Overweight and with a metabolism out of whack? Here’s the mathematical model for a personalised diet

We see it every day. The same diet does not work for everyone. So, for those struggling with being overweight or obese, diabetes and metabolic problems, focusing solely on calorie intake and energy expenditure through physical activity to understand what is happening and how much one can influence excess weight may be too simplistic. For a truly personalised and sustainable diet over the long term, we cannot therefore limit ourselves to the simplification of counting the calories in each food, just as calculating macronutrients such as fats, carbohydrates and proteins may not be enough either. More is needed to move beyond a reductionist view centred exclusively on energy balance.

Today, this approach seems outdated. We need tools that allow us to truly understand what happens within each organism, revealing the differences that can then explain why a dietary model succeeds or fails. The key subjective difference lies in the invisible: food passes through a living microbial ecosystem that can influence the number of calories actually absorbed. So what now? Looking to the future, an answer may come from an original mathematical model developed by experts at Arizona State University and published in PLOS One. It is called DAMM, an acronym standing for digestion, absorption and bacterial metabolism, and it tracks the ‘hidden’ part of the digestive and absorption process, going beyond the calories listed in tables. Or rather. It virtually tracks food as it travels through the digestive tract, estimating what the body absorbs directly, what reaches the colon, and how gut bacteria help transform the remaining material into products that are either absorbed or excreted, thereby determining how much energy we actually ‘take in’. The aim of the tool is to promote an understanding of the real effects of dietary patterns on individuals with diabetes, obesity and other metabolic conditions.

“Digestion is not just a human process, but a collaboration between our body and the trillions of microbes that live in the gut – explains Rosa Krajmalnik-Brown, one of the authors of the paper, in a university press release. The study was carried out in collaboration with experts from the AdventHealth Translational Research Institute in Orlando, together with Bruce Rittmann and Taylor Davis. “DAMM provides us with a powerful new tool for quantifying the contribution of these microbial partners to human health and energy balance, and for highlighting the importance of adequately nourishing gut microbes.” In short, whilst estimating actual energy intake may also rely on Atwater’s parameters—using a method that multiplies the quantity of proteins, carbohydrates and fats present in food by the average value of metabolisable calories per gram of each, a significant difference can arise when these data are integrated with the specific characteristics of each individual’s gut microbiota. This is because it would be important to understand how different diets nourish gut microbes, or how these microbes produce compounds such as short-chain fatty acids from fibre and other undigested food in the colon.

The study compared healthy adults who followed two ‘tailored’ diets. One diet focused particularly on the gut microbiota, providing more fibre and resistant starch (and therefore fewer processed foods and foods with larger particle sizes), whilst the other favoured processed foods and those with smaller particle sizes. People following this second diet – a Western-style diet – consumed around 116 calories more per day than those following a high-fibre diet. Yet the group following a high-fibre diet did not report feeling any more hungry. Result: the higher energy intake had no impact on feelings of hunger, and for this reason, ‘nutrition’ that also takes the microbiota into account appears crucial to explaining this result. This is why the DAMM model is so valuable; it offers the opportunity to quantitatively link human metabolism to the metabolism of the microorganisms present in the colon and was used to assess what occurred in the two populations under study.

The DAMM model begins by breaking down the diet into the classic macronutrients: proteins, carbohydrates and fats. It then estimates how much usable energy from these components is absorbed in the upper digestive tract. Next, the material is virtually tracked through to the colon, where bacteria break down the remaining food components that were not digested in the previous stages. In this process, they produce short-chain fatty acids, which can be absorbed through the colon and utilised by the body as additional calories. The model also takes into account methane production by certain methanogenic strains. The microbial contribution is significant. The model estimated that short-chain fatty acids absorbed by the colon contribute, on average, around 140 calories per day, or approximately 7.4% of total usable energy. Around 85% of usable energy comes from the upper gastrointestinal tract, whilst around 15% comes from the lower gastrointestinal tract, where microbial activity plays a central role.