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Abstract Protein powder is one of the nutritional supplements most commonly consumed, by professional and amateur athletes and also by those taking exercise for non-sporting purposes. During its manufacture, the thermal treatment involved can reduce the nutritional value of the product, an aspect that, to date, has received little research attention. The present study was undertaken to determine the damage suffered by protein powder supplements during heat treatment, according to the concentration of blocked lysine, obtained from measures of furosine and total lysine. This study analyses 52 samples of protein powder in which the main sources of protein were concentrates and isolates of whey proteins, either intact or hydrolysed. Half of the supplements analysed contained over 6% of blocked lysine but only 9% presented values exceeding 20%. The supplements with the highest concentrations of blocked lysine were whey hydrolysates and peptides (12%), while the lowest concentrations were recorded for whey isolates and caseins. In these nutritional supplements, the labelled carbohydrate contents could be a useful indirect indication of thermal damage to whey supplements. The concentration of furosine, without taking into account total lysine, can only be used with respect to whey protein isolates to determine heat damage to protein powder supplements. Highlights • Half of the 52 supplements analysed contained over 6% blocked lysine. • The PPS with the highest levels of blocked lysine were whey peptides and hydrolysates. • The PPS with the lowest levels of blocked lysine were whey isolates and caseins. • Blocked lysine was only obtained accurately from total lysine and furosine content. [ABSTRACT FROM AUTHOR]