Does Bertrand's rule apply to macronutrients?

Abstract

It has been known for over a century that the dose-response curve for many micronutrients is non-monotonic, having an initial stage of increasing benefits with increased intake, followed by increasing costs as excesses become toxic. This phenomenon, termed Bertrand's rule, is widely assumed not to apply to caloric macronutrients. To date this assumption has been safe, owing to the considerable methodological challenges involved in coaxing animals to over-ingest macronutrients in a way that enables the effects of specific food components to be isolated. Here we report an experiment which overcomes these difficulties, to test whether the second phase (incurring costs with excessive intake) applies to carbohydrate intake by the generalist-feeding caterpillar Spodoptera littoralis. The results showed that excess carbohydrate intake caused increased mortality, thus extending Bertrand's rule to macronutrients.

Figure 1

Effects of dietary macronutrient concentration on (a) performance (development rate×survival), (b) micronutrient intake, (c) cellulose intake and (d) macronutrient intake by sixth stadium S. littoralis caterpillars. In (d), the solid horizontal line represents the level of protein intake which was self-selected by caterpillars over the same developmental period and provided maximum performance (i.e. the target intake), while the dashed horizontal line represents the target intake for carbohydrate (Simpson et al. 2004). Symbols for nutrient concentrations are: L, lowest; ML, medium low; MH, medium high; H, highest.

Figure 2

Effects of diet macronutrient content on (a) mortality, (b) development time and (c) growth of S. littoralis fed one of four diet pairings. Symbols for nutrient concentrations are: L, lowest; ML, medium low; MH, medium high; H, highest.