The carbon footprint of motorized transport modes per unit of travel length encompasses the unit share of the vehicle lifetime emissions, that of the transport infrastructure and those of the motor energy, considered from both "well to tank" and "tank to wheel". In the active modes of transport, i.e. Walking and Cycling, the counterpart of motor energy is human energy. It comes with two kinds of carbon flows: the carbon footprint of food intake-which we call the Food to Body componentand the carbon dioxide emissions of respiration-say the Body to Foot component. The article provides a model in simple mathematical form to assess those carbon flows per unit length. It involves the modal speed in (i) the Metabolic Equivalent of the Task which gives rise to the energy and carbon flows, and (ii) the ratio of time spent to length travelled. The carbon footprint of food intake varies widely depending on the person food diet. In a numerical study, the Food to Foot carbon emissions of Walking, Cycling, e-scooter riding and car driving are estimated and compared to the rest of modal carbon footprint. Under average food diet as of France in the 2010s, the inclusion of F2F in modal footprints changes the ranking of the modes according to the carbon footprint per unit length.