When researchers did a brain scan of people being shown pictures of food and non-food items, they found certain brain areas that were more active in responsive to food vs. non-food pictures following weight loss than in people who had not lost weight (including the reward systems and systems responsible for decision-making functions). Other studies showed that some of these same brain areas had similar patterns of neuronal activation in response to food in individuals who had been successful at sustaining weight loss and individuals who had been subjected to a prolonged fast. The reviewers consider that these results demonstrate that reduced weight is accompanied by changes in the emotional response to food and its rewarding properties, coupled with decreased “restraint”. The reviewers do not comment on the similarity between successful weight maintenance and fasting, sadly.
They go on to discuss activity in a brain area called the hypothalamus. The hypothalamus contains neurons that mediate both hunger and satiety (feeling full). Following weight loss, there is a decrease in hypothalamic activity and it is thought that this essentially takes the brakes off eating resulting in weight gain. Now, interestingly, the studies by Mark Mattson (who featured in Dr Mosley's Horizon programme) have shown that intermittent fasting increases activity in the hypothalamus link.
The reviewers then turn to leptin. Leptin is one of around 50 currently known molecules that are capable of influencing eating and/or energy expenditure. The authors say that the main function of leptin is to tell the brain that energy stores are below a critical level (for that individual). This “threshold” is determined by genetic, developmental and extant metabolic circumstance. A decline in leptin concentrations below this critical level makes the body try to correct the low energy reserve by increasing fat stores. Injecting leptin into people maintaining a 10% weight reduction reversed the reduced metabolic rate and the hunger feelings and improved the ability to judge how much food had been eaten.
So, can we speculate on whether intermittent fasting might be different from other diets with respect to being able to keep the weight off? I'll try, but of course, the story is a lot more complicated than I've laid out above so I could well be wrong! I think that, yes, it is possible that intermittent fasting may be different. First, we have the changes in the hypothalamus seen by Mark Mattson, second, we know that the day after fasting, instead of overeating as the theory above suggests, mostly we don't overeat, and third, we know that while leptin levels decrease during fasting, they rise again on feeding. I think we have some reason to hope, but of course time will tell...