Citrus fruits, like many other kinds of fruits, contain citric acid. This is part of the reason they taste sour or tart. The citric acid in citrus technically contains calories, but since you take up so little of it into the cells, it has a negligible effect.
Citric Acid
Citric acid is a small molecule made up of carbon, hydrogen and oxygen. Because it's acidic, it tastes sour. The sour flavor receptors on your tongue actually respond to acid. Unlike many acids, however, citric acid is safe to eat. For this reason, it's a common additive in processed foods, both to add tartness and as a preservative. It's also common in nature, however, and occurs in many fruits, including the citrus fruits for which it's named.
Quantities
The amount of citric acid in citrus fruit varies by the type of fruit, and within type, the cultivar. Generally speaking, the more sour citrus fruits contain more citric acid per mass of fruit, and larger fruits contain more total citric acid than smaller fruits. According to Long Island University, oranges contain 1.5 g of citric acid per fruit, while grapefruits contain 5.4 g per fruit. Lemons, despite their small size, contain 3 g per fruit, and limes contain 1.8 g per fruit.
Importance
The fact that citrus fruits contain citric acid is of minimal nutritional significance. According to the Alcohol Tobacco Tax and Trade Bureau in a publication titled "Calories in Flavored Malt Beverages," citric acid contains 2.5 calories per gram. This, however, is a measure of calories per gram that your cells take up, and you don't absorb much of the citric acid you consume into the cells.
Filtration
The reason you absorb so little of the citric acid you eat into the cells is that while you can take it up into the bloodstream, it's water-soluble, according to a 1999 article published in the scientific journal "Seminars in Nephrology." You filter most of it from the bloodstream into the urine. As such, while citric acid contributes to the flavor of citrus fruit, it affects the nutritional value of the fruit to an insignificant degree.
References
- Long Island University: Citrus
- "Calories in Flavored Malt Beverages"; Alcohol Tobacco Tax and Trade Bureau; 2004
- "Seminars in Nephrology"; Citrate Transport by the Kidney and Intestine; A. Pajor; 1999
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