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WEST LAFAYETTE, Ind. – “Here are the nuts,” said Drew Sayer, a graduate student in nutrition science, before shoving me into the MRI machine, flat on my back. “Chew them. Swallow them. And don’t move your head.”

I moved my head, which blurred the resulting images. But if all goes well in the coming weeks, researchers at Purdue University will have stacks of brain scans with crystal-clear views inside the minds of their test subjects – while they were eating nuts. These images could help answer a timely question: Do nuts really merit the hype they’ve been getting as a guilt-free indulgence?

The reports about their many benefits have come thick and fast: studies finding that people who eat nuts (tree nuts like cashews, almonds and pistachios, along with their legume pal, the peanut) live longer and healthier lives, with less risk of chronic ailments like heart disease, respiratory problems and Type 2 diabetes.

But perhaps the most startling news is that nuts may help in maintaining a healthy weight. Research has found that people can snack on modest amounts of them without gaining pounds, and that nuts can even help in slimming down.

This dieting power is particularly hard to fathom when you consider that nuts pack 160 to 200 calories in each tiny ounce, not even a handful. And most of those calories come from fat. Ounce for ounce, cashews and pecans and walnuts are loaded with more calories than many of the processed foods being blamed for the surge in obesity. In the conventional wisdom, a dieter’s best friends are watery foods like celery and carrot sticks.

One of the country’s leading nutrition scientists, Richard Mattes of Purdue, has been exploring this seeming paradox and has some intriguing, if still uncertain, findings.

Nuts have several big things going for them, Mattes said. For starters, even a small amount can make you feel full. Scientists call this feeling satiety; it is a busy field in food research and marketing these days, given the way that snacking has become a sort of fourth meal, adding an estimated 580 calories to the average person’s daily consumption.

Why do nuts appease the appetite so well? Mattes pointed to several studies.

“They’re high in protein, and protein is satiating,” he said. “They’re high in fiber, and fiber is satiating. They’re rich in unsaturated fats, and there is some literature that suggests that has satiety value. They’re crunchy, and that would suggest just the mechanical aspect of chewing generates a satiety signal.”

Snacking on nuts makes it likely that you will eat less later in the same day, according to some research. That decrease in consumption can make up for many of the nuts’ calories – as much as three-fourths of them, studies have shown.

Nuts are also resistant to digestion, thanks to the tough walls of their cells. As much as one-fifth of the fat in nuts never gets absorbed by the body, Mattes estimated in a 2008 paper published by the Journal of Nutrition. He noted some weaker evidence that nuts may cause people to burn a little more energy as they simply sit around.

Even the fat that the body absorbs from nuts tends to be virtuous. It’s mostly unsaturated fat, with lesser amounts of the saturated type whose excessive consumption has been associated with heart disease.

But even the best foods come with caveats, and nuts have several to consider. Their high fat content generates the powerful allure that food scientists call mouth feel, making it tempting to wolf down a lot of them. Two cups of mixed nuts can pack 1,600 calories.

“When I travel, I take nuts,” said Barbara Rolls, director of the Laboratory for the Study of Human Ingestive Behavior at Penn State and the author of “The Ultimate Volumetrics Diet.” “They’re compact and energy-dense, great when you’re stuck on a runway for hours. But I do find them easy to overeat.”

Also, diets commonly fail (and quickly) when people become bored, or feel deprived, and turn back to less-nutritious foods. Can nuts even come close to holding their own against the Oreo?

This is where Mattes’ current study comes in. He is trying to determine whether nuts have staying power as a substitute for other snacks. Will they remain appealing if people eat them every day, or be tossed aside in favor of chips or cookies?

The 20 volunteers will be asked to eat a handful of almonds every day for 12 weeks. Their brains will be scanned at the start, midway through and at the end of the trial, to see if their pleasurable response changes over time.

The more pleasure they get, the more neurological activity will appear in the insular cortex, amygdala, dorsal striatum and other parts of the brain known as the reward centers because they send signals of pleasure when we do things that keep us, or our species, going, like eating and having sex. (The scans, known as functional MRIs, don’t measure neural activity, but rather changes in blood flow, which is a good indicator.)

To quote the old candy-bar slogan, sometimes you feel like a nut; sometimes you don’t.

“It’s been our experience that some people like them less and less over time,” Mattes said. “And some learn to like them more and more, so that they get unhappy when the trial is over and their free supply gets cut off.”