Peripheral blood mononuclear cells (PBMC) have a great potential for nutrition and obesity studies. PBMC reflect the nutritional response of key organs involved in energy homeostasis maintenance, which is altered in the obese state. Here, we aimed to determine the usefulness of PBMC as a source of early markers of obesity. To that purpose, we analysed whether PBMC could reflect the insensitivity to changes in feeding conditions associated with obesity during the development of this pathology. Expression of key genes central to energy metabolism was measured by Q-PCR in PBMC samples of normoweight (control) and cafeteria-fed (obese) rats in feeding, fasting and refeeding conditions. Samples were obtained monthly from 2 (beginning of cafeteria diet-feeding) to 6 months of age. In general terms, expression of genes related to fatty acid synthesis (Fasn, Srebp1) and adipogenesis (Pparg) decreased with fasting and increased with refeeding. Conversely, the expression of a key gene regulating beta-oxidation (Cpt1a) and the gene for an orexigenic neuropeptide (Npy)-in accordance with their metabolic role-increased with fasting and decreased with refeeding. This expression pattern disappeared in obese rats, in which insensitivity to feeding conditions was observed after only 1 month of cafeteria diet-feeding. Thus, during development, PBMC accurately reflect nutritional regulation of energy homeostasic genes and the insensitivity to feeding associated with obesity, even in the earlier stages with a low degree of overweight. For this reason, this set of blood cells could constitute a potential source of biomarkers of early homeostatic imbalance which would be useful in nutrition studies that could help prevent the occurrence of obesity.