Appetite and stress

The tendency to emotional overeating or malnutrition is not inherited

Alyona Sukhoputova, "Elements"

Fig. 1. When appetite increases in response to some experiences, we talk about emotional overeating, and when, on the contrary, it decreases, we talk about emotional malnutrition. Photos from the website deti.mail.ru

A large-scale project is underway in the UK to study eating behavior using the twin method. One of the first studies within the framework of this project showed that both overeating and malnutrition in response to negative emotions are practically not genetically fixed. It also turned out that the same children can both undereat and overeat – and it depends on the situation. As a result, it became clear that changes in appetite in response to emotions in children are formed under the influence of their families.

One of the questions that researchers often ask is what is the heritability of some individual trait. A sign here means any property of the body: from the color of the hair and the tan acquired this summer to a tendency to aggression or a changeable appetite. For example, some people during an emotional experience begin to eat more than usual, some – less, and someone's appetite does not depend on the mood in any way. Questions arise: why is this happening? are genes to blame for this or upbringing? As a rule, there is no unambiguous answer, since every organism is a product of the interaction of genes with external conditions during its development.

Whenever you encounter headlines like "The obesity gene has been found," you need to remember that the true picture is much more complicated: there are no genes that alone would be the "obesity gene," the "aggression gene," etc. The work of all these genes depends on external conditions, on the work of other genes, on various accidents, in particular eventually. So, for example, some "tall growth gene" (for example, hmga2 with cytosine) will not be able to manifest itself with serious malnutrition in childhood, and the "black hair gene" (normal mc1r) will be powerless in case of complete baldness. At the same time, understanding how much genetics determines our appearance, behavior and, most importantly, diseases is simply necessary for the development of medicine: for the treatment and prevention of diseases, for risk assessment during family planning. That's where this very heritability comes in handy.

To be precise, heritability is the proportion of phenotypic variability in a population due to genetic variability. Simply put, – how much on average a trait depends on genes. If the heritability is equal to one, then the trait under study does not depend on external conditions at all and is completely formed under the influence of genes. Such an ideal situation is almost impossible, but, for example, about such a complex trait as gender in mammals, we can say that it is almost 100% determined by genes. But there will be exceptions here too. Heritability equal to zero, when the trait is completely determined by the environment, is much more common. So, it does not depend on genes, for example, the presence or absence of a scratch on your left wrist. Most of the cases of interest to researchers are somewhere between these two extreme options. There are no exact boundaries, but it is customary to talk about low, medium and high heritability. In some studies, low heritability is considered to be up to 20%, in others – up to 40%, and high heritability can start from 40-70%. Everything depends on the studied feature and is known, so to speak, in comparison.

How to understand what is in a complex organism from genes, and what from the environment? A number of ingenious ways to assess heritability rely on a relatively rare event for people – the birth of twins. If the twins are monozygotic, that is, they developed from the same egg and have 100% the same genes, then all the differences between them can be attributed to the action of the environment. Twins who were separated in infancy, despite the difference in environmental conditions, show common signs (which have high heritability), as well as – despite genetic identity – different signs (they have low heritability). Dizygotic twins, who developed due to fertilization of two different eggs with two different spermatozoa, have about 50% of common genes (as do "simple" siblings). It is important that if they were not separated in childhood, then most likely they grew up in almost the same conditions, which cannot be said about children born in different years. It turns out that the differences in the environment of children's twin pairs of any type are approximately comparable, but the genetic differences are twice as high in dizygotic couples. Therefore, if a certain trait differs between dizygotic twins more often than between monozygotic twins, then it's about genes.

In connection with the described possibilities of the twin method, scientists are trying to attract as many pairs of twins as possible for research, because more statistics are needed to obtain more accurate data. So, the Gemini project was created in the UK. From March to December 2007, the founders of the project contacted all families in England and Wales who had twins during this period and asked them to participate in a number of studies. A total of 2402 families (4804 twins) agreed to participate. Among these pairs there were 749 monozygotic and 1616 dizygotic, the type of the remaining 37 pairs of twins is unknown. It may seem strange that for some couples belonging to dizygotic or monozygotic twins is not established. But in fact, it is really surprising that for the remaining two thousand (!) pairs, this affiliation is established. To conduct a complete DNA comparison in all these cases is still an unaffordable task from a financial point of view. And determining the zygosity of infants and young children by appearance is difficult not only for researchers, but also in many cases for the parents themselves (Fig. 2). After all, the younger the age, the more children resemble each other.

Fig. 2. Gemini from the Gemini project. What do you think, in which of the photos are identical twins? Photos from the website geministudy.co.uk

In order to be able to conduct mass studies not only on adult twins, but also on children, a questionnaire was developed for parents to help establish the zygosity of children from 1 year old. There are 18 questions in it and the main focus is on physical signs, about which it is known that they have high heritability (like the shape of the earlobe). In addition, parents are asked questions about the ability to distinguish their children, for example, by a photo. Already during the development of the questionnaire, its high efficiency was shown: the test results coincided with the DNA analysis data by 95% (T. S. Price et al., 2000. Infant zygosity can be assigned by parental report questionnaire data). However, the organizers themselves tested this test for accuracy by sequencing the DNA of 81 pairs of twins from the project. They got a complete match of the results.

As part of the Gemini project, various surveys of parents about the eating behavior of their children were also conducted. In 2012, 1,027 families took a 35-question test on their children's eating habits (CEBQ). The questions in it are: "Does your child eat more when he is annoyed?" or "Does the child's appetite decrease when he is upset?" etc. They are aimed at identifying the tendency to overeating or malnutrition during an emotional experience.

These two eating habits have interested scientists in connection with their possible impact on health: the first habit, both in children and adults, is associated with overweight, the second with a lack of weight. However, studies confirm only the association with overweight (L. Webber et al., 2009. Eating behavior and weight in children), while there is almost no data on the consequences of emotional malnutrition in the literature. It is still not known why these habits are formed. Someone considers them innate, someone - the result of imitation of parents or the consequence of excess or lack of food in infancy, etc. The authors of the work decided first of all to find out what is the heritability of inclinations to these habits: how much they are formed by genes, and how much by the environment.

It turned out that there is a significant correlation between the propensities to overeating and malnutrition in five-year-olds, that is, often the same child is simultaneously prone to both. Why he chooses this or that action in each particular case is not yet clear. The questionnaire by "emotional experience" means different things, for example, sadness or irritation, so it is quite possible that different types of behavior manifest themselves in response to different emotions.

Differences in eating habits between monozygotic twins living together turned out to be very low. The researchers calculated that no more than 2% of the tendency to emotional overeating or malnutrition is determined by external conditions that are different for twins of the same family. Most importantly, these differences turned out to be almost the same for twins in monozygotic and dizygotic pairs. The small difference, which was still caught, indicates only a 7 percent contribution of genes for both types of behavior. This is unexpectedly small. And the contribution of external conditions common to twins – the habits of parents and their parenting methods, the financial situation of the family, etc. – as a result amounted to 91%.

This truly grandiose project demonstrated that the tendency to change eating behavior in response to emotional experience is formed in children in the family, and not under the influence of genes. Perhaps a special role here is played by the parental strategy to encourage or soothe the child with delicious. Some correlations of this strategy with emotional overeating have already been shown for school-age children (S. Steinsbekk et al., 2016. Emotional Feeding and Emotional Eating: Reciprocal Processes and the Influence of Negative Affectivity). Excessive attention to what a child eats and how much he eats also correlates with emotional overeating. Less is known about the formation of an emotional decline in appetite, but the data of the work under discussion speak about the commonality of the two eating habits. It seems that both one and the other side of this coin is formed in children by their family, and by the same methods. The article does not give recommendations on parenting, but it makes you think about it.

Source: Herle et al., Emotional over-and under-eating in early childhood are learned not inherited // Scientific Reports. 2017.

Portal "Eternal youth" http://vechnayamolodost.ru  15.09.2017