Is this accurate? We're not quite sure.

We have one standard metric in this country for the amount of energy in a given quantity of food: the calorie. Just about every food you can find in the grocery store, aside from fresh produce, comes branded with a calorie count. The new health-care law will soon require chain restaurants to post the caloric content of standard menu items.

There's just one problem: The methodology for determining caloric content, developed about a century ago, may not be all that accurate. That is what scientists are learning as they try to answer what seems like a pretty simple question: How many calories does an almond have?

Usually, food scientists do this in a fairly no-nonsense way: They light food on fire, and see how much energy is produced as a result (measured by how hot the water around the aforementioned burning food gets). You can play around with a virtual bomb calorimeter -- the apparent device of choice for burning foods -- here, and see the energy output for some hamburgers and tomatoes.

Using that system, scientists have assigned a caloric value for every gram of protein (4 calories), carbohydrate (4 calories) and fat (9 calories).

We now use these calorie measurements pretty much everywhere. But researchers have had a lot of trouble actually confirming that these coefficients are accurate.

"During this past century, there have been few, if any, studies reporting on the energy value of a whole food within a mixed diet that could confirm the accuracy" of the coefficients, a team of USDA researchers write in this month's Journal of American Clinical Nutrition.

They decided to try out a new method of measuring caloric content, using almonds as their guinea pig. They ran a controlled study in which some people ate almonds, and others didn't. Then they measured how much fat was exiting the participants' bodies -- you can figure out where they turned to get that metric.

What they found, as described by study author David Bear: "When people are consuming nuts, the amount of fat in the feces goes up. And that suggests that we're not absorbing all the fat or calories that's in the nut."

In other words, there's might be a whole lot of fat in almonds that shows up in a bomb calorimeter, but a good amount of it never gets absorbed by the body. As a result, the researchers concluded that almonds actually have 20 percent fewer calories than we currently think.

This study applied only to almonds, although other research has suggested that different nuts -- including the pistachio -- could also have fewer calories than we think, for much the same reason.

Unsurprisingly, the almond lobby was encouraged by these results. The Almond Board of California says it is "now working with government agencies to determine what these study results may mean for future consumer education about almonds."

(h/t: The Salt)