Does Fermentation Affect Calories? The Surprising Truth

November 12, 2025 •

6 min read

Fermentation is a food preservation technique used for millennia, but according to scientific studies, the microorganisms involved consume carbohydrates, which can slightly decrease the overall calorie count. This process involves more than just a reduction in energy; it fundamentally alters the macronutrient profile, leading to complex changes in the final product. So, does fermentation affect calories in your favorite foods like yogurt and sourdough? The answer is a bit more nuanced than a simple yes or no.

Quick Summary

Fermentation can slightly reduce the caloric content of food as microorganisms consume carbohydrates for energy. However, the impact on overall calories is often minimal, with more significant changes occurring in the macronutrient profile, anti-nutrient levels, and nutrient bioavailability. The type of fermentation and food matrix determines the exact change in caloric value.

Key Points

Microbes Consume Carbohydrates: During fermentation, microorganisms consume sugars for energy, which slightly lowers the food's carbohydrate and calorie count.
Caloric Density Can Change: In alcoholic fermentation, the introduction of ethanol can increase caloric density, while lactic acid fermentation tends to result in a minimal reduction.
Minimal Overall Calorie Impact: For most fermented foods, the net effect on total calories is relatively small and not the primary nutritional consideration.
Improved Nutrient Bioavailability: Fermentation increases the body's ability to absorb essential vitamins and minerals by breaking down anti-nutrients like phytic acid.
Altered Macronutrient Profile: The process changes the food's overall macronutrient composition, not just its caloric value, enhancing protein quality and creating new bioactive compounds.
Health Benefits Beyond Calories: The main advantages of fermented foods come from probiotic enrichment and improved digestibility, which contribute to better gut health.
Type of Fermentation Matters: The specific changes to the food's calorie content depend entirely on the type of fermentation, such as lactic acid, alcoholic, or acetic acid.
Consider the Whole Picture: Focusing solely on calorie changes overlooks the complex and beneficial nutritional and functional changes that fermentation brings to food.

The Science Behind Fermentation and Calorie Changes

Fermentation is a metabolic process where microorganisms, such as bacteria, yeasts, and molds, break down organic compounds like carbohydrates in the absence of oxygen. This breakdown, known as anaerobic respiration, produces various byproducts, including organic acids, alcohols, and gases like carbon dioxide. The energy required for the microorganisms' metabolic activity is drawn from the food's substrates, directly impacting its caloric value.

How Microorganisms Consume Energy

During fermentation, microbes use the energy stored in carbohydrates (sugars) to fuel their growth and metabolic processes. This means that the total amount of available carbohydrates decreases over the course of fermentation. For example, during the fermentation of grape juice into wine, yeast consumes the fruit sugars, converting them into alcohol and carbon dioxide. The resulting product has less sugar, but the alcohol itself adds calories, with one gram of alcohol containing about 7 calories. This creates a trade-off where some calories from carbohydrates are lost, but new calories from alcohol are added.

Lactic Acid Fermentation vs. Alcoholic Fermentation

The caloric changes in a food item depend heavily on the type of fermentation occurring. The two most common types are lactic acid fermentation and alcoholic fermentation, and their impact on calories is distinct.

Lactic Acid Fermentation: In this process, microorganisms, primarily lactic acid bacteria, convert sugars into lactic acid. A great example is yogurt, where bacteria ferment the lactose in milk. The breakdown of lactose slightly lowers the total sugar content and, by extension, the caloric value. The final product will have slightly fewer calories from carbohydrates than the unfermented milk, but the change is generally small. This process also occurs in sauerkraut and kimchi, where the fermentation of vegetable sugars results in a slightly lower-calorie finished product.
Alcoholic Fermentation: This process, driven by yeast, converts sugars into ethanol (alcohol) and carbon dioxide. This is how beer, wine, and spirits are made. While the yeast consumes some of the original sugar, the alcohol it produces is calorically dense (7 calories per gram). The total calories in the final product depend on the starting sugar content and the final alcohol percentage. For instance, a beer with a higher alcohol by volume (ABV) will have more calories, even if the residual sugar is low. A portion of the mass is also lost as gaseous carbon dioxide, which contains no calories.

Comparison of Fermented vs. Unfermented Foods

To illustrate how fermentation affects calories and nutrient profiles, consider the differences between common fermented and unfermented foods. The table below outlines some key comparisons, focusing on macronutrient changes.


Feature	Fermented Food Example (Yogurt)	Unfermented Food Example (Milk)	Key Difference
Carbohydrates	Lower due to microbial consumption of lactose.	Higher lactose content.	Microorganisms break down sugars, reducing total carbs.
Proteins	Higher digestibility due to partial protein breakdown.	Intact milk proteins.	Fermentation can enhance protein bioavailability.
Probiotics	Contains beneficial live bacterial cultures.	No live cultures in pasteurized milk.	Fermentation introduces gut-healthy microbes.
Vitamins	Can have increased B vitamins synthesized by microbes.	Standard vitamin content.	Some microorganisms can produce certain vitamins.
Taste	Tangy or sour from lactic acid.	Creamy and sweet from lactose.	New flavors and aromas are developed during fermentation.

What Really Drives Calorie Changes?

The change in calories is not as simple as subtracting the energy the microbes consume. Several interconnected factors influence the final caloric count and nutritional density:

Dry Matter Loss: As microorganisms consume carbohydrates and produce gaseous byproducts like CO2, the overall dry weight of the food can decrease. This can lead to a relative concentration of other nutrients, like protein and fat, in the final product. For example, during the fermentation of vegetables like cucumbers, water content is reduced, concentrating the remaining nutrients.
New Compounds: Fermentation introduces new compounds that can have their own caloric value. As seen with alcoholic fermentation, ethanol is a major caloric component. In other cases, organic acids produced might slightly alter the energy profile.
Digestibility and Bioavailability: The impact on calories is not just about the absolute numbers but also about how the body can utilize them. Fermentation breaks down complex molecules, making some nutrients more readily absorbed. It also reduces anti-nutrients like phytic acid in grains and legumes, which can otherwise bind to minerals and make them unavailable to the body. Therefore, even if the caloric difference is slight, the enhanced digestibility can be a significant health benefit.
Changes in Sugar Profile: Microbes often consume specific sugars, like glucose, preferentially. This can change the ratio of simple sugars to other compounds, affecting the food's taste and potentially its glycemic impact. The breakdown of starches into simpler sugars early in fermentation means that what remains is not the same as what was there originally.

Caloric Impact in Popular Fermented Foods

Kombucha

Kombucha is a fermented tea made using a SCOBY (Symbiotic Culture of Bacteria and Yeast). The yeast and bacteria consume the sugar added to the sweet tea, converting it into various organic acids and a small amount of alcohol. The total calorie count is lower than the original sugary tea, but the amount varies depending on the fermentation time and the initial sugar content. Less sugar means fewer calories, so a longer fermentation generally results in a drier, less sugary kombucha with fewer calories.

Sourdough Bread

The sourdough process involves a long fermentation of flour by wild yeast and lactic acid bacteria. During this time, the microbes consume starches and sugars in the flour. This reduces the overall amount of available carbohydrates and can lower the bread's glycemic index, meaning it causes a slower rise in blood sugar. The final calorie count is slightly reduced compared to a conventional, non-fermented bread, although not enough to make it a low-calorie food.

Yogurt

In the production of yogurt, bacteria ferment the lactose (milk sugar) into lactic acid. This process provides the yogurt's signature tang and slightly lowers its carbohydrate and calorie content compared to the milk it started from. The overall caloric effect is minimal, but the change in sugar profile and the addition of probiotics are significant nutritional benefits.

Conclusion: The Bigger Nutritional Picture

While fermentation does affect calories, the impact is often minor and highly dependent on the type of food and fermentation process. For most fermented foods, the caloric reduction is minimal. However, focusing solely on the calorie count misses the much bigger picture. Fermentation offers significant nutritional benefits that go beyond simple energy changes. It enhances the bioavailability of essential vitamins and minerals, reduces anti-nutrients, and introduces beneficial probiotic cultures that support gut health. Therefore, the main takeaway is not about counting calories but appreciating the transformative power of fermentation on overall nutritional quality.

When considering fermented foods for your diet, think less about the slight calorie change and more about the enhanced nutrient profile, improved digestibility, and probiotic effects that contribute to better gut health. This ancient practice proves that sometimes, the best food science happens on a microbial scale.

Does Fermentation Affect Calories: A Quick Summary

Energy Consumption: Microorganisms consume carbohydrates during fermentation, which slightly reduces the total caloric content of the food.
New Caloric Compounds: In alcoholic fermentation, some calories from carbs are replaced by alcohol, which has its own caloric value.
Minimal Impact: The overall change in calories is usually not significant enough to drastically alter a food's energy density.
Bioavailability: A key effect is improved nutrient digestibility and increased bioavailability of vitamins and minerals.
Diverse Outcomes: The caloric impact varies by fermentation type and food, from the small decrease in yogurt to the caloric complexity of beer.
Enhanced Nutrition: The true benefit of fermentation lies in its ability to enhance a food's nutritional value, not its minor effect on total calories.

Frequently Asked Questions

Fermenting bread dough for a longer period can slightly reduce the total calories by breaking down some of the carbohydrates into simpler compounds. However, the caloric change is typically not significant enough to make sourdough a 'diet' food.

Kombucha generally has significantly fewer calories and less sugar than conventional soda. The fermentation process allows the bacteria and yeast to consume a large portion of the added sugar, leaving a final product that is less sweet and less calorically dense.

Fermentation doesn't necessarily increase the fat content. In some cases, as microorganisms break down carbohydrates, the total dry weight decreases, leading to a relative concentration of other macronutrients, including fats. However, the absolute amount of fat does not increase through fermentation alone.

Yes, if the fermentation process produces alcohol, it can increase the calorie count of a beverage. For example, in beer and wine production, the conversion of sugar to alcohol adds calories, as alcohol is calorically dense (7 calories per gram).

Fermented foods are often recommended for weight management due to other benefits, not a significant calorie reduction. They can improve gut health, enhance metabolism, and increase satiety, which can support weight loss efforts.

Yes, fermentation can lower the glycemic index of some foods, particularly sourdough bread. Organic acids produced during the process can slow down gastric emptying and starch hydrolysis, leading to a smaller spike in blood glucose.

Yes, the microorganisms (probiotics) in fermented foods consume some of the food's carbohydrates and sugars as their energy source during the fermentation process itself. This is the primary reason for the slight caloric reduction in the final product.