Obesity is associated with elevation of circulating levels of several amino acids [1], but the mechanism of this elevation is unclear. Type of dietary protein influences risk of obesity [2], suggesting that specific amino acids could contribute to regulating weight gain. Plasma concentrations of cysteine (tCys), but not of other sulfur amino acids, correlate strongly with BMI [3, 4] fat mass [5], and odds of obesity [4] in men and women. Increase or decrease of tCys over 6 years is associated with change of BMI in the same direction [6].

Massive weight loss in bariatric surgery patients had minimal effects on tCys [7], suggesting a direction of causality from tCys to BMI. Rodent studies demonstrate that cysteine is a powerful modulator of adiposity and glucose, lipid and energy metabolism [8, 9]. Collectively, evidence shows that cysteine may be a modifiable, hitherto unrecognized risk factor for obesity [10].

Preliminary data shows that tCys is strongly associated with adiposity in overweight children. There is no data on association of tCys with obesity phenotypes in a general childhood population, or with obesity genotypes.