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Monday, May 26, 2014

The Mystery of Pregnancy Cravings


And Jacob took him rods of green poplar, and of the hazel and chestnut tree; and pilled white strakes in them, and made the white appear which was in the rods.And he set the rods which he had pilled before the flocks in the gutters in the watering troughs when the flocks came to drink, that they should conceive when they came to drink. And the flocks conceived before the rods, and brought forth cattle ringstraked, speckled, and spotted.And Jacob did separate the lambs, and set the faces of the flocks toward the ringstraked, and all the brown in the flock of Laban; and he put his own flocks by themselves, and put them not unto Laban's cattle.And it came to pass, whensoever the stronger cattle did conceive, that Jacoblaid the rods before the eyes of the cattle in the gutters, that they might conceive among the rods.(Genesis 30:37-41)

Have you ever heard stories of pregnant women craving pickles and ice cream? Or seen the media portraying pregnant women demanding that their partners get Ben and Jerry’s ice cream at 3am? Such anecdotes become more outlandish - pregnant women fighting in Trader Joe’s over the last box of multi-grain waffles, losing control at the smell of chocolate, and driving through hailstorms to get curly fries. As we cross cultural boundaries, the specifics of the stories change, but cultures almost universally offer some sort of explanation as to why cravings exist and legitimize the occasional bizarre craving behavior.

When Queen Jane Seymour became pregnant in 1537, King Henry VIII did not have a male heir to the throne since his two previous wives had been unable to bear children. With a vested interest in the stability of Jane’s pregnancy, Henry went to great lengths to import live quail from France in order to satisfy the Queen’s cravings, since tradition stipulated that satisfied cravings made for safer pregnancies. This grand gesture seemed to pay off, as future heir Edward VI was born later that year. However, in a strange twist of fate, Jane died twelve days later. Rumor had it that her extravagant cravings had caused her death.

Why should science concern itself with in pregnancy cravings? From the perspective of advancing knowledge, we simply do not know what causes pregnancy cravings. Societies have furnished causal explanations that made sense within their cultural context for millennia. However, science has had difficulty evaluating these claims since a craving cannot be easily quantified or measured. From a more practical standpoint, being able to study cravings scientifically would allow us to better understand the state of the body during pregnancy and offer nutritional guidelines that may improve maternal diets.

Most women experience food cravings at some point during pregnancy. While these cravings might be for a specific type of cuisine or unusual food, they typically include the desire for sweet, salty, spicy or fatty foods. Sometimes, even nonfood products serve as cravings. While these cravings are most likely the result of hormonal and metabolic changes that take place in a woman’s body, some propose that the baby’s sex can be revealed based on the nature of these cravings.



Woman ususally have cravings when they are pregnant. 

Cravings are a well-known and expected symptom of pregnancy. Several theories exist for such food related urges. Hormonal changes are known to alter taste and smell, which might explain the desire for specific, sometimes unusual foods. A temporary nutritional deficiency because of the high nutrient needs of the baby is also a possibility. A lack or shortage of specific nutrients might lead to the heightened need to consume these nutrient-rich foods. Food intake is also known to be connected to women’s emotions. During pregnancy, women might crave specific foods, consciously or unconsciously, as a response to emotional needs. Another explanation is that cravings serve as an early means of deciphering the baby’s sex as certain foods correlate to the sex of the baby.

Medical Interpretations: Hormones and Orexigenic Agents

With advances in the sciences, we can begin to bridge the gap between culture and scientific theory using the physiology behind pregnancy cravings. During pregnancy, women are resistant to leptin, an endocrine growth factor. Leptin reduces appetite and stimulates metabolism by interacting with receptors at the “appetite center” of the hypothalamus. Without leptin, multiple orexigenic pathways, or pathways that stimulate hunger, are left unopposed. This causes neuropeptide Y, one of the most potent orexigenic agents known, to have significantly decreased immunoreactivity during pregnancy, and this is thought to contribute to increased food intake.

“Neuropeptide Y increases hunger.

Studies have also been performed to understand women’s changing sense perception during pregnancy. Some of the first signs of pregnancy include “increased sensitivity to odors” and “a metallic taste in your mouth.”Abnormal smell and/or taste perception is reported in roughly 75% of women.It has been hypothesized that these altered gustatory and olfactory thresholds are caused by a change of neuronal activity within the female brain. Because of uterine expansion during pregnancy, adjacent uterine and gustatory neurons may be interacting with one another.In any case, cravings appear to be affected by smell and taste sensitivity, and thus have a direct biological basis.

Challenges for Research

While science has shown that the physiological changes accompanying pregnancy cravings have a neuronal and hormonal basis, it has yet to offer a strong theory as to how these cravings have come about. Is the Western notion of causality due to “wisdom of the body” valid? In order to determine if Queen Seymour’s cravings were a result of a nutritional deficiency, it would by hypothetically necessary to establish that (a) she had such a deficiency (b) quail had a nutrient that satisfied it and (c) the deficiency preceded her consumption of quail, so as to exclude reverse causation. Since this question involves a human subject whose cravings cannot be manipulated in a laboratory setting, population-based nutritional epidemiology may hold the answer.

“Nutritional epidemiology may hold the answer to our craving questions.

I am applying the conceptual methodology outlined above to my current thesis project. In order to measure a nutritional deficiency (part a), I could theoretically use biochemical blood or tissue indicators to measure the amounts of various nutrients in the blood. However, analyzing these biochemical indicators on a large scale would require expensive blood analyses, and some nutrients such as total fat have no indicators. Thus, nutritional epidemiologists utilize noninvasive instruments like food records and dietary journals to calculate the consumption of nutrients based on what food was eaten. In order to find out if the food craved contains the nutrient (part b), its nutrient profile would be determined from nutritional databases. Finally, the deficiency must precede the craving in order to determine a causal relationship (part c).

While we do not yet know the mechanisms that led to Queen Jane’s cravings, comparative cross-cultural studies have shed insight on the myriad of causal interpretations offered. Advances in nutritional epidemiology also hold promise for assessing the validity of the “wisdom of the body” hypothesis, which would lead to better nutritional guidelines regarding maternal diets. Until then, it’s probably best to satisfy that craving for quail!

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