The Neurobiology of the Obesity Epidemic

I recently read an interesting review paper by Dr. Edmund T. Rolls titled "Taste, olfactory and food texture reward processing in the brain and the control of appetite" that I'll discuss in this post (1). Dr. Rolls is a prolific neuroscience researcher at Oxford who focuses on "the brain mechanisms of perception, memory, emotion and feeding, and thus of perceptual, memory, emotional and appetite disorders." His website is here.

The first half of the paper is technical and discusses some of Dr. Rolls' findings on how specific brain areas process sensory and reward information, and how individual neurons can integrate multiple sensory signals during this process. I recommend reading it if you have the background and interest, but I'm not going to cover it here. The second half of the paper is an attempt to explain the obesity epidemic based on what he knows about the brain and other aspects of human biology.

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Food Variety, Calorie Intake, and Weight Gain

Let's kick off this post with a quote from a 2001 review paper (1):

Increased variety in the food supply may contribute to the development and maintenance of obesity. Thirty-nine studies examining dietary variety, energy intake, and body composition are reviewed. Animal and human studies show that food consumption increases when there is more variety in a meal or diet and that greater dietary variety is associated with increased body weight and fat.

This may seem counterintuitive, since variety in the diet is generally seen as a good thing. In some ways, it is a good thing, however in this post we'll see that it can have a downside.
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Neuronal Control of Appetite, Metabolism and Weight

Last week, I attended a Keystone conference, "Neuronal Control of Appetite, Metabolism and Weight", in Banff. Keystone conferences are small, focused meetings that tend to attract high quality science. This particular conference centered around my own professional research interests, and it was incredibly informative. This post is a summary of some of the most salient points.

Rapid Pace of Scientific Progress

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Why Do We Eat? A Neurobiological Perspective. Part VIII

In the (probably) last post of this series, I'll take the pieces that I've gradually outlined in previous posts, and put them together into a big-picture, common-sense framework for thinking about human eating behavior, and why we eat more today than ever before.

Why is Eating Behavior Regulated?

Let's start at the most fundamental level. To be competitive in a natural environment, organisms must find rational ways of interacting with their surroundings to promote survival and reproduction. One of the most important elements of survival is the acquisition of energy and chemical building blocks, either by photosynthesis, or (in the case of animals) eating other organisms. This imperative drove the evolution of rational food seeking behaviors long before the emergence of humans, mammals, reptiles, amphibians, fish, worms, and even eukaryotes (organisms with nuclei).

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Why Do We Eat? A Neurobiological Perspective. Part VII

Welcome back to the series, after a bit of a hiatus! In previous posts, we covered the fact that humans eat because we're motivated to eat, and many things can motivate us to eat. These include factors related to energy need (homeostatic factors), such as hunger, and factors that have little to do with energy need or hunger (non-homeostatic factors). These many factors are all processed in specialized brain 'modules' that ultimately converge on a central action selection system (part of the reward system); this is the part of you that decides whether or not to initiate eating behaviors.

This will be somewhat of a catch-all post in which I discuss cognitive, emotional, and habit influences on food intake. Since these factors are not my specialty, I'll keep it brief, but I don't mean to suggest they aren't important.

Food 'Cost'

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Why Do We Eat? A Neurobiological Perspective. Part V

In previous posts, I explained that food intake is determined by a variety of factors that are detected by the brain, and integrated by circuits in the mesolimbic system to determine the overall motivation to eat. These factors include 'homeostatic factors' that reflect a true energy need by the body, and 'non-homeostatic factors' that are independent of the body's energy needs (e.g. palatability, habit, and the social environment).

In this post, we'll explore the hedonic system, which governs pleasure. This includes the pleasure associated with food, called palatability. The palatability of food is one of the factors that determines food intake.

The Hedonic System

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Why Do We Eat? A Neurobiological Perspective. Part III

In the first post, I explained that all voluntary actions are driven by a central action selection system in the mesolimbic area (the reward system). This is the part of you that makes the decision to act, or not to act. This system determines your overall motivation to obtain food, based on a variety of internal and external factors, for example hunger, the effort required to obtain food, and the sensory qualities of food/drink. These factors are recognized and processed by a number of specialized 'modules' in the brain, and forwarded to the reward system where the decision to eat, or not to eat, is made. Researchers divide food intake into two categories: 1) eating from a true energy need by the body (homeostatic eating), e.g. hunger, and 2) eating for other reasons (non-homeostatic eating), e.g. eating for social reasons or because the food tastes really good.

In the second post of the series, we explored how the brain regulates food intake on a meal-to meal basis based on feedback from the digestive system, and how food properties can influence this process. The integrated gut-brain system that accomplishes this can be called the satiety system.

In this post, we'll explore the energy homeostasis system, which regulates energy balance (energy in vs. energy out) and body fatness on a long term basis.

The Energy Homeostasis System

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Why Do We Eat? A Neurobiological Perspective. Part II

In the last post, I explained that eating behavior is determined by a variety of factors, including hunger and a number of others that I'll gradually explore as we make our way through the series. These factors are recognized by specialized brain 'modules' and forwarded to a central action selection system in the mesolimbic area (the reward system), which determines if they are collectively sufficient cause for action. If so, they're forwarded to brain systems that directly drive the physical movements involved in seeking and consuming food (motor systems).

The term 'homeostasis' is important in biology. Homeostasis is a process that attempts to keep a particular factor within a certain stable range. The thermostat in your house is an example of a homeostatic system. It reacts to upward or downward changes in a manner that keeps temperature in a comfortable range. The human body also contains a thermostat that keeps internal temperature close to 98.6 F. Many things are homeostatically regulated by the body, and one of them is energy status (how much energy the body has available for use). Homeostasis of large-scale processes in the body is typically regulated by the brain.

We can divide the factors that determine feeding behavior into two categories, homeostatic and non-homeostatic. Homeostatic eating is when food intake is driven by a true energy need, as perceived by the brain. For the most part, this is eating in response to hunger. Non-homeostatic eating is when food intake is driven by factors other than energy need, such as palatability, habitual meal time, and food cues (e.g. you just walked by a vending machine full of Flamin' Hot Cheetos).

We can divide energy homeostasis into two sub-categories: 1) the system that regulates short-term, meal-to-meal calorie intake, and 2) the system that regulates fat mass, the long-term energy reserve of the human body. In this post, I'll give an overview of the process that regulates energy homeostasis on a short-term, meal-to-meal basis.

The Satiety System (Short-Term Energy Homeostasis)


The stomach of an adult human has a capacity of 2-4 liters. In practice, people rarely eat that volume of food. In fact, most of us feel completely stuffed long before we've reached full stomach capacity. Why?

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More Thoughts on Macronutrient Trends

I had a brief positive exchange with Gary Taubes about the NuSI post. He reminded me that there's an artifact (measurement error) in the USDA data on fat consumption in the year 2000 when they changed assessment methods. Here are the USDA data on macronutrient consumption since 1970, corrected for loss (28.8%) but not corrected for the artifact:

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Calories and Carbohydrate: a Natural Experiment

In the lab, we work hard to design experiments that help us understand the natural world. But sometimes, nature sets up experiments for us, and all we have to do is collect the data. These are called "natural experiments", and they have led to profound insights in every field of science. For example, Alzheimer's disease is usually not considered a genetic disorder. However, researchers have identified rare cases where AD is inherited in a simple genetic manner. By identifying the genes involved, and what they do, we were able to increase our understanding of the molecular mechanisms of the disease.

The natural experiment I'll be discussing today began in 1989 with the onset of a major economic crisis in Cuba. This coincided with the loss of the Soviet Union as a trading partner, resulting in a massive economic collapse over the next six years, which gradually recovered by 2000.

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Two Great Quotes About Obesity (technical)

By Dr. Hans-Rudolf Berthoud, from a recent paper, "The Neurobiology of Food Intake in an Obesogenic Environment" (1). I came across it because it cites my review paper (2). My perspective on obesity is similar to his. From the abstract:

The modern lifestyle with its drastic changes in the way we eat and move puts pressure on the homoeostatic system responsible for the regulation of body weight, which has led to an increase in overweight and obesity. The power of food cues targeting susceptible emotions and cognitive brain functions, particularly of children and adolescents, is increasingly exploited by modern neuromarketing tools. Increased intake of energy-dense foods high in fat and sugar is not only adding more energy, but may also corrupt neural functions of brain systems involved in nutrient sensing as well as in hedonic, motivational and cognitive processing.

And a nice one from the conclusions:
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