Sweetness may be in your genes

The next time you cast a disparaging glance at the overweight guy dumping six spoonfuls of sugar into his morning coffee, consider this: He may be less sensitive to sugar than you are - and genes might be the reason why.

A specific set of as-yet-unidentified genes also is largely responsible for the similarity in how we perceive natural vs. artificial sweeteners, according to a study by researchers from Australia and Philadelphia.

"I think one thing we know very clearly is that everyone likes sugar," said Robert Lustig, a professor of pediatrics at the University of San Francisco School of Medicine, who was not involved with the new research. "There is no one on the planet that doesn't like it. It's hardwired into our DNA, and this study corroborates that."

Watching laboratory mice at the Monell Chemical Senses Center in Philadelphia 15 years ago, behavioral geneticist Danielle Reed, a coauthor of the new study, was struck by radical differences in how much sugar each rodent liked in its water.

She wondered whether inborn differences might explain the variation, just like differences in hearing and vision that are known to be genetic. Did humans also differ in their preference and sensitivity to sweetness? And were those differences determined by our genes?

To find out, Reed, who is still at Monell, and colleagues at the Queensland Institute of Medical Research analyzed data from a long-term study of Australian twins who had been used previously to study bitter tastes.

They followed 243 pairs of identical twins, who share a nearly identical set of genes; 452 pairs of fraternal twins, who share roughly half their genes, as non-multiple-birth siblings do; plus 511 unrelated individuals, for almost 15 years. They tested the groups on taste sensitivity to the natural sugars fructose and glucose, as well as the noncaloric artificial sweeteners aspartame and NHDC.

The study design allowed the researchers to tease out what might be genetic from what might be determined by environmental factors, like eating a diet laden with sugary foods. Using twins automatically controlled for age, since the intensity of taste for sweetness (and bitterness) declines over time.

Participants in the study, published last week in the journal Twin Research and Human Genetics, ranged in age from 12 to 26.

There is a myth that eating a lot of foods high in sugar can change how one perceives sweetness, which might lead to consuming more sweets. In fact, the researchers found that what you eat has a relatively small influence on your sensitivity for sweetness.

"Seventy-four percent of all foods in the U.S. have sugar added for sweetness," said Lustig. This study "provides us with the smoking gun for the reason why the food industry uses sugar so indiscriminately."

We're born to appreciate it.

"Americans have become accustomed to a very high level of sweetness across a broad range of foods," said Kelly Brownell, an obesity expert and professor of public policy at the Sanford School of Public Policy at Duke University. Sugar is added to "obvious foods like ice cream, but also to not-so-obvious things like ketchup."

Since the inherent taste for sweetness is determined by a single set of genes, it benefits the food industry to use that as a hook, Lustig said.

The study also suggests that it doesn't seem to matter whether the food industry uses real sugar or sugar substitutes.

It had been unknown if different molecular pathways responded to real sugar vs. artificial sweeteners. The researchers were surprised to learn that those who perceived the natural sugars as not so sweet experienced the sugar alternatives the same way, a finding that hints at a shared pathway.

What this study suggests is that some people find sweetness - real or artificial - more rewarding than others, Brownell said. "It's possible that one could identify such people as high-risk early in life and be more careful about their diet."

Brownell also thinks that the findings might lead to some biological discovery that will make sugar less rewarding for people. "Anything that can be done to reduce the amount of sugar in the American diet will be most helpful," he said.

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