A fruit fly sits in a test tube and endures five different troubles: hunger, thirst, heat, cold and too much salt. Each trial lasts until it stops moving. It used to seem that all these abilities to hold up under pressure were linked, like musicians in one orchestra: if one instrument plays louder, the others follow. A new diet experiment has shown there is no conductor for this orchestra. The instruments can be tuned one at a time, 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 seen in calm conditions do not predict how they will respond to a change in nutrition.
Previously it was unclear whether different types of stress resistance form a single system and whether they can be changed individually 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. The fly in the test tube that 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.