Scientists Looked at Dietary Fats and Health Problems. Their Findings May Change How You Eat.

ONE OF THE most divisive nutrition topics right now—and there are a lot of them—is fat.

Most of the recent arguments have centered on the supposed dangers of seed oils. Now, saturated fats like butter and even beef tallow have entered the chat. The two topics are linked: The anti-seed-oils faction has turned to saturated fats as a replacement, based on the belief that these fats are more “natural.”

Which brings us to a new study in JAMA Internal Medicine, which looked at the association between varying consumption levels of either butter or plant-based oils (aka seed oils), and mortality levels, heart disease, and cancer incidence over the long term.

Using data from the Nurse’s Health Study, a large-scale study started in 1976, researchers analyzed the dietary habits of more than 220,000 adults. They collected the data through participants’ food frequency recalls to determine the amount of butter and/or plant-based oils in the participants’ diets over the course of roughly 30 years. (Researchers included safflower, soybean, corn, canola, and olive oils in the “plant-based” oils group.)

The results were consistent with what previous research has already shown: People who ate the most butter had a 12% higher risk of developing cancer, and a 15% higher risk for all-cause mortality.

Conversely, the subjects who ate the most plant-based oils had a 16% lower risk for all-cause mortality, a 6% lower risk of death from heart disease, and 11% lower risk for cancer, which compounded the more plant-based oils were consumed.

Researchers also found that substituting just 10 grams of butter a day—which is less than a tablespoon—for plant-based oil, led to a 17% reduction in cancer deaths and all-cause mortality.

These conclusions seem aligned with what other studies have found.

However—and this is a big HOWEVER—there are some caveats.

First, it’s virtually impossible to draw a definitive link between specific diets—especially in a population—and health outcomes over time. Scientists just can’t keep people in a controlled lab environment for years of their lives. For long-term research like this, we have to rely on self-reported food intake, and at least some of that data may be inaccurate: People can forget, omit, or even lie about what they’ve been eating.

Second, other factors may muddle the link. People who eat more butter, for instance, may also do more of other things that increase their risk for disease. They may eat a more processed diet, be more sedentary, or be more likely to smoke.

Third, people’s diets tend to change over time. (While researchers had participants fill out food frequency forms every four years, is this enough data to show the true nature of each person’s intake? Maybe or maybe not.

Fourth, all of the participants in the study were nurses. Do they represent the general population? Also maybe or maybe not.

Despite the potential flaws in nutrition research as a whole, dietitians like myself look for reproducible effects and patterns. If we keep getting the same results over a series of similar studies, it’s likely that there is a true effect happening. In this case, it’s highly likely that a diet high in saturated fats is associated with a greater risk for disease and death.

This doesn’t mean we all need to avoid saturated fat; it simply means that a diet containing a mix of fats is probably the best for health (with the exception of trans fats, which are definitely harmful).

This means favoring unsaturated fats (including seed oils like canola and soybean) and adding in some saturated fats like butter.

Balance.

It might not be the next sexy food trend, but it’s the most effective and sustainable.