Nutrient labels on food products in the United States list their percentages based on a 2000 calorie per day diet. However, energy requirements can vary significantly based on age, sex, weight, height, physical activity, and base metabolic rate, ranging between 1000 to 4000 calories.
Another number on labels is the portion, or suggested serving size,
which is also arbitrary and should be looked at closely. A package
stating 90 calories on the front might show 90 calories per serving
with three servings, so 270 total calories.
The Merck Manual states 1,600 calories per day are needed for young
children and sedentary women; 2,000 for active adult women and
sedentary men; and 2,400 for active adolescent boys and young men.
These values are also a bit arbitrary, because the average person
uses different amounts of energy almost daily. One day you might go
for a hike and the next sit on the couch watching football. One
month you could lose 5 pounds and increase your base metabolic rate
in the process. The next month with the same caloric intake gain 10
pounds and decrease your base metabolic rate. Bottom line, the
numbers are arbitrary and should be used with caution.