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Good fat vs bad fat: Study links one type of fat to a higher risk of stroke

However, scientists have long struggled to prove which kind of fat matters.

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Increasing your uptake of fat from vegetable sources and decreasing the amount from red meat is the takeaway from research presented this week at the American Heart Association’s Scientific Sessions.

The researchers, led by Fenglei Wang, a postdoctoral fellow at the Harvard T.C. Chan School of Public Health, analyzed data from 117,136 participants in two hulking studies of healthcare professionals that tracked their health outcomes over 27 years.

The participants completed questionnaires to track the amount, source, and types of fat in their diets. The research is preliminary, the study has not yet been published, and is the latest to tackle the contentious subject of “good fats” versus “bad fats.”

The researchers say theirs is the first study to parse out stroke risk by food type. In a statement, Wang said: “Our findings indicate the type of fat and different food sources of fat are more important than the total amount of dietary fat in the prevention of cardiovascular disease including stroke.”

But using this information comes with a caveat: It comes atop a heap of research that tries to nail down which fats can be linked to diseases, one full of contradicting evidence and no consensus. While Wang and colleagues recommend replacing lard and beef fat with non-tropical vegetable oils like olive oil, corn, and soybeans, other scientists aren’t convinced there’s an association between intake of specific types of fat and stroke risk.

“There is mixed evidence,” Amanda Nash, a dietician who works on projects for the Heart and Stroke Association of Canada, tells Inverse. However, she still advises avoiding saturated fats for stroke risk. “The best strategy is to look at your overall diet, particularly at ultra-processed foods.”

LONGEVITY HACKS is a regular series from Inverse on the science-backed strategies to live better, healthier, and longer without medicine. Get more in our Hacks index.

Science in action — Wang and colleagues say this research suggests switching animal fat for vegetable fat may reduce stroke risk

The study participants were 97 percent white, 63 percent women, with an average age of 50. Every four years during the 27-year-long study they answered questionnaires about what they ate. This information was used to determine the consumption of fats; a limitation to this study is that it is observational and dietary intake was self-reported.

Those in the highest quintile of red meat intake were 16 percent more likely to have a stroke than those in the lowest quintile. That’s the result of the one-fifth who ate the most red meat compared to the one-fifth who ate the least.

Alternatively, those who ate the most vegetable fat and other sources of polyunsaturated fats were 12 percent less likely to experience a stroke than those who ate the least. Again, we’re talking about quintiles: The one-fifth who ate the most vegetable fat compared to the one-fifth who ate the least.

Nuts are a dense source of vegetable fat.Shutterstock

Polyunsaturated fats are not synonymous with vegetable fats but are similar enough that they were grouped together in the study. Polyunsaturated fats come from both vegetable sources (like walnuts, corn oil, sunflower seeds, soybean oil) and from fish (salmon, mackerel, herring, and trout).

Dairy fat did not have any impact on risk of stroke in the study.

The debate over saturated fats

If you have read a health article about “good fats and bad fats,” saturated fats were probably one of the bad ones (and trans fats the even-worse one). Red meats are full of saturated fats and they are also found in fretted-over foods like baked goods, whole-fat dairy, palm oil, and fried food.

Nutritionists and cardiologists view them with trepidation because of their supposed link to a certain kind of cholesterol and have recommended switching them with “good fats,” monounsaturated and polyunsaturated fats.

The full scientific record is more nuanced. A diet high in saturated fats has been shown to increase low-density lipoprotein (LDL) cholesterol. If you have ever read a health article about “good cholesterol and bad cholesterol,” that was probably the bad one. It builds up as plaque in the arteries, which can lead to some kinds of heart disease.

However, all these links from this one type of fat to the internal mechanisms of the body to risk for disease are tenuous enough that some scientists have pushed back against broad warnings about saturated fat. One large analysis from 2010 found no clear link between saturated fats and heart disease.

Saturated fats and stroke

Researchers who have looked for a link between saturated fats and stroke have come up with even less.

An ischemic stroke (the most common kind) occurs when a blood vessel supplying blood to the brain is blocked by a clot. The risk for stroke is increased by common heart disorders and high blood pressure.

Naturally, one would think if a high-saturated fat diet leads to a build-up of low-density lipoprotein (LDL) it would mean thinner passageways for the blood and more risk of a clot.

That is enough for many stroke prevention organizations to recommend a diet low in saturated fats. But generally, researchers who tried to link a fat type with stroke risk have not come up with a conclusion.

A 2003 analysis, published in the BMJ, which used a portion of the data set also utilized by the research team who are presenting at the American Heart Association meeting this week did “not support associations between intake of total fat, cholesterol, or specific types of fat and risk of stroke in men.” Women were not part of the study.

Red meat, a prime source of saturated fat, has been causing controversy in health circles for 60 years.Darius Dzinnik / 500px/500Px Plus/Getty Images

Oddly, some studies correlated saturated fat with lower stroke risk.

A 2018 study from Japan with the goal of differentiating saturated fat risk and stroke risk between Japanese and non-Japanese men found that Japanese men with diets high in saturated fat had lower stroke risk. However, “this favorable effect of saturated fat was not observed in non-Japanese,” the study authors write. The difference could be genetic or related to the Japanese diet.

A meta-analysis done in 2019 and published in Nutrition, Metabolism and Cardiovascular Diseases scoured and complied scientific literature on the subject and also found that saturated fat consumption lowered the risk of stroke.

How this affects longevity — The contradictory messaging — which mirrors contentious fights among scientists about red meat — is no doubt frustrating to a health-minded consumer.

According to the authors of this new research — which was not provided to Inverse for review — the findings suggest one should reduce their consumption of red and processed meat, avoid the fatty parts of unprocessed meat, and replace animal fats with non-tropical vegetable oils.

This is aligned with the 2021 dietary guidance from the American Heart Association, which suggests people:

  • “Choose healthy sources of protein (mostly plants; regular intake of fish and seafood; low-fat or fat-free dairy products; and if meat or poultry is desired, choose lean cuts and unprocessed forms).”
  • “Use liquid plant oils rather than tropical oils and partially hydrogenated fats.”

But considering the scientific debate over linking fat types to stroke specifically, what to do with this information ultimately comes down to individual health needs and how one wants to balance the research available. Commenting on this research, other experts told NBC News this study doesn’t mean someone needs to become vegan, but it does jibe with what’s generally known: swapping one red meat meal a week for an unprocessed vegetarian option is good for heart health.

As Nash said, the best strategy for a healthy life is to pay attention to your overall diet.

HACK SCORE — 🍖🍖 (2/10 meat on the bones)

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