DNA is the genetic material that makes us who we are, determining our physical characteristics and even helping to shape our personality. There are many ailments that have a strong hereditary component—Alzheimer’s, Huntington’s Disease, cancers and diabetes among others—and the risk of suffering them is passed down from our parents through our DNA.

But we’re finding out that our DNA isn’t always set in stone. Now, a team of researchers from Northwestern University led by anthropology professor Thom McDade have shown that DNA can also be modified by your environment during childhood. What’s more, the authors conclude in the journal Proceedings of the National Academy of Sciences, those modifications can affect how or when you develop certain illnesses during adulthood.

Their investigation followed more than 500 children in the Philippines and found that certain childhood situations can create modifications in genes associated with inflammation, which affects how prone we are to suffer from certain illnesses. Specifically, these factors included socioeconomic status, the prolonged absence of a parent, the duration of breastfeeding, birth during the dry season, and exposure to microbes in infancy.

But what exactly do the findings mean?

DNA is, in essence, a really long text made up of a 4-letter alphabet that our cells use as an instruction manual for making proteins. The order in which the letters are arranged (the DNA sequence) defines the genes that a person has, which remain the same throughout that person’s body. Despite that, only some genes (or sentences in the DNA text) are necessary for each cell type to function.

If genes are sentences within the DNA text, epigenetic marks are like differently colored highlighters that indicate which genes a cell should express (importantly, they do not change the sequence of the DNA). The most important of these marks is methylation, or the addition of a methyl group to the DNA molecule, which promotes or inhibits the expression of certain proteins depending on which gene it is on and where on the gene it is located.

“We could have genes in our bodies that might lead to some bad outcomes or adverse health outcomes, but if those genes are silent, if they’re turned off due to epigenetic processes, that can be a good thing,” explains McDade, principal author of the PNAS study.

McDade adds that, for the most part, once a gene is methylated it remains permanently methylated. Although it is not quite clear how a person’s childhood environment causes the methylation of some genes, it is possible to investigate its effect.

How environment impacts health

Inflammation—the body’s reaction to infections and wounds—plays a central role in human health. It is an important player in many fatal diseases related to old age, like diabetes, cardiovascular diseases and dementia. What’s more, there is increasing evidence that inflammation levels during pregnancy can affect the baby’s weight, or influence whether a baby is born prematurely.

The body must be able to mount an inflammatory response against different threats and threat levels. McDade compares the job of inflammation to the job of firefighters.

Let’s assume the fire is an infection or an injury and the fire department is the inflammatory response. You want the fire department to come as fast as possible and to use the least amount of water to put out any fire, and then you want them to leave. You don't want them to come into your house with more firefighters than needed and to hose everything down to put out a small fire; nor do you want them to show up to a massive fire with just a bucket of water. Think of the potential damage in either scenario.

The researchers focused on this bodily function for two reasons. First, previous research has shown that childhood environments can cause improper regulation of inflammation during adulthood. Second, they had access to literally lifetimes’ worth of data from a cohort of babies in the Philippines that they could mine for methylation and inflammation data.

This cohort comprised over 3,000 pregnant women recruited in the Philippines in 1983. These women came from all different walks of life: They differed in access to clean water or a roof over their heads, whether they lived in an urban or a rural area, and whether they came into frequent contact with animals. From the data, they looked at over 500 of those women in order to figure out if their child’s environment growing up led to epigenetic modifications to their DNA—and later to a change in inflammatory proteins in their blood in adulthood.

Once their children were born, the investigators kept track of them and of the environments they were exposed to throughout their lives. Once they turned 21, the investigators took a blood sample that they used to measure the DNA methylation throughout their genome, as well as inflammation-related proteins that have been previously associated with cardiovascular diseases and other aging-related diseases.

The authors determined that the childhood environment of these youths affected the level of inflammation-related proteins (biomarkers) in their blood during adulthood, likely as a result of methylation of some of their inflammation-related genes. The dysregulation of these proteins can affect health and risk of disease.

The nutritional, microbial, psychological and social environments that children are exposed to growing up are critical for their physiology and health later in life, says McDade. As to the effects of specific childhood environments, he pointed to prolonged breastfeeding, exposure to microbes, and an abundance of family assets that led to better regulation of the inflammatory proteins.

In turn, the prolonged absence of a parent, the lack of exposure to microbes, and the lack of family assets were predictive of a higher dysregulation of the inflammatory proteins.

This is not the first time research has shown that a child’s environment growing up can help determine his or her future health. This isn’t even the first time that scientists have linked environment to DNA methylation and methylation to health (these studies have been done in mice). This is, however, one of the first and most complete investigations that show that epigenetic modifications created by the environment have lasting effects on human health.