1. This paper considers the possibility that the key determinant of leaf age feeding preferences in foliovores is not the concentration of either nutrients or secondary metabolites, but is the ratio of the two. 2. In some fast-growing plants, nitrogen is most heavily defended by defensive toxins in the young leaves. This empowers the use of a simple model of leaf-age preference, based on the conflict between maximizing nutrient intake and minimizing toxicosis. 3. Young leaves are particularly valuable, not only because they lock up nitrogen, but also because their assimilative value is high. We calculate the loss of value due to herbivory and find that, if the herbivore is moderately intolerant of toxicity and feeds selectively on its predicted optimal leaf age, the costs of damage are greatly reduced. However, efficient toxin distribution protects the plant only if it grows rapidly, so that the well protected young foliage retains a high value. 4. The trends are reconcilable with observed leaf-age preferences of both polyphagous and oligophagous species. There is, as yet, little empirical evidence to substantiate the model, but it may be useful for future studies to focus on the toxin:nutrient ratio as a potentially important determinant of feeding preferences.