There's a new candidate in the century-old quest for perfect, guiltless sweetness.

I encountered it at the annual meeting of the Institute of Food Technologists, a combination of Super Bowl, Mecca, and Disneyland for the folks who put the processing in processed food.

It was right in the middle of the vast exhibition hall, at the Tate & Lyle booth. This is the company that introduced the British Empire to the sugar cube, back in 1875. A century later, it invented sucralose, aka Splenda. "We have a deep understanding of sweetening," says Michael Harrison, Tate & Lyle's vice president of new product development.

This year, his company launched its latest gift to your sweet tooth. It's called allulose.

"This is a rare sugar. A sugar that's found in nature," Harrison explains.

Chemically speaking, it's almost identical to ordinary sugar. It has the same chemical formula as fructose and glucose, but the atoms of hydrogen and oxygen are arranged slightly differently.

And that slight difference means that my body won't turn this sugar into calories.

A few feet from us, people are lining up to taste it, in cups of chocolate and vanilla soft serve ice cream.

I try it. It's good. Sweet.

That's not so unusual, of course. There are plenty of low-calorie sugar substitutes. But Harrison says most of them wouldn't work in this ice cream. Because sugar, and allulose, do more than deliver sweetness: They also keep the ice cream from freezing solid.

"It allows that ice cream to be soft-served. It's a smooth, creamy texture," Harrison says. "It's bringing the functionality of sugar because it is sugar."

Tate & Lyle has now come up with a way to manufacture this rare sugar in large quantities.

To my ears, it all sounds practically perfect. All the pleasure of sugar with none of the pain.

Could this finally be a free lunch?

I called George Fahey, a nutrition expert at the University of Illinois.

Fahey is actually a fan of allulose. He signed off on a report to the Food and Drug Administration, arguing that allulose is safe. (Relying in part on that report, the FDA considers allulose, also called psicose, "generally recognized as safe.")

But Fahey says that there's also good reason to be careful with low-calorie sugar substitutes like allulose. The same quality that makes them attractive can also make them quite unpleasant.

Our bodies don't digest them, he says. "They travel right through the small intestine and get into the large bowel."

They're just dietary fiber, Fahey says. Which is good. We need more fiber. But, he says, "the bad news is, you have to be very cautious about how much you eat of this stuff." Because once it goes into the large bowel, all the bacteria that live there may start feasting on it. In other words, the allulose might ferment, releasing gas and creating painful intestinal problems.

UPDATE: August 25, 7:30 p.m. — Tate & Lyle tells us that this doesn't happen, at least not very much, with allulose. This particular low-calorie sugar, the company says, "is absorbed by the small intestine and excreted in the urine without being significantly metabolized." According to one study, in which 14 people consumed allulose, more than two-thirds of it was excreted in the urine. The rest presumably went into the large bowel, but according to Tate & Lyle, it did not appear to be very fermentable.)

Our original story continues below.

But this is what happens to lactose — the sugar in milk — when people who are lactose-intolerant consume it. Lacking the lactase enzyme, they can't digest lactose. For them, lactose is, in fact, a low-calorie sugar, but eating it is not pleasant at all.

"When we come up with a carbohydrate that excapes intestinal digestion, you have to worry a lot about what happens when it gets into the large bowel," says Fahey.

The companies that are planning to use allulose in foods have assured the Food and Drug Administration that they won't use a lot of it. Allulose will never replace all the ordinary sugar in food.

Such small amounts of allulose, they say, won't cause any problems.

But there also could be other effects that aren't as well understood. Robert Margolskee, director of the Monell Chemical Senses Center in Philadelphia, says that when a once-rare carbohydrate like allulose enters the food supply, it could affect the living community of microbes in our gut — the so-called microbiome.

"That would be another issue that one would want to know about with any of these compounds," Margolskee says. "What do they do to the microbiome? And then what does that do to the person?"

The effect could be positive. It could also be not so positive.

A few believe that effects on the microbiome could help explain a curious observation. In some studies, people who ate lots of sugar and people who ate lots of low-calorie sweeteners both seemed more prone to obesity and type II diabetes.

There are other theories that may also explain this. Perhaps people who've just consumed low-calorie sweeteners then feel entitled to add some additional high-caloric foods to their plate.

But the bottom line? No free lunch, probably.

Sorry.

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