake in response to dietary restriction, resulting in increased craving for high-caloric food leading to weight regain [2].
Weight loss decreases thyroid hormone levels while increasing hypothalamic-pituitary-adrenal axis activity, leading to decreased fat metabolism and increased appetite and fat storage. Calorie restriction may lead to decreased sympathetic nerve activity and reduces epinephrine and norepinephrine levels causing a decrease in heart rate and a general shift in metabolic activity, counteracting weight loss ambitions [2].

RESTRICTING CALORIES
CAN CAUSE DEPRESSION

Groups of patients who lose comparable amounts of weight by different means (changes in life style vs. gastric surgery) exhibit different changes in brain activity upon food cues. Dieting patients have higher activity in brain regions involved in food cue processing (similar to what is known about obese in contrast to normal-weight patients). This may lead to increases in reward, perception, and preoccupation with food intake; mechanisms probably leading to weight regain [3].
The changes do not stop at the functional level but there are even physical alterations. An imaging study identified increased white matter volumes in brains of obese patients in contrast to normal-weight controls. Interestingly, dieting and weight loss reverses some of these changes [4].

You're not Yourself When You're on a Diet
Another common side effect of dieting and calorie restriction is an increasing tendency to be in a bad mood, feeling a bit off and tetchy, all of which significantly interfere with dieting ambitions. A study in adolescent women has shown that calorie restriction may indeed cause depression due to decreased brain serotonin levels. Calorie restriction leads to low levels of tryptophan, a crucial precursor of serotonin synthesis and an essential amino acid that must be supplied in our diets. This occurs especially in cases of extreme dieting and anorexia nervosa (see also "Can Surgery Treat a Mental Illness?" on page 12) and causes symptoms such as depressed mood and impulsivity (this also happens to people who eat high-calorie but low-nutrient foods such as processed snacks; see also "Our Moods, Our Foods" on page 8 [1,5].

Weight Loss Surgery Induces Plasticity
Weight change-related plasticity in our brains is not a one-way road. Some obese patients undergo Roux-en-Y gastric bypass surgery to lose weight, a procedure that involves constructing a shortcut from the esophagus to the small intestine, skipping the passage of food through the stomach. Studies found changes in brain activity in these patients leading to increasing satiety and energy expenditure. These changes also occurred after shifting food preferences from high-fat and high-sugar options to healthier choices [4].
Thanks to such findings, analogues of anorexic gut homones, such as glucagon-like peptide 1 and peptide tyrosine tyrosine, are regarded as the next big development in the field of obesity treatment. These peptides are released in response to ingested food and signal satiety towards the the aforementioned pathways [3]. Furthermore, there are ongoing investigations on whether targeted manipulation, for example of dopaminergic signaling in the dorsal striatum by virus-mediated knockdown of leptin receptors in the ventral tegmental area, is able to modulate food intake [6]. Until such approaches are validated and developed, the most efficient way to ensure a healthy body weight is through adopting an active lifestyle and a balanced diet.


[1] Haleem, Behav Pharmacol, 2012
[2] Ochner et al, Physiol Behav, 2013
[3] Behary and Miras, Exp Physiol, 2014
[4] Haltia et al, J Clin Endocrinol Metab, 2007
[5] Jacka et al, Aust N Z J Psychiatry, 2010
[6] Hankir et al, Trends Endocrinol Metab, 2015


by Bettina Schmerl, PhD Student AG Shoichet
this article appeared originally September 2015 in Vol.08 Issue 3 "Food for Thought"