A diagnosis method for detecting inflammatory bowel diseases comprising Ulcerative Colitis and Crohn's Disease, the method comprising the following steps: a) providing, as input data, a Raman spectrum of a protein extract of a fecal sample; b) isolating, from said Raman spectrum, the band in a wavenumber range of about 1550-1750 cm -1, which defines the amide-I vibration modes; c) calculating the second derivative profile of said band of Raman spectrum, and identifying the minima of said profile; d) performing a deconvolution of said band, setting a number of sub-bands (1,2,3,4) equal to the number of minima of the profile of said second derivative, obtaining a number of sub-bands (1,2,3,4) equal to at least said number of minima, each sub-band having a respective center-frequency ω i and a respective area A i, with i = 1, 2,... n; e) associating the sub-bands (1,2,3,4) obtained in step d), based on the center-frequency ω thereof, with the corresponding vibration modes of the protein secondary structures comprising the vibration modes of the disordered protein secondary structures of amide-I and the vibration modes of the higher frequency anti-parallel β-sheet protein secondary structures; f) calculating the ratio R of the area of the sub-band, associated with the vibration modes of the disordered protein secondary structures of amide-I, with the area of the sub-band associated with the vibration modes of the higher frequency anti-parallel β-sheet protein secondary structures; g) verifying that the ratio R is greater than or equal to a first threshold value to confirm that the protein extract of the fecal sample belongs to a patient suffering from an inflammatory bowel disease between Ulcerative Colitis and Crohn's Disease. a) providing, as input data, a Raman spectrum of a protein extract of a fecal sample; b) isolating, from said Raman spectrum, the band in a wavenumber range of about 1550-1750 cm -1, which defines the amide-I vibration modes; c) calculating the second derivative profile of said band of Raman spectrum, and identifying the minima of said profile; d) performing a deconvolution of said band, setting a number of sub-bands (1,2,3,4) equal to the number of minima of the profile of said second derivative, obtaining a number of sub-bands (1,2,3,4) equal to at least said number of minima, each sub-band having a respective center-frequency ω i and a respective area A i, with i = 1, 2,... n; e) associating the sub-bands (1,2,3,4) obtained in step d), based on the center-frequency ω thereof, with the corresponding vibration modes of the protein secondary structures comprising the vibration modes of the disordered protein secondary structures of amide-I and the vibration modes of the higher frequency anti-parallel β-sheet protein secondary structures; f) calculating the ratio R of the area of the sub-band, associated with the vibration modes of the disordered protein secondary structures of amide-I, with the area of the sub-band associated with the vibration modes of the higher frequency anti-parallel β-sheet protein secondary structures; g) verifying that the ratio R is greater than or equal to a first threshold value to confirm that the protein extract of the fecal sample belongs to a patient suffering from an inflammatory bowel disease between Ulcerative Colitis and Crohn's Disease.