In this second post of the series, I want to take a closer look at the neural and psychological causes and underpinnings of food addiction.
Because the obese condition can be reached several ways, and because not all instances of excessive food intake can be regarded as dysfunctional, it would not be suitable to group these two entities into a subclass of addiction. Addiction requires compulsive action and lasting changes on the neural level. Therefore, as a distinct phenotype of obese individuals, compulsive overeaters may very well be classified as “addicts.”
The dysfunctional behavior and neural maladaptations of compulsive overeaters adhere to DSM-IV criteria and parallel drug-addicted patients sufficiently enough to implicate substance addiction as playing the key role in their non-homeostatic physical and mental states. Food can be addictive. Just as some individuals can recreationally use drugs, many people can casually/passively eat to meet their caloric requirements. However, similar to the compulsive drug-taking and seeking behavior seen in addicts, food inspires compulsive behaviors in a distinct class of individuals, consciously and unconsciously driving their thoughts and actions to the point where homeostasis is no longer an issue: compulsive overeating is an addiction. The DSM-IV defines substance-dependence (addiction) as exhibiting three or more of the following in regards to a substance of abuse, which in this case, is food: tolerance, withdrawal, consuming large amounts over a long period of time, having unsuccessful efforts to stop use or to cut down, time spent in obtaining, thinking about, and consuming the substance replaces social, occupational, and recreational activities, and lastly, exhibiting continued use despite adverse consequences. With this in mind, we can relatively easily classify compulsive overeating as a substance-dependence disorder, or addiction.
In empirical research, compulsive overeaters display characteristics of developing tolerance; they’ve been found to progressively require more and more food to achieve the desired “drugged” effect (e.g. “the sugar high”) as time goes on (Davis & Carter, 2009). Much of the human evidence of this comes from clinical anecdotes given by the patients themselves. Picot and Linfield (2003) also found that higher body weight correlated with the frequency and severity of bingeing episodes. This may indicate that as level of obesity increases, the disorder worsens. More indirect evidence of tolerance can be found in animal studies. Avena et al. (2008) found that rodents on a high sugar chow diet exhibit addictive-like behaviors, increasing daily sugar intake over a period of 28 days. The rats also exhibit opioid-like withdrawal symptoms when given the opioid antagonist naloxone (3 mg/kg). Symptoms include anxiety, teeth chattering, tremor, head shakes, and a drop in body temperature (Avena et al, 2008) suggesting that natural rewards such as sugar act on similar neural substrates as illicit drugs. And with all of the warnings from medical practitioners as well as nutritional education in the public school systems from a young age, it would be absolutely inappropriate to assume that compulsive overeaters were not aware of their behaviors’ adverse affects on their health. They continue compulsive overeating despite awareness of how detrimental it is. Donald Hebb describes eating as being a learned behavior that is reinforcing because it reverses unpleasant bodily signals and comes to be associated with environmental cues which can trigger craving (Hebb, 1949). Food is considered a natural reward. Food, as a substance of abuse, becomes a rather complicated matter because, unlike illicit drugs, food is intrinsically necessary for survival. However, in excess, it can have such adverse effects as to lead to diabetes, heart disease, and consequently, premature death. When viewed through the scope of evolution, food cravings can be seen as a necessary biological mechanism for survival in unpredictable environments. Early hunter-gatherer societies never knew how long it would be before they found their next meal. It was not uncommon to go for days without food, or with very little food. In these cases, the human body has developed in such a way that it will store fat for energy, especially after large meals. When food enters the body and blood glucose levels rise, insulin is released and fat is no longer used as an energy source. The body preserves it for later use. It instead utilizes the energy from the incoming calories, carbohydrate being the most readily consumed. This “fat-preservation” mechanism is what allowed our ancestors to survive for long periods of time between meals. However, in our current nutritional condition, food is readily available. Not only that, it is also calorically dense, with most processed varieties being high in sugar as well. The steady influx of sugar into the human body keeps blood glucose levels high, never giving it the chance to use fat stores as fuel. Instead, excess fat is stored and over time accumulates until the individual is in the obese state. Many individuals also become insulin resistant, resulting in Type II Diabetes.
So what is the difference between liking and wanting food? And why can making such a distinction help develop a better understanding of compulsive overeating as an addiction disorder? First, to differentiate between the two, liking of food refers to the pleasure one gets from eating the food. Wanting food refers to appetite. Excessive wanting leads to compulsion. There is no doubt that food addicts experience compulsive food-seeking/consuming behavior. But why do they do so, when others don’t? To answer this question, we look to Robinson and Berridge (2000) and their incentive-sensitization theory. According to sensitization theory, the drug/food will have an increased effect on the reward system following repeated doses/eating. Individuals with easily sensitized brains will therefore be more likely to develop an addiction. The critical prediction made by incentive-sensitization is that the brains of addicted individuals will no doubt contain a region or pathway that has been sensitized by drugs, or in our case food. To add to it, sensitization is dose dependent, meaning the higher the dose, the more pronounced the sensitization. Unhealthy meals, such as a Super-Sized Value Meal, and binge episodes may constitute consuming thousands of calories, definitely a large amount of food. Therefore, according to this theory, sensitization would be greatly increased in over eaters. The main point here is that when sensitized, the food/drug ‘wanting’ produces compulsive patterns of seeking-behavior. Herein lies the critical connection. Certain individuals are more easily sensitized than others; as the doses increase, sensitization increases, creating a heightened wanting for food; this wanting thus manifests in compulsive food-seeking behavior: addiction. They outline two different types of wanting: implicit and explicit wanting (Finlayson et al, 2003). These involve separate processes but they complement each other to contribute to food preference. Implicit wanting is how hard an individual is willing to work for food. This is the independent risk factor for overconsumption. The harder someone is willing to work to get the food, the more likely they are to eat too much of it. Food has a heightened value to them. Explicit wanting refers to an individual’s desire to eat a specific kind of food. Preference for high fat, high sugar foods would reflect explicit wanting.
Incentive-sensitization is one of several popular theories for why people compulsively over eat. It is important to realize that the processes leading to food consumption are not all explicit. Implicit processes play just as much of a role, if not more, in food seeking behavior. With this in mind, is eating necessarily under voluntary control? We usually see it as such because of utilization of musculature to obtain and consume food. We can perceive our outstretched arm as our hand reaches for the food. But knowing what we do about wanting, the importance of implicit wanting cannot be overlooked, because without it, explicit wanting would not matter. If the individual is not willing to work to get food, say get off the coach, then of course they will not care about the type of food they eat. So what drives implicit wanting? On the surface, we might say hunger, because it does. However, this doesn’t account for why someone will continue to eat past feeling ‘full.’ There must be a relationship between the hedonics of food and bodily homeostasis, with hedonics having the ability to override the innate desire for a homeostatic internal environment. Our knowledge of liking and wanting in food-related behaviors is relatively limited. So, in order to strengthen our understanding, it helps to draw upon separate theoretical models for guidance.
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