Gillum now thinks FGF21, like leptin—another hormone that regulates appetite—may suppress the neural response to rewards, both in terms of the desire to seek them and consume them“We are still working on why the liver would have evolved mechanisms to do this, but hypothesize it may be to limit excess sugar consumption—either to promote diet diversification or to prevent the [problematic] effects of excess sugar intake,” Gillum says.
I think the thinking on this is so muddy when it comes to reward. Want to decrease reward? Reduce dopamine. Or give people a drug the blocks endocannabinoid signalling. Food intake as well as general motivation to move decreases. Fighting obesity by depression.
Lean, healthy, annoying people who are spontaneously this way with no special attention to diet. What can these people do? They can go to a birthday party, have one slice of pizza, one bite of cake and ice cream--and actually enjoy that little treat, where most of us would probably just be too frustrated to really enjoy that.
I become annoying if my diet is sufficiently ketogenic. If I'm not eating this way, even if I'm eating fairly keen Atkins, then I'll binge on cheese, peanuts, pepperoni, I'll overeat highly palatable low carb foods, and find them unsatisfying. But if my diet has been tight, I can fit these foods into my macros, I can be happy with an ounce of cheese or nuts or peanuts, I can feel satisfied--which to me is the definition of reward. If anything I think the problem is that people will seek reward in a bag of potato chips--but the chips are actually low reward, because it takes such a large amount to reach satisfaction, or reward.
In liver FGF21 expression is regulated by PPARα and levels rise substantially with both fasting and consumption of ketogenic diets.
Stephan Guyunet has said that both a low fat diet and a low carb diet work because they are low-food reward. I realize "food reward" is a technical term, but I wish it had some relation to the word as used in English, is it too much to ask that it not mean the opposite of common use? Anyways, I don't think he's right. It's entirely possible that somebody will stop eating tasteless, low fat food at a lower calorie intake because there's just no reward to be had in eating much of it. But they might eat less of a low carb or ketogenic food because they're satisfied with less.
My bias may be showing here, it's entirely possible that there are forms of low fat diet could take that are more rewarding, by my rogue layman's definition, than the old low-fat snackwell's and chocolate chip and caramel rice cakes route.