A study from Stanford University puts another nail in the coffin of the increasingly discredited saturated fat theory of heart disease.
This recent data analysis also undermines the long-standing but scientifically dubious idea that high-carb diets are healthier than high-fat diets.
The new findings come from a Stanford University team who analyzed data from their prior DIETFITS clinical weight-loss trial, whose results we reported in Stunning Study Upsets a Big Diet Debate.
The original DIETFITS trial showed that the fat-carb balance of a diet matters much less to weight control than a decision to heavily favor whole foods over refined, processed foods, especially sugars and white flour goods.
The outcomes of the DIETFITS trial were bolstered by those of a subsequent clinical trial, called the Framingham State Food Study, which found that people burned more calories on low-carb diets than on low-fat diets (see Low-Carb Diets Won Tough Clinical Test).
Let’s turn our attention to the results of the Stanford University team’s recent analysis of data from their DIETFITS clinical trial, and then pursue its important implications.
Stanford team’s analysis of clinical data undermines saturated fat myth
The Stanford team analyzed data from a subgroup of 210 low-carb dieters in the year-long DIETFITS trial, which included 609 male and female participants between the ages of 18 and 50, who were randomly assigned to either a low-carb or low-fat diet (Chiu S et al. 2019).
And their analysis revealed that low-carb diets with higher average percentages of calories from saturated fat than advised by U.S. health authorities (i.e., no more than 10% of calories) produced healthier blood cholesterol profiles.
The Stanford team concluded that relatively high dietary intakes of saturated fats significantly improved blood lipid profiles, provided they are part of a diet low in carbs (especially sugars and white flour goods) and relatively high in whole, unprocessed vegetables.
Contrary to conventional wisdom, the members of the low-carb diet group who consumed the highest proportions of calories from saturated fats had healthier blood-lipid profiles:
Blood levels of triglycerides are raised more by sugars and other refined carbs than by dietary fats, including saturated fats — which matters because high triglyceride levels are a bigger risk factor for cardiovascular disease than the large, fluffy, benign forms of LDL cholesterol whose blood levels are raised by saturated fats.
As study co-author Christopher Gardner, Ph.D., noted, “If one of these people were to go to their doctor the doctor might see an increase in saturated fat percent from 10% to 15% and be concerned. But ... they have higher HDL levels, stable LDL levels, and lower levels of triglycerides.”
(Oddly, as we’ll explain, the Stanford team didn’t seem to recognize the full implications of their findings: see “Stanford team underplayed significance of their findings”, below.)
According to Gardner, the bottom line is that the low-carb dieters whose diets contained the highest proportions of saturated fat had healthier blood lipid profiles than people assigned to the low-fat diet group.
“An increase [in saturated fat consumption] to 15 percent [of daily calories] exceeds the guidelines, but I want this finding to put people who are on a low-carb diet, and their doctors, more at ease about the percent of saturated fat consumption,” said Gardner.
It’s important to note that the relatively high proportions of saturated fat in the low-carb diet group (i.e., more than 10% of calories) were still low in comparison with demonstrably heart-healthy traditional diets — as documented in the inconvenient studies that've been studiously ignored by advocates of the saturated fat theory of heart disease.
And we wish that — as discussed below — Dr. Gardner’s team had highlighted the compelling new evidence that the predominant types of LDL cholesterol particles in your blood (large, fluffy, and benign versus small, dense, and dangerous) matter much more than your total blood levels of LDL cholesterol.
Stanford team underplayed significance of their findings
Why do we say that the Stanford team appear to miss the broader implications of their findings?
To answer that question, we need to take a quick look at the current evidence regarding dietary fats and cardiovascular disease.
Current U.S. Dietary Guidelines for Americans call for limiting saturated fat to 10% of calories — but that advice is clearly contradicted by the available evidence.
Critically, reviews of the epidemiological (population) and clinical evidence have not linked the total amount or proportion of saturated fat in a person’s diet to their risk for cardiovascular disease (Zock PL et al. 2016).
That said, when someone’s diet contains relatively high amounts of saturated fats, most — but not all — the available evidence suggests that replacing 5% of dietary saturated fat with polyunsaturated fat reduces the risk of coronary heart disease by about 10% (Wang DD et al. 2016; Praagman J et al. 2016).
Don’t replace saturated fat with refined carbs
Demonization of dietary fats from the late 1980s through the 1990s led many people to favor low-fat and fat-free foods, many of which were packaged foods high in white flour and sugars.
That disastrously counterproductive campaign worsened the very problems — rising rates of obesity, cardiovascular disease, and diabetes — that prominent academic researchers mistakenly said it would solve.
In contrast, there’s evidence that replacing some dietary saturated fat with whole grains — rather than refined carbohydrates — can reduce cardiovascular risks, probably due to their abundance of fibers and antioxidants (Li Y et al. 2016).
Accordingly, the authors of a recent evidence review urged public health officials to recommend reductions in dietary sugars and white flour goods: “When saturated fats are replaced with refined carbohydrates, and specifically with added sugars (like sucrose or high fructose corn syrup), the end result is not favorable for heart health.” (DiNicolantonio JJ et al., 2016)
Saturated fat primarily raises benign forms of LDL cholesterol
In Cholesterol Picture Gets Clearer, we reported on the evidence that different forms of LDL cholesterol have very different effects on cardiovascular health, ranging from innocuous to dangerous.
In reality, the effects of most saturated fats on blood cholesterol are relatively benign. For example, although palmitic acid — the most common saturated fat in the American diet — raises total LDL levels, most of that rise takes the form of benign, large-fluffy LDL particles.
And — like some other common saturated fats — palmitic acid raises blood levels of “good” HDL cholesterol: a fact that usually goes unmentioned by public health authorities.
Further, many people are surprised to learn that the predominant fat in beef and pork isn’t a saturated fat — instead it’s oleic acid, the same monounsaturated fat that predominates in olive oil:
What about dairy foods?
The authors of every recent evidence review have concluded that whole milk and whole dairy foods don’t worsen blood lipid profiles or cardiovascular risks, and that fermented dairy foods like yogurt tend to exert positive effects.
In contrast, low-fat dairy foods — which have higher proportions of sugars (e.g., lactose) due to the removal of fat — are not so benign, because they raise triglyceride levels (Lordan R et al. 2018).
All polyunsaturated fats aren’t equal
While diets relatively high in saturated fats aren’t unhealthful, diets containing excessive amounts of polyunsaturated omega-6 fat are unhealthful.
Unfortunately, the average American’s diet is awash in vegetable oils high in omega-6 fats (i.e., soy, corn, safflower, sunflower, and cottonseed). Oils much lower in omega-6 fats and higher in “neutral” monounsaturated fats include olive oil, canola oil (which has significant amounts of omega-3 fat), and high-oleic sunflower oil.
Replacing saturated fats with large amounts of polyunsaturated omega-6 fats from vegetable oils, nuts, and seeds creates a pro-inflammatory condition in the body, which is linked to cardiovascular disease diabetes, dementia, and more.
Unfortunately, the authors of many major diet-health studies failed to distinguish between omega-6 and omega-3 polyunsaturated fats, which exert very different effects on the body's inflammation-control system.
Because the standard American diet contains too few omega-3s and far too many omega-6 fats, replacement of saturated fats with even more omega-6 fats from vegetable oils is a clearly unhealthful, pro-inflammatory tactic. For more on that, see Vegetable Oils Debunked for Heart Disease and our Omega-3/6 Balance page.