Page 54 - MI-2-4

Microbes & Immunity Management of obesity

the mechanisms in humans ambiguous. The most energy absorption. A mathematical model indicated that,
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compelling evidence regarding the significance of SCFAs although intestinal transit time was not directly correlated
in human energy balance comes from a study conducted with metabolizable energy, it was essential for explaining
by Canfora et al. They administered colonic infusions interindividual variability in metabolizable energy. 26,39
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of SCFA combinations in men with obesity (n = 12) Research indicates that colonic transit time correlates
at concentrations and ratios similar to those achieved with fecal energy losses, with individuals exhibiting faster
with a high-fiber diet. The study revealed enhancements transit times experiencing reduced fecal energy losses.
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in fat oxidation, energy expenditure, and peptide YY, A study comparing diets rich versus poor in microbiota-
accompanied by elevated lipolysis due to SCFA colonic accessible carbohydrates found no difference in colonic
infusion. While fermentation products, such as SCFAs transit time by diet; however, transit time accounted for
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serve as readily absorbable energy sources for both bacteria 5% of the diversity in the gut microbiome. Additional
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and the host, the quantitative effect on energy balance gastrointestinal physiological traits may influence
remains uncertain and likely varies among individuals. energy absorption. Gastric emptying regulates the rate
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SCFAs (propionate, acetate, butyrate) are produced through at which nutrients are delivered to the small and large
bacterial fermentation of dietary fiber. Increased SCFA levels intestines, thereby influencing satiety and body weight.
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in obese individuals are linked to a higher abundance of Consequently, it may affect the absorption of microbially
Firmicutes and H -utilizing methanogenic archaea. SCFAs derived energetic substrates. Nonetheless, the role of gastric
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can influence energy expenditure in conflicting ways – emptying in energy absorption has not been established in
some studies suggest they suppress fasting-induced adipose either rats or humans. 26,49 An augmented intestinal mucus
factor, reducing fat oxidation. Conversely, butyrate can layer may result in diminished energy absorption by the
enhance mitochondrial activity, stimulate thermogenesis, host. This corresponds with findings that Western diets
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and increase fatty acid oxidation in brown adipose tissue. 43 are linked to increased energy absorption and a microbial
community more inclined to utilize the mucus layer as
7.4. Overall contribution to obesity an energy source. A diminished mucus layer correlates
The gut microbiota contributes to obesity progression with increased intestinal permeability, which may be
through both elevated digestible energy intake (via the mechanism enhancing the absorption of energetic
enhanced nutrient absorption and fermentation) and substrates. 26,32 Higher GLUT2 expression elevates serum
reduced energy expenditure (due to bile acid depletion glucose levels, stimulating key transcription factors
and uncertain SCFA effects). While SCFAs show potential (SREBP1, ChREBP) that drive lipid accumulation.
for promoting fat burning, their role remains controversial Increased SCFAs, especially acetate, provide pre-cursors
and requires further research. 44 for fatty acid and cholesterol synthesis, contributing to
lipid buildup.
7.5. Lipid synthesis and storage
Altered gut microbiota in obese individuals influences 7.8. Inflammation and lipid storage
lipid synthesis and storage through several mechanisms. Elevated levels of LPS in obese individuals induce
metabolic endotoxemia, leading to chronic inflammation.
7.6. Bile acid reduction and lipogenesis This inflammatory state increases the expression of pro-
A decrease in Bacteroides and Lactobacillus leads to lower inflammatory cytokines, such as IL-6 and tumor necrosis
bile acid levels, weakening FXR activation in the liver. factor-alpha (TNF-α), thereby disrupting insulin signaling
This results in increased SREBP1c expression, promoting and contributing to insulin resistance and excess fat
hepatic de novo lipogenesis. Similarly, reduced fibroblast storage. In addition, LPS promotes adipocyte pre-cursor
growth factor 19 signaling further enhances lipogenesis. 45 proliferation, further amplifying fat accumulation. 51
7.7. Digestible energy absorption and lipid 7.9. Gut microbiota and fat regulation
synthesis The gut flora may also influence bodily energy reserves
A key physiological element that may influence variations through alterations in energy expenditure. To date,
in energy absorption is intestinal transit time. Colonic human studies have not demonstrated a correlation
transit time exhibits significant variability across between the gut microbiome and energy expenditure. 26,32
individuals and influences gut microbial metabolism, as The extent of anaerobic microbial thermogenic activity
it determines the duration during which colonic bacteria in humans remains unknown. Microbial thermogenesis
can ferment food substrates. The interplay between gut cannot be quantified using the indirect calorimetry
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bacteria and colonic transit may be significant for human techniques currently employed to assess human energy

Volume 2 Issue 4 (2025) 46 doi: 10.36922/MI025160036