Modifying the Dietary Carbohydrate-to-Protein Ratio Alters the Postprandial Macronutrient Oxidation Pattern in Liver of AMPK-Deficient Mice

A New Murine Undernutrition Model Based on Complementary Feeding of Undernourished Children Causes Damage to the Morphofunctional Intestinal Epithelium Barrier

BACKGROUND

Complementary feeding is critical in establishing undernutrition. However, experimental undernourished diets do not represent the amount of nutrients in the complementary diets of undernourished children.

OBJECTIVES

To develop, validate, and evaluate the impact of a new murine model of undernutrition on the intestinal epithelium, based on the complementary diet of undernourished children from 7 countries with low-socioeconomic power belonging to the Malnutrition-Enteric Diseases (MAL-ED) cohort study.

METHODS

We used the difference in the percentage of energy, macronutrients, fiber and zinc in the complementary diet of children without undernutrition compared with stunting (height-for-age Z-score < -2) for the MAL-ED diet formulation. Subsequently, C57BL/6 mice were fed a control diet (AIN-93M diet) or MAL-ED diet for 28 d. Weight was measured daily; body composition was measured every 7 d; lactulose:mannitol ratio (LM) and morphometry were evaluated on days 7 and 28; the cotransport test and analysis of intestinal transporters and tight junctions were performed on day 7.

RESULTS

The MAL-ED diet presented -8.03% energy, -37.46% protein, -24.20% lipid, -10.83% zinc, +5.93% carbohydrate, and +45.17% fiber compared with the control diet. This diet rapidly reduced weight gain and compromised body growth and energy reserves during the chronic period (P < 0.05). In the intestinal epithelial barrier, this diet caused an increase in the LM (P < 0.001) and reduced (P < 0.001) the villous area associated with an increase in FAT/CD36 in the acute period and increased (P < 0.001) mannitol excretion in the chronic period.

CONCLUSIONS

The MAL-ED diet induced undernutrition in mice, resulting in acute damage to the integrity of the intestinal epithelial barrier and a subsequent increase in the intestinal area during the chronic period. This study introduces the first murine model of undernutrition for the complementary feeding phase, based on data from undernourished children in 7 different countries.

Role of liver AMPK and GCN2 kinases in the control of postprandial protein metabolism in response to mid-term high or low protein intake in mice

PURPOSE

Protein synthesis and proteolysis are known to be controlled through mammalian target of rapamycin, AMP-activated kinase (AMPK) and general control non-derepressible 2 (GCN2) pathways, depending on the nutritional condition. This study aimed at investigating the contribution of liver AMPK and GCN2 on the adaptation to high variations in protein intake.

METHODS

To evaluate the answer of protein pathways to high- or low-protein diet, male wild-type mice and genetically modified mice from C57BL/6 background with liver-specific AMPK- or GCN2-knockout were fed from day 25 diets differing in their protein level as energy: LP (5%), NP (14%) and HP (54%). Two hours after a 1 g test meal, protein synthesis rate was measured after a ¹³C valine flooding dose. The gene expression of key enzymes involved in proteolysis and GNC2 signaling pathway were quantified.

RESULTS

The HP diet but not the LP diet was associated with a decrease in fractional synthesis rate by 29% in the liver compared to NP diet. The expression of mRNA encoding ubiquitin and Cathepsin D was not sensitive to the protein content. The deletion of AMPK or GCN2 in the liver did not affect nor protein synthesis rates and neither proteolysis markers in the liver or in the muscle, whatever the protein intake. In the postprandial state, protein level alters protein synthesis in the liver but not in the muscle.

CONCLUSIONS

Taken together, these results suggest that liver AMPK and GCN2 are not involved in this adaptation to high- and low-protein diet observed in the postprandial period.

Duckweeds: their utilization, metabolites and cultivation

Duckweeds are floating plants of the family Lemnaceae, comprising 5 genera and 36 species. They typically live in ponds or lakes and are found worldwide, except the polar regions. There are two duckweed subfamilies-namely Lemnoidea and Wolffioideae, with 15 and 21 species, respectively. Additionally, they have characteristic reproduction methods. Several metabolites have also been reported in various duckweeds. Duckweeds have a wide range of adaptive capabilities and are particularly suitable for experiments requiring high productivity because of their speedy growth and reproduction rates. Duckweeds have been studied for their use as food/feed resources and pharmaceuticals, as well as for phytoremediation and industrial applications. Because there are numerous duckweed species, culture conditions should be optimized for industrial applications. Here, we review and summarize studies on duckweed species and their utilization, metabolites, and cultivation methods to support the extended application of duckweeds in future.

Macronutrient, Energy, and Bile Acid Metabolism Pathways Altered Following a Physiological Meal Challenge, Relative to Fasting, among Guatemalan Adults

BACKGROUND

The healthy human metabolome, including its physiological responses after meal consumption, remains incompletely understood. One major research gap is the limited literature assessing how human metabolomic profiles differ between fasting and postprandial states after physiological challenges.

OBJECTIVES

Our study objective was to evaluate alterations in high-resolution metabolomic profiles following a standardized meal challenge, relative to fasting, in Guatemalan adults.

METHODS

We studied 123 Guatemalan adults without obesity, hypertension, diabetes, metabolic syndrome, or comorbidities. Every participant received a standardized meal challenge (520 kcal, 67.4 g carbohydrates, 24.3 g fat, 8.0 g protein) and provided blood samples while fasting and at 2 h postprandial. Plasma samples were assayed by high-resolution metabolomics with dual-column LC [C18 (negative electrospray ionization), hydrophilic interaction LC (HILIC, positive electrospray ionization)] coupled to ultra-high-resolution MS. Associations between metabolomic features and the meal challenge timepoint were assessed in feature-by-feature multivariable linear mixed regression models. Two algorithms (mummichog, gene set enrichment analysis) were used for pathway analysis, and P values were combined by the Fisher method.

RESULTS

Among participants (62.6% male, median age 43.0 y), 1130 features (C18: 777; HILIC: 353) differed between fasting and postprandial states (all false discovery rate-adjusted q < 0.05). Based on differing C18 features, top pathways included: tricarboxylic acid cycle (TCA), primary bile acid biosynthesis, and linoleic acid metabolism (all Pcombined < 0.05). Mass spectral features included: taurine and cholic acid in primary bile acid biosynthesis; and fumaric acid, malic acid, and citric acid in the TCA. HILIC features that differed in the meal challenge reflected linoleic acid metabolism (Pcombined < 0.05).

CONCLUSIONS

Energy, macronutrient, and bile acid metabolism pathways were responsive to a standardized meal challenge in adults without cardiometabolic diseases. Our findings reflect metabolic flexibility in disease-free individuals.

Protein Status Modulates an Appetite for Protein To Maintain a Balanced Nutritional State-A Perspective View

Protein sufficiency is tightly controlled through different sensing and signaling processes that modulate and adapt protein and energy metabolism and feeding behavior to reach and maintain a well-balanced protein status. High-protein diets, often discussed in the context of body weight management, usually activate anorexigenic pathways, leading to higher satiety, decreased food and energy intake, and decreased body weight and adiposity. Diets marginally low in protein (3-8% energy) or marginally deficient in some indispensable amino acid more often activate orexigenic pathways, with higher appetite and a specific appetite for protein, a response that leads to an increase in protein intake to partially compensate for the deficit in protein and amino acid. Diets severely deficient in protein (2-3% energy as protein) usually depress food intake and induce lower weight and lower fat mass and lean tissues that characterize a status of protein deficiency. The control of protein sufficiency involves various peripheral and central signals, including modulation of both metabolic pathways at the periphery as well as central pathways of the control of food and protein intake, including a reward-driven specific sensitivity to the protein content of foods.

Liver GCN2 controls hepatic FGF21 secretion and modulates whole body postprandial oxidation profile under a low-protein diet

General control nonderepressible 2 (GCN2) is a kinase that detects amino acid deficiency and is involved in the control of protein synthesis and energy metabolism. However, the role of hepatic GCN2 in the metabolic adaptations in response to the modulation of dietary protein has been seldom studied. Wild-type (WT) and liver GCN2-deficient (KO) mice were fed either a normo-protein diet, a low-protein diet, or a high-protein diet for 3 wk. During this period, body weight, food intake, and metabolic parameters were followed. In mice fed normo- and high-protein diets, GCN2 pathway in the liver is not activated in WT mice, leading to a similar metabolic profile with the one of KO mice. On the contrary, a low-protein diet activates GCN2 in WT mice, inducing FGF21 secretion. In turn, FGF21 maintains a high level of lipid oxidation, leading to a different postprandial oxidation profile compared with KO mice. Hepatic GCN2 controls FGF21 secretion under a low-protein diet and modulates a whole body postprandial oxidation profile.

Animal Models of Undernutrition and Enteropathy as Tools for Assessment of Nutritional Intervention

: Undernutrition is a major public health problem leading to 1 in 5 of all deaths in children under 5 years. Undernutrition leads to growth stunting and/or wasting and is often associated with environmental enteric dysfunction (EED). EED mechanisms leading to growth failure include intestinal hyperpermeability, villus blunting, malabsorption and gut inflammation. As non-invasive methods for investigating gut function in undernourished children are limited, pre-clinical models are relevant to elucidating the pathophysiological processes involved in undernutrition and EED, and to identifying novel therapeutic strategies. In many published models, undernutrition was induced using protein or micronutrient deficient diets, but these experimental models were not associated with EED. Enteropathy models mainly used gastrointestinal injury triggers. These models are presented in this review. We found only a few studies investigating the combination of undernutrition and enteropathy. This highlights the need for further developments to establish an experimental model reproducing the impact of undernutrition and enteropathy on growth, intestinal hyperpermeability and inflammation, that could be suitable for preclinical evaluation of innovative therapeutic intervention.

Low-protein and methionine, high-starch diets increase energy intake and expenditure, increase FGF21, decrease IGF-1, and have little effect on adiposity in mice

Low-protein diets most often induce increased energy intake in an attempt to increase protein intake to meet protein needs with a risk of accumulation as fat of the excess energy intake. In female adult BALB/c mice, a decrease in dietary casein from 20% to 6% and 3% increased energy intake and slightly increased adiposity, and this response was exacerbated with soy proteins with low methionine content. The effect on fat mass was however limited because total energy expenditure increased to the same extent as energy intake. Lean body mass was preserved in all 6% fed mice and reduced only in 3% casein-fed animals. Insulin response to an oral glucose tolerance test was reduced in soy-fed mice and in low-protein-fed mice. Low-protein diets did not affect uncoupling protein 1 and increased fibroblast growth factor 21 (FGF21) in brown adipose tissue and increased FGF21, fatty acid synthase, and cluster of differentiation 36 in the liver. In the hypothalamus, neuropeptide Y was increased and proopiomelanocortin was decreased only in 3% casein-fed mice. In plasma, when protein was decreased, insulin-like growth factor-1 decreased and FGF21 increased and plasma FGF21 was best described by using a combination of dietary protein level, protein-to-carbohydrate ratio, and protein-to-methionine ratio in the diet. In conclusion, reducing dietary protein and protein quality increases energy intake but also energy expenditure resulting in an only slight increase in adiposity. In this process, FGF21 is probably an important signal that responds to a complex combination of protein restriction, protein quality, and carbohydrate content of the diet.

AMPK Re-Activation Suppresses Hepatic Steatosis but its Downregulation Does Not Promote Fatty Liver Development

Nonalcoholic fatty liver disease is a highly prevalent component of disorders associated with disrupted energy homeostasis. Although dysregulation of the energy sensor AMP-activated protein kinase (AMPK) is viewed as a pathogenic factor in the development of fatty liver its role has not been directly demonstrated. Unexpectedly, we show here that liver-specific AMPK KO mice display normal hepatic lipid homeostasis and are not prone to fatty liver development, indicating that the decreases in AMPK activity associated with hepatic steatosis may be a consequence, rather than a cause, of changes in hepatic metabolism. In contrast, we found that pharmacological re-activation of downregulated AMPK in fatty liver is sufficient to normalize hepatic lipid content. Mechanistically, AMPK activation reduces hepatic triglyceride content both by inhibiting lipid synthesis and by stimulating fatty acid oxidation in an LKB1-dependent manner, through a transcription-independent mechanism. Furthermore, the effect of the antidiabetic drug metformin on lipogenesis inhibition and fatty acid oxidation stimulation was enhanced by combination treatment with small-molecule AMPK activators in primary hepatocytes from mice and humans. Overall, these results demonstrate that AMPK downregulation is not a triggering factor in fatty liver development but in contrast, establish the therapeutic impact of pharmacological AMPK re-activation in the treatment of fatty liver disease.

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Hepatic AMPK deficiency is not sufficient to trigger fatty liver development

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Re-activation of downregulated AMPK in fatty liver normalizes hepatic lipid content

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Hepatic AMPK activation both inhibits lipogenesis and stimulates fatty acid oxidation

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AMPK activation modulates lipid metabolism via a transcription-independent mechanism

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Small-molecule AMPK activators enhance metformin effects on hepatic lipid metabolism

Nonalcoholic fatty liver disease is a highly prevalent component of metabolic syndrome, for which treatment options are limited. Downregulation of the energy sensor AMPK is viewed as a pathogenic factor in the development of fatty liver. However, we show here hepatic AMPK suppression is not sufficient to promote hepatic lipid accumulation, indicating that the decreases in AMPK activity associated with hepatic steatosis may be a consequence, rather than a cause, of changes in hepatic metabolism. In contrast, we found that pharmacological re-activation of downregulated AMPK in fatty liver is sufficient to normalize hepatic lipid content. Thus, these results establish the therapeutic impact of pharmacological AMPK re-activation in the treatment of fatty liver disease.