Last Saturday I linked to a story from Consumer Reports that highlighted the state of the evidence on fat. That story seemed to upset some folks and so I reached out to the article's authors, Kevin Lomangino, the editor of Clinical Nutrition Insight and asked him if he would be willing to expand on his piece and also to cover what the term, "state of the evidence" means as it would seem that some of the comments left that Saturday were left by folks who aren't familiar with the concept that rather than meaning "fact", it means this is what the data looks like today.
More Facts On Fats
Revisiting Saturated Fat
Is saturated fat bad for you? A growing number of people think that the answer to this question is no. But to my mind, this question can’t really be answered without first considering a second question: What are you comparing it with?
If you reduce your saturated fat intake, chances are very good that you’ll be replacing those calories with something else. And if that “something else” is refined carbohydrate, then evidence increasingly shows that you won’t improve your health and might be worse off. On the other hand, if you replace those saturated fat calories with unsaturated fat instead, then there’s consistent evidence that you may well reap some cardiovascular benefits from the change.
As noted in the Consumer Reports piece, a recent Cochrane Group meta-analysis, widely regarded as the “gold standard” in critical evaluation of medical evidence, found a 14% reduction in cardiovascular events across randomized controlled trials that involved reduction and/or replacement of saturated fat with unsaturated fat (including both mono and polyunsaturates). Another meta-analysis that looked specifically at studies replacing saturated fat with polyunsaturated fat (trials involving monounsaturated fat were excluded) found a 19% reduction in coronary heart disease events.
Admittedly, the reduction is not that big, which is something we tried to communicate by providing the absolute number of events that occurred in these studies (77 per 1000 people on a regular diet vs. 66 per 1000 on a reduced saturated fat diet in the Cochrane review). Still, the difference is substantial enough to provide meaningful benefits if large numbers of people carried out the recommendations.
These studies have limitations, of course, and some researchers say there’s not enough proof to conclude that replacing saturated fat with polyunsaturated fat is causally related to heart benefits. Then again, diet studies are hard to do and almost always yield messy evidence. It’s unlikely that better studies will be available any time soon to guide our thinking. Meanwhile, a variety of prestigious groups have looked at the evidence (see this and this), including epidemiologic and biomarker studies, and concluded that it all points to a benefit for replacing saturated fat with unsaturated fat. That’s a well-supported stance based on the current state of the evidence.
Benefits of Omega-6
Commenters also raised concerns about our endorsement of vegetable oils that are high in linoleic acid, an omega-6 polyunsaturated fatty acid. This reflects longstanding worries that high intakes of omega-6 are pro-inflammatory, and that increased intake may well lead to higher risk of cardiovascular disease and other adverse health outcomes.
As the article pointed out, this is an area where research is rapidly evolving and could quickly swing one way or the other based on new evidence. Nevertheless, studies conducted to date don’t show that we have much to worry about with linoleic acid. Despite theoretical concerns about inflammation, recent systematic reviews found no evidence that higher linoleic acid intake increases inflammatory biomarkers or their precursors. And as noted above, a meta-analysis of clinical trials showed cardiovascular benefits from replacing saturated fat with polyunsaturated oils. In these studies, participants replaced saturated fat primarily with oils rich in linoleic acid such as soybean oil, corn oil, and safflower oil.
As some researchers have pointed out, there may be a difference between studies that increased linoleic acid exclusively and those that also simultaneously increased intakes of omega-3 fatty acids such as alpha-linolenic acid, EPA, and DHA. They say that the increased intake of omega-3s in these studies may have negated the harmful effects of linoleic acid and obscured its risks. This view is supported by a recent analysis that unearthed old data from one of these studies – the Sydney Diet Heart Study, originally conducted in the 1960s – that replaced saturated fat exclusively with safflower oil, which is 75% linoleic acid and has no omega-3s. The authors reported a higher rate of death from cardiovascular disease in the safflower oil group compared with controls who consumed their usual diet (17.2% vs. 11%).
This study hadn’t yet been published at the time our Consumer Reports piece was released, and I think it will take some time before these results are fully digested. However, early reaction to the study has raised concerns about its validity – particularly the fact that the margarine consumed by participants in the safflower oil group apparently had considerable amounts of trans fat, which might have contributed to the cardiovascular harm seen in that group.
Personally, I wouldn’t fault anyone for being a little more cautious about safflower oil in the wake of these results. But it’s hard to see how the results implicate linoleic acid in general, as similar studies involving soybean oil, which is about 50% linoleic acid and has only a small amount of omega-3s (about 7%), have shown cardiovascular benefit. Also, you’d have to work very hard to obtain 15% of your daily energy from safflower oil as they did in this study. (The average intake of linoleic acid in the U.S. is currently 6.7%, less than half that level.) If you “choose a variety of plant-based oils, plus low-mercury fish such as salmon twice a week” as recommended by Consumer Reports, you can be confident of getting a good mix of unsaturated oils that are likely to be protective.
What About Hexane?
Lastly, some commenters expressed concern about hexane, an industrial solvent that is used to process many cooking oils, including soybean oil (as well as the defatted soy protein flakes that are used in veggie burgers and other soy foods). As we noted in the article, hexane is a neurotoxin, and testing has reported trace amounts residual hexane in soybean oil (about 10 parts per million). While that may sound disturbing, there’s really no evidence that such minute exposures pose any kind of health risk. Hexane has been shown to cause toxicity in places like shoe factories and furniture finishing shops, where workers were breathing in much higher amounts of hexane and absorbing it through their skin from rags. But these types of exposure are likely more hazardous than oral consumption, because they bypass the liver.
According to an EPA toxicological assessment, there has never been a study of oral exposure to hexane in humans, and very few studies in animals. John L. O’Donoghue, VMD, PhD, a toxicologist who was a coauthor on one of these rodent studies, told me that they had to give “very large quantities” of hexane before they observed any neurotoxic effects. How much are we talking about? This Slate article estimates you’d have to eat the equivalent of more than 1 million veggie burgers a day to approximate this level of exposure! O’Donoghue told me, “I don’t know of any reason to consider [hexane in cooking oil] a problem.” With that being said, I realize that we can’t definitively rule out adverse effects from chronic long-term exposure to small amounts of hexane, because the studies simply haven’t been done. And you may also wish to avoid hexane-processed oils for other reasons, such as worker safety and air pollution concerns. Still, in a world full of risks, hexane in cooking oil doesn’t rank as priority for personal health. You can avoid hexane by choosing expeller-pressed oils, but realize that these products will cost quite a bit more than hexane-processed oil, because the extraction process is much less efficient.
Why is fat so controversial? In my mind, it’s an example what Marion Nestle called the conflict between science- and belief-based perspectives. Many people believe that vegetable oils simply can’t be good for you, because they haven’t been a significant part of our diet for more than a few decades. They are much more comfortable eating the foods that we’ve always eaten, which more often than not will be animal-based and higher in saturated fat. It’s a valid view, but the science—at least for now—does not support this approach. The verdict could change if and when we get more high-quality research on this topic.
Meanwhile, the overall science-based message is not to worry too much about one specific food or nutrient. Where reasonable and feasible, replace butter and solid fats in your diet with a variety of plant oils such as olive, canola, soybean, and peanut. Eat fatty fish twice a week. Limit processed junk food that is usually rich in saturated fat and refined carbohydrate, and fill up most of your plate with fruits and vegetables instead.
Kevin Lomangino is editor of Clinical Nutrition Insight, a monthly evidence-based newsletter for physicians and dietitians, and a reviewer for HealthNewsReview.org. He tweets as @Klomangino.