Maybe you know a smoker who never exercises and is living happily into old age. Or perhaps you read about a veggie-loving marathon runner who had a heart attack in middle age.
These kinds of stories can’t help but make you think: If your health is simply written into your genes, what’s the point of all that exercise and healthy eating? Why not just do what you want?
But, says Laura Zimmermann, MD, medical director of the Rush University Prevention Center, these stories stick with us because they are unusual. They are examples of what scientists call “outliers.”
The reality is that, for most people, lifestyle habits like eating healthy foods and getting regular exercise have a big effect on age-linked health problems like heart disease, type 2 diabetes, cancer, and Alzheimer’s disease, Zimmermann says.
So it’s only about lifestyle then? Not exactly.
Your genes, Zimmerman says, can raise your risk for many diseases. But they do not typically work alone. And it’s rarely a single gene that determines whether you will get a disease or not. More often you inherit some genetic characteristics that make you more likely to get a disease, she says.
And even that’s not written in stone. That is, these genetic characteristics (genetic predisposition) might not affect you at all unless they are triggered by certain aspects of your surroundings (environmental factors) or your lifestyle.
These “epigenetic changes” influence the way your genetic material, or DNA, works in your body. An epigenetic change happens when lifestyle or environmental factors cause a particular gene to turn “on” or “off.” In the case of cancer, for instance, such changes might turn on a gene that enables abnormal cells to grow. Or the changes could turn off a gene that would suppress their growth.
To make matters more complicated, each gene doesn’t have just one switch. Or even a half dozen switches. “There might be hundreds or thousands,” says John Kelly, MD, MPH, president of the American College of Lifestyle Medicine.
This complexity makes it harder for scientists to figure out exactly which epigenetic change is to blame and exactly how it raises your risk for a particular disease. But experts are confident that lifestyle factors including poor diet, smoking, and lack of exercise play a major role. “They are actually driving gene expression into negative territory,” says Kelly.
They play such a big role in so-called “diseases of aging” – like diabetes and heart disease, Kelly says, that they are better described as “diseases of toxic lifestyle over time.”
For example, heart disease is the No. 1 killer of both men and women in the U.S. Yet some research shows it may be possible to prevent 80% of heart disease. Why?
“I have a colleague who says that coronary heart disease is a foodborne illness,” says Kelly. “And he’s right! For the vast majority of people, it’s caused by food and can be reversed by food.”
Your health habits and environment interact with your genes in similar ways for other conditions, including type 2 diabetes, obesity, high blood pressure, and to a lesser extent and some types of cancer.
Type 2 diabetes tends to run in families. There are several genetic variations including KLF14, ENPP1, and numerous others that increase the risk of developing diabetes by up to 30%. But dietary factors, including how much alcohol you drink, whether you’re a smoker, and how active you are all have the potential to tip the balance one way or the other, perhaps by turning a switch on a gene on or off.
An estimated 9 out of 10 cases could be prevented through healthy lifestyle changes.
The landmark Diabetes Prevention Program found that people could reduce their risk of developing diabetes by about 65% with a careful diet and exercise plan compared to just 35% with the blood-sugar lowering drug metformin.
There’s little doubt that genetic factors play a role in many cancers. Women who have a BRCA1 or BRCA2 gene mutation, or instance, have a 45%-72% chance of developing breast cancer in their lifetime, which is much higher than average. But many women without any known genetic mutations also get breast cancer.
Perhaps more importantly, there’s evidence that good lifestyle habits can help lower cancer risk for people with and without a genetic predisposition to various types of cancer.
While a healthy lifestyle alone won’t prevent all cancers, mounting evidence suggests that it plays an important role. A study led by Cancer Research UK found that about 4 out of every 10 cancers could be prevented by smart lifestyle choices like not smoking, maintaining a healthy weight, and eating a healthy diet. Limitations on sugar, alcohol, and time in the sun may also help.
It’s true that certain mutations to the APOE gene raise the risk of Alzheimer’s disease. If you have one copy of a version called APOE4, you’re two to three times more likely than average to develop this condition. People who’ve inherited two copies might have 12 times the average risk.
But it’s possible and even common to get Alzheimer’s if you don’t have an APOE mutation. While other, not-yet-discovered genetic factors might play a role, research suggests that lifestyle is a big factor. A meta-analysis of several studies, published in the Lancet Neurology, found that about a third of cases are caused by factors you may be able to control like diet and regular exercise.
Again, research shows diet and exercise are very important. While most healthy diets should help, the MIND (Mediterranean-DASH Intervention for Neurodegenerative Delay) Diet – which emphasizes brain-boosting foods like vegetables, berries, fish, and olive oil – seems best.
Scientists still have a long way to go to unpack the complex interplay between your genes and your environment. Zimmermann is careful to point out that the impact of these factors varies between diseases and from person to person. There’s no doubt, she says, that in some cases, you can do everything right and still develop serious illness or disease.
The best you can do, says Zimmermann, is try to change the factors that are in your control, like diet, exercise, regular medical checkups, and taking your medication as prescribed.
