The researchers discovered that cutting out two amino acids common in animal proteins—methionine and cysteine—caused the mice to burn significantly more energy.[1][2] The increase in heat production was almost as strong as continuous exposure to low temperatures.[1] The mice did not eat less or exercise more, but simply created more heat in their beige fat.[1] In experiments, they genetically modified mice to stop producing cysteine and fed them a diet without this amino acid, which resulted in a loss of up to 30% of body weight in 7 days.[2] Low levels of cysteine induced rapid fat loss by activating a network of interconnected biological pathways and deficient in coenzyme A, which coordinates metabolic reactions.[2] This discovery suggests that diet itself can activate the body's calorie-burning mechanism.[1]