Sugar is the enemy, according to a growing body of research, and not just because it rots our teeth and adds padding to our thighs.
The real danger is fructose -- a main ingredient in table sugar, high fructose corn syrup, and fruit -- that actually gets into our cells and alters metabolism.
The findings may help to explain how our nation's excessive consumption of sweetened foods is contributing to growing rates of obesity, heart disease, high blood pressure, diabetes and more -- in a way that has nothing to do with sugar's rich source of empty calories.
What's more, there may be deep evolutionary roots that explain sugar's power over our bodies. Many millions of years ago, according to new research, our ape ancestors developed mutations that made it easy for them to get fat from eating fructose.
At the time, the mutation was a good thing. It allowed our ancestors to survive seasonal periods of famine when the fruit trees went bare.
Today, the mutation makes a year-round, fructose-filled diet dangerous to our health, said Richard Johnson, chief of the division of Renal Diseases and Hypertension at the University of Colorado, Denver, and author of "The Sugar Fix: The High-Fructose Fallout That Is Making You Fat and Sick."
Both sugar and high-fructose corn syrup are equally bad, he said, because both contain about the same amount of fructose.
"This mutation that occurred 15 million years ago could explain why we're fat today," Johnson said. "It doesn't mean we all become obese. It doesn't mean everyone is going to get diabetes. It does mean that all of us are more susceptible to being fat than most other mammals."
As rates of diabetes and obesity increased in the 1960s, Johnson said, an idea emerged that some ancient mutation might have occurred during a time of famine, increasing people's ability to become fat easily when food became plentiful again. But only recently have scientists found evidence to explain what that mutation might be and how it might work.
In a paper published in January in the journal Transactions of the American Clinical and Climatological Association, and a more detailed paper set for publication in November, Johnson and colleagues identify just such a mutation that affects how our bodies deal with uric acid, an ordinary byproduct of metabolism.
The mutation evolved 15 million years ago, during a period of starvation. One hundred percent of us have it.
While many of the details have yet to be published, Johnson said that the mutation led to an increase in how much uric acid our bodies produce after eating fructose, while also lengthening the amount of time that uric acid sticks around after a sweet treat.
The result, he said, is inflammation and an increased ability for cells to become fat. In other words, uric acid works within cells to amplify sugar's ability to cause obesity.
Even on a calorie-restricted diet, he said, animals that eat too much sugar develop insulin resistance, an early sign of diabetes. Other trials in people have shown that lowering uric acid levels lowered their blood pressure.
"We need to start thinking about sugar in a completely different way," said Robert Lustig, a pediatric endocrinologist at the University of California, San Francisco.
Lustig agrees with Johnson that uric acid might drive high blood pressure, but he isn't convinced that uric acid explains all of sugar's power to induce obesity, diabetes and cardiovascular disease. Instead, he thinks the story is more complicated than that.
In several papers, including one published in the Journal of the American Dietetic Association, Lustig and colleagues report striking similarities in the health effects of both sugar and ethanol, the active ingredient in beer, wine and liquor.
Fatty-liver disease, for example, is common in alcoholics, and it appears in more than a third of obese kids and nearly half of obese adults. In these obese patients, Lustig said, the liver looks exactly like the liver of an alcoholic with the disease, even when the patient doesn't drink.
Like alcohol, his recent study found, sugar is also habit-forming and possibly addictive.
More than half of the American population is overdosing on sugar, he added. The American Heart Association recommends a maximum of six teaspoons per day of added sugars for women and nine teaspoons for men. But with so much sugar and high fructose corn syrup in processed foods, Lustig said, education alone will never succeed at getting people down to slash the sweets from their diet.
"We need to have sugar policies the same way we have ethanol policies," he said. "And until we do, don't expect the obesity epidemic to get any better."