“Despite its anti-obesity effects, BSO did not exert any detrimental effects on bones.”
Efforts to improve metabolic health through dietary interventions often come with trade-offs. Some approaches that reduce obesity or extend lifespan in laboratory models can also negatively affect other tissues, including bone.
One example is sulfur amino acid restriction (SAAR), a diet low in methionine and lacking cysteine that has repeatedly shown strong anti-obesity effects in animal studies. However, despite these promising metabolic benefits, SAAR has also been associated with reduced bone mineral density, weaker bones, and increased marrow fat accumulation.
This has led researchers to ask whether the metabolic benefits of SAAR can be separated from its harmful skeletal effects.
A new research paper was published in Volume 18 of Aging-US, titled “D, L-Buthionine-(S, R)-sulfoximine recapitulates the anti-obesity effects of sulfur amino acid restriction without the associated deleterious effects on bone in male mice.” The researchers investigated whether those metabolic benefits could be achieved without the same harmful effects on bone. The study was led by first author Naidu B. Ommi and corresponding author Sailendra N. Nichenametla from the Orentreich Foundation for the Advancement of Science Inc., in collaboration with Dwight A. L. Mattocks from the same institution and Mark C. Horowitz from the Yale University School of Medicine.
Understanding the Trade-Off
SAAR has attracted attention because of its strong anti-obesity effects in laboratory animals. But the same diet can also weaken the skeleton. In previous studies, SAAR reduced fat mass while increasing bone marrow adipocytes and decreasing bone strength. This complicates the idea of using SAAR as a long-term metabolic intervention without first understanding why those bone-related side effects occur.
The researchers focused on cysteine restriction and glutathione metabolism. Cysteine is a sulfur-containing amino acid and a key building block of glutathione, an important molecule involved in antioxidant defense, redox balance, and cell signaling. Because SAAR removes cysteine from the diet, the authors wanted to determine whether cysteine restriction was responsible not only for the anti-obesity effects, but also for bone-related side effects.
Testing a Different Approach
To investigate this, the team studied obese male mice fed high-fat diets under different conditions. One group received a control diet, another received the SAAR diet, a third received the SAAR diet with N-acetylcysteine (NAC), and another received the control diet with D, L-buthionine-(S, R)-sulfoximine (BSO), a compound that inhibits glutathione biosynthesis.
The results showed a clear difference between the dietary intervention and the pharmacological approach. Mice on the SAAR diet had lower trabecular and cortical bone mineral density, fewer osteoblasts, reduced bone strength, and more marrow adipocytes. However, mice treated with BSO did not show these harmful skeletal effects, even though BSO reproduced several anti-obesity effects seen with SAAR.
NAC also reversed the bone-related changes caused by SAAR, suggesting that cysteine restriction was a major driver of the skeletal side effects.
Bone, Fat, and Cysteine Restriction
One of the most important parts of the study is the connection between bone-forming cells and marrow fat. Osteoblasts, which build bone, and marrow adipocytes, which store fat inside bone marrow, can arise from related skeletal progenitor cells. When more of these cells shift toward fat formation, bone formation can decline.
In the SAAR-fed mice, the researchers observed fewer osteoblasts, weaker bone structure, and more marrow fat. When NAC was added, many of these effects were reversed. This supported the idea that cysteine restriction plays a central role in the bone loss associated with SAAR.
BSO, however, behaved differently. Although it affected body composition, it did not reduce bone mineral density, weaken mechanical strength, or increase marrow adipocytes in the same way as SAAR.
Why BSO May Act Differently
The finding that BSO did not harm bone was especially important. The authors suggest that this may be due to tissue-specific effects. In other words, BSO may lower glutathione more strongly in some tissues than in others. The paper notes that bone marrow may be more resistant to glutathione depletion by BSO than tissues such as the liver or kidney.
This could help explain why BSO was able to produce anti-obesity effects without reproducing the bone damage seen with SAAR. Still, the authors were careful to emphasize that more research is needed before BSO can be considered for broader therapeutic use. The authors also note that long-term studies will be necessary to better understand potential toxicity and tissue-specific effects.
Looking Ahead
This study is preclinical and was conducted in male mice, so the findings cannot yet be applied directly to humans. Future studies will need to examine long-term safety, effects in female mice, tissue-specific responses, optimal dosing, and possible off-target effects.
Still, the findings point to an important idea: the metabolic benefits of sulfur amino acid restriction may be separable from its harmful effects on bone. If researchers can better understand that separation, it may become possible to design safer interventions for obesity, aging, and metabolic health.
Conclusion
This study provides new insight into how sulfur amino acid metabolism, cysteine restriction, glutathione biology, obesity, and bone health are connected. By showing that BSO can reproduce anti-obesity effects without the bone deterioration seen with SAAR, the findings point toward a possible new direction for future research in nutrition, aging, and metabolic disease.
This study provides the first evidence that CysR mediates the adverse effects of the SAAR diet on bone health, while BSO induces beneficial changes in body composition without detectable adverse effects on bone.
Click here to read the full research paper published in Aging-US.
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