It can be tricky to determine the actual calorie content of foods. Most calculations are done using the Atwater system, developed over 100 years ago, which showed that carbs and protein have 4 calories and fat has 9 calories in 1 gram. You can think of a gram as about the weight of an average paper clip. Comparing the Atwater system to human digestibility experiments, a number of research studies have shown that humans do not digest all the fat calories in nuts.
A recent research study showed that one ounce of walnuts has 146 calories rather than the 185 calories calculated by the Atwater approach, 21% fewer calories than listed in most calorie databases.
Both walnuts and almonds have gotten skinnier and it is likely that further research will show that this calorie reduction will hold true across all nut varieties. That is why studies have continuously shown that people who regularly eat a reasonable serving of nuts are consistently thinner than people who rarely eat nuts. A study which followed over 11,000 adults showed that those who ate 1 ounce of any type of nuts daily were thinner and had a lower risk for obesity.
