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Vegetables, Vegetable Oils and Dairy Fat? Please Don't Eat the Headlines!

Conventional wisdom is made to be challenged -- that, in fact, is what science is for. Science never rests, and those of us who profess devotion to it are obligated to move with it. But spitting convention in the eye is probably another matter.
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One of the particular fringe benefits of what I do is that my interactions almost every week include leading experts in my field from all around the world. Sometimes that's face to face at a conference that convenes a few of us, or on rare and special occasions, assembles a who's who. More often, we're just swapping electrons as people do these days, but sharing ideas just the same. This past week was no exception.

So it is I can say with reasonable authority that the recent spate of wildly gyrating nutrition headlines did not cause (m)any of the world's leading nutrition experts to change what they eat. To the best of my knowledge, none of my vegetarian colleagues renounced their leafy greens when headlines suggested that vegetarianism, overnight, had become bad for us. No one I know not already inclined to eat full-fat dairy suddenly broke out the whipped cream and went wild when headlines canonized dairy fat. And I am aware of no one who stopped drinking daily mugs of corn oil when it got slapped with a skull and crossbones, mostly because I don't know anyone who was drinking mugs of corn oil in the first place.

That, in the proverbial nutshell, is the punch line here. Now, let's deal with the details.

By far the most challenging, and strangest, entry in the current cluster of headline-generating provocations is a study just published in the British Medical Journal, highlighting the liabilities of corn oil. Of course, the headlines run amok as they always do, going so far as to suggest that the Mediterranean diet is in doubt now. The study had nothing whatever to do with the traditional Mediterranean diet, and corn oil is not a feature of that diet, but the imperative of headlines is, clearly, titillation -- not accuracy. Moving on.

This paper was, in effect, a disinterment of data. The researchers went back some 50 years to dig up unpublished data from the Minnesota Coronary Experiment, originally designed to determine if replacing saturated fat with unsaturated vegetable oil would lower cholesterol, and thereby lower mortality.

According to the current authors, the prevailing view has long been that this intervention -- a concentrated dose of corn oil, rich in the omega-6 fat, linoleic acid -- does, in fact, lower both cholesterol and mortality. Their additional analyses purportedly change that, showing that corn oil did lower cholesterol, but not mortality.

The study authors, with one of whom I have been privileged to confer directly via email, suggest that their findings highlight what may have been an overlooked hazard of linoleic acid, namely that it may lower cholesterol, but is apt to increase oxidation. They suggest that oxidized LDL results, propagating atherosclerosis.

I have conferred as well with other colleagues, leading experts in cardiovascular and nutritional epidemiology, and explored some of the important limitations of this study. The study population, limited to nursing home and mental hospital residents, may have included people with significant illness at baseline. The mortality rates did not differ significantly between intervention and control diets, so it may be that mortality in this cohort had as much or more to do with baseline health than with the intervention.

The new analysis is limited to study participants who remained in the trial for at least a year, and that was less than 25 percent of those originally randomized. That extremely high rate of attrition is a very considerable study limitation. Of note, even a year is a very short interval for a study of atherosclerosis, which develops over decades, let alone coronary events and deaths. Whatever their respective liabilities, neither cream nor corn oil tends to kill us quickly.

Despite the inevitable headline hyperbole, this study did not show higher mortality in the corn oil group in conjunction with their significant cholesterol reduction. Rather, it found a correlation between falling blood cholesterol levels and mortality in both groups, and this was significant in those over age 65. Cholesterol tends to fall in older people with serious illness, and unintended weight loss, which in turn, unsurprisingly, tend to predict mortality.

We could spend more time on the fine points of this study, but the most useful perspective may be from a little altitude. Remember hearing that tilapia was worse for you than bacon? This, too, was hyperbolic nonsense, but as tends to be true of all such media distortions, it was based on something. It was based on the recognition that an excess of omega-6 fat, and an imbalance in dietary fatty acids, is bad.

While the BMJ paper is new, the study was indeed run 50 years ago, before that knowledge was established. The intervention did not test the effect of a switch to a balanced array of vegetable oils, or even one very salutary oil such as olive, let alone a switch to wholesome foods rich in beneficial oils, such as nuts, seeds, avocado, and fish. Rather, it tested something that nobody expert in nutrition is recommending: an extremely high dose of omega-6, linoleic acid. Whether or not our memory extends beyond today's headlines seems forever in question, but we already got the memo that this is a dubious proposition.

With all due respect to the BMJ authors, I personally found it a bit odd that they stated the following: "A key component of dietary guidelines has long been to replace saturated fat with oils rich in linoleic acid..." The current Dietary Guidelines for Americans, let alone the far better 2015 Dietary Guidelines Advisory Committee Report, make no such recommendation. Searching for any mention of linoleic acid, I found it only in the appendices in the context of descriptions and definitions, not in any of the actionable guidance. While the current Dietary Guidelines do recommend limiting saturated fat intake, the replacement encouraged is a balance of healthful oils, such as olive, and the fats found natively in, as noted, nuts, seeds, avocado, and fish.

More relevant, perhaps, are the first formal Dietary Guidelines in the U.S., released in 1980, and thus most proximal to the era from which the BMJ data issue. These guidelines encouraged limiting fat intake altogether, along with saturated fat in particular. They certainly did not encourage chugging corn oil. As an aside, but an important one to the historical revisionists who pretend to have discovered the harms of excess sugar in an epiphany yesterday morning, the 1980 Dietary Guidelines included explicit advice to limit sugar intake in the proverbial (and in this case, literal) short list (#5, to be exact).

Turning now to the two other recent provocations, vegetarian diets and dairy fat, I concede any level of detailed scrutiny here could turn this into a very long column. Fortunately, I have addressed both already, vegetarian diets here and here, and dairy fat here. If I may trouble you to exercise your clicking finger, we may spare ourselves loquacious repetition. I will, however, revisit highlights.

Headlines related to the dairy study suggest we should dive head first into the first fondue pot we find. In reality, there was a very modest association between certain, specific, and rather esoteric fatty acids in dairy and a lower incidence of diabetes. There were many potential confounders.

The study might not mean that full-fat dairy confers any protection against diabetes, but it suggests it could. If it does, it is likely related to satiety -- fullness -- and defense against weight gain. That benefit, in turn, if real, likely pertains only in the context of a prevailing American diet engineered to undermine satiety in the first place. A specific satiety benefit of dairy fat in the context of any diet of wholesome foods in some sensible combination is fairly unlikely, and certainly unimportant. Most of the world's contenders for "best diet" laurels either exclude dairy, or include it at very low levels. The one obvious exception, the DASH diet, explicitly emphasizes low- and non-fat dairy. Let's just say the dairy fat case is far from closed.

Finally, the headlines related to the health effects of vegetarian diets were such a massive, absurd distortion of the actual study that I thought I must be misreading one or the other. Consequently, I conferred directly with the senior study author before publishing my incredulity. Dr. Brenna confirmed my impression: this was some of the most irresponsible journalism we had ever witnessed. Given what passes for journalism these days, perhaps especially in the domain of diet and health, that's really saying something.

Let's turn now to the implications of all this, and then bring it to a close.

As I've had occasion to note (or lament) before, our answers are seldom any better than our questions. So, if we ask: which is better, (a) eating pepperoni pizzas every day, or (b) drinking jugs of corn oil? -- the obvious answer is (c) none of the above. Similarly, if we ask, which is better -- (a) being stabbed through the heart or (b) being clubbed over the head -- again the right answer is (c) an all-expense-paid vacation for two to Tahiti. The questions, alas, allow for the correct answer in neither case.

Why do we ask dubious questions about diet and health in the first place? THAT is a very good question, and one with, apparently, a range of satisfying answers. Sometimes, the question was quite sound when posed some years ago, and has matured into silliness between then and now, when the data are finally ready for prime time. That seems to be the case for a study of high-dose linoleic acid, a hypothesis no one would be inclined to test today.

Sometimes what seems a silly question is really all about ulterior motives, and a hidden agenda. Sometimes, there is an inclination to challenge convention, and be heard above the din.

Conventional wisdom is made to be challenged -- that, in fact, is what science is for. Science never rests, and those of us who profess devotion to it are obligated to move with it. But spitting convention in the eye is probably another matter.

The conventional wisdom of today may be wrong, but it was as hard earned as tomorrow's will be, and much of it will prove correct. As noted, there is a cottage industry these days in pointing out the conventional neglect of sugar as a dietary concern. But sugar was a salient concern in the Dietary Guidelines 40 years ago! Allowing for that truth is apparently inconvenient for the current crop of iconoclasts, conspiracy theorists, and self-promoting renegades.

As bad as answers tend to be to bad questions, headlines tend to be far worse than even the worst answers to the worst questions. The reason for this is obvious. The media want our attention, not our understanding. Headlines are not about education. They serve the mantra: afflict the comfortable, comfort the afflicted. The fine print presumably goes on: if you have to turn facts into gobbledygook to sell copy or top the Nielsen ratings, don't lose any sleep over it.

We heard that vegetarian diets were suddenly bad for us, and all the nutrition experts I know kept eating vegetables. We heard that vegetable oil was suddenly bad for us, and nobody I know stopped chugging corn oil, because no one I know had ever started. We heard about the relative advantages of full-fat dairy, but only those already inclined that way chose to skim this as cream. None of this nihilism is because nutrition experts are impervious to the challenges of evolving science. Quite the contrary, for any who deserve to be called expert in the first place. Rather, it's because they know that headlines are a stupefyingly poor representation of that science.

In other words, we didn't get indigestion from the latest spate of hyperbolic headlines, because we never swallowed them in the first place. And neither should you.

-fin

Director, Yale University Prevention Research Center; Griffin Hospital