Does Fermentation Change Calories? The Surprising Answer

January 3, 2026 •

4 min read

According to a review published in the journal Annals of Microbiology, fermentation generally decreases the carbohydrate content of foods like cereals, which directly influences their final caloric value. The metabolic activity of microorganisms means that yes, fermentation does change calories, but the overall effect can be complex and is often smaller than many people assume.

Quick Summary

Fermentation causes microorganisms to consume carbohydrates, leading to a slight reduction in calories. The metabolic process converts sugars into acids, alcohol, and gas, altering the final energy content. The total calorie change varies widely depending on the food and specific microbes used.

Key Points

Slight Calorie Reduction: Microorganisms consume carbohydrates during fermentation, which slightly lowers the final calorie count.
Carbohydrate Conversion: Yeasts and bacteria transform sugars and starches into organic acids, alcohol, and carbon dioxide, altering the energy content.
Improved Digestibility: While fat and protein calories don't change much, fermentation can enhance their bioavailability and digestibility.
Varies by Food: The extent of calorie change depends on the food's composition, the type of microbe used, and the fermentation time.
Nutritional Benefits: The biggest benefit of fermentation is not calorie reduction but the creation of probiotics and enhanced nutrient profiles.
Complex Analysis: Calculating calories in fermented foods is complex; simple theoretical calculations based on raw ingredients are not accurate.

The Core Science: Why Fermentation Alters Caloric Content

Fermentation is a metabolic process in which microorganisms like bacteria and yeast convert carbohydrates (sugars and starches) into organic acids, gases, or alcohol. These microbes use the carbohydrates as their primary energy source to survive and multiply. When they consume these caloric-rich compounds, they transform them into new molecules that may or may not be as calorie-dense. For example, in yeast fermentation, glucose is converted to ethanol and carbon dioxide. The CO2 is a gas that escapes, taking some mass and potential energy with it. The ethanol also has a different caloric density than the original sugars.

The Microorganism's Caloric Consumption

Consider the simple chemical reaction for glucose fermentation: one molecule of glucose ($C6H{12}O_6$) is converted into two molecules of ethanol ($C_2H_5OH$) and two molecules of carbon dioxide ($CO_2$). Microbes perform this conversion to generate a small amount of ATP (cellular energy) for themselves. This consumption means the finished product will have less total energy than the sum of its raw ingredients. While the energy conversion is inefficient compared to full aerobic respiration, it still results in a measurable calorie reduction.

What Happens to Macronutrients?

While fermentation primarily targets carbohydrates, it can also have secondary effects on other macronutrients:

Carbohydrates: This is where the most significant changes occur. Sugars (like lactose in milk or sucrose in tea) are consumed, and complex starches are broken down, leading to a reduced carbohydrate count.
Fats and Proteins: The caloric content of fats and proteins generally remains stable. However, fermentation can increase the digestibility and bioavailability of proteins by breaking them into smaller, more easily absorbed amino acids and peptides. This improved digestibility, while not changing the raw calorie count, can affect how the body absorbs and uses the nutrients.
Bioactive Compounds: Fermentation can increase the levels of other beneficial compounds, such as certain vitamins (especially B vitamins), antioxidants, and bioactive peptides, which are not related to calorie counts but enhance overall nutritional value.

Fermentation's Impact on Common Foods

Yogurt and Kefir: Lactic acid bacteria ferment lactose (milk sugar) into lactic acid. This process reduces the overall sugar content, resulting in a slightly lower calorie count than the milk it was made from. For low-fat or non-fat yogurt, the change is primarily due to the sugar conversion, but for full-fat dairy, the fat content remains the main calorie source.

Sourdough Bread: The wild yeasts and bacteria in a sourdough starter ferment the flour's starches and sugars. This reduces the fermentable carbohydrate content of the dough. Some of the alcohol produced during fermentation evaporates during baking, further reducing the final energy content. The resulting bread may also be easier to digest due to the pre-digestion of starches.

Beer: In brewing, yeast ferments the sugars from grains into alcohol and CO2. Both the CO2 and the yeast biomass are removed, resulting in a product with fewer calories from carbohydrates. While alcohol itself contains calories (7 kcal per gram), the total calorie count is often less than what would be in the unfermented sugary mash.

Kombucha: This fermented tea relies on a SCOBY (Symbiotic Culture of Bacteria and Yeast) to ferment sugar. The microbes convert the sugar into organic acids and a small amount of alcohol. This significantly reduces the total sugar content, making kombucha less caloric than the sweetened tea it started as.

Sauerkraut and Kimchi: Lactic acid fermentation of vegetables consumes sugars to produce lactic acid. The calorie change is usually minor, but some studies have observed slight fluctuations, sometimes even an increase in caloric density due to water loss and concentration of other components. The major benefit is the creation of probiotics and other beneficial compounds.

Comparing Calorie Estimates: Raw vs. Fermented Foods


Food Type	Raw Calorie Impact	Fermented Calorie Impact	Notes
Dairy (Milk)	Rich in lactose sugar.	Lowered due to lactose conversion to lactic acid.	Calorie reduction is primarily from carb reduction.
Grains (Dough)	High in starches and sugars.	Slightly lower due to yeast consuming carbs.	Alcohol also produced; some evaporates during baking.
Vegetables (Cabbage)	Very low in sugar.	Can remain similar or slightly fluctuate.	Minor sugar conversion; main change is in nutrient profile.
Sweet Tea	High in added sugars.	Significant reduction as sugar is fermented.	Fermented into kombucha; sugar is converted to acids.
Legumes (Soybeans)	Moderate protein/fat/carb.	Bioavailability and digestibility increase.	Calorie change is minimal; nutritional quality is enhanced.

Key Factors Influencing Calorie Changes

The Food Substrate: The starting food's composition dictates what's available for microbes to consume. A sugary drink will have a much larger calorie change than a low-carb vegetable.
Microbial Strain: Different strains of bacteria and yeast have varying metabolic pathways and efficiencies. This can influence the type and amount of end products formed, affecting the final calorie count.
Fermentation Duration: The longer the fermentation process, the more carbohydrates the microbes will consume, generally leading to a greater reduction in total calories.
Aerobic vs. Anaerobic Conditions: Oxygen presence or absence can change the metabolic pathways used by microbes, altering the end products and energy conversion efficiency.
Processing After Fermentation: For foods like bread, baking affects the final calorie count by evaporating alcohol. For raw ferments like kimchi, no further processing occurs, so the change is final.

Conclusion: Fermentation and the Bigger Nutritional Picture

While it is true that fermentation does change calories, the change is often a small reduction, not a dramatic one. The primary takeaway for nutrition is not the slight shift in caloric count, but the profound shift in nutritional quality. By breaking down carbohydrates, improving nutrient bioavailability, and producing probiotics, fermented foods offer significant health benefits that are much more impactful than a marginal calorie difference. Therefore, incorporating fermented foods into your diet should be viewed as a way to enhance your overall nutritional intake and gut health, rather than a strategy for significant calorie reduction. For more on the nutritional impacts of food processing, readers can consult resources like this USDA Agricultural Research Service Study.

Frequently Asked Questions

Not necessarily. While most fermented foods show a slight calorie reduction due to carbohydrate consumption, the change can be minimal. In some rare cases, like certain concentrated vegetable ferments, the caloric density might slightly increase.

This is unlikely for most foods. However, the concentration of other components during fermentation, paired with water loss, could theoretically result in a higher caloric density per gram in some products, though this is not a significant effect.

The calorie change in yogurt is relatively minor and primarily comes from the bacteria consuming the milk's lactose. The final calorie count is more dependent on the initial milk fat content and any added sugars.

Yes, sourdough bread typically has a slightly lower caloric content than regular bread made with the same ingredients. This is because the wild yeast and bacteria in the starter consume some of the carbohydrates in the flour during the longer fermentation process.

Yes, for fermented doughs, baking cooks off the alcohol produced during fermentation, which contributes to the final calorie count. The heat also evaporates some water, concentrating the remaining nutrients.

Yes, a significant amount of the sugar in the sweet tea is consumed by the SCOBY during fermentation. The longer the fermentation time, the more sugar is consumed, resulting in a less sweet, lower-calorie kombucha.

The calorie count changes because the microorganisms used for fermentation—like bacteria and yeast—metabolize carbohydrates for their own energy. They convert these sugars and starches into other compounds like lactic acid, alcohol, and CO2, altering the food's overall energy content.