Products that claim to be calorie-free may actually contain a small amount of calories, according to the Food and Drug Administration. To qualify as a zero-calorie food or beverage, a product must provide 5 calories or fewer per serving.
Diet Coke is produced by the Coca-Cola Company. An 8 oz. serving provides zero calories, according to the company's website. One serving also provides zero grams each of fat, carbohydrates and protein and contains 40mg of sodium.
Ingredients in Diet Coke include carbonated water, caramel color, aspartame, phosphoric acid, potassium benzoate, natural flavors, citric acid and caffeine. Diet Coke also contains phenylalanine.
The Mayo Clinic says that the artificial sweetener aspartame is the source of phenylalanine in diet soda, and that this ingredients also occurs naturally in foods that are high in protein. Although consuming phenylalanine is safe for most people, individuals who have the rare condition phenylketonuria, or PKU, should avoid diet sodas.