The quest for effective weight loss strategies has led researchers to explore various dietary components and their impact on metabolism. Among these, the amino acid L-cysteine has emerged as a promising area of investigation. Recent studies have shed light on the potential role of L-cysteine in promoting weight loss through various mechanisms, including the induction of thermogenesis in adipose tissue and the suppression of appetite.
The Role of Essential Amino Acids
In 1937, William C. Rose's groundbreaking work identified nine essential amino acids (EAAs): histidine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Cysteine is also essential in animals with mutations in either cystathionine γ-lyase (CSE, also known as CTH and CGL) or cystathionine β-synthase (CBS), enzymes of the trans-sulfuration pathway. Extensive research has examined the effects of removing individual EAAs, shedding light on their roles in metabolism, energy expenditure, and weight and fat loss.
Cysteine Deprivation and Rapid Weight Loss
The growing interest in diets that induce weight loss prompted a comparison of restriction of each individual EAA and cysteine. Findings revealed that cysteine deficiency induces the most weight loss compared with all other EAAs, resulting in a 30% reduction of body weight within 7 days. Experiments have elucidated a coordinated mechanism underlying this phenomenon, characterized by the rapid induction of ISR and oxidative stress response (OSR), accompanied by increased GDF15 and FGF21, and a reduction in CoA levels resulting in metabolic inefficiency, therefore offering insights for potential intervention in metabolic diseases and body-weight control.
Experimental Evidence
To investigate the impact of cysteine deprivation on weight loss, researchers conducted experiments on mice with and without the Cse gene, which is involved in cysteine synthesis. Cysteine deprivation in Cse knockout (KO; Cse−/−) mice, but not in heterozygous and wild-type (WT) mice, led to the largest weight loss compared with other EAAs. A cysteine-free (no-Cys) diet induced weight loss exclusively in Cse−/− mice, indicating that depletion of newly absorbed and synthesized cysteine is necessary for the effect. Female mice displayed slightly lower weight loss on day 1, a difference that remained constant. Weight loss was completely prevented by supplying cysteine through either N-acetylcysteine (NAC) or GSH (which is broken down to cysteine in the gut). Restoration of H2S, a degradation product of cysteine, did not prevent weight loss. Microbiota alterations also did not explain weight loss, as antibiotic-treated Cse−/− mice and Cse−/− mice co-housed with Cse+/− mice had similar weight-loss profiles. Weight loss was still most pronounced for cysteine deficiency, with only a 2.7% change from the 22 °C condition.
Food Consumption and Caloric Restriction
Given that diets deficient in EAAs induce food aversion behavior, the daily food consumption of Cse−/− and control heterozygous mice fed a no-Cys diet was monitored compared with a control diet. Cse−/− mice on the no-Cys diet exhibited a 30% reduction in daily food intake, from 3.5 g to 2.4 g per day, while no difference was observed in Cse+/− mice on the control or no-Cys diets (3.4 g in both). This food aversion, and the resultant caloric restriction (CR), could independently lead to rapid weight loss. However, while CR of 2.1 g per day led to a weight loss of only 15-16% in the control mice, the Cse−/− mice on the no-Cys diet experienced a substantial 31.5% weight loss within 1 week. Thus, at least 15% of the weight loss in Cse−/− mice could not be explained by reduced food intake alone. For other EAAs, such as tryptophan and phenylalanine, the entire weight loss was accounted for by reduced food intake, further emphasizing the unique effect of cysteine deprivation.
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Reversibility of Weight Loss
Notably, after mice on the no-Cys diet were reverted to a standard chow diet, they regained approximately two-thirds of the lost weight within 2 days and fully recovered within 4 days. When returned to the no-Cys diet, the mice promptly resumed losing weight at a similar rate, which was once again reversed immediately after reintroduction to the standard chow. This pattern highlights the high reversibility of cysteine-deprivation-induced weight loss without apparent detrimental effect.
Metabolic Changes and Fat Burning
To further characterize the weight loss, metabolic and behavioral assessments of Cse−/− and Cse+/− mice were conducted. After adapting to CR with a control diet (2.1 g per day, given between 2 and 3 pm throughout the experiment), the mice were placed into metabolic cages and, 2 days later, were switched to the no-Cys CR diet. The shift to the no-Cys diet immediately triggered weight loss in the Cse−/− mice, with a 10% decrease over 3 days, compared with a 0% change for Cse+/− mice. Notably, there were no significant differences in locomotion and movement between the Cse−/− and Cse+/− groups under any condition, indicating that the weight loss is not attributable to increased physical activity in Cse−/− mice, and that they do not exhibit lethargy.
Respiratory Exchange Ratio and Fat Content
The respiratory exchange ratio (RER), which shows whether mice are selectively burning fat or carbohydrate, progressively decreased from day 1 to day 3 in Cse−/− animals on a no-Cys diet, suggesting increased usage of fat as fuel. DEXA scans revealed a substantial reduction in fat content in Cse−/− mice on day 7 of the no-Cys diet. No differences were observed between Cse−/− and Cse+/− mice when provided with the control diet.
Histological Studies of Adipose Tissue
Histological studies of white adipose tissue revealed that Cse−/− mice deprived of cysteine exhibited higher fat loss from individual adipocytes by day 3 and, by day 7, there was near complete depletion of fat content throughout the tissue. Caspase-3 staining on day 7 revealed that, despite substantial fat loss, there was no detectable adipocyte cell death.
Browning of White Adipose Tissue
However, by day 3 in Cse−/− mice on a no-Cys diet, a notable proportion of adipocytes contained multiple small fat droplets instead of a single large droplet, resembling brown/beige adipose tissue. Immunostaining of the fat pad for UCP1 revealed robust browning of white adipose tissue, which on day 3 was faster and more pronounced than previously reported after 4 weeks of CR. Moreover, there was a rapid loss of visceral fat in Cse−/− mice on a no-Cys diet.
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Muscle and Liver Histology
There were no significant differences in muscle (quadricep) histology across the four groups. In the liver, there was no significant fat accumulation or apparent pathology. These findings may explain the lack of any effects on movement and the ease with which Cse−/− mice on a no-Cys diet can recover.
Transcriptional Responses to Cysteine Depletion
To gain insights into the molecular responses triggered by cysteine deprivation, bulk RNA-sequencing (RNA-seq) analysis of liver (which exhibits the highest expression of Cse), muscle (the most abundant tissue) and adipose tissue (the tissue most impacted by cysteine loss) was conducted. To distinguish responses specific to cysteine deprivation, a group exposed to a tryptophan-deficient diet was included. After a 3-day CR control diet, Cse−/− and Cse+/− mice were shifted to either CR control, CR no-Cys or CR tryptophan-free (no-Trp) diets, and tissues were collected after 2 days.
Gene Ontology Enrichment Analysis
Gene Ontology (GO) enrichment analysis of genes upregulated in the liver of Cse−/− mice on a no-Cys diet revealed prominent categories such as ‘cellular response to xenobiotic stimulus’, ‘GSH’ and ‘small molecule’ metabolic processes. The latter category revealed strong upregulation of genes associated with ISR, including amino acid synthesis and one-carbon metabolism (Mthfd2, Pycr1, Asns, Psat1), tRNA charging, amino acid transporters (Slc7a1, Slc1a4, Slc3a2) and various stress-response genes (Fgf21, Gdf15, Ddit3, Trib3, Atf5, Atf6). Genes in the ‘cellular response to xenobiotic stimulus’ and ‘glutathione metabolic processes’ categories (Nqo1, Gstm1-Gstm4, Gsta1, Gsta2, Srxn1) are characteristic of NRF2-regulated OSR.
Cholesterol and Lipid Metabolism
The liver has a central role in the metabolism of fatty acids (FA) and triglycerides (TGs), shifting between synthesis and breakdown in response to the fed and fasted states. Several GO categories related to cholesterol and lipid metabolism were also enriched.
L-Cysteine as an Anorectic Agent
High-protein diets promote weight loss and subsequent weight maintenance, but are difficult to adhere to. The mechanisms by which protein exerts these effects remain unclear. Studies tested the effects of a range of amino acids on food intake in rodents and identified l-cysteine as the most anorexigenic. Using rodents the effect of l-cysteine on food intake, behavior and energy expenditure was further studied. The effect on neuronal activation in the hypothalamus and brainstem was investigated before investigating its effect on gastric emptying and gut hormone release. L-Cysteine dose-dependently decreased food intake in both rats and mice following oral gavage and intraperitoneal administration. This effect did not appear to be secondary to behavioral or aversive side effects. L-Cysteine increased neuronal activation in the area postrema and delayed gastric emptying. It suppressed plasma acyl ghrelin levels and did not reduce food intake in transgenic ghrelin-overexpressing mice. Repeated l-cysteine administration decreased food intake in rats and obese mice.
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Effects on Food Intake and Behavior
Of the amino acids investigated, l-cysteine reduced food intake to the greatest extent following both oral and intraperitoneal administration. Oral administration of l-cysteine dose-dependently decreased food intake in rats and mice. L-cysteine to rats significantly reduced feeding behavior without altering behaviors indicative of illness or nausea. Oral gavage administration of l-cysteine at doses up to 4 mmol kg−1 did not cause CTA in rats.
Neuronal Activation and Gastric Emptying
L-cysteine significantly reduced cFLI in the lateral hypothalamic area (LHA). L-cysteine significantly increased cFLI in the area postrema compared with water-treated controls. L-cysteine significantly reduced gastric emptying 30 min after administration to rats.
Ghrelin Suppression
L-cysteine, plasma levels of acyl ghrelin were significantly reduced compared with water-treated rats. L-cysteine also significantly reduced plasma acyl ghrelin levels compared with saline-treated animals, but not GLP-1 or PYY levels following oral or IP administration.
Human Studies
L-cysteine reduced feelings of hunger compared with glycine-treated controls as measured by visual analogue scales (VAS). L-cysteine significantly reduced plasma acyl ghrelin levels at 45 min post administration compared with levels following vehicle and glycine treatment, the time point at which the largest cysteine-induced change from baseline for ‘How pleasant would it be to eat' and ‘How much could you eat' occurred. L-cysteine had no effect on plasma GLP-1 and PYY.
Repeated Administration
Repeated administration of l-cysteine over a period of five nights significantly reduced cumulative food intake in lean rats compared with water and glycine-treated controls. L-cysteine-treated animals had lost significantly more weight than water and glycine-treated controls on days 2 and 3.
Cysteine Depletion and Adipose Tissue Thermogenesis
Consuming fewer calories is largely accepted as a way to improve health and lose weight, but a recently published study in Nature Metabolism points to a specific sulfur-containing amino acid cysteine as a key component in weight loss. Pennington Biomedical researchers Dr. Eric Ravussin and Dr. Krisztian Stadler contributed to the study in which they and colleagues examined cysteine and discovered that it triggered the transition of white fat cells to brown fat cells, which are a more active form of fat cells that burn energy to produce heat and maintain body temperature. “In addition to the dramatic weight loss and increase in fat burning resulting from the removal of cysteine, the amino acid is also central to redox balance and redox pathways in biology,” said Dr. Stadler, who directs the Oxidative Stress and Disease laboratory at Pennington Biomedical.
Human Trials
For the human trials, researchers examined fat tissue samples taken from trial participants who had actively restricted calorie intake over a year. When examining the fat tissue samples, they looked for changes in the thousands of metabolites, which are compounds formed when the body breaks down food and stores energy. The tissue samples came from participants in the CALERIE clinical trial, which recruited healthy young and middle-aged men and women who were instructed to reduce their calorie intake by an average of 14% over two years.
Animal Models
In the animal models, researchers provided meals with reduced calories. The researchers found that those who reduced their calorie intake had lower levels of cysteine and a reprogrammed metabolic system. “What we found in the mice was that cysteine deprivation converted white fat into brown fat,” says Dixit.
Brain Signals and Fat Metabolism
To better understand this dramatic change, the researchers measured brain activation, looking for any areas that were more or less active than usual when there was no access to cysteine. The nerves of the sympathetic nervous system, once activated, released a chemical called norepinephrine into the fat tissue, which then induced the flip from white to brown fat, the researchers found.
Cysteine's Role in Cellular Metabolism
Led by researchers at NYU Grossman School of Medicine, the study reveals key details about how cells process fuels like carbohydrates and fats (metabolism), and how cysteine depletion affects tissues. Scientists had previously been unable to study its function directly. That said, the study authors say it is worth considering that fruits, vegetables, and legumes contain much lower levels of cysteine and its precursor, the sulfur-containing amino acid methionine, than red meat.
Oxidative Phosphorylation and Stress Responses
Specifically, cysteine deprivation disrupted oxidative phosphorylation, the main process for producing adenosine triphosphate (ATP), the molecule that serves as cells’ energy currency. Further, the team found that cysteine restriction activates both the integrated stress response (ISR), a signaling network that restores cellular balance after stress, and the oxidative stress response (OSR), which is triggered by higher levels of reactive oxygen species (ROS) following depletion of glutathione, the body’s primary antioxidant.