The naturally thin often have a hard time understanding the struggles of the very many who battle with overweight and obesity. Why is losing weight -- and especially keeping it off -- so very hard? What is it that drives us to obesity, and what can we do about it? And why has obesity spread like an epidemic in the last few decades?
A new article in Obesity Reviews explains that obesity is caused by two distinctive mechanisms, and understanding these different causes might help with targeting a more tailored solution.
Body weight regulation
Our body likes stability. It regulates its temperature, blood pressure, blood sugar and electrolytes, and for the most part also our adult weight. Most people fluctuate around a "set point." You may lose weight due to stress or illness, gain some over the holidays, but usually these pounds come back and go away once life goes back to normal.
Researchers have been studying how weight is regulated for several decades, and it's a very complex system of hormones (the most famous are leptin and insulin), neurons, nutrients and feedback mechanisms from the gut that keep this steady state. In order to keep weight stable we regulate intake and energy expenditure, so that when weight is lost we'll eat and/or lower our activity; when we gain weight appetite will diminish and our cells might burn more energy, even at rest.
The set point helps explain the two types of obesity this review proposes.
Overeating for hunger
In what's called metabolic obesity, that set point is in play, just at a higher point. The body "defends" this higher set point and will drive hunger and energy expenditure so that when pounds are shed they slowly creep up towards that elevated set point unless a lot of effort is put in to fight the body's basic tendency.
An elevated set point is inherited and determined by our genes, but if that were the entire story metabolic obesity couldn't explain the tripling obesity rates we've experienced in the past decades -- the population's genetic makeup hasn't changed in this time period.
And indeed, early childhood influences and environmental factors can tune the weight set point in susceptible individuals. Experiments in rats show that when they are forcefully overfed or underfed with regular food for a while, they gain and lose weight respectively, but once they return to eating at will their weight goes back to normal. But if the same rats are fed tasty, high calorie food for a significant while, about half of them will become obese, and their set point will set at a higher weight. They will now defend this higher weight point even when returned to a regular rat diet.
The same mechanism seems to be present in humans. Some people's set point will change in an obesogenic environment, while others are resistant to it.
Overeating for pleasure
The other mechanism for obesity isn't regulated by hunger and satiety, but rather by emotions, desires and rewards. Hedonic obesity is governed by liking and wanting food, and by seeking pleasure from it. Whether people overeat because food is more highly rewarding and pleasurable for them, or rather because they need more food stimuli in order to feel the same amount of pleasure thin people get with less food, the end result is that they eat above what their body needs metabolically, and their eating has less to do with energy balance. Satiety signals are overridden and the body's set point doesn't matter.
How can we tell the two overeating patterns types apart?
A person with metabolic obesity would have normal metabolic rates and would burn no more than the expected number of calories appropriate for his body size. On the other hand, with hedonic obesity, metabolism is revved up -- the body is above its weight set point and will try to burn off the extra energy. Behavior around food might also differ, with food addiction and compulsive over-eating more typical of hedonic obesity.
In reality it's very hard to test for such differences, it's tough to tell the two types apart, and both patterns often interact and interplay in the same person.
The authors believe that knowing the underlying mechanism would help with treatment. Hedonic obesity would be treated with behavioral and lifestyle interventions, while those would not work as well for metabolic obesity, in which drugs (of which, sadly, there are very few) and other forms of intervention would be of more benefit. Bariatric surgery works well for both types.
High-calorie processed foods are the common pathway
It's hard to tease apart metabolic vs. hedonic causes for obesity, and impossible to change the underlying genetics leading to each susceptibility. Both mechanisms explain why it's so hard to lose weight and keep it off -- body and mind fight really hard, with multiple redundant defense layers -- to protect from weight loss.
But both of these causes flourish in the modern food environment in which we're surrounded by foods that are sweet, salty and fat, attractive and addictive and at the same time so easy to get. Exposure to these foods affects our metabolism and can up-regulate the internal homeostat that determines body weight; having these foods around, seeing ads and marketing for them wherever we go, also affects our desire and craving for food.
The processed food diet is a common pathway to both types of obesity, and its spread helps explain why obesity has soured.