The FAST method is based on the determination of maximal fluorescence emission when exciting at 330-350 nm, which corresponds to molecular structures formed between reducing sugars or oxidizing lipids and lysine residues of proteins. This fluorescence is dependent on heat treatment and related to protein nutritional loss. Applied to a soluble extract of the food and corrected for the protein concentration of the solution obtained, using Trp fluorescence, the method allows to calculate the FAST index (FI), an indicator of the nutritional damage during heat process. The method, firstly validated on milk samples, is demonstrated here to well correlate with lysine damage on various food products, such as heat-treated milk and breakfast cereals, essentially modified by the Maillard reaction, and roasted soybean or cooked salmon, where interactions between oxidizing lipids and proteins better take place. Independently on the food product or the type of heat process, the FAST index appears always well correlated (r2: 0.84-0.98) to the lysine loss, the latter being estimated by determination of acid-released lysine, fluorescamine-reactive lysine or infrared. Shortly, roasted corn flakes appeared to be more damaged than extrudated flour (FI 100 and lysine blockage 40% instead of 55 and 30%), condensed milk more than UHT milk (FI 150 and 85% of acid-released lysine instead of 80 and 94%), and steam-cooked salmon much less than pan-fried (FI 28 instead of 372). Roasted soy can reach FI of more than 300 corresponding to chemical lysine loss of 40% and poultry-digestive lysine loss of 100%. As a conclusion, the FAST method, once precisely calibrated with pertinent nutritional indicators, should be of great interest for controlling or adapting a process in order to ensure a better nutritional quality for the food product.