Wednesday, March 26, 2008

Every Sweet Hath its Sour

People, along with many other animals, have a preference for sweet foods. This is putting it mildly, as our love of sugary sustenance has immensely influenced our culture, economy, and health. Even our vocabulary has been affected by an affinity for sugar, as the word “sweet” itself has a positive connotation, in English and other languages (e.g. la dolce vita).

But, our predilection for sweetness has come with a cost, as evidenced by a worldwide prevalence of obesity that is over 300 million. Obesity is not, as is sometimes implied, a condition that suddenly appeared within the last fifty years. The rate of obesity is, however, growing at an alarming pace, and has been for several decades. This has led to a similar increase in obesity-related illnesses, like type 2 diabetes, hypertension, and heart disease.

Why would evolution have left us with a preference for sweet foods, when it is these very same foods that make us fat and unhealthy? Part of the answer may lie in the fact that the environment in which our evolutionary ancestors lived didn’t have 64 oz. fountain sodas, frosting-covered donuts, and candy aisles. In their hunter-gatherer societies, food was much more scarce, so while we spend our days counting calories, they spent theirs searching for them. Sweet-tasting foods are usually an indication of high caloric content. They are also generally not poisonous, making them doubly valuable to a primitive food gatherer. So, a predilection for them would have been adaptive at one time, and may be an evolved mechanism. This hypothesis is reinforced by a widespread partiality for sweetness throughout much of nature.

A group of researchers from Duke University Medical Center have conducted a study, however, that calls into question the idea that a penchant for high-sugar foods is based on the ability to taste their sweetness. The group genetically engineered a line of mice that lack the ability to taste sweetness. They then exposed the mice to sugar water and water containing sucralose, a noncaloric sweetener. The “sweet-blind” mice demonstrated a preference for the actual sugar water. The preference appeared to be based not on sweetness, but calorie content.

This still fits in with the idea that the proclivity for high-calorie foods is an adaptive trait, but without the ability to taste sweetness as an indicator of the water’s calorie content, how did the mice know which water to drink? The researchers examined the brains of the mice and found that their reward system was activated by the caloric level of the water—independent of taste. The high-calorie sugar water raised dopamine levels and stimulated neurons in the nucleus accumbens, an area of the brain thought to be integral in reinforcing the value of rewarding experiences.

This activation of the reward system is one that seems to be separate from the hedonic aspect of pleasure. The affinity of the sweet-blind mice for high calorie water may represent the involvement of metabolic awareness in the reward system. This implies the brain’s understanding of "reward" is at a much deeper biological level than that which we normally associate with the word. It also is further indication of a separation between the hedonic and reinforcing aspects of the reward system (see the previous post on dopamine).

This finding could have real-world implications in helping to battle the obesity epidemic. If high-calorie foods are rewarding in and of themselves, it may help to explain our nation’s addiction to items that contain calorically fulsome additives, like high-fructose corn syrup. Reducing the prevalence of such additives could decrease the rewarding value of the food they are in, and thus reduce consumption.

It’s imperative that something is done soon to curb the rising rates of obesity. Our propensity toward heftiness may be partly due to a once evolutionarily adaptive trait that has become maladaptive in our modern environment. Thus, our difficulty in making the adjustment illustrates the power of genetics and evolution. But it should also remind us that evolution might be having a powerful effect right now. If so, it is being aided by fast food, mini-marts, and billions of dollars of advertising, and a society that may be too complacent to pay attention to the ramifications.

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