Branched-chain amino acids (BCAA) belong among nutrients strongly linked with insulin sensitivity (IS) measures. Their exact effect on IS appears to be dependent on various conditions that remain to be better defined . The aim of the current study was to investigate effects of chronic increase of BCAA intake on IS in two groups of healthy subjects differing in their basal consumption of BCAA, i.e. vegans and omnivores. Interventional trial was designed where vegans and omnivores were separately interveined with 15 (women) or 20 (men) grams of BCAA daily. Examinations of participants are intended to be done at baseline, after the intervention and wash-out period. Examinations (anthropometry, hyperinsulinaemic-euglycaemic clamp, arginine test) and blood analyses will be performed at baseline, after the intervention and after a 6 month wash-out period. Samples of subcutaneous abdominal adipose tissue (AT) and skeletal muscle (SM, vastus lateralis) obtained before and after the intervention will be used for various analyses (mRNA levels of selected metabolic markers, fatty acid composition, mitochondrial activity).