In a nutshell:
"Specific neurobiological systems and diet intake are functionally linked within a positive feedback loop, whereby a specific diet stimulates particular brain neurochemicals that in turn stimulate further consumption of that same diet. This diet-neurochemical-diet feedback process, while appropriate for producing overeating and gorging under conditions when food is scarce, helps to explain the eating and body weight disorders that develop when sugar- and fat- rich foods are abundant."A positive feedback loop. So... Eat sugar, crave sugar. Eat fat, crave fat (and alcohol, see below).
- Dr. Sarah Leibowitz, Rockefeller University Laboratory of Behavioral Neurobiology
Some brain chemicals and their actions:
Norepinephrine - stimulates food intake, preferential to carbs, secreted at weaning and high corticosterone
Neuropeptide Y - stimulates food intake (counteracted by leptin), preferential to carbs, secreted at weaning and high corticosterone
Galanin - stimulates food intake, preferential to fat esp. saturated and to alcohol, secreted at puberty and estrogen-rich periods in a woman's cycle
Enkephalin (opioid) - stimulates food intake, preferential to fat esp. saturated and to alcohol
Dynorphin (opioid) - stimulates food intake, preferential to fat esp. saturated and to alcohol
Orexin - stimulates food intake, preferential to fat esp. saturated and to alcohol
Serotonin - reduces appetite and produces satiety
Dopamine - reduces appetite and produces satiety
Fast Response Times
"These effects of diet on the hypothalamus are found to be amazingly rapid, occurring within the context of a single day — or even a single meal."Early Exposure to Imbalanced Diets Sets One Up For Obesity
"In a particularly exciting animal model developed and characterized in our laboratory, we have demonstrated that sugar- or fat-rich diets introduced early in life, even during pregnancy, can produce profound changes in circulating nutrients and brain neurochemical systems. These changes persist over time, even after the diet is returned to a balanced mixture, and they result from an increase in cell proliferation and neurogenesis in specific peptide systems that ultimately cause the offspring to overeat and become obese as adults."Fat And Alcohol Linked
"There exists a positive feedback loop between these peptides [galanin, enkephalin, dynorphin, orexin] and alcohol intake, which is similar to that seen with dietary fat and may be involved in promoting the over-consumption of alcohol as well as a fat-rich diet. ... Together, this evidence suggests for the first time that alcohol and fat intake during a meal may synergize to produce larger meals and greater alcohol consumption."So... Eat fat, crave fat and alcohol. Eat alcohol, crave fat and alcohol.
High Carbohydrate Diet May Reduce Appetite, Feeding, And Body Weight
Some of Leibowitz's newer research seems to be a leap, or maybe a fine-tuning of her earlier work. She focused on a chemical in the brain called Hypothalamic Huntingtin-associated protein 1 (Hap1). Levels of Hap1 are reduced by a high-carbohydrate diet. Lower levels of Hap1 "mediate the feeding-inhibitory action of insulin in the brain," and can result in a decrease of food intake and body weight.
Hypothalamic Huntingtin-Associated Protein 1 As A Mediator Of Feeding Behavior, Nature Medicine, April 2006
That a high-carb diet could result in weight loss reminded of this study...
A Low-Fat Vegan Diet Improves Glycemic Control and Cardiovascular Risk Factors in a Randomized Clinical Trial in Individuals With Type 2 Diabetes, Diabetes Care, 2006
...which saw people with type 2 diabetes who were eating a high-carb diet (over 70% of their calories from carbs) lose twice as much weight as people eating the lower-carb, higher-fat ADA diabetes diet. (My review: High-Carb, Low-Fat Diet For Diabetes)