Food Reward: a Dominant Factor in Obesity, Part I

A Curious Finding

It all started with one little sentence buried in a paper about obese rats. I was reading about how rats become obese when they're given chocolate Ensure, the "meal replacement drink", when I came across this:

...neither [obesity-prone] nor [obesity-resistant] rats will overeat on either vanilla- or strawberry-flavored Ensure.

The only meaningful difference between chocolate, vanilla and strawberry Ensure is the flavor, yet rats eating the chocolate variety overate, rapidly gained fat and became metabolically ill, while rats eating the other flavors didn't (1). Furthermore, the study suggested that the food's flavor determined, in part, what amount of fatness the rats' bodies "defended."

As I explained in previous posts, the human (and rodent) brain regulates the amount of fat the body carries, in a manner similar to how the brain regulates blood pressure, body temperature, blood oxygenation and blood pH (2). That fact, in addition to several other lines of evidence, suggests that obesity probably results from a change in this regulatory system. I refer to the amount of body fat that the brain defends as the "body fat setpoint", however it's clear that the setpoint is dependent on diet and lifestyle factors. The implication of this paper that I could not escape is that a food's flavor influences body fatness and probably the body fat setpoint.

An Introduction to Food Reward

The brain contains a sophisticated system that assigns a value judgment to everything we experience, integrating a vast amount of information into a one-dimensional rating system that labels things from awesome to terrible. This is the system that decides whether we should seek out a particular experience, or avoid it. For example, if you burn yourself each time you touch the burner on your stove, your brain will label that action as bad and it will discourage you from touching it again. On the other hand, if you feel good every time you're cold and put on a sweater, your brain will encourage that behavior. In the psychology literature, this phenomenon is called "reward," and it's critical to survival.

The brain assigns reward to, and seeks out, experiences that it perceives as positive, and discourages behaviors that it views as threatening. Drugs of abuse plug directly into reward pathways, bypassing the external routes that would typically trigger reward. Although this system has been studied most in the context of drug addiction, it evolved to deal with natural environmental stimuli, not drugs.

As food is one of the most important elements of survival, the brain's reward system is highly attuned to food's rewarding properties. The brain uses input from smell, taste, touch, social cues, and numerous signals from the digestive tract* to assign a reward value to foods. Experiments in rats and humans have outlined some of the qualities of food that are inherently rewarding:

• Fat
• Starch
• Sugar
• Salt
• Meatiness (glutamate)
• The absence of bitterness
  
• Certain textures (e.g., soft or liquid calories, crunchy foods)
• Certain aromas (e.g., esters found in many fruits)
• Calorie density ("heavy" food)
  

We are generally born liking the qualities listed above. In addition, aromas and flavors that are associated with these qualities can become rewarding over time. For example, beer tastes terrible the first time you drink it because it's bitter, but after you drink it a few times and your brain catches wind that there are calories and a drug in there, it often begins tasting good. The same applies to many vegetables. Children are generally not fond of vegetables, but if you serve them spinach smothered in butter enough times, they'll learn to like it by the time they're adults.

The human brain evolved to deal with a certain range of rewarding experiences. It didn't evolve to constructively manage strong drugs of abuse such as heroin and crack cocaine, which overstimulate reward pathways, leading to the pathological drug seeking behaviors that characterize addiction. These drugs are "superstimuli" that exceed our reward system's normal operating parameters. Over the next few posts, I'll try to convince you that in a similar manner, industrially processed food, which has been professionally crafted to maximize its rewarding properties, is a superstimulus that exceeds the brain's normal operating parameters, leading to an increase in body fatness and other negative consequences.


* Nerves measure stomach distension. A number of of gut-derived paracrine and endocrine signals, including CCK, PYY, ghrelin, GLP-1 and many others potentially participate in food reward sensing, some by acting directly on the brain via the circulation, and others by signaling indirectly via the vagus nerve. More on this later.

Upcoming Talks

I'll be giving at least two talks at conferences this year:

Ancestral Health Symposium; "The Human Ecological Niche and Modern Health"; August 5-6 in Los Angeles. This is going to be a great conference. Many of my favorite health/nutrition writers will be presenting. Organizer Brent Pottenger and I collaborated on designing the symposium's name so I hope you like it.

My talk will be titled "Obesity; Old Solutions to a New Problem." I'll be presenting some of my emerging thoughts on obesity. I expect to ruffle some feathers!

Tickets are going fast so reserve one today! I doubt there will be any left two weeks from now.


TEDx Harvard Law; "Food Policy and Public Health"; Oct 21 at Harvard. My talk is tentatively titled "The American Diet: a Historical Perspective." This topic interests me because it helps us frame the discussion on why chronic disease is so prevalent today, and what are the appropriate public health measures to combat it. This should also be a great conference.

Obesity and the Fluid-in, Fluid-out Therapy for Edema

I recently attended a lecture by Dr. Arya M. Sharma here at the University of Washington. Dr. Sharma is a Canadian clinician who specializes in the treatment of obesity. He gave the UW Science in Medicine lecture, which is a prestigious invited lecture.

He spent a little bit of time pointing out the fallacy behind conventional obesity treatment. He used the analogy of edema, which is an abnormal accumulation of fluid in the body.

Since we know that the amount of fluid contained in the body depends on the amount of fluid entering the body and the amount of fluid leaving the body, the treatment for edema is obvious: drink less, pee more.

Of course, this makes no sense. It doesn't address the underlying cause of edema and it will not help the patient. Yet we apply that exact same logic to fat loss. Since the amount of energy contained in the body (in the form of fat) depends on the amount entering and the amount leaving, the solution is easy: eat less, move more. Well, yes, if you can stick to that program it will cause fat loss. But that's equivalent to telling someone with edema to drink less water. It will cause a loss of fluid, but it won't correct the underlying problem that caused excessive fluid retention in the first place.

For example, if you have edema because your heart isn't pumping effectively (cardiac insufficiency), the heart is the problem that must be addressed. Any other treatment is purely symptomatic and is not a cure.

The same applies to obesity. If you don't correct the alteration in the system that causes an obese person to 'defend' his elevated fat mass against changes*, anything you do is symptomatic treatment and is unlikely to be very effective in the long term. My goal is to develop a method that goes beyond symptomatic treatment and allows the body to naturally return to a lower fat mass. I've been doing a lot of reading and I have a simple new idea that I feel confident in. It also neatly explains the results of a variety of weight loss diets. I've dropped a few hints here and there, but I'll be formally unveiling it in the next couple of months. Stay tuned.


* The body fat homeostasis system. The core element appears to be a negative feedback loop between body fat (via leptin, and insulin to a lesser degree) and the brain (primarily the hypothalamus, but other regions are involved). There are many other elements in the system, but that one seems to set the 'gain' on all the others and guides long-term fat mass homeostasis. The brain is the gatekeeper of both energy intake and energy expenditure, and unconscious processes strongly suggest appropriate levels for both factors according to the brain's perceived homeostatic needs. Those suggestions can be overridden consciously, but it requires a perpetual high degree of discipline, whereas someone who has been lean all her life doesn't require discipline to remain lean because her brain is suggesting behaviors that naturally defend leanness. I know what I'm saying here may seem controversial to some people reading this, because it's contrary to what they've read on the internet or in the popular press, but it's not particularly controversial in my field. In fact, you'll find most of this stuff in general neuroscience textbooks dating back more than 10 years (e.g., Eric Kandel and colleagues, Principles of Neuroscience).