Glutamine could help people with obesity reduce inflammation of fat tissue and reduce fat mass, according to a new study at Karolinska Institutet in Sweden and the University of Oxford in the U.K.
The researchers also show how glutamine levels can alter gene expression in several different cell types. However, more research is needed before glutamine supplementation may be recommended as a treatment for obesity. The study is published in the journal Cell Metabolism.
Glutamine is an important amino acid with many key functions such as providing energy and maintaining good intestinal health.
It also has anti-inflammatory effects on for example white blood cells and T-cells that are important for the immune system.
In the current study, the researchers examined how the metabolic processes differed in fat tissue collected from the abdomen of 52 obese and 29 non-obese women.
They identified glutamine as the amino acid that displayed the largest differences when comparing the two groups.
People with obesity had on average lower levels of glutamine in their fat tissue than normal-weight people. Lower glutamine-levels were also associated with larger fat cell size and higher body fat percentage independently of body-mass index (BMI), according to the study.
“Our results suggest that treatment with glutamine could be of value against obesity and insulin resistance,” says Mikael Ryden, professor and senior physician at the Department of Medicine in Huddinge, Karolinska Institutet, and the study’s corresponding author.
“We know, however, that glutamine is also important for cell division and the metabolism of cancer and therefore, more research on possible long-term side effects is needed before glutamine may be recommended as a dietary supplement to help treat obesity and its complications.”
The researchers also showed through a combination of animal and cell analyses that glutamine levels influenced the expression of different genes and that low glutamine levels induced an increase in the expression of pro-inflammatory genes in the fat tissue.
Obese mice injected with glutamine for two weeks had less fat tissue inflammation than mice who received a control saline solution.
Their body fat mass, fat cell volume and blood glucose levels were also reduced. In an analysis of cultured human fat cells, the expression of pro-inflammatory genes and the lipid content were attenuated after incubation with increasing concentrations of glutamine.
The largest effect was observed after treatment with 5-20 millimolar (mM) glutamine for 11 days, according to the study.
The researchers also studied in detail what happens inside the fat cell when glutamine levels are altered.
They found that glutamine impacts a mechanism called O-GlcNAcylation that can control epigenetic changes, that is changes in gene expression caused by environmental and lifestyle factors rather than by alterations in our underlying DNA sequence.
People with obesity had higher levels of O-GlcNAcylation in their fat tissue while mice and human cells treated with glutamine had lower levels of O-GlcNAcylation in the cell nucleus.
“Our study shows that glutamine is anti-inflammatory in the fat tissue by changing the gene expression in several different cell types,” says Mikael Ryden.
“This means that a lack of glutamine, which may occur during long-term obesity, could lead to epigenetic changes that fuel inflammation in the body.”
Further research is needed to fully understand which genes and cellular processes are affected the most, according to the researchers.
Obesity has become a significant public health problem worldwide, and it is associated with various comorbidities [1,2,3,4,5]. Obesity is considered a low-grade inflammatory disease and the degree of inflammatory status correlates positively with the development of insulin resistance and type 2 diabetes mellitus [6,7,8]. The white adipose tissue has a primary role sensing and managing energy homeostasis [7].
Consistently, human and rodents studies demonstrated that, under a positive energy balance, the white adipose tissue triggers an immune response, which develops low-grade inflammation milieu, associated with infiltration of immune cells [7,8,9].
Additionally, products from intestinal microbiota as lipopolysaccharides (LPS) can also contribute to this inflammatory state in obesity [10,11,12].
Many efforts have been made to prevent and treat obesity and to reduce the low-grade inflammatory status, including hypocaloric diets combined or not combined with physical activity and drugs in humans [13].
However, the complexity and time spent in this treatment often lead to weight recovery [14]. Thus, it will be useful if an obesity treatment strategy takes into account single nutrient supplementation.
Diets with glutamine (Gln) supplementation have aroused interest since they can mitigate the release of cytokines, reduce organ damage, and improve survival of mice and humans with endotoxemia [15,16,17,18,19,20].
However, studies that evaluated the potential of oral glutamine supplementation decreasing body weight and fat mass in an obese human being are scarce [21,22,23].
Chronic oral glutamine supplementation was shown to improve fasting blood glucose and A1c as well as reduced body fat and waist circumference (WC) in type 2 diabetic individuals [23].
In another study, oral glutamine supplementation for four weeks was able to reduce body weight (BW) and WC, but not insulinemia and HOMA-IR in overweight and obese female patients [22].
However, none of these studies investigated possible pathophysiological mechanisms of how glutamine supplementation could contribute to weight and adipose mass reduction and improve metabolic parameters.
Insulin has a potent lipogenic effect on adipose tissue [7]. This effect is related to the tyrosine phosphorylation of the insulin receptor by insulin, which induces PI3K/Akt pathway activation that leads to glucose transport, and lipogenesis afterwards [7,8].
Mice with specific insulin receptor disruption on fat (FIRKO mice) are protected from obesity induced by a high fat diet [24]. On the other hand, animals on the high fat diet usually displayed insulin resistance in the liver and muscle but not in the adipose tissue [25,26].
These data argue that, in some specific conditions, the lack of insulin effects in adipose tissue may protect from obesity and some of its comorbidities. Previous cell culture studies suggested that glutamine was able to reduce insulin action in adipocytes, but not on L6 muscle cells [27,28,29]. Accordingly, the lack of effect of insulin on the adipose tissue through glutamine supplementation might be beneficial in reducing lipogenesis and, hence, fat accumulation in vivo.
Thus, we combined human and animal models to deeper understand the mechanisms by which glutamine may reduce obesity and its comorbidities.
Herein, we aimed to investigate (1) whether chronic oral Gln supplementation may alter anthropometric, metabolic parameters and also the inflammatory profile in overweight and obese humans in a proof of concept study. We also attempted to investigate (2) whether chronic Gln supplementation via gavage may alter these same parameters in high-fat diet (HFD) rats, which are integrated with an investigation of insulin action and signaling in specific tissues as liver, muscle, and adipose tissue. We attempted to understand, at a molecular level, this beneficial effect of glutamine.
Discussion
Here, we showed that glutamine supplementation to rats treated with HFD reduces weight gain and improves insulin action and signaling in the liver and muscle. These beneficial effects seem to be associated with tissue-specific insulin resistance in adipose tissue, which prevents the increase in adipose mass in these mice. In a proof of concept study, we also showed that, in humans who are overweight or obese, oral Gln supplementation reduces waist circumference in parallel with a reduction in circulating LPS, which suggests a modulation of microbiota and/or an intestinal barrier.
It is important to mention that we used alanine as a control to give the same amount of calories to the group, which received glutamine. In addition, alanine was chosen because it is the second most abundant circulating amino acid, is produced in muscle and metabolized in the liver as glutamine, and transports ammonia from the muscle to the liver to produce urea [17]. Furthermore, alanine did not significantly affect muscle metabolism or renal and hepatic function [35].
Although in humans, Gln supplementation reduced waist circumference, it did not change body weight and BMI of overweight and obese subjects. The absence of changes on body weight and BMI in humans may be due to the short duration of supplementation.
However, waist circumference measurements are a viable and reliable way to measure abdominal fat mass and represent mostly visceral adipose tissue [36,37]. Visceral adipose tissue loss is associated with improved insulin resistance, which decreases the risk of type 2 diabetes development [37,38,39,40].
Both overweight humans and animals on high fat diet with glutamine supplementation reduced circulating lipopolysaccharide levels. The gastrointestinal tract is the main source of LPS, because of its massive bacterial load compared to other anatomical sites [41] and there is a direct and strong association between plasma LPS levels and the degree of intestinal permeability, in different pathological conditions [42].
This translocation of LPS into the plasma leads to binding of LPS to TLR4, which will induce the activation of the NF-κB pathway in different tissues, which results in systemic inflammation [43,44,45,46] and insulin resistance [42]. In mice, the subcutaneous infusion of LPS induces glucose intolerance, insulin resistance, and a very interesting increase in body weight accompanied by increases in adipose tissue [11,47].
The mechanism by which glutamine reduces circulating LPS levels is not completely understood but might involve modulation of intestinal microbiota and or changes in intestinal permeability.
Recently, our group demonstrated that L-glutamine altered gut microbiota by decreasing the Firmicutes/Bacteroidetes ratio [21], which is considered a biomarker for obesity.
Since we used the same protocol to supplement L-glutamine in the present study, we can, thus, suggest that glutamine may have modulated the microbiota.
Taken together, these data with our results suggests that glutamine supplementation may induce changes in intestinal microbiota and in intestinal permeability, which may account for reduced circulating levels of LPS in humans and the animal model, which contributes to improved insulin action.
Previous data demonstrated that in the first months of HFD administration occurred insulin resistance in the liver and muscle, but not in the adipose tissue of rats and mice [25,26]. This tissue-specific protection from insulin resistance in adipose tissue may have an important role in the development of obesity because it allows insulin to increase glucose uptake and synthesize triglycerides in this tissue [7].
However, the inflammatory process that develops in adipose tissue with infiltration of macrophages and the consequent increase in circulating cytokines certainly contribute to worsening the systemic insulin resistance [48,49,50].
In this regard, previous data showed that tissue-specific knockout of the insulin receptor in adipose tissue (FIRKO mice) protects the animal from diet-induced insulin resistance [24].
Based on previous data that glutamine supplementation may induce insulin resistance in adipose cell lines but not in muscle cell lines [27,28,29], we used glutamine supplementation to induce insulin resistance in the adipose tissue of an HFD animal.
Our data showed that we could develop a model of adipose tissue-specific insulin resistance in rats by protecting these animals against diet-induced systemic insulin resistance, which is similar to what was described for the FIRKO mice.
We also investigated the molecular mechanism by which glutamine induces insulin resistance only in adipose tissue and reduces glucose uptake and triglycerides synthesis in this tissue.
Several mechanisms have been described to explain insulin resistance, and low-grade inflammation represents one of the most important of these mechanisms in obesity [48,49].
However, the adipose insulin resistance induced by glutamine was not related to inflammation, because we found a reduced circulating LPS in humans and the animal model. Furthermore, in adipose tissue of rats, there was a decrease in NF-κB activity, which indicates a clear reduction in an inflammatory milieu.
The insulin resistance in adipose tissue of animals on a high fat diet supplemented with glutamine was associated with a reduction of ~50% of fat mass, which suggests that these mechanisms may be linked. One possibility is that GLN increases the activity of the hexosamine pathway (HBP) [51].
GLN is an intermediary substrate to generate glucosamine-6-phosphate via HBP, which is further metabolized to UDP-GlcNAc (UDP-N-acetylglucosamine). UDP-GlcNAc causes posttranslational modification of proteins, which leads to lower lipogenesis and reduced adipose mass [51].
It is well known that the hexosamine pathway can induce insulin resistance through direct post-translational modification of key insulin signaling proteins, via O-linked glycosylation on serine and threonine residues with the GlcNAc moiety [51].
Our results demonstrating increased IRS1/O-GlcNAc association in adipose tissue suggest that O-linked GlcNAc post-translational modification may induce insulin resistance in this tissue.
Another important point that should be considered in animals supplemented with glutamine is the increase in adiponectin levels associated with an increase in energy expenditure and a decrease in RER, which indicates that these animals were largely using fatty acids as an energy source.
In addition, to act as a glucose-lowering adipokine [52,53,54,55,56,57], adiponectin also increases fatty acids oxidation [58]. Our data showing an increase in circulating adiponectin levels after glutamine supplementation may explain the increased energy expenditure and fat oxidation, which reinforces the protection from diet-induced obesity in these animals.
Our study has some limitations such as the short time of glutamine supplementation in humans and the absence of a gold standard method to measure insulin sensitivity, and of an oral glucose tolerance test in humans. In addition, 24-hour dietary recall and our assessment of the physical activity level have limitations.
Our data show that glutamine supplementation, in animals and humans, was accompanied by an increase in plasma glutamine levels and in rat tissue by an increase in IRS1/O-GlcNAc in adipose tissue but not in the liver and the muscle tissue.
This action of glutamine mainly in adipose tissue may be a consequence of differential mechanisms involved in the glutamine transport among tissues [59], but, certainly, this point deserves a deep investigation.
Conclusions
In summary, our data showed that glutamine supplementation could reduce the insulin action and glucose uptake in fat and adipose mass, which improves the insulin action and signaling in the liver and muscle of rats. In addition, a proof of concept study showed that, in humans, glutamine supplementation for a short time is accompanied by a reduction in waist circumference, in circulating LPS. Our preliminary data suggest that further investigations with glutamine supplementation should be performed for longer periods. In addition, it would be interesting to investigate whether glutamine is also able to induce insulin resistance in the adipose tissue of humans, which may have some long-term detrimental consequences.
More information: Paul Petrus, et al. “Glutamine links obesity to inflammation in human white adipose tissue,” Cell Metabolism, online December 19, 2019.
