Highlights:
Palatable human food is the most effective way to cause a normal rat to spontaneously overeat and become obese, and its fattening effect cannot be attributed solely to its fat or sugar content.
But modern food is calorically dense – it contains many more calories than predicted per unit mass – and fiber-poor. This fools the brain into thinking that we’re eating less than we really are, and shuts down the system that would normally make us feel full once we’ve had enough. Simultaneously, the extremely high level of food reward tricks the brain into thinking that this food is especially nutritionally valuable and that it should relax its normal constraints.
In the 1940s, scientists discovered that if they damaged the ventromedial hypothalamic nucleus (VMN) of rats, the rats would basically never stop eating, becoming grotesquely obese. They theorized that the VMN was a “satiety center” that gave rats the ability to feel full; without it, they would feel hungry forever.
In 1965, some scientists locked people in a room where they could only eat nutrient sludge dispensed from a machine. Even though the volunteers had no idea how many calories the nutrient sludge was, they ate exactly enough to maintain their normal weight, proving the existence of a “sixth sense” for food caloric content. Next, they locked morbidly obese people in the same room. They ended up eating only tiny amounts of the nutrient sludge, one or two hundred calories a day, without feeling any hunger. This proved that their bodies “wanted” to lose the excess weight and preferred to simply live off stored fat once removed from the overly-rewarding food environment.
The theory they came up with to explain the results was this: fat must produce some kind of satiety hormone, saying “Look, you already have a lot of fat, you can feel full and stop eating now”.
It’s the leptin-VNM-feedback system (dubbed the “lipostat”) that helps people match their caloric intake to their caloric requirements so impressively.
Just as a well-functioning lipostat is very good at keeping people normal weight, a malfunctioning lipostat is very good at keeping people obese. Fat people seem to have “leptin resistance”, sort of like the VMN-lesioned rats, so that their bodies get confused about how much fat they have.
Now he will have to be 300 lbs before his brain “believes” he is the right weight and stops encouraging him to eat more. If he goes down to a “mere” 250 lbs, then he will go into the same semistarvation neurosis as Ancel Keys’ experimental subjects and become desperately obsessed with food until they get back up to 300
How do we connect this with the other half of the book, the half with food reward and satiety and all that?
Calorie-dense, highly rewarding food may favor overeating and weight gain not just because we passively overeat it but also because it turns up the set point of the lipostat. This may be one reason why regularly eating junk food seems to be a fast track to obesity in both animals and humans…focusing the diet on less rewarding foods may make it easier to lose weight and maintain weight loss because the lipostat doesn’t fight it as vigorously. This may be part of the explanation for why all weight-loss diets seem to work to some extent – even those that are based on diametrically opposed principles, such as low-fat, low-carbohydrate, paleo, and vegan diets. Because each diet excludes major reward factors, they may all lower the adiposity set point somewhat.
Some researchers believe the low fiber content of the diet precipitates inflammation and obesity by its adverse effects on bacterial populations in the gut (the gut microbiota). Others propose that saturated fat is behind the effect, and unsaturated fats like olive oil are less fattening. Still others believe the harmful effects of overeating itself, including the inflammation caused by excess fat and sugar in the bloodstream and in cells, may affect the hypothalamus and gradually increase the set point. In the end, these mechanisms could all be working together to promote obesity. We don’t know all the details yet, but we do know that easy access to refined, calorie-dense, highly rewarding food leads to fat gain and insidious changes in the lipostat in a variety of species, including humans
Personally, I believe overeating itself probably plays an important role in the process that increases the adiposity set point. In other words, repeated bouts of overeating don’t just make us fat; they make our bodies want to stay fat.
Leptin doesn’t just correlate with body fat levels; it also responds to short-term changes in calorie intake. So if you overeat for a few days, your leptin level can increase substantially, even if your adiposity has scarcely changed (and after your calorie intake goes back to normal, so does your leptin).
I did do some research in mice suggesting that unrefined, simple food does reverse the brain changes and the obesity. I don’t claim that it’s all attributable to the blandness though– the two diets differed in many respects (palatability, calorie density, fiber content, macronutrient profile, fatty acid profile, content of nonessential nutrients like polyphenols).
Genetically privileged people may have very finicky lipostats that immediately burn off any extra calories they eat, and which never become dysregulated. Genetically unlucky people may have weak lipostats which fail to defend against weight gain, or which are too willing to adjust their set point up in the presence of an unhealthy food environment.
A few weeks ago Guyenet announced The Bland Food Cookbook, a collection of tasteless recipes guaranteed to be low food-reward and so discourage overeating. It was such a natural extension of his philosophy that it took me a whole ten seconds to realize it was an April Fools joke.
I asked him, and he responded that:
If I thought enough people would actually be capable of following the diet, I would consider making such a cookbook non-ironically. The second point I want to make here is that there are many ways to lose weight, and deliberately reducing food reward is only one of them. You could also exercise, eat a low-calorie-density diet, eat a high-protein diet, restrict a macronutrient, restrict animal foods, restrict plant foods, eat nothing but potatoes. Most approaches overlap with a low-reward diet to varying degrees
