There're two reasons you need to eat; to gain materials (nutrients: protein, vitamins, minerals, etc.) your body is built from and needs to function, and for energy (calories: carbohydrates, fats...) to power your metabolism and activity. Kinda like a car. You build one from steel, aluminum, copper, plastic, silicon, rubber, but it doesn't go without gas (or electricity). Just about all the foods you eat are some combination of both material and energy but vary in the relative amounts. Some have a high energy content but little in the way of nutrients. The original intent of trail mix was to provide tasty and convenient calories during energy intensive hiking, so has energy dense constituents. Nuts and dried fruit (particularly without added sugar) are "healthy" in that they provide nutrients along with calories, but are energy dense, so are fine for small snacks...or while engaging in strenuous activities....