ARS scientists have found that walnuts contain fewer calories than the amounts now assigned to them by the U.S. Department of Agriculture (USDA).Image courtesy of ARSAgricultural Research Service (ARS) scientists have found that walnuts have fewer calories than previously thought.

David J. Baer and his colleagues in Beltsville, Maryland, placed 18 people on controlled diets and fed them two different diets for 3 weeks, in a random order. One diet contained walnut halves and pieces (42 grams), and the other diet was identical, but without the walnuts. Scientists collected the participants' stools for a week and measured the total amounts of energy, protein and fat they contained. When researchers subtracted out the calories excreted by the participants while they were eating walnuts, they found that a typical 28-gram serving actually contains 146 calories, 21 percent fewer than the 185 calories currently assigned by the U.S. Department of Agriculture (USDA).

The current system for calculating calories generally works well for estimating calories in mixed diets involving several foods, but not so well for estimating calories in certain foods individually, including tree nuts, Baer said. In the 1950s, scientists grouped walnuts and other tree nuts with other plant-based foods, such as dry beans, legumes, and peas, and estimated the calories of energy those foods contained in each gram of protein, carbohydrate, and fat.

The cell walls of nuts and other plant foods need to be broken before the nutrients inside the cells can be digested, but people don't always completely chew nuts before swallowing them. Baer speculates that the differences between his revised calorie estimates and what appears on nut container labels could be because when people eat nuts, they don't fully open the nut's cell walls when they chew them. But few studies have focused on how the human body absorbs energy from individual foods and while tree nuts are now attracting interest from researchers, few scientific studies have focused specifically on digestion of tree nuts until recently.

The study was partially funded by the California Walnut Commission, and the findings are consistent with recent studies showing that eating walnuts and other types of tree nuts, as part of a healthy diet, can improve cardiovascular health and reduce the risk of obesity.

ARS is USDA's chief intramural scientific research agency.