What are these mysterious pregnancy food cravings? Why do pregnant women experience them? Is it the body demanding what it needs, or is it just an unhinging of secret yearnings, normally frowned upon and now exonerated by the virtue of pregnancy?
Unfortunately for us ice cream lovers, the second option seems to be the current dominant view. In fact, if you search for scholarly articles about pregnancy and food cravings you will immediately come across titles such as “Intake of sweets, snacks and soft drinks predicts weight gain in obese pregnant women” and “Early pregnancy cravings, dietary intake, and development of abnormal glucose tolerance”. In other words, pregnancy cravings seem to cause unhealthy weight gain and related undesirable health conditions.
At the same time though, it has been established that pregnancy alters women’s sense of smell and taste. As many other pregnant women, I can testify that for the past few months I have been experiencing a heightened sense of smell that has been modulating my food preference to no small degree. Doesn’t it stand to reason that these changes in sensation might lead to food cravings? Well…I’m not sure…so when trying to investigate this problem, I spoke to Carlos Ribeiro, a principal investigator at Champalimaud Centre for the Unknown. Dr Ribeiro uses flies to study the neural mechanisms that control feeding behaviour and recently published a study looking at salt craving in mated female flies.
“In one of the projects in the lab we studied what happens in the brain during pregnancy to bring about food cravings. More specifically, we asked how does the brain know what the body needs and then, what changes in the brain’s perception of the world that leads females to seek out certain foods during pregnancy?”
To pursue the answers to these challenging questions, Carlos Ribeiro’s team chose to study the neural basis of changes in nutrient intake in female fruit flies after mating. “Nutrition is a highly complex topic. To understand how the brain regulates nutrient intake, you need to work on an organism that gives you access to a lot of diverse technologies. In that respect, the fruit fly is unbeatable” said Dr Ribeiro. “We wanted to exploit these tools to uncover how the female fly’s food preferences changed after mating. Many mammalian species increase their preference for salt during pregnancy; but until now it was not known if the fruit fly shared this behavior.”
Surprisingly, even though different levels of salt directly influenced offspring production, the researchers discovered that the salt craving was not based on the precise physiological needs of the body.
Through a series of experiments, the researchers revealed, for the first time, not only that flies indeed share mammals’ inclination towards salt during pregnancy, but also that higher dietary salt levels result in increased offspring production.
Surprisingly, even though different levels of salt directly influenced offspring production, the researchers discovered that the salt craving was not based on the precise physiological needs of the body. On the contrary, “even if their egg production was disabled, mated females showed increased salt preference” said Dr Ribeiro.
It appears that the brain of the female knows that she will need more salt to produce eggs and so it automatically changes the way the animal perceives salt to allow it to ingest higher amounts of this important nutrient. Just as in humans, the ‘tongue’ of the female fly becomes much more responsive to the taste of salt, leading her to prefer saltier food. The key question for the researchers was then: “What is the biological mechanism that leads to this change in salt sensation in pregnant animals?”
According to the leading researcher in this study, Samuel Walker, a molecule called sex peptide, injected during mating by the male into the female, manipulates female taste perception (Figure 1). “The molecule activates neurons in the female’s uterus. From there, we found that a short chain of neuronal interactions signals the brain to ‘dial up’ the salt preference.”
Figure 1. During mating the male injects a molecule called sex peptide into the female which manipulates taste perception.
By piecing together this complicated puzzle, the researchers were able to demonstrate that flies display salt-craving similarly to mammals, and that this craving plays an important role in their reproductive abilities. They were also able to identify the trigger for salt-craving and map several steps in the neural circuitry that brings about this behavioral change. “Now”, concluded Dr Ribeiro, “we move on to the next question, which is to identify how the brain’s response to salt changes after mating to bring about this cross-species behavior. We will continue using the fruit fly, an organism that is unbeatable in its array of genetic tools, essential to understand a topic as complex as nutrition.”
So what does this all mean for pregnant women?
Is it time for a salty pretzel? Maybe not. This study actually makes me think that perhaps, in some cases, the body doesn’t know what it needs. Maybe pregnancy just puts our brains in an evolutionary programmed gear that might not be as relevant as it used to be some time ago. This is probably especially true when it comes to women living in the western world who have access to rich and diverse foods that offer a wide range of nutrients. Not to mention that salt is an abundant nutrient in the western diet and is in fact associated with negative health consequences, such as increased blood pressure, when consumed in excess. So I would take my cravings with a pinch of salt and just try to keep a balanced diet which I am sure will keep my body and baby well enough.
For the original study see: Walker SJ, et al. (2015) Postmating circuitry modulates salt taste processing to increase reproductive output in Drosophila. Current Biology 25(20):2621–2630.
Liad Hollender works at the Science Communication office at the Champalimaud Neuroscience Programme.
Edited by: Ivo Marcelo (section editor), Clara Ferreira (editor-in-chief)