A Drosophila fly sits in a test tube and endures five different troubles: hunger, thirst, heat, cold and too much salt. Each trial lasts until the fly stops moving. It used to seem that all these abilities to hold up under pressure were linked, like the musicians of one orchestra: if one instrument plays louder, the others follow along. A new diet experiment has shown that this orchestra has no conductor. The instruments can be tuned separately, simply by changing the ratio of protein to carbohydrates in the food.
Scientists fed larvae and adult flies two kinds of food with the same calorie content but a different balance of protein and carbohydrates. One food was protein-rich, the other carbohydrate-rich. It turned out that protein-rich food in adulthood makes flies more resistant to heat and salt, while carbohydrate-rich food makes them more resistant to hunger, but only in females. At the same time, hunger, which under normal conditions is only weakly linked to the other stresses, responded to diet in the opposite direction from heat and salt. The links between stresses that show up under calm conditions do not predict how they will respond to a change in nutrition.
A larval diet had a weaker effect, but its effects persisted into adulthood and were often the opposite of the adult ones. This means that early nutrition leaves a trace that later shows up in stress responses. Physiologically, the protein diet raised the protein content of the body and the carbohydrate diet raised fat, and these states matched different profiles of stress resistance. But the core made up of dehydration, salt and heat was not explained by the simple water content of the body.
Previously it was unclear whether different kinds of stress resistance are tied into a single system and whether they can be changed separately through nutrition. Now it has become clear that stress resistance is not unified: different stresses respond to diet in different ways, and these responses do not match how the stresses correlate with one another. For the biology of aging this means that stress resistance is not a single measure of "health" but a set of separate axes that can be tuned through nutrition independently. This is an important methodological lesson: if we see that two stresses always rise or fall together, that does not mean they are governed by one mechanism.
The results will help interpret aging experiments more precisely, since stress resistance is often used there as a measure of health. Now it is clear that one stress does not stand in for another and that nutrition can change the picture. A fly in a test tube that has survived hunger but not heat is a reminder: an orchestra without a conductor does not play from a shared score but from separate notes, and those notes can be rewritten.
Flies fed a protein-rich diet lost a larger percentage of their dry weight during starvation, even though they were heavier. So the reserve was bigger, but it was used less efficiently, as if you had a full tank of gas but the engine burned through it faster.
Not published on arXiv. Here are its page, the PDF and the formalization: you can check it yourself, not only read our account.