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Kerodon rupestris, a folivorous rodent endemic to the Caatinga of Brazil, exhibits complex modifications of diet in response to changes in the absolute abundance of 10 food species when their relative abundances remain equal. We evaluated the correlation between a suite of 12 nutrients (energy, ash, carbon, nitrogen, phosphorous, sulphur, zinc, manganese, calcium, magnesium, potassium, and sodium) and the consumption of leaves from 10 different plant species. Regardless of the taxonomic composition of the diet, the proportional representation of all nutrients but sodium was constant during phases of low and high abundance of foods. The uptake of sodium increased by a factor of 2.84, even though total consumption increased only by a factor of 2.04. From multiple stepwise-regression analysis, we suggest that nitrogen, calcium, and potassium contributed significantly to food selection. Carbon and sodium also were correlated with consumption, but excluded from the multiple-regression results because of their intercorrelation with other nutrients. The median-log Levene’s test was significant, indicating differences in the relative variation in uptake of the 12 nutrients. At high abundance of resources, sodium exhibited the most consistent daily uptake of all the nutrients, suggesting that it may constrain the maximization of other essential dietary elements. Sodium may play a role in the water balance of this arid-land rodent.