(CNN)We've long known that "bad" fats aren't the healthiest choices in our diets, but the latest news from Harvard is lethal: Eating too much saturated or trans fat can increase your risk of dying.
Good fats can cut risk of death by 27%, study says
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The silver lining? You can reverse some of that risk by making healthier fat choices.
"This study is by far the most detailed and powerful examination of the relationship between different types of dietary fats and mortality," said Dr. Frank B. Hu, professor of nutrition and epidemiology at the Harvard T. H. Chan School of Public Health. "Our study demonstrates that not all fats are created equal, and eating healthy unsaturated fats at the expense of unhealthy saturated and trans fats is an important way to live a longer and healthier life."
Hu and his fellow researchers analyzed the eating habits of more than 126,000 men and women over a 32-year period from 1980 to 2012, checking in with each participant every two to four years about the amount and type of fat in their diets.
All of the people in the study started off with no signs of cancer, type 1 or 2 diabetes, or cardiovascular disease. In each questionnaire, participants were asked how often they consumed portions of up to 150 types of fatty foods, as well as the types of margarine, fat or oil they used to prepare dishes. The researchers then compared those results against death rates.
Hu and his colleagues found that although eating more saturated fat and trans fats was associated with an increase in mortality, eating more polyunsaturated and monounsaturated fats lowered the risk of death. In fact, the study found that if people replaced a mere 5% of their calorie intake from "bad" fats with polyunsaturated fats, they could reduce their risk of death by 27%. If those calories came from monounsaturated fats, the risk of mortality dropped by 13%.
Why weren't monounsaturated fats as protective? Perhaps the two good fats have different biological effects, Hu said. Or perhaps it's the American diet.
"A large proportion of food sources of monounsaturated fat in the typical American diet are animal-sourced, such as dairy and red meats," Hu said, pointing out that those are also major sources of saturated fats. "Therefore, current analysis may not be able to completely distinguish the benefits of monounsaturated fat from the effects of food source and saturated fats."
But the study didn't stop there. The researchers also compared fat intake to different causes of death, including cardiovascular disease, cancer, neurodegerative disease and respiratory disease. Some of their findings were "new and interesting," Hu said.
For example, people who ate more healthy fats had a lower risk of dying from neurodegenerative and respiratory disease, but both of those causes of death increased significantly with higher trans fat intake. The risk of dying from respiratory disease also increased significantly for people who ate more saturated fat.
"These results need to be studied further and replicated in other populations," Hu said. "The major takeaway is that the types of fat in our diet are more important than the total amount of fat."
However, think twice about adding carbs instead. The study also found that replacing fat calories with carbohydrates was not significantly protective against mortality.
Polyunsaturated fats contain essential fats your body can't produce by itself, such as omega-6 and omega-3 fatty acids. You must get these essential fats through the food you eat. Some of the best sources are nuts, seeds, fatty fish, algae and leafy greens.
One polyunsaturated fat, an omega-6 fatty acid called linoleic acid, was shown in the Harvard study to be especially protective against death by cancer and coronary artery disease, Hu said.
Prior studies showed linoleic acid to reduce total and bad cholesterol, and to be associated with better blood pressure and insulin sensitivity. Though some studies have connected too much omega-6 with inflammation in the body, others find no such link.
Linoleic acid is found in sunflower, soybean and safflower oils, as well as nuts and seeds. Walnuts, Brazil nuts and peanuts are excellent sources, as are safflower, pumpkin and squash seeds.
Another key polyunsaturated fat, the omega-3 fatty acid called alpha-linolenic, was not associated with "all-cause mortality," Hu said, but "interestingly, we found that alpha-linolenic acid was protective against death due to neurodegenerative disease."
Alpha-linolenic acid is found in vegetable oils, such as flaxseed oil, canola oil and soybean oils, and walnuts as well as green leafy vegetables.
Monounsaturated fats are typically liquid at room temperature but when chilled begin to turn solid. This healthy fat can lower bad cholesterol levels and contribute vitamin E, which many Americans are missing. The most famous example of a monounsaturated fat is olive oil, a key player in the Mediterranean diet, often touted as one of the healthiest in the world. Other good sources include avocados, peanut butter and many nuts and seeds.
"Bad" fats include trans fats and saturated fats. Saturated fats are common in the American diet, in red meats and full-fat dairy. Research in this area has been mixed, with some studies finding that whole-milk dairy products could be linked to less body fat and obesity, possibly because they promote feelings of fullness and therefore less is consumed. A recent study associated dairy and butter with lower rates of diabetes, although it still found a 1% higher risk of death per tablespoon of butter.
Trans fats are the major culprit when it comes to health problems. Though they can be found naturally in small amounts in meat and dairy products, the major sources are from artificial trans fats used in processed foods.
The Food and Drug Administration banned trans fats from products in 2013, but they can still be found in many foods like crackers, cookies, doughnuts, muffins, pies and cakes, often in trace amounts that quickly add up. Research from the Harvard School of Public Health has showed that the risk of heart disease rises by 23% for every 2% of calories obtained from trans fats.
"There have been some positive trends in the U.S. diet over the past two decades," Hu said. "However, saturated fat is still over-consumed at the expenses of healthy fats. Thus, there is still a long way to go to improve the quality of fats and the overall diet quality in the U.S. population."