Fasting May Change Brain's Hunger Response
Written by Editor   
Tuesday, May 13, 2014 12:08 PM

Intermittent fasting is an alternative to calorie restriction used primarily in lab experiments. It usually involves placing animals on a 24-hour on, 24-hour off feeding cycle, and usually results in weight loss.  Several studies have suggested that it promotes a number of significant improvements in health.  "We concluded that improvements in cardiovascular risk factors and cardiovascular and neuroendocrine stress adaptation occur in response to [intermittent fasting]," one study reported.  Intermittent fasting caused changes in the hypothalamus of rats that may explain the low feeding efficiency, reduced body mass, and overeating seen in these animals, researchers reported.

Though most of the research on the benefits of intermittent fasting has been done in lab animals, research is starting to be done in humans, and the results are similar to those seen in animals.

Researchers found increased expression of the orexigenic neurotransmitters agouti-related peptide (AGRP) and neuropeptide Y (NPY), which stimulated appetite and energy conservation, in a group of rats fed on a 24-hour fasting and feeding cycle for 3 weeks.  The neurotransmitters are produced by a "subpopulation" of neurons in the arcuate nucleus.  Increased expression of the neurotransmitters was seen even on feeding days, posing an explanation for the overeating pattern of the intermittent fasting rats on those days, they explained.

On eating days, the rats gorged when offered food, eating approximately 53% of their daily food consumption within the first 2 hours.

On fasting days, the rats exhibited higher metabolic rates, lower efficiency of energy use, and enhanced lipid oxidation.

Overall, the intermittent fasting animals ate approximately 20% less food, and gained less, than control animals. Weight gain for intermittent fasting animals was 22.50 g versus 50.39 g for control rats.  More notably, there was a decrease in liver mass in intermittent fasting animals during fasting days, which was possibly a consequence "of the utilization of hepatic glycogen and lipids as energetic sources."

In addition to the increase in orexigenic neurotransmitters AGRP and NPY, the researchers found that mRNA levels of thyrotropin releasing hormone (TRH) decreased in intermittent fasting animals during fasting, but returned to control levels on feeding days.  "Since TRH controls the levels of circulating thyroid hormones and thermogenesis, this modification may be related to the observed increase in feeding metabolic rates for [intermittent fasting] animals."

"Overall, the changes in hypothalamic function promoted by [intermittent fasting] explain both the overeating patterns displayed by these animals and, possibly, the lower energy conversion efficiency resulting in lower weight gain," the authors said.