An apple a day keeps the doctor away — or so we thought.

New research from the University of Southern California suggests young non-smokers with fruit- and veggie-packed diets may face a higher lung cancer risk than those with less healthy habits.

“These counter-intuitive findings raise important questions about an unknown environmental risk factor for lung cancer related to otherwise beneficial food that needs to be addressed,” Dr. Jorge Nieva, a medical oncologist and lead investigator of the study, said in a press release.

In the study, Nieva and his colleagues analyzed 187 lung cancer patients under 50, who dished out details on their demographics, diet, smoking history and diagnosis.

Notably, most had never lit up — and their cancers were biologically different from those typically linked to smoking.

The researchers evaluated the quality of the participants’ diets using the Healthy Eating Index (HEI), scoring them on a scale of 1 to 100.

They found that the patients’ average score was 65, well above the national average of 57.

They were also piling their plates high with fruits, vegetables and whole grains. On average, participants consumed 4.3 servings of dark green vegetables and legumes and 3.9 servings of whole grains daily.

For comparison, the typical American adult eats about 3.6 servings of greens and legumes and just 2.6 servings of whole grains a day.

The researchers also found that young women who don’t smoke are diagnosed with lung cancer more often than men in the same age group.

Interestingly, women in the study also tended to consume more fruits, vegetables and whole grains, and had higher HEI scores than men.

“Our research shows that younger non-smokers who eat a higher quantity of healthy foods than the general population are more likely to develop lung cancer,” Nieva said.

Researchers are still exploring why a diet high in fruits and vegetables might be linked to an increased risk of early-onset lung cancer, but they have one leading theory: pesticides used in agriculture.

Farmers spray these chemicals to protect their crops from insects, weeds, fungus, disease and rodents, boosting yields and helping ensure a steady food supply.

But their widespread use has also raised concerns about potential effects on human health.

In general, pesticide exposure through food, water and air is thought to be limited, with safety thresholds established by regulatory agencies like the EPA and WHO to protect public health.

However, research suggests that long-term, daily exposure could contribute to serious health issues.

While effects vary depending on the type of pesticide, they have been associated with a higher risk of Parkinson’s, Alzheimer’s and several types of cancer, as well as reproductive, developmental and metabolic disorders.

Just look at agricultural workers, who are regularly exposed to high levels of pesticides and have been found to have higher rates of chronic illness, including lung cancer.

Because non-organic fruits, vegetables and whole grains can carry more pesticide residue than foods like dairy, meat and heavily processed fare — staples of the average American diet — researchers believe this could help explain the unexpected link.

The findings may also shed light on the changing face of lung cancer.

Once considered a disease largely affecting older male smokers, the landscape is shifting. Cigarette smoking rates in the US have fallen by more than 70% since the 1960s, and overall lung cancer cases have followed, but one group is bucking the trend.

Across the country, non-smokers under 50 are seeing rising rates, especially women.

In the US, up to 20% of lung cancer cases — about 40,000 diagnoses each year — occur in people who have never smoked or smoked fewer than 100 cigarettes in their lifetime. Women account for roughly two-thirds of those cases.

Still, Nieva stressed that more research is needed to confirm any link between pesticide exposure and lung cancer in younger people.

After all, researchers didn’t directly measure pesticide levels in the foods participants ate. Instead, they estimated exposure using existing data on average pesticide levels in food categories such as fruits, vegetables and grains.

Looking ahead, the team wants to examine pesticide levels in patients through blood or urine samples, hoping to gauge whether certain chemicals are more strongly associated with lung cancer risk than others.

“Our hope is that these insights can guide both public health recommendations and future investigation into lung cancer prevention,” Nieva said.

This doesn’t mean you should steer clear of the produce aisle. A diet rich in fruits, vegetables and whole grains is still widely seen as a cornerstone of good health, helping lower the risk of cancer and other diseases.

But there are steps you can take to reduce the amount of pesticides you’re consuming while researchers probe whether they may play a role in lung cancer risk.

That includes peeling or trimming fruits and vegetables to help remove residue from the outer skin, as well as thoroughly washing produce.

Studies have also found that pesticide levels can be reduced by anywhere from 10% to 80%, depending on the method of cooking and food processing used.