Food labels make calories seem simple. They show the number of calories per serving, which is calculated based on how much fat, carbohydrates and protein the food contains.
But inside the body, digestion is far more complicated. Food passes through a living microbial ecosystem that can influence how many of those calories people actually absorb.
A new mathematical model developed by Arizona State University researchers takes a closer look at that hidden part of digestion. The model, called DAMM — for digestion, absorption and microbial metabolism — follows food through the digestive tract, estimating what the body absorbs directly, what reaches the colon and how gut microbes help process the remaining material into products that are either absorbed or excreted.
The model could eventually help researchers better understand obesity, diabetes and other metabolic disorders by showing how different diets affect both the human body and the microbial community inside the colon.
With more research, it could become a tool to help health care providers design more personalized diets for their patients.
"Digestion is not just a human process — it is a collaboration between our bodies and trillions of microbes living in the gut," said Professor Rosa Krajmalnik-Brown . "DAMM gives us a powerful new way to quantify how those microbial partners contribute to human health and energy balance, and also point at the importance of properly feeding our gut microbes."
Krajmalnik-Brown directs the Biodesign Center for Health Through Microbiomes . She is also professor with the School of Sustainable Engineering and the Built Environment at ASU.
She is joined by ASU colleagues, including Professor Bruce Rittmann , who directs the Biodesign Swette Center for Environmental Biotechnology and is a Regents Professor of environmental engineering at ASU, and first author Taylor Davis, an ASU graduate research assistant. The work was carried out in collaboration with AdventHealth Translational Research Institute in Orlando, Florida.
The research appears in the current issue of the journal PLOS One.
Beyond the calorie count
For more than a century, scientists have relied on Atwater parameters to estimate the energy people get from food, measured in calories. The method multiplies the amount of protein, carbohydrates and fat in the food by the average metabolizabled calories per gram of each.
The system is simple and useful, but it does not capture the microbial side of digestion, including how different diets feed gut microbes or how those microbes produce compounds such as short-chain fatty acids from fiber and other undigested food in the colon.
The new research builds on a controlled diet study that examined how the gut microbiome affects human energy balance. The gut microbiome is the vast community of bacteria and other microbes living in the digestive tract.
In the study, healthy adults consumed two carefully designed diets: a microbiome-enhancer diet rich in fiber and resistant starch (less processed foods and foods with larger particle size) versus a more typical Western diet lower in those components (more processed foods and smaller particle size). People eating the Western diet absorbed about 116 more calories per day than the people eating the high-fiber diet. Yet the high-fiber group did not feel hungrier.
"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," Rittmann said.
Tracking hidden energy
DAMM starts by splitting a diet into the nutrients that make up the protein, carbohydrates and fat; then, it estimates how much usable energy of those components is absorbed in the upper digestive tract.
Next, it follows the material into the colon, where gut microbes break down the remaining food components that escaped earlier digestion. In the process, they produce short-chain fatty acids, which can be absorbed through the colon and used by the body as additional calories. The model also accounts for methane production by certain microbes known as methanogens.
That microbial contribution is meaningful. The model estimated that short-chain fatty acids absorbed from the colon contributed an average of about 140 calories per day, or roughly 7.4% of total usable energy. About 85% of usable energy came from the upper gastrointestinal tract, while about 15% came from the lower gastrointestinal tract, where microbial activity plays a central role.
When researchers tested DAMM against results from the controlled diet study, it came closer than the standard Atwater approach to estimating how many calories people actually absorbed from food. The standard method tended to underestimate absorbed calories, while DAMM produced estimates that more closely matched the study measurements.
A diet-sensitive model
The model also captured meaningful differences between the high- and low-fiber diets. The microbiome-enhancer diet delivered more fermentable material to the colon, where microbes could convert it into short-chain fatty acids.
DAMM predicted higher short-chain fatty acid production on this diet, mirroring the general pattern seen in the clinical trial, where higher levels of short-chain fatty acids were observed in serum and fecal measurements.
Short-chain fatty acids are more than byproducts of digestion. They are made when gut microbes ferment fiber and other food components that were not absorbed earlier in digestion, and some of the fatty acids can be absorbed by the body and used as calories. But this is only one part of the overall calorie balance. The microbiome-enhancer diet led to fewer net calories absorbed overall, even as it increased microbial activity and short-chain fatty acid production.
DAMM helps researchers separate these pieces of the process, showing what the body absorbs directly, what microbes produce and what is ultimately absorbed or excreted.
"The DAMM model is more than just a tool for characterizing diet," Davis said. "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."
