According to the FDA, a product can be labeled calorie free if it has fewer than 5 calories per serving. It can be labeled low calorie if it has 40 calories or less per serving or per 50g of the food.
A product can be labeled fat free if it has less than 0.5g of fat per serving. It can be labeled as saturated fat free if it has less than 0.5g of saturated fat per serving and the level of trans-fatty acids does not exceed 1% of total fat.
Similar claims follow a similar rule (e.g. sodium free).
So a food can still be labeled "calorie free" even if in actuality it contains 4.999999999 calories per serving. Same goes with fat free products if it has 0.49999999g of fat per serving.
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