Should you dip your bread in oil or spread butter over it? Will those who consume oil rather than butter be healthier? Will they live longer?

One way to explore such a question is to compare populations that consume more oil with those that consume more butter. France and Italy immediately come to mind. French cuisine traditionally incorporates a substantial amount of butter (in 2024 Frenchmen consumed 8.38 kilograms of butter per person compared with only 2.4 kilograms in Italy), while Italians consume much more olive oil than Frenchmen (9.08 liters vs 1.60 liters annually).

The comparison between the two countries is tempting because France and Italy are similar in many ways: Both are European nations with Mediterranean coastlines, rich cultural traditions, and a shared history that dates back to the Roman Empire–after all, Rome wasn’t built in a day and neither was Paris.

With so many similarities and such a sharp difference in butter and oil consumption, France and Italy seem like the perfect case study for understanding how the consumption of these ingredients affects health and longevity.

It turns out that the average Italian lives just a bit longer than the average Frenchman. The average life expectancy in Italy is 84.04 years compared to 83.95 years in France. The difference is small, about a month of life, and probably isn’t statistically significant.

Thinking about all that, I almost slipped into a fatalistic mood. If the health effects of oil versus butter boil down to a single month of life, does it matter whether I dip my bread in oil or spread it with butter? No! It doesn’t matter! Then, I read more about the subject and my perspective quickly changed.

What changed my perspective was a recent study by Yu Zhang and his colleagues published in JAMA Internal Medicine on March 6, 2025. The study followed a total of 221,054 individuals for up to 33 years. The researchers collected information from three large databases: two Nurses’ Health Studies and the Health Professionals Follow-up Study. The participants were men and women who were healthy at the time the study began. They were asked to report every 4 years–using structured questionnaires–on their intake of butter (added at the table and during cooking), and their consumption of plant-based oils (including safflower, soybean, corn, canola, and olive oil).

The participants were then divided into four equal groups based on their butter and oil consumption. Participants in the group with the highest butter intake had a 15% higher risk of total mortality compared with those in the lowest butter consumption. In contrast, participants in the group with the highest intake of total plant-based oils had a 16% lower mortality than those in the lowest intake of total plant-based oils.

The researchers also found that higher consumption of butter was associated with higher cancer mortality and that higher plant-based oils were associated with lower risks of cancer and cardiovascular disease (including heart attacks and strokes). They estimated that substituting 10 grams of daily butter intake with an equivalent amount of total plant-based oils would be associated with a 17% reduction in total mortality and a 17% reduction in cancer mortality.

In their conclusions, Yu Zhang and his colleagues suggested that “Substituting butter with plant-based oils may confer substantial benefits for preventing premature deaths.”

Yu Zhang’s study is observational. Like all observational studies, it can only indicate a correlation rather than a cause-and-effect relationship. Still, the study is powerful. It is vast in scope, with a very large number of participants followed for many years. Its conclusions are therefore likely to point us in the right direction.

Remember my attempt to draw a conclusion about the effect of butter and plant-based oils on longevity based on limited information about Italy and France? Yu Zhang’s study reminds us that our reliance on intuition, limited observations, and half-baked analysis is often misleading. A deeper dive and analysis are more likely to bring us closer to the truth.

Shahar Madjar, MD, MBA, is a urologist and an author. He practices in Michigan, at Schoolcraft Memorial Hospital in Manistique, and in Baraga County Memorial Hospital in L’Anse. Find his books on Amazon. Contact him at smadjar@yahoo.com.