Reply to “Pitfalls of measuring feeding rate in the fruit fly Drosophila melanogaster”

Carvalho et al. reply:

In their Correspondence, Wong et al.¹ argue that dye labeling is not a satisfactory technique to address the issue of compensatory feeding and try to generalize this inadequacy to all food labeling methods. While some valid points are raised, others are off the mark.

As the authors point out, long-term measurements made with food labels reflect not only ingestion, but also internal capacity for the label and elimination rates. Capacity is a major limitation of non-absorbable dyes but not of ³²P or ¹⁴C radiolabeling. Whereas dye measurements quickly plateau¹, ³²P and ¹⁴C levels accumulate near-linearly for several days^2,3, indicating that internal capacity is not rate-limiting over 24 h, the time point used in our study². In contrast, as emphasized in our report, the absorbable nature of isotopes precludes us from discerning whether compensation takes place at the level of intake or absorption. Using the capillary feeder (CAFE), a direct, real-time assay of ingestion in undisturbed animals involving no food labels, it has been recently demonstrated that flies can sense nutrient variation and adapt their intake accordingly⁴, supporting the behavioral over the metabolic mechanism for compensatory feeding. As for elimination, the fact that diluted media stimulate excretion⁵ suggests that compensatory feeding may be even more dramatic than our results indicated. However, this does not directly address isotope turnover. It will be interesting to dissect this issue using methodology developed since the publication of our report⁶.

Radiolabeling and the CAFE are currently the most reliable and sensitive assays of feeding behavior in adult Drosophila melanogaster. With the advent of these methods, it is, in our opinion, no longer justifiable to infer feeding rate from surrogate assays such as egg laying or indirect behavioral observations⁷.