A food or drink can be labeled “0 calories” because it either contains so little energy that your body can’t meaningfully use it, or because labeling rules allow rounding down. Under FDA regulations, any food with fewer than 5 calories per serving can legally be listed as zero on the label. That single rule explains most of what you see on store shelves, from diet sodas to cooking sprays to certain condiments.
But behind that labeling threshold, there are real biological reasons why some ingredients deliver sweetness, texture, or bulk without providing energy. The mechanisms vary, and they’re worth understanding.
The Labeling Rule That Makes Zero Possible
The FDA requires calorie counts to be rounded to the nearest 5-calorie increment for foods containing up to 50 calories per serving. Anything below 5 calories per serving can be expressed as zero. This means a product could contain 4.9 calories per serving and still show “0” on the Nutrition Facts panel. A cooking spray, for instance, sets its serving size at a fraction-of-a-second burst. That tiny amount of oil does contain calories, but the serving is small enough to slip under the 5-calorie cutoff.
The European Union uses a different system. A food can only be called “energy-free” if it contains no more than 4 kilocalories per 100 milliliters. For tabletop sweeteners, the limit is 0.4 kilocalories per portion that provides the same sweetness as a teaspoon of sugar. Either way, “zero calories” almost never means literally zero molecules of energy. It means the amount is too small to matter nutritionally.
Your Body Can’t Break Down What It Can’t Digest
Calories come from your body breaking chemical bonds in food and capturing the released energy. If your digestive system can’t break those bonds, the food passes through you without delivering calories, no matter how much of it you eat. This is the core principle behind most truly low-calorie ingredients.
Dietary fiber is the most familiar example. Humans lack the enzymes to break apart the bonds holding fiber together, so most of it travels through the stomach and intestines undigested. Konjac-based products like shirataki noodles are almost entirely made of a soluble fiber called glucomannan. Your gut bacteria can ferment a small fraction of soluble fiber into short-chain fatty acids (which do provide a tiny amount of energy), but the vast majority exits your body intact.
The same logic applies to certain engineered fats. Olestra, a fat substitute used in some snack chips, is built from sucrose bonded to fatty acids in a configuration too large for digestive enzymes to dismantle. It behaves like fat in cooking and in your mouth, but it passes through the digestive tract without being absorbed. Zero calories from fat, in a product that tastes fatty.
How Sweeteners Deliver Taste Without Energy
High-intensity sweeteners work by being hundreds of times sweeter than sugar, so you need almost none of them. Sucralose is roughly 600 times sweeter than table sugar. Aspartame is about 200 times sweeter. Saccharin ranges from 200 to 700 times sweeter, depending on the formulation. When a can of diet soda needs only a few milligrams of sweetener to taste as sweet as one made with 39 grams of sugar, the caloric contribution is negligible.
Stevia-based sweeteners take a slightly different path. The sweet compounds in stevia leaves, called steviol glycosides, aren’t broken down by any digestive enzymes in the upper gastrointestinal tract. They arrive in the colon fully intact, where gut bacteria partially break them down. The byproducts are absorbed and quickly cleared through urine. The net caloric contribution is effectively zero.
Sugar Alcohols and Novel Sugars
Not every zero-calorie sweetener is synthetic or plant-derived. Some are chemically similar to sugar but behave differently in your body. Erythritol is a sugar alcohol that tastes mildly sweet and is found naturally in some fruits. Unlike regular sugar, which provides 4 calories per gram, erythritol provides zero. Your small intestine absorbs it rapidly, but instead of metabolizing it for energy, your body routes it to the kidneys and excretes it in urine within about 24 hours. It never gets converted into usable fuel.
Allulose is a newer option that sits in a different category. It’s a rare sugar that exists in small quantities in figs, raisins, and maple syrup. It tastes and behaves like sugar in recipes, but your body processes very little of it. The FDA recognizes allulose at 0.4 calories per gram, one-tenth the energy of regular sugar. That’s low enough that many products made with allulose can round down to zero calories per serving, depending on how much is used.
Why “Negative Calorie” Foods Don’t Exist
You’ve probably heard the claim that celery burns more calories to digest than it contains. It’s an appealing idea, but research consistently shows it’s not true. Digesting and processing any meal costs your body roughly 5 to 15 percent of that meal’s energy. Even for extremely low-calorie, high-fiber foods like celery, the math doesn’t work out to a net loss.
Researchers tested this directly using celery as the food source. After accounting for energy lost through waste and the cost of digestion, the subjects still retained about 24 percent of the meal’s energy. When the same calculations were applied to nine other foods commonly called “negative calorie,” each one still provided a net energy gain of roughly 64 percent of the calories consumed. Even in the most generous scenario, where all fiber energy is assumed lost, these foods still deliver about 49 percent of their caloric content.
That doesn’t mean these foods are unhelpful for weight management. A stick of celery has about 6 calories. Eating it takes time, provides fiber, and contributes to fullness. The more accurate framing is that these are “negative budget” foods: eating them makes it easier to end the day with fewer total calories than your body burned. But they are not literally zero or below zero in energy.
Putting It All Together
Zero-calorie labels come from three overlapping realities. First, labeling regulations allow rounding down when a serving contains fewer than 5 calories. Second, some ingredients are structured in ways your digestive system simply can’t unlock for energy, so they pass through you unused. Third, certain sweeteners are so intensely sweet that the amount needed to flavor a food is calorically meaningless. Most zero-calorie products rely on a combination of these factors: a tiny serving size, an ingredient your body can’t metabolize, and a regulatory framework that rounds the remainder to zero.