September 20, 2013

Scientists Improve Calorie Estimation Method

U.S. Department of Agriculture (USDA) scientists have improved the method for estimating calories in tree nuts, showing that there are fewer calories in pistachios and almonds than previously thought. The modified method should also work well for other foods, according to the scientists with the Agricultural Research Service (ARS), USDA's in-house research arm.

Chewing begins the digestive process of liberating nutrients from food. This process is necessary before nutrients are considered "bioaccessible." In theory, the fat within some hard foods is not completely absorbed because it's difficult to digest the food's cell walls, which contain the fat.

Physiologists David Baer and Janet Novotny at the ARS Beltsville Human Nutrition Research Center in Beltsville, Md., fed 16 healthy adults pistachios at three different levels: none, 1.5 ounces per day, and three ounces per day, along with a background or "base" nut-free diet. The volunteers ate each pistachio level for 18 days. Researchers collected and analyzed urine and stool samples from all feeding periods. This analysis consisted of measuring calories in the foods that were fed to volunteers (energy in) and measuring the same foods' excreted remains (energy out).

Novotny, also a mathematician, wrote a series of algebraic equations to evaluate data from the biological samples and to isolate and measure the calories specifically supplied by the pistachios separately from the base diet consumed. “The base diet always consisted of the same foods and composition, thus allowing us to tease out the caloric value of the single target food,” says Novotny.

The study suggests that the caloric value of pistachios has likely been overestimated by about 5 percent, because the fat from the nuts wasn't completely absorbed by the intestinal tract. The study, published in the British Journal of Nutrition in 2011, was supported by USDA and Paramount Farms, Inc., Los Angeles, with an improved method and reported in the September 2013 issue of Agricultural Researchmagazine.

Baer and Novotny conducted a similar human clinical trial in which they fed the volunteers three different levels of whole almonds as part of a carefully controlled diet for an 18-day period. According to food labels, almonds provide 168 calories per 1-ounce serving, but the researchers found that the same serving actually provided 129 calories when computed by the modified method. This difference was due in large part to loss of undigested fat, protein, and carbohydrate in the stools, according to the authors.

The study showed that it is possible that the total number of available calories from certain whole nuts, and perhaps other similar foods, may be lower than originally estimated. The study, published in the American Journal of Clinical Nutrition in 2012, was supported by USDA and Almond Board of California.

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