DIETS: a simple and sensitive assay to measure and control the intake of complex solid foods, like high-fat diets, in Drosophila
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Curated by eLife
eLife assessment:
This work describes an easily implemented method for measuring solid food intake in Drosophila, which is necessary for studying the consumption of experimentally challenging diets, such as high-fat foods, as well as their nutritional impacts on the organism. It is a valuable technical contribution with solid evidence supporting the conclusions, filling a significant gap in the field.
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Abstract
The fruit fly Drosophila melanogaste r offers a powerful model to study how diet affects the body and brain. However, existing methods for measuring their food intake often rely on dyes or tags mixed with food, which can be inaccurate due to how the flies absorb and eliminate them. Capillary-based assays like CAFE directly measure consumption, but only work with liquids and shorten fly lifespan. Additionally, capillary assays are incompatible with delivering viscous foods like high-fat diets. Even solidified high-fat diets tend to be sticky death traps for flies. Another longstanding challenge for fly researchers is that dietary restriction in flies involves diluting food, leading to compensatory feeding. To address these shortcomings, we have developed DIETS, a sensitive feeding assay that can be implemented even in low-resource settings. DIETS eliminates the need for labels and directly weighs the solid food consumed by small groups of flies over extended periods of hours to weeks. It allows us to deliver precise amounts of food to flies and implement accurate dietary restrictions. Importantly, DIETS is compatible with studying energy-dense high-fat diets. Using DIETS, we observed that, unlike a high-sugar diet, an isocaloric high-fat diet did not improve the flies’ ability to withstand starvation, even though they consumed more calories and had higher fat deposition.
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eLife assessment:
This work describes an easily implemented method for measuring solid food intake in Drosophila, which is necessary for studying the consumption of experimentally challenging diets, such as high-fat foods, as well as their nutritional impacts on the organism. It is a valuable technical contribution with solid evidence supporting the conclusions, filling a significant gap in the field.
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Reviewer #1 (Public Review):
Summary:
Thakare et al propose a gravimetric method to evaluate feeding from solid food in Drosophila adults that can be used to evaluate the nutritional impact of high-fat food.
Strengths:
This method is new and fills a gap in the methods used in Drosophila research.
Weaknesses:
The data presented address a number of questions that are mainly interesting for people needing to reproduce such experiments. The work could be improved by being presented within a broader scope.
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Reviewer #2 (Public Review):
Summary:
Thakare et al. present the DIETS assay for quantifying food consumption in adult Drosophila. DIETS measures food intake by weighing fly food before and after feeding. Technically, this is a well-designed, executed, and analyzed study. The interpretations are generally conservative and justified by the results. Although the results aren't always consistent with other published studies, which might reflect some of the unique conditions of the DIETS assay, the technique can clearly distinguish between some expected differences in food intake. Although lifespan is shortened in the DIETS chamber, the method seems robust for various time scales up to a week. DIETS adds another useful and versatile tool for fly researchers interested in studying feeding behavior.
Strengths:
The authors test various …
Reviewer #2 (Public Review):
Summary:
Thakare et al. present the DIETS assay for quantifying food consumption in adult Drosophila. DIETS measures food intake by weighing fly food before and after feeding. Technically, this is a well-designed, executed, and analyzed study. The interpretations are generally conservative and justified by the results. Although the results aren't always consistent with other published studies, which might reflect some of the unique conditions of the DIETS assay, the technique can clearly distinguish between some expected differences in food intake. Although lifespan is shortened in the DIETS chamber, the method seems robust for various time scales up to a week. DIETS adds another useful and versatile tool for fly researchers interested in studying feeding behavior.
Strengths:
The authors test various conditions, including food presentation, surface area, and humidity (by changing the food cup distance to an agar base) to demonstrate an optimized technique for quantifying consumption. Under these conditions, evaporation is generally limited to <10%.
The authors use DIETS to validate diverse feeding paradigms, including the published effects of temperature, food dilution, and intermittent fasting on food intake.
Weaknesses:
The studies to optimize and test the DIETS assay are technically rigorous and well-designed. However, the results reveal some weaknesses or potential caveats of the assay. As highlighted below, how much nutrition flies are actually obtaining may be misestimated due to vapor diffusion, and crowding/competition for food. This appears largely acceptable though, since the 'group' measurement can still clearly distinguish between expected feeding differences under different conditions, but it likely reduces accuracy, which may be important in some studies, and probably nullifies the effectiveness of using DIETS to restrict caloric intake.
It is my understanding that flies suck out nutrients from the medium, leaving behind the agar/cornmeal matrix. This seems consistent with the images in Figure S2B, where the spheroidal medium in the food cup maintains its shape as it shrinks, but there don't seem to be any pits or holes from fly consumption. Given that flies in DIETS consume a significant portion of the available food, it seems that the food concentration at the medium surface may be changing throughout the experiment. This may also make it challenging to use other common fly food ingredients, such as cornmeal, much of which is indigestible.
Similarly, vapor diffusion is expected between the agar bed and food cup (which the authors observed; in line 385), which may further affect assay accuracy, especially in comparisons between foods with different osmolarity.
In DIETS, increased feeding is observed with increased flies per chamber, but this is not observed in other techniques, such as EX-Q (Wu et al. 2020). It is unclear whether sensitivity to adult density is a DIETS-specific feature, or if adult density instead directly affects food intake estimates using DIETS (e.g., by affecting chamber humidity).
In another example, there is a ~300% difference in absolute feeding when the DIETS food cup is presented in different formats (Figure 3C). Again, it is unclear whether food presentation has an inherently greater effect in DIETS, or if the measurements themselves are highly sensitive to the environment.
Although the control of total food mass given to the animals is a novel feature of the assay, the likely differences in nutrient intake between individuals (and shortened lifespan) in a DIETS chamber makes this a challenging method to use to study caloric restriction. The shortened lifespan likely stems from the high adult density per vial, which has been explored in previous publications (e.g., Pearl in the 1920s; Mueller in the 1990s).
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