People who buy organic food are usually convinced it is better for their health, and they are willing to pay for it. But evidence of the benefits of eating organic has been lacking, until now.

A French study that followed 70,000 adults, most of them women, for five years reported that the most frequent consumers of organic food had 25 percent fewer cancers overall than those who never ate organic. Those who ate the most organic fruits, vegetables, dairy products, meat and other foods had a particularly steep drop in the incidence of lymphomas, and a significant reduction in postmenopausal breast cancers, said the study, which was published in JAMA.

The magnitude of protection surprised the researchers. “We did expect to find a reduction, but the extent of the reduction is quite important,” said Julia Baudry, the study’s lead author and a researcher with the Center of Research in Epidemiology and Statistics at the Sorbonne in Paris. She noted that the study does not prove an organic diet causes a reduction in cancers, but strongly suggests “that an organic-based diet could contribute to reducing cancer risk.”

Nutrition experts from Harvard University who wrote an accompanying commentary expressed caution, however, criticizing the researchers’ failure to test pesticide residue levels in participants in order to validate exposure levels. But they called for government bodies like the National Institutes of Health to fund research to evaluate the effects of an organic diet, saying there is “strong enough scientific rationale, and a high need from the public health point of view.”

“From a practical point of view, the results are still preliminary, and not sufficient to change dietary recommendations about cancer prevention,” said Dr. Frank B. Hu of Harvard’s T.H. Chan School of Public Health. He said it was more important for Americans to simply eat more fruits and vegetables, whether the produce is organic or not, if they want to prevent cancer.

The only other large study that has asked participants about organic food consumption with reference to cancer was a large British study from 2014. While it found a significantly lower risk of non-Hodgkin’s lymphoma among women who said they usually or always ate organic food, it also found a higher rate of breast cancers in the organic consumers — and no overall reduction in cancer risk.

The authors of that study, known as the Million Women study, said at the time that wealthier, more educated women, who were more likely to purchase organic food, also had risk factors that increase the likelihood of having breast cancer.

For food to be certified organic by the Department of Agriculture, produce must be grown without the use of most synthetic fertilizers and pesticides and may not contain genetically modified organisms. Meat must be produced by raising animals fed organic food without the use of hormones or antibiotics. U.S. sales of organic food increased to $45.2 billion last year, the Organic Trade Association said.

A representative of the Alliance for Food and Farming, a group that seeks to allay public concerns about pesticides, said consumers should not worry about cancer risks from consuming conventionally grown fruits and vegetables, citing “decades of peer-reviewed nutritional studies.”

For the new study — paid for by public and government funds — researchers recruited 68,946 volunteers who were 44, on average. The vast majority, 78 percent, were women.

Participants provided information — more detailed that the Million Women study — about how frequently they consumed 16 types of organic foods. The researchers asked about a wide range of foods, including fruits, vegetables, dairy and soy products, meat, fish and eggs, as well as grains and legumes, bread and cereals, flour, oils and condiments, wine, coffee and teas, biscuits and chocolate and sugar, and even dietary supplements.

They also provided information about their general health, occupation, education and income.

The study found that the most frequent consumers of organic food had 76 percent fewer lymphomas, with 86 percent fewer non-Hodgkin’s lymphomas, and a 34 percent reduction in postmenopausal breast cancers.

The reductions in lymphomas may not be all that surprising. Epidemiological studies have consistently found a higher incidence of some lymphomas among people like farmers who are exposed to certain pesticides through their work. One reason an organic diet may reduce breast cancer risk is because many pesticides are endocrine disrupters that mimic estrogen function, and hormones play a causal role in breast cancer.