Vitality

Pass The Salt: Why Pregnant Flies Crave Salt, And How That May Also Hold True For Humans

flies
The reproductive behavior of the fly may reveal why pregnant women get strange cravings. steve p2008 CC BY 2.0

Bizarre cravings are a popular pregnancy symptom, and scientists have long hypothesized that different cravings signified the specific nutritional needs of the developing fetus or the changing mother’s body. However, according to a recent study on fruit flies, salt cravings in pregnant females was actually more closely related to the flies’ reproductive behaviors than their nutrition, and the same may be true for humans.

Salt cravings are one of the most common pregnancy cravings women experience worldwide, but researchers are not entirely sure why. Common theories suggest the cravings are the body’s way of retaining more water, or how it restores salt reserves lost during frequent urination and morning sickness. Recently, researchers at the Champalimaud Centre for the Unknown in Lisbon, Portugal investigated both the root and the reasoning behind salt cravings in pregnant fruit files with the hope of shedding light onto human taste preferences. The team chose to focus specifically on the neural reasons for these changes in nutrient intake, specifically in female fruit flies after mating.

"Nutrition is a highly complex topic. To understand how the brain regulates nutrient intake, you need to work in an organism that gives you access to a lot of diverse technologies. In that respect, the fruit fly is unbeatable," lead researcher Dr. Carlos Ribeiro explained in a recent press release.

For the study, the team used a number of tests to discover exactly how the female flies’ food preferences changed after mating, and to see if, like many other mammalian species, pregnancy increased their desire for salt. Test results revealed that flies shared the mammalian inclination toward salt during pregnancy. Most importantly, there was a direct correlation between the insects’ salt intake and the amount of eggs they were able to produce, suggesting that the purpose of the fly’s increased desire for salt was to enhance reproductive abilities.

The team not only discovered the biological purpose of this desire for salt in pregnant flies, but also figured out what stimulates the cravings. Although many people believe pregnancy cravings are a subliminal cry for certain nutritional needs from either the unborn child or the mother, the team discovered this was not true in the case of the flies. Even when the females’ ability to produce eggs was disabled, females continued to show increased preference for salt after mating. This suggested that the salt cravings were independent of the actual needs of the mother’s and offspring’s.

Instead, the increased desire for salty food was caused by a molecule called sex peptide, which the male fly transfers to the female during intercourse. "The molecule activates neurons in the uterus of the female. From there, we found that a short chain of neuronal interactions signals the brain to 'dial up the salt preference.'"

Although this study was based strictly on fly models, Ribeiro believes the study suggests “the existence of unifying biological principles underlying this behavior that could be traced across species,” from flies to elephants to humans. However, this is only one piece to the puzzle, and the team now plans to work on identifying the exact brain response after mating that leads to these strange salt cravings.

Source: Walker SJ, Corrales-Carvajal VM, Ribeiro C.Postmating circuitry modulates salt taste processing to increase reproductive output in Drosophila. Current Biology. 2015

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