gut microbiome diet

Digestible and Metabolizable Energy Intake in Humans: a Systematic Review

6 February 2026

Yoshimura E, Oi N, Abe K, Nishida Y.

Summary

Here is a summary of the research study for a health-conscious reader. ### **Plain-Language Summary** This review examined how many calories our bodies truly absorb from food, since not everything we eat is used for energy. The researchers found that what and how much you eat significantly impacts calorie absorption, with high-fiber foods and nuts consistently leading to fewer calories being absorbed. Aging and certain diseases can also reduce the body’s efficiency at capturing energy from food. ### **Key Findings** * **High-Fiber Foods & Nuts:** Diets rich in fiber and tree nuts consistently decrease the proportion of calories your body absorbs. This means more energy is lost in waste. * **Overeating:** When people overeat, they lose more total calories in their waste, but the *percentage* of calories absorbed from their food remains relatively stable, suggesting the body adapts. * **Aging and Disease:** Older adults and individuals with specific conditions (like short bowel syndrome) absorb significantly fewer calories from their food. * **Time-Restricted Eating (TRE):** The evidence for TRE’s effect on calorie absorption is inconclusive, with different studies showing conflicting results. ### **Practical Takeaways for Nutrition and Longevity** The "calories in" part of the energy equation is more complex than a food label suggests. Including high-fiber foods and nuts in your diet can reduce the net calories you absorb, which may support healthy weight management. As we age, our ability to absorb energy can decrease, making it crucial to focus on nutrient-dense foods to prevent deficiencies and support overall health. ### **Study Limitations** The review included a small number of studies with varying methods, making direct comparisons difficult. More standardized research is needed to confirm these findings across different groups of people.

Abstract

Understanding digestible energy intake (DEI) and metabolizable energy intake (MEI) is essential for elucidating human energy balance. The absolute of DEI refers to gross energy intake minus fecal energy loss (EL), whereas MEI further accounts for urinary EL. This systematic review aimed to synthesize the findings from studies that utilized bomb calorimetry to measure DEI and/or MEI (PROSPERO CRD42021230982). Medical Literature Analysis and Retrieval System Online (via PubMed), the Cochrane Library, Cumulative Index to Nursing and Allied Health Literature, and Scopus were searched for articles published between January 1973 and July 2024. Human studies (adults aged ≥18 y) were included without restrictions on study design. Data were descriptively summarized according to dietary conditions, including overeating, undereating, high-fiber diets, tree nut intake, time-restricted eating (TRE), medication use, and disease status. Twenty-three studies were included. Overeating generally increased absolute fecal ELs; however, proportional DEI and MEI remained relatively stable, suggesting adaptive responses. High-fiber diets and tree nut intake consistently lowered the proportions of DEI and MEI, indicating that dietary composition affects energy absorption efficiency. Results for TRE were inconsistent, with 1 study showing increased fecal EL and another reporting no significant changes. Aging and disease, particularly short bowel syndrome and home parenteral nutrition dependence, were associated with markedly reduced proportions of DEI and MEI. Despite methodological variability across studies, this review highlighted that both dietary quantity and composition significantly influence energy absorption. Furthermore, limited evidence suggested that aging and diseases impair energy absorption. Future studies using standardized protocols and randomized controlled trials are warranted to clarify the determinants of DEI and MEI across diverse populations. This study was registered at PROSPERO as CRD42021230982.

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