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Överkalix parish in northern Sweden lies above the Arctic Circle, with an inhospitable climate characterized by long, cold winters and short summers. The community has always been small; even today there are fewer than 4000 residents. During the 1800s and early 1900s, there were few roads in the region, and transportation during the winter was limited by the ice and snow. Because of its extreme northern location, growing crops was a constant challenge. When crops failed, the remoteness of the region limited the import of food and people starved. Crops failed frequently throughout the 1800s (for example, in 1800, 1812, 1821, and 1829) and the period of 1831–1836 was marked by total crop failure and extreme hardship. However, because of the unpredictability of weather, years of successful harvest and food abundance often followed these bad years.

In the 1980s, researchers became interested in the effects of feast and famine on the long-term health of the people of northern Sweden. They wanted to determine whether food shortages that people experienced in their childhood affected the future health of their descendants. By combing through harvest statistics, grain prices, and other historical facts, the researchers were able to determine the availability of food in the area throughout the nineteenth and early twentieth centuries. They also scrutinized the health records of the inhabitants; this was made possible by the availability of centralized medical records that Sweden maintains on all its citizens.

The researchers focused on the health of three groups of people, born in 1890, 1905, and 1920. They examined the life span of these individuals and looked at their risk of dying from cardiovascular disease and diabetes. They then tracked down their parents and grandparents, determined the availability of food when they were children, and looked for correlations between the diet of the parents and grandparents, and the health of their descendants.

What the researchers found was startling. Individuals whose parents and grandparents were exposed to food shortages as children lived longer than individuals whose ancestors were exposed to food excesses. On the other hand, people whose ancestors grew up during times of abundant food died at an earlier age and were more likely to die from cardiovascular disease and diabetes. For example, if a paternal grandfather had access to excess food as a child, his grandchildren were four times more likely to die from diabetes than the grandchildren of people who were not exposed to excess food during childhood.

How can the amount of food available to a person during childhood affect the health of his or her children and grandchildren living 20 to 60 years later? One of the principles of modern genetics is that our genes are stable (except for rare mutations) and not altered by the environment, so how can diet influence the traits of descendants for two generations? The effect of the grandfathers diet on later generations was particularly remarkable. Mothers provide their offspring with the cytoplasm of the egg and a uterine environment, as well as genes, but through their sperm fathers contribute only a set of paternal genes to their offspring.

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The researchers proposed that the effect they observed occurred through epigenetics: changes to chromatin and DNA that are heritable but do not involve alteration of the base sequence of the DNA. Epigenetic inheritance was not envisioned by Mendel nor, until recently, by most modern geneticists, but epigenetic processes appear to play an important role in the inheritance of many phenotypes. Today the study of epigenetics is the focus of intensive research.

Another question that this study raises is why would the adverse conditions of famine during childhood lower the risk of dying from cardiovascular disease and diabetes in future generations, while conditions of excess food increase the risk? One might expect just the opposite, that nutritional stress during childhood would increase the risk of dying, while excess food would lower it. Evolutionary biologists have proposed an explanation for this relationship, which has also been observed in other studies. This explanation, termed the thrifty phenotype hypothesis, is based on the assumption that information about the parental environment may be useful to the offspring, allowing them to respond in ways that increase their own survival and reproduction. This hypothesis proposes that when environmental conditions are poor for the parent, they are likely to persist and also to be poor for the offspring. Therefore, when the parent experiences hard times, natural selection favors parents that produce metabolically thrifty offspring—offspring that eat as much as possible when food is available, minimize energy expenditure, and hoard calories—because the parent’s environment predicts that there will be little food available to the offspring. This strategy was probably advantageous in the distant past, before agriculture, but it often backfires in modern society. Eating all you can, minimizing energy expenditure, and hoarding calories when food is plentiful often leads to obesity, heart disease, and diabetes, as was seen in the children and grandchildren of the people of Överkalix.