Sept. 3, 2024 – You probably already know that a diet heavy in fruits, vegetables, and other plant-based foods is better for you than one that skews toward meat and dairy. But up to now, dietary research has failed to explain the precise impact of specific kinds of dietary fats on health.
A new study looked at how eating fats from plants, compared to fats from animals, affects how long people live. Researchers found what we already suspected: A diet higher in fruits, vegetables, grains, and plant-based oils offers better protection against death from all causes and diseases of the heart and blood vessels (cardiovascular disease).
And how much you consume matters, too. People who consumed more fats from plants had a 9% lower risk of death from all causes and a 14% lower risk of death from cardiovascular disease than those who ate significantly less of these foods.
On the other hand, people who consumed a lot of animal-based fat, including fat in meat, dairy products, and eggs, had a higher risk of death from all causes and death from cardiovascular disease than those who ate less. Comparing the top consumers of animal-based fat to the lowest found those in the top tier had a 16% higher overall death risk and a 14% increased cardiovascular disease risk.
Data From Long-Term Health Study
The more than 400,000 people in the study were part of the National Institutes of Health-AARP Diet and Health Study. In the group used for the dietary fats research, there were slightly more men than women, and the average age of the subjects was 61. They were enrolled in 1995 and were followed through 2019.
When they enrolled, the people in the study filled out a questionnaire that included questions about their dietary habits, broken down to 124 dietary items and portion sizes. Total dietary fat intake included both plant sources (such as grains, nuts, legumes, and vegetable oils) and animal sources (such as red and white meat, dairy foods, and eggs).
During the 24 years of follow-up, 185,111 deaths were recorded, including 58,526 from cardiovascular disease (45,634 for heart disease and 10,877 for stroke). The researchers linked these deaths to the dietary information in the baseline questionnaire in order to calculate diet-related death risks, after adjusting for a number of other factors that may have contributed to the deaths as a whole.
In addition, the authors looked at fats in certain food groups to determine their links to death risks. On the plant side, for example, they found that a higher intake of fat from beans and legumes was not linked to any of the mortality outcomes.
A higher consumption of fat from dairy products and eggs showed increased risks of overall mortality, including from cardiovascular diseases, while eating greater amounts of fat from white meat was tied to lower risks. Fat from red meat carried higher mortality risks; in contrast, higher fat consumption from fish was not significantly linked to an increase in the risk of death overall or death from diseases of the heart and blood vessels.
Links to Personal Traits
The researchers also examined whether making dietary changes affects death risks. The answer? A resounding “yes.”
Replacing just 5% of calories from total animal fat, red meat fat, dairy fat, or egg fat with the same amount of total plant fat was tied to 4% to 24% lower risk of overall deaths and a 5% to 30% lower risk of deaths from diseases of the heart and blood vessels, researchers said.
Another takeaway: Replacing plant-based fat with fish and white meat fat did not lower death risk, they said.
Because of the fatty plaques that can build up in human arteries over time, altering one’s diet can have very different impacts on heart health, depending on when in life a person makes that change, said Walter Willett, MD, a professor of epidemiology and nutrition at the Harvard T.H. Chan School of Public Health. It takes a long time for these plaques to diminish, so the earlier one moves to a more plant-based diet, the better, he said.
Study co-author Demetrius Albanes, MD, senior investigator in the National Cancer Institute’s Division of Cancer Epidemiology and Genetics, agreed with Willett. “It probably takes a long time to reverse cardiovascular plaques,” he said. “So it’s better to change your diet earlier, under the advisement of clinicians, while avoiding drastic and extreme types of diets.”
Possible Limitations of the Study
A major drawback of the paper, Willett said, is that the underlying NIH-AARP study asked people about their diets only in the baseline questionnaire. Some of the people in the study, he said, undoubtedly changed their diets during the 24 years of follow-up, but those changes were not put into the data used in the dietary fats study. Therefore, he said, an increasing number of errors were introduced into the data used in the analysis, weakening the link between participants’ diets and the deaths of a portion of the study group.
“If we only use the baseline survey, we only see a weak signal among the random noise,” he said.
Another problem with the study’s findings, Willett said, is that in the 1990s, when the people were enrolled, trans fat was starting to be removed from foods that contained plant oils. The FDA banned partial hydrogenation of plant oil, a type of food processing that creates trans fats, in 2018. Since most plant-based foods are no longer partially hydrogenated, he said, “this adds to error because up to 30%-40% of the plant fats [in the baseline diets] were trans fats, and these seriously increase the risk of cardiovascular disease, which would greatly underestimate the benefit of plant fats.”
Although the study’s authors say they statistically controlled for the decrease in trans fats, Willet said, they could not have done this very well “because the food composition databases were not keeping up with the changes.”
Between those two issues and problems with the validity of the questionnaire, he said, “the correlations they describe [in the study] will lead to about a 2-fold underestimation of the effect of diet [on mortality]. Putting all sources together, they are probably underestimating the effects of diet by about 4-fold or even more.”
Albanes responded that the study’s methodology was valid and that most studies of this type “can only collect the dietary data at baseline.” Any changes in diet over the years
“might have added some noise to our estimates” but, if so, their estimates of the risks from animal fats “might be underestimates.”
“The trans fat issue is real, and there are questions about how that would have impacted our follow-up and the mortality in our population. That’s continuing to be explored, given that it’s a more recent development.”
In any case, Albanes said, if the impact of eating more plant-based fats or animal-based fats on the risk of death was underestimated in the study, that only underlines the importance of its central findings.
Dietary Recommendations
Albanes believes that the study’s data on the mortality risks of various food groups is solid enough to be used in diet recommendations. Among the plant-based fat sources he’d recommend, besides fruits and vegetables, are fats from grain foods (bread, pasta, etc.) and vegetable oils, including olive, canola, and corn oils.
Grains don’t have much fat except in their germs, Willett pointed out. “Almost all plants have fat to protect their seeds, and that fat is full of antioxidants.”
Albanes agreed. “When we say fat from grains, that’s across all the grain foods that were asked about on the original questionnaire. That will vary, whether it’s a whole-grain bread that includes the germ portions, versus refined wheat or white bread.” While the study didn’t break out the effects of whole-grain bread vs. more processed breads, he said the former is better for your health.
