Why Diets Fail (Because You're Addicted to Sugar) (12 page)

We see this phenomenon in the laboratory as well. Some studies suggest that laboratory animals may prefer the taste of carbohydrates to protein,
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and even prefer carbohydrates (glucose) to protein when they do not taste the sweetness that is usually attributed to its appeal.
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Also, even when animals don’t taste the sweetness of the sugar, they still show an increased dopamine response to it compared to protein.
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Thus, it has been suggested that we may have strong and complex food preferences; even if carbohydrates don’t taste great, they may still be pleasurable for us because of postingestive factors, such as signals that our gut sends to our brain, which may reinforce our consumption of them. This might explain why nonsweet carbohydrates, such as breads and pastas, are also problematic for some individuals.

DOPAMINE DOESN’T WORK ALONE

There are other reasons that support the notion that sugars could have addiction-like effects on the brain. For example, there is evidence from both human studies and laboratory studies that indicates that brain systems that regulate feeding also have a role in regulating the addiction processes. As we just mentioned, dopamine
has a prominent role in addiction. However, studies show that dopamine is affected by many other things in the brain that regulate food intake. For example, in the hypothalamus (a key region of the brain related to food intake), the neurotransmitter orexin is associated with increasing food intake, but it turns out that orexin also sends information directly to the dopamine neurons in the reward systems and can promote drug seeking and addiction.
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There are also some satiety, or fullness, signals that circulate in the body that are associated with drug seeking. Leptin, which is a hormone that is secreted from fat cells in the body, normally works as a satiety signal by traveling to the brain to tell us to stop eating (because we have a lot of fat to store and don’t need to eat more). However, when people are overweight or obese, they may become insensitive to leptin, and the receptors in the brain that normally detect these high levels of it no longer work correctly. Thus, people who have a lot of fat stored continue to eat as if they are lean. Interestingly, there are leptin receptors in the reward regions of the brain, located on dopamine neurons, and activation of these receptors can promote sensitivity to drugs of abuse.
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A similar story is emerging for the hormone insulin, which is released from the pancreas in response to spikes in blood glucose.
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The bottom line is that chemicals that are associated with feeding can also affect brain regions that make us want to use drugs. This adds another layer of biochemical support for the idea that certain foods may have properties of addiction.

RESEARCH STUDIES OF SUGAR ADDICTION

Most of the science suggesting that excessive intake of sugars can result in signs that resemble an addiction comes from laboratory studies conducted with rats. Although we look very different, rats are actually very good models of humans because we have similar
brain systems and neural circuitry. Rats are used to study many disorders and diseases that affect humans, including those related to feeding behaviors and addictions. Pharmaceutical drugs that are developed for humans are almost always tested in rodents before they find their way into clinical trials, as rodents are generally thought to provide a good indication of what is likely to happen in humans in similar situations.

Why are rat studies important? Researchers using animal models don’t have to worry about the potential influences of the environment, media, or cultures on the behaviors or brain changes they observe. It is a great way to isolate the variables of interest and minimize outside influences that could affect the results.

Several studies have been designed to assess whether aspects of addiction, such as tolerance, exist in rats offered access to sugar-rich foods or sugar solutions. Most of the time, these studies are designed to mimic the pattern of food access that might be seen in humans. Rats will have free access to water as well as a “healthy” food alternative (that is, their standard, nutritionally complete rat chow—which is sort of the equivalent to dog food, but for rats). The sugar-rich food is offered as a supplement, or treat, so that it is not the sole source of calories for the animal, and they are not forced to eat it. In some cases, this “treat” is available twenty-four hours a day, but in other cases, the rats get restricted access to it (that is, perhaps just twelve hours a day). This limited access schedule, coupled with the fact that the sugar-rich food is offered a few hours after the time that the rats typically eat (so they are hungry), leads the rats to binge on the sugar-rich food when it becomes available. After just a few weeks on this feeding schedule, the behaviors shown by the rats demonstrate some noteworthy similarities to drug addiction.
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Rats that binge on a sugar solution consume a large amount of it when it becomes available each day, and over the course of several weeks, the amount of sugar-rich food that the rats consume increases
markedly. So, already we can see that rats that overeat sugar show two symptoms that are characteristic of addiction: bingeing (evidenced by consuming a large amount of the substance in a short period of time) and tolerance (evidenced by increased consumption of the substance over time).

Another behavior apparent in rats with binge access to sugar that mirrors drug addicts is withdrawal. When deprived of sugar, rats show signs of withdrawal similar to those with opiate withdrawal; they show signs of anxiety by remaining stationary and hiding in corners of mazes, both of which are textbook examples of anxiety in rats. Other symptoms include teeth chattering, body shaking, and physiological changes, such as diarrhea and changes in body temperature. They also show signs of craving, such as ingesting more sugar when it is reintroduced following a period of deprivation. Similarly, when rats are trained to press a lever to get access to sugar, rats that previously binged on sugar that have been deprived of it will press that lever repeatedly to turn on a light above the lever that is normally on when the sugar is available, even if they don’t actually get to drink any of the sugar. Thus, these rats are even willing to work for
cues
that signal sugar, highlighting how powerful food cues (such as the image that we see on a hamburger wrapper or the smell of french fries) can be when we encounter them in our environment. Collectively, these behaviors all mimic what happens in rats and people when they are addicted to drugs of abuse.

Another parallel between binge consumption of sugar and drug addiction that has been noted in rat studies is the “gateway” effect. Addiction to one drug can lead to
cross-sensitization
, meaning an increase in the likelihood that one will also become addicted to other drugs. This concept is most commonly discussed when referring to the use of drugs such as nicotine or alcohol, which some suggest may lead to an increased vulnerability to use “harder” drugs, such as cannabis, cocaine, or heroin. Well, it turns out that
cross-sensitization is not reserved just for drugs of abuse; rats that overeat sugar show a cross-sensitization to other foods, particularly those high in fat, as well as drugs of abuse, such as alcohol, amphetamine, and cocaine. Thus, it appears that overeating sugars makes animals more sensitive to the effects of drugs of abuse later on. This makes theoretical sense; recall that we only have one brain reward system, and that our brain doesn’t have separate systems for drugs versus foods. So, if we prime the brain system to be “addicted” to one particular substance, other addictions may ensue. As we discussed in the beginning of this chapter, there is evidence of this from research studying alcoholics, who tend to have a sweet tooth.
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To summarize, evidence from animal studies shows that overeating of sugars can produce a state that looks much like an addiction to drugs of abuse. Also noteworthy is the finding that animals with a history of overeating sugars more readily acquire a taste for “real” drugs later on. Changes in the brain can result from certain eating behaviors, which may then perpetuate them, creating a cycle. For example, taking certain drugs can cause people to feel a sense of pleasure or a high due to a reaction in the brain, and this positive experience may lead a person to continue to engage in that behavior, forming a cycle of addiction. Changes in brain dopamine systems have been observed in animals with a history of overeating sugars along with the addiction-like behaviors discussed earlier, and these brain changes are similar to those seen with drugs of abuse.

DOPAMINE AND EATING

Food is a natural reward and is inherently pleasurable, especially when we are hungry. Thus, it is natural for food to cause a dopamine release, but this release is normally short-lived, and it is more associated with the novelty of the food. Here is why: In addition to its role in pleasure and reward, dopamine also has an important role
in learning and orienting (that is, paying attention). Have you ever consumed a food that you normally eat (for example, milk), and it tasted a little funny? Not necessarily bad, but just different? Normally, when this happens our reaction is to step back and assess the situation. Is the milk expired? Was it sitting out on the countertop for too long? A funny taste can stop us dead in our tracks and make us think about the food we are eating and reassess whether it is a good or bad idea to continue to eat it. Here is why: Our primitive ancestors had a biological drive to eat food, but getting food was not as easy as it is today (there were no supermarkets or fast-food chains), so when they acquired food, they would eat a lot of it. So, if they were to stumble upon a berry patch and they had never tasted those berries before, their dopamine systems would be activated, causing them to pay attention to what was happening in regard to this new taste, in case it were to make them sick. This way they would know to never eat it again. Rotten or poisonous food can be deadly, even in modern times, so our dopamine systems make sure that we are aware of the situation that surrounds eating new tastes, just in case.

Our brain has a lot to do to keep us moving, so in order to function efficiently, it needs to conserve energy and resources. Since we need to eat regularly, it wouldn’t be very efficient for our brains to orient us in this way when eating every single morsel of food. That is why the release of dopamine is normally limited to “risky” eating, such as when a new taste is involved. Once we are used to the taste and our brains know that it is safe, the dopamine release in response to eating it decreases. This is a key neurochemical difference between food and drugs of abuse: dopamine release typically decreases following repeated exposure to a food, but it is always released when someone takes a drug.

However, studies show that when palatable foods, such as those rich in sugar, are consumed in excess, the pattern of dopamine release
that is typically observed with food changes: dopamine is released in response to a sugar binge even when sugar is no longer a novel food.
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It is as if the brain starts to treat sugar like a drug. Thus, overeating certain foods has been shown to have the potential to change the landscape of the brain, causing alterations in dopamine functioning that can affect food-intake behavior and undoubtedly contribute to the compulsion to continue to overeat.

It should be noted that there are many studies in laboratory animals that lend support to the idea that sugar-rich foods can have properties of addictions. Studies have been conducted using sugar solutions, solid foods that are high in sugar, and even foods that people tend to like to eat, such as Oreos and M&M’s. In addition to the findings described above, when rats have access to these types of foods and overeat them, many engage in behaviors that are atypical, such as running over an electrified grid in order to get access to M&M’s.
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Thus, there is a growing body of evidence coming from laboratory animal studies that supports the notion that sugar-rich foods can have properties of addiction.

FOOD ADDICTION STUDIES IN HUMANS

It is important to note that not all people are addicted to sugar—many people can eat sugars and sugar-rich foods in moderation as part of a healthy diet and have no associated problems. This is similar to alcohol: if you are not an alcoholic, you can enjoy a glass of wine now and then, but you can stop drinking at any time, and you don’t feel the same urge to drink that leads an alcoholic to imbibe. However, as we have seen, sugar is pervasive in modern Western diets, meaning that it takes a conscious effort and careful consideration to regulate our sugar consumption. It is relatively easy to overeat sugar-rich foods, and doing so for an extended period of time may put some
people at risk for becoming dependent on them, which can lead to more and more overeating. We also have the added disadvantage that, if we become addicted to sugar, it can be tough to manage due to the constant food cues and pressures we have regarding food.

While lab-based research with rats has allowed scientists to uncover a lot of information, both behavioral and in terms of what happens in the brain, there have been several studies conducted in humans that also support the idea that overeating palatable foods can result in a state that resembles an addiction.
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Humans eat a variety of different types of foods, many of which are rich in fats and sugars and are pleasurable to eat. In excess, however, consuming these foods may alter brain mechanisms responsible for processing rewards, potentially resulting in a hijacked brain reward system, similar to what is seen among individuals addicted to drugs, as well as the data from laboratory animal studies discussed previously.