Calorie absorption: New model calculates how much we take in through the gut

A new mathematical model reveals that calories consumed are far more complicated than what is written on food labels. When food passes through the living microbial ecosystem in our guts, it may influence how many of those calories are actually absorbed. 

The digestion, absorption, and microbial metabolism (DAMM) model follows food as it travels through the digestive system, estimating what is absorbed directly or reaches the colon. It also observes how gut microbes help process the remaining foods that are either absorbed or excreted.

The researchers say the DAMM model may help researchers to gain a more comprehensive understanding of metabolic diseases, obesity, and diabetes by showing how diet impacts the human body and the colon’s microbial community.

“Digestion is not just a human process — it is a collaboration between our bodies and trillions of microbes living in the gut,” says study co-author Rosa Krajmalnik-Brown, director of the Biodesign Center for Health Through Microbiomes and professor at the School of Sustainable Engineering and the Built Environment at Arizona State University (ASU), US. 

“DAMM gives us a powerful new way to quantify how those microbial partners contribute to human health and energy balance, and also points to the importance of properly feeding our gut microbes.” 

High vs. low fiber

The report, published in Public Library of Science One, builds on a controlled diet study that examined how the gut microbiome affects energy balance. 

The Western diet group consumed an average of 116 more calories daily compared to the high-fiber group.It included healthy adults who consumed two diets: a microbiome-enhanced one rich in fiber and resistant starch, or a Western diet, which is typically low in those components and has more processed foods.

The Western diet group consumed an average of 116 more calories daily compared to the high-fiber group. 

“What is truly unique about the DAMM model is that it quantitatively links human metabolism to the metabolism of the microorganisms in the colon in a way that matches the results from the clinical study and provides fundamental insight into how the microbial community works in partnership with the human host,” says Bruce Rittmann, who directs the Biodesign Swette Center for Environmental Biotechnology and is a regents professor of environmental engineering at ASU. 

The authors note that for over a century, scientists have relied upon Atwater parameters to estimate the energy people get from food, measured in calories. It multiplies the amount of protein, carbohydrates, and fat in the food by the average metabolized calories per gram of each.

While being a simple system, it does not capture the microbial side of digestion. This includes how different diets feed gut microbes or how the microbes produce compounds such as short-chain fatty acids (SCFAs) from fiber and other undigested foods in the colon.

Splitting nutrients

The DAMM model “splits up” the nutrients making up the food’s protein, carbohydrate, and fat content. Next, it estimates how much usable energy of those components is absorbed in the upper digestive tract and then follows the food into the colon.

The DAMM model “splits up” the nutrients making up the food’s protein, carbohydrate, and fat content.That is where the gut microbes break down the remaining components from the remaining food that “escaped” — not previously digested. In this process, SCFAs are produced, which may be absorbed through the colon and then used by the body as additional calories. The authors note that the DAMM model also takes methane production by certain microbes — methanogens — into account.

The model estimated that the SCFAs absorbed from the colon accounted for 140 calories daily, approximately 7.4% or total usable energy. Meanwhile, 85% of usable energy came from the upper digestive tract and 15% from the lower.

The DAMM method has been shown to be closer to estimating how many calories people absorb from food compared to the Atwater approach, which tended to underestimate absorbed calories.

It was also able to more accurately show differences between a high and low fiber diet. The diet high in fiber provided more fermentable material to the colon, allowing microbes to convert it into SCFAs.

The high-fiber diet also resulted in fewer calories absorbed, even when microbial activity and SCFA production were equal.

“The DAMM model is more than just a tool for characterizing diet,” says first author Taylor Davis, an ASU graduate research assistant. “It’s a framework designed to evolve. As we discover more on how diet, metabolism, and the microbes interact, the new insights can be incorporated into the model, allowing it to grow with us as we learn.”