Dietary Conditions

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For normal rearing conditions, we use standard cornmeal-agar-sugar fly food (15). For specialized food, such as palmitic acid, soy, and beef-containing recipes, we followed a protocol that was derived from Driver and colleagues (24). The food recipes that we followed from Driver and colleagues (24) contains cornmeal, agar, oatmeal, and either sucrose or an isocaloric amount of palmitic acid, tofu, or beef (see Subheading 2.4., Buffers and Preparation of Foods). Most aging laboratories use sucrose-yeast food (no corn meal or oat meal) for better control of the nutrititve content.

Other dietary considerations are diets that have identical phosphate levels, protein levels, lipid levels, glycemic index, and so on, but vary in the source of these components. We have not yet performed experiments in Drosophila with these parameters controlled, but they are important considerations depending on the nutritional question being asked. For example, in mammalian models, intake of a low-phosphate diet stimulates transepithelial transport of inorganic phosphate (Pi) in the small intestine, which is associated with a change in the apical localization of NaPi cotransporters (69). A low-Pi diet can also lead to an increase in the level of vitamin D3 absorbed in the small intestine in mammals (69).

The source of protein or lipids, whether from meat or soy, can also potentially affect metabolism, so iso-protein or iso-lipid diets are other considerations. For example, according to Ascencio et al. (70), "The consumption of soy protein was shown to reduce blood lipids in humans and other animal species. Furthermore, it was shown that the ingestion of soy protein maintains normal insulinemia" (70). Lipids can be considered either "good" (such as polyunsaturated fatty acids) or "bad" (such as saturated fatty acids), so iso-lipid diets are a third consideration. For example, Tatematsu et al. (2005) have shown "Canola oil (Can), as well as some other oils, shortens the survival of SHRSP rats compared with soybean oil (Soy)" (71). It might be worthwhile to determine whether specific protein or lipids have similar effects in Drosophila.

Finally, depending on the goals of a particular nutrigenomics study, other considerations must also be made about the food components. One might want to consider glycemic index (GI), which is related to the speed in which nutrients are digested and absorbed, or glycemic load (GL), which describes the total GI content of the diet. Simple sugars have a high GI, whereas complex carbohydrates have a low GI. According to Colombani, in a review on the importance of GI on human nutrition, "It is claimed that low-GI and -GL diets favorably affect many noncommunicable diseases that are prevalent in developed countries, including type II diabetes, insulin resistance, obesity, cardiovascular disease (CVD), and cancer" (72). Although, to our knowledge, it has not yet been investigated, it is likely that GI is also important for regulating metabolic processes such as disease resistance or insulin signaling in Drosophila.

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