Washington: Calculating the calorie content of tree nuts like walnuts and almonds based on methods developed over a century ago should be reviewed as they are "inaccurate", according to a top US government researcher.


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Scientists from the US Department of Agriculture (USDA) this month announced that they found that walnuts have 21 per cent fewer calories than previously thought.


Historically, the calorie value for walnuts, almonds and pistachios have been determined using the Atwater factors, which were developed in the late 19th century and calculates metabolisable energy, or energy available to the body, for many foods, Dr David J Baer, Supervisory Research Physiologist at the USDA's Agricultural Research Service, said.


Baer's research found that the metabolisable energy of walnuts was 21 per cent less than that predicted by the Atwater factors.


"This is a third study that we have published investigating the calorie content of tree nuts. We have previously found that for pistachios and almonds, the current approach for calculating the calorie content of nuts (using the Atwater factors) also is inaccurate," Baer said.


He said the discrepancy found by the researchers could be attributed to evolving methods for calculating calories in foods.


Scientists in the 1950s grouped walnuts and other tree nuts with other plant-based foods, such as dry beans, legumes and peas, and estimated that each gram of protein or carbohydrate in those foods contained an average of 4 calories of energy, while each gram of fat contained an average of 9 calories.


Baer said the system can work well for estimating calories in mixed diets involving several foods, but not so well for estimating specific calories for certain foods individually, such as tree nuts. Few studies over the years have focused on how individual foods are absorbed by the body, and little scientific data has been collected on tree nuts, he said.


Baer found previously that the number of calories in almonds was 20 per cent less than the standard used for labeling. For pistachios, it was five per cent less.


At the same time, Baer said further studies are needed, but the differences between the calorie levels found in those study results and what appears on the labels could indicate that people don't completely chew nuts before swallowing them or that it is difficult to fully digest the nut's cell walls.


"We're just beginning to understand how many foods are absorbed by the body, and nuts are one of the foods now attracting increasing interest," he said.


The study on walnut's calorie content was published this month in The Journal of Nutrition.