Exciting research1 is showing that the amino acid cysteine may hold the key to a quicker route to your physique goals.
We already know that the amino acids from protein can support weight loss in various ways, like helping to build more muscle to support our metabolism and increasing satiety.
However, new research1 has revealed another link. The study published in Nature Metabolism has uncovered an unexpected biological loophole that turns fat storage into fat usage by reducing – not adding – a specific amino acid.
The findings show that a diet low in cysteine reprograms fat to burn more calories, triggering weight loss and reduced inflammation.
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The weight loss switch
In a process commonly known as ‘browning’, restricting cysteine flicks a metabolic switch, triggering fat-storing white fat cells to convert to metabolically active brown fat cells.
These brown fat cells burn energy and produce heat in a process known as adipose tissue thermogenesis, which burns more calories overall.
In the animal trial part of the study, the researchers from Pennington Biomedical fed mice a high-fat diet, and they gained weight, as expected.
However, when the researchers removed cysteine and blocked the mice’s ability to produce it internally, they returned to a healthy weight with reduced inflammation while continuing to consume the same amount of high-fat food.
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Cut cysteine and calories
Previous research2 has also determined that a metabolic pathway that regulates how much cysteine is available in the body behaves differently in participants who also moderately (15%) reduced their calorie intake.
This finding emerged from the Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy (CALERIE-II) clinical trial, the first controlled study of calorie restriction in healthy humans.
The researchers discovered that completely removing cysteine from the body accelerated fat burning—essentially removing the brakes from metabolism.
When researchers in the latest study followed up on CALERIE-II participants, they found that those who reduced their calorie intake had lower cysteine levels, and their metabolic system had been reprogrammed.
As such, the newly discovered “metabolic switch” offers potential future strategies for weight management that go beyond just reducing calories, with controlled cysteine-restriction a potential approach to enhance health by reducing excess body fat.
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How to lower cysteine
Cysteine is a sulphur-containing amino acid made in the body from methionine, an amino acid found in high amounts in animal proteins.
To reduce cysteine in your diet, it is best to decrease your methionine intake by cutting back on animal proteins, particularly meat, poultry, and eggs. Lowering your sodium intake and drinking more water are other ways to lower.
These dietary guidelines are encapsulated in the sulphur amino acid restriction (SAAR) diet3.
While not a mainstream diet, the SAAR diet is being researched for its potential health benefits, particularly in relation to ageing, metabolism, and certain chronic diseases.
In addition to the removal of the animal products already mentioned, this vegan-based diet may also exclude certain high-sulphur plant-based foods, such as grains, nuts, and some vegetables.
However, the SAAR diet is not about cutting out protein entirely. Rather, it is about making smart choices, with a focus on plant-based foods and a careful selection of lower-sulphur animal products, such as fish and some dairy.
READ MORE | Ditch the calorie counting to reach your goals
Beyond calorie cutting
The more we learn about how different eating approaches can boost our metabolism and help with weight management, the more intriguing cysteine research becomes.
Scientists are starting to wonder if tweaking cysteine levels might be another tool in the toolkit for supporting healthy weight and metabolism. The real benefit would be less reliance on counting calories or following strict diets.
References:
- Lee AH, Orliaguet L, Youm YH, Maeda R, Dlugos T, Lei Y, Coman D, Shchukina I, Andhey PS, Smith SR, Ravussin E, Stadler K, Chen B, Artyomov MN, Hyder F, Horvath TL, Schneeberger M, Sugiura Y, Dixit VD. Cysteine depletion triggers adipose tissue thermogenesis and weight loss. Nat Metab. 2025 Jun;7(6):1204-1222. doi: 10.1038/s42255-025-01297-8. Epub 2025 Jun 3. PMID: 40461845; PMCID: PMC12198010.
- Stewart TM, Bhapkar M, Das S, Galan K, Martin CK, McAdams L, Pieper C, Redman L, Roberts S, Stein RI, Rochon J, Williamson DA; CALERIE Study Group. Comprehensive Assessment of Long-term Effects of Reducing Intake of Energy Phase 2 (CALERIE Phase 2) screening and recruitment: methods and results. Contemp Clin Trials. 2013 Jan;34(1):10-20. doi: 10.1016/j.cct.2012.08.011. Epub 2012 Sep 5. PMID: 22981898; PMCID: PMC3525758.
- Varghese, A., Gusarov, I., Gamallo-Lana, B. et al.Unravelling cysteine-deficiency-associated rapid weight loss. Nature 643, 776–784 (2025). https://doi.org/10.1038/s41586-025-08996-y.
Author: Pedro van Gaalen
When he’s not writing about sport or health and fitness, Pedro is probably out training for his next marathon or ultra-marathon. He’s worked as a fitness professional and as a marketing and comms expert. He now combines his passions in his role as managing editor at Fitness magazine.
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