Have Scientific Studies Redeemed Saturated Fats?
There are few nutrients as controversial as saturated fat. For decades, we’ve been warned that saturated fat is a dangerous substance capable of raising our cholesterol, clogging our arteries, and sending us to an early grave. But then, a major 2010 meta-analysis pooled the available research and found “no significant evidence” for connection between saturated fat intake and coronary heart disease, stoke or total cardiovascular disease. A large 2015 meta-analysis further showed no association between saturated fat intake and all-cause mortality, cardiovascular disease mortality, coronary heart disease, ischemic stroke, or type 2 diabetes. A 2016 meta-analysis showed that butter had an overall very minor to neutral impact on all-cause mortality, cardiovascular disease and type 2 diabetes, suggesting no need to emphasize either increasing or decreasing butter consumption within dietary guidelines. And a 2020 meta-analysis showed that dietary saturated fat reduces stroke risk; for every 10 grams per day of saturated fat, risk of stroke decreased by 6%. As a result of these and similar studies, some health-conscious communities have gone all-in on saturated fat, while others still fear it.
However, not all studies show a neutral health impact from saturated fats. For example, a 2021 meta-analysis showed a non-linear relationship between all-cause mortality and saturated fat intake, plateauing above 11% of calories coming from saturated fats. This study highlighted an increased cancer mortality risk with consumption of saturated fats; for every 5% of total calories coming from saturated fats, cancer-related deaths increased by 4%.
This all leads us to the question: Are high intakes of saturated fat good or bad? Or, is this a case where moderation is really the best strategy?
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Is It STILL Beneficial To Moderate Saturated Fat?
While the current USDA Dietary Guidelines for Americans recommend limiting saturated fat intake to 10% of total calories or less, this threshold has been widely criticized for not accurately reflecting the current state of scientific evidence.
Certainly, consuming more monounsaturated fats, especially heart-health oleic acid (abundant in olives, avocados and their oils) reduces cardiovascular disease risk. In fact, these healthy fats and oils are Foundational Foods on Nutrivore. That being said, the benefits of consuming more monounsaturated fats doesn’t automatically imply harm to consuming saturated fats.
While the data is much more mixed, there’s also some evidence that consuming more polyunsaturated fats might also be beneficial. For example, a 2010 meta-analysis evaluated the potential benefits of replacing dietary saturated fat with polyunsaturated fats and showed that replacing 5% of total calories worth of saturated fats with polyunsaturated reduced coronary heart disease by 10%, although this study has been criticized. A 2021 meta-analysis showed that replacing dietary saturated fat with polyunsaturated fat may be even more helpful for reducing cardiovascular disease risk people with type 2 diabetes, although the authors highlight the low quality of evidence and the need for more studies on this topic. And, linoleic acid has a potent cholesterol-lowering effect via several mechanisms: increasing bile acid production and cholesterol breakdown, upregulating the LDL receptor, moving LDL cholesterol out from the blood and into body tissues, and decreasing the conversion of very low density lipoprotein (VLDL) to LDL.
It’s also worth noting that high saturated fat intake negatively impacts gut health. In mice, diets containing high amounts of saturated fat (from palm oil) cause an overflow of fat into the distal intestine, leading to an increase in the Firmicutes-to-Bacteroidetes ratio (which is associated with obesity). Even though we’re not mice, a similar effect can likely occur in humans and lead to harmful changes in mucosal gene expression. In fact, some research in humans has correlated gut dysbiosis and obesity-promoting gut microbiomes with diets high in saturated fat and low in fiber. Saturated fat also has a well-established role in increasing gut permeability. In a study of mice fed different high-fat diets (60% fat) emphasizing saturated fat, omega-3 fats, or omega-6 fats, the saturated fat diet was the only one to increase intestinal permeability, largely driven by a rise in hydrogen sulfide-producing bacteria.
One way that people reduce saturated fat intake is with an overall low-fat diet, however swinging the pendulum to the other extreme also isn’t healthful. Low-fat diets in conjunction with low levels of serum cholesterol (which tend to go hand-in-hand) have been linked to a variety of health conditions, including depression and suicide (low-fat diets may impair serotonin receptors by decreasing the fats in nerve-cell membranes), anxiety, aggression, other violent behavior, premature death, and even cancer (fat and cholesterol are important for the integrity of cell membranes). The Accepted Macronutrient Distribution Ranges (AMDR) for fat is 20% to 35% of total energy.
However, there is one group of people who do seem to benefit from moderate saturated fat intake, those with ApoE4 genes.
ApoE4 Carriers
Even though saturated fat has pretty much been redeemed on the heart disease front, there’s one subset of the population that might genuinely need to limit their intake for the sake of heart-health: ApoE4 carriers!
The ApoE lipoprotein (coded by the ApoE gene) plays a major role in metabolizing and transporting cholesterol and saturated fat. All of us have two copies of the ApoE gene, which can be a combination of any of three variants: ApoE2, ApoE3, or ApoE4. While ApoE2 and ApoE3 carriers generally don’t have a noteworthy response to eating saturated fat, it turns out that ApoE4 carriers are a different story!
A number of studies show that compared to the other variants, ApoE4 carriers see a much higher spike in LDL cholesterol from eating large amounts of saturated fat (without a rise in HDL to match). And, they’re the group most likely to benefit from lower-saturated-fat diets, since decreasing their saturated fat intake causes a sharp decline in LDL cholesterol and an improvement in the HDL/LDL ratio (non-E4 carriers usually don’t see that ratio improve from going on a low-fat diet).
The scary news about ApoE4 is that for people who are born with at least one copy (about 20% of the population), the risk of heart disease (as well as Alzheimer’s disease) is much higher than the rest of the population. So, managing heart health should be on the forefront of these people’s minds… which means being conservative with saturated fat intake, and not going gung-ho on things like buttered coffee and tons of bacon. (To find out if you’re an ApoE4 carrier, a number of genetic testing services are available. Your doctor may be able to order an ApoE test for you as well, especially if you have a family history of heart disease and Alzheimer’s disease.)
People with familial hypercholesterolemia are also often advised to consume a low-saturated fat diet; however, in this case, the evidence for benefit is lacking. See for example this 2021 review.
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What Really Matters: Stearic to Palmitic Acid Ratio
A number of studies have been conducted specifically on the effects of stearic acid versus palmitic acid, given these two fats’ dietary abundance and the significant differences in how they affect blood cholesterol. Human trials have shown that when palmitic acid replaces stearic acid in the diet, fasting LDL levels go up, whereas when stearic acid replaces dietary palmitic acid, fasting LDL levels go down. One trial of postmenopausal women tested the effects of diets enriched with stearic acid, palmitic acid, or oleic acid, and found that stearic acid had similar effects as oleic acid when it came to fasting LDL levels, whereas palmitic acid had less favorable LDL-raising effects compared to the other two fatty acids.
What’s more, these fats also differentially impact the blood lipid landscape immediately after eating—a phase called the “postprandial period.” Given that we spend the majority of the day in a postprandial state rather than a fasted state (because each meal we eat leads to physiological changes lasting many hours, during which our bodies adapt to the influx of nutrients!), postprandial blood lipids can actually have a significant impact on cardiovascular risk, and are just as valuable to study as fasting blood lipids. One randomized crossover trial fed participants diets rich in stearic acid or palmitic acid for four weeks each, and found that participants’ post-meal triglyceride levels and apolipoprotein B48 levels (a marker of intestinal chylomicrons) were significantly lower after the stearic acid diet period than the palmitic acid diet period.
Palmitic Acid
Palmitic acid is a long-chain saturated fatty acid that serves as a major energy source, both due to its abundance in our diet and its storage in our fat tissue. It has the most potent LDL-raising capacity out of all the saturated fats, and has cellular signaling functions that can promote inflammation, metabolic dysfunction, tumor growth, and neurological harm. As a result, excess consumption may raise the risk of cardiovascular disease, diabetes, certain cancers, obesity, and neurodegenerative diseases like Alzheimer’s and Parkinson’s.
Stearic Acid
Stearic acid is a long-chain fatty acid abundant in the modern diet; it can be partially converted to the monounsaturated fat oleic acid. Unlike most other saturated fats, stearic acid doesn’t raise (and might even slightly lower!) LDL cholesterol levels, and has an overall neutral effect on most cardiovascular risk factors. It also acts as a signaling molecule affecting mitochondrial function and lipid oxidation, potentially giving it a protective role in nervous system diseases, muscular disorders, and even aging. It’s also demonstrated some anti-cancer activity, although more research is needed in humans.
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