Maybe you know a smoker who never exercises and lives happily into old age. Or maybe you’ve read about a vegetable-loving marathon runner who had a myocardial infarction in middle age.

These types 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 exercising regularly have a big effect on age-related health problems such as heart disease, type 2 diabetescancer and Alzheimer’s disease, says Zimmermann.

So it’s just about lifestyle? Not quite.

Your genes, Zimmerman says, can increase your risk of many diseases. But they usually don’t work alone. And rarely is it 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 contract a disease, he says.

And even that is not written in stone. That is, these genetic characteristics (genetic predisposition) may not affect you at all unless they are caused by certain aspects of your environment (environmental factors) or your lifestyle.

These “epigenetic changes” influence the way your genetic material, or DNA, functions in your body. An epigenetic change occurs when lifestyle or environmental factors cause a particular gene to be “turned on” or “off.” In the case of cancer, for example, such changes could activate a gene that allows abnormal cells to grow or the changes could change. off a gene that would suppress their growth.

To complicate matters further, each gene does not have a single switch. Or even half a dozen switches. “There may be hundreds or thousands,” says John Kelly, MD, MPH, president of the American College of Lifestyle Medicine.

This complexity makes it more difficult for scientists to determine exactly which epigenetic change is to blame and exactly how it increases the risk of a particular disease. But experts are confident that lifestyle factors, such as poor diet, smoking and lack of exercise, play a role. “They’re actually pushing gene expression into negative territory,” Kelly says.

They play such an important role in so-called “diseases of aging,” such as diabetes and heart disease, Kelly says, that they are best described as “diseases of a toxic lifestyle over time.”

For example, heart disease is the leading cause of death among men and women in the US. However, some research shows that it may be possible to prevent 80% of heart disease. Because?

“I have a colleague who says coronary heart disease is a foodborne illness,” Kelly says. “And you are right! “For the vast majority of people, it is caused by food and can be reversed with food.”

Food affects your health directly through nutrition. It also has an indirect link through heart disease risk factors such as obesity and high blood pressure. For example, there are more than 300 genetic variants that could increase the risk of high blood pressure. But even with high-risk genes, you can often reduce your risk of heart disease by a third with a healthy diet and regular exercise. (You may also need medication to lower your blood pressure; check with your doctor about this.)

For optimal heart health, experts recommend a plant-based diet. That doesn’t necessarily mean you have to become vegan or even vegetarian, says Zimmermann. “The goal is to replace some processed foods with whole foods, including fruits and vegetables.” Look for whole grains, lean proteins (including nuts and seafood), and check the packaging for added salt and sugar. Talk to your doctor if you’re not sure how to design your own heart-healthy diet.

To exercise, experts recommend 150 minutes of moderate physical activity per week. That’s 2 hours and 30 minutes a week, less than 30 minutes a day. And you don’t have to run the Boston Marathon. A walk around the block, a little gardening, or even dancing should do the trick.

Spend this little time and you can reduce your risk of a number of diseases, whether you have a genetic predisposition to any of them or not. And yet, Zimmermann says, most people simply don’t do it.

Smoking is one of the biggest risk factors for heart disease. And that applies whether you’re genetically predisposed to heart disease or not. If you smoke, consider quitting, especially if you have heart disease or are at risk for it.

Your doctor can also help you find a program that will help you give up smoking.

Your health habits and environment interact with your genes in similar ways in other conditions, such as type 2 diabetes, obesity, high blood pressure, and, to a lesser extent, some types of cancer.

Type 2 diabetes tends to run in families. There are several genetic variations, including KLF14, ENPP1 and many others, that increase the risk of developing diabetes by up to 30%. But dietary factors, including how much alcohol you drink, whether you are a smoker, and your activity level, have the potential to tip the scale one way or another, perhaps turning a gene on or off.

It is estimated that 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 blood sugar-lowering medication metformin.

There is no doubt that genetic factors play a role in many cancers. Women who have a mutation in the BRCA1 or BRCA2 gene, for example, have a 45% to 72% chance of developing breast cancer during their lifetime, which is much higher than average. But many women without known genetic mutations also get breast cancer.

Perhaps most importantly, there is evidence that good lifestyle habits can help reduce the risk of cancer in people with and without a genetic predisposition to various types of cancer.

While a healthy lifestyle alone does not prevent all cancers, increasing evidence suggests it plays an important role. A study led by Cancer Research UK found that around 4 in 10 cancers could be prevented through smart lifestyle choices, such as not smoking, maintaining a healthy weight and eating a healthy diet. Limiting sugar, alcohol, and time in the sun can also help.

It is true that certain mutations of the APOE gene increase the risk of Alzheimer’s disease. If you have a copy of a version called APOE4, you are two to three times more likely than average to develop this condition. People who have inherited two copies could have a 12-fold increased risk.

But it is possible and even common to suffer from Alzheimer’s if you do not have an APOE mutation. While other as-yet-undiscovered genetic factors could play a role, research suggests that lifestyle is an important factor. A meta-analysis of several studies, published in the lancet neurology, found that about a third of cases are caused by factors you can control, such as diet and regular exercise.

Once again, research shows that diet and exercise are very important. While most healthy diets should help, MIND Diet (Mediterranean Intervention-DASH for Neurodegenerative Delay) – which emphasizes brain-boosting foods like vegetables, berries, fish and olive oil – seems best.

Scientists still have a long way to go to unravel the complex interaction between genes and the environment. Zimmermann is careful to note that the impact of these factors varies between diseases and from person to person. There’s no doubt, he says, that in some cases you can do everything right and still develop serious illnesses.

The best thing you can do, Zimmermann says, is to try to change the factors that are under your control, such as diet, exercise, regular medical checkups, and taking medications as prescribed.