Short-term macronutrient-specific diets induce distinct metabolic changes detectable in blood and saliva.

BACKGROUND: Tools to non-invasively capture dynamic metabolic responses to diet in humans remain limited. Saliva has emerged as an accessible complementary biofluid, but its responsiveness to diet-induced metabolic adaptations remains insufficiently characterized. METHODS: In this controlled crossover study, twelve healthy adults completed three 7-day dietary interventions enriched in protein, lipids, or carbohydrates, each preceded by a Mediterranean-style washout. Fasting blood and stimulated saliva samples were collected at baseline and after each intervention, with an additional salivary assessment at day 4. Blood biochemical markers and salivary biochemical and physicochemical parameters were analyzed. Temporal responses were assessed using repeated-measures ANOVA, and selected associations between within-participant changes from baseline to day 7 were evaluated using Spearman rank correlations, with Holm adjustment across the three focal associations. RESULTS: Macronutrient-specific diets induced distinct and coordinated metabolic responses detectable in both blood and saliva. The high-protein diet increased blood urea nitrogen and salivary ammonium and reduced blood uric acid. Within-participant changes in blood urea nitrogen and salivary ammonium were positively associated (Spearman ρ = 0.620, P = 0.032). The high-carbohydrate diet reduced salivary pH and buffering capacity, but their associations with changes in HOMA-IR did not reach statistical significance. The high-lipid diet reduced circulating GOT and LDH without significant changes in the evaluated salivary parameters. CONCLUSIONS: Short-term dietary modulation induces distinct and non-identical biochemical responses in blood and saliva. The parallel nitrogen-related responses observed following the high-protein diet support further investigation of saliva as a complementary biofluid for studying dietary adaptations. However, the exploratory associations do not establish equivalence or robust concordance between salivary and blood biomarkers.