How and When Do Insects Rely on Endogenous Protein and Lipid Resources during Lethal Bouts of Starvation? A New Application for 13C-Breath testing

Abstract

Most of our understanding about the physiology of fasting and starvation comes from studies of vertebrates; however, for ethical reasons, studies that monitor vertebrates through the lethal endpoint are scant. Insects are convenient models to characterize the comparative strategies used to cope with starvation because they have diverse life histories and have evolved under the omnipresent challenge of food limitation. Moreover, we can study the physiology of starvation through its natural endpoint. In this study we raised populations of five species of insects (adult grasshoppers, crickets, cockroaches, and larval beetles and moths) on diets labeled with either 13C-palmitic acid or 13C-leucine to isotopically enrich the lipids or the proteins in their bodies, respectively. The insects were allowed to become postabsorptive and then starved. We periodically measured the δ13C of the exhaled breath to characterize how each species adjusted their reliance on endogenous lipids and proteins as energy sources. We found that starving insects employ a wide range of strategies for regulating lipid and protein oxidation. All of the insects except for the beetle larvae were capable of sharply reducing reliance on protein oxidation; however, this protein sparing strategy was usually unsustainable during the entire starvation period. All insects increased their reliance on lipid oxidation, but while some species (grasshoppers, cockroaches, and beetle larvae) were still relying extensively on lipids at the time of death, other species (crickets and moth larvae) allowed rates of lipid oxidation to return to prestarvation levels. Although lipids and proteins are critical metabolic fuels for both vertebrates and insects, insects apparently exhibit a much wider range of strategies for rationing these limited resources during starvation.

More than just sugar: allocation of nectar amino acids and fatty acids in a Lepidopteran

“…Palmitic acid comprises approximately 30% (21.8-46.5%) of the fatty acids in insects used for human consumption [36] and has proven to be an effective tracer in insects [26,37]. We mixed 1 mg of palmitic acid with 40 ml of industrial food emulsifier (TWIN w 20, Sigma Aldrich, USA) in a 1.5 ml screw-top microcentrifuge tube and homogenized it for 1 min at 3000 r.p.m.…”

Sex differences in the utilization of essential and non-essential amino acids in a Lepidoptera

“…In contrast, some insects preferentially oxidize proteins even when carbohydrates and lipids are readily available (McCue et al, 2015). It is not clear whether protein oxidation strategies differ between the sexes.…”

The many roles of fats in overwintering insects

“…Although we are not aware of any studies that have used stable isotope-based approaches (e.g. McCue et al, 2015;Levin et al, 2017), we suggest that these approaches could be used to determine overwinter fuel use. Alternatively, enzyme activities should give some clues to overwinter fuel use: for example, lipolysis-related enzymes of the freeze-avoidant gall moth Epiblema scudderiana show increased activity in winter, indicating a shift to lipid metabolism (Rider et al, 2011).…”

“…A more specific comparison regarding the relationship between starvation and overwintering energetics is necessary to examine the role of lipid consumption in each. In general, starved insects seem to switch from carbohydrate-to lipid-(and protein-)fuelled metabolism during the early stages of starvation (Hill and Goldsworthy, 1970;Auerswald and Gäde, 2000), and variation in the timing and thresholds for that switch appear to be associated with variations in starvation tolerance (McCue et al, 2015). This observation may apply directly to overwintering insects (although temporal shifts in the fuel source have not been well-explored), but other strategies, such as the resorption of oocytes seen in some grasshoppers (Lim and Lee, 1981), are unlikely to be used by overwintering insects.…”