Trans-fats have been in the news sporadically in recent years, thanks largely to the bans passed by the New York City Health Department and the Indiana State Fair. Even consumers who don’t read the news have undoubtedly become familiar with the term because of food manufacturers labeling their products “0 Trans Fats!” (often with a small-print “per serving” which usually means there are trans fats in the product, just less than .5 g per serving) or “Trans-fat free!” whether or not they ever contained trans-fats in the first place.

But there still seems to be considerable confusion about what trans-fats are and why they might be bad for your health, which has likely been complicated by the long, stupid demonization of fats qua fats and saturated fats as a supposed cause of high cholesterol and heart disease.

Trans-fats are trans isomers of fatty acids, and although they occur naturally in small amounts in meat and dairy products from ruminants like cows and goats (in the form of vaccenic acid), the primary source of trans-fats in most Americans’ diets is hydrogenated vegetable oils. Most vegetable oils are composed primarily of unsaturated fats, which tend to be liquid at room temperature. In the early 20th Century, when the U.S. started importing soybeans as a source of cheap protein, soybean oil became readily available as a byproduct and was far cheaper than butter or lard. However, liquid and unsaturated fats get rancid much more quickly than solid fats, have a lower smoke point, and were unsuited to many American culinary traditions—biscuits and pastry crusts or all “short breads”* absolutely depend on solid fats to create their flaky texture, as explained in the note at the end.

Hydrogenation, a process first developed by French and German chemists around the turn of the century,  provided the solution: heating the liquid, unsaturated fats in the presence of hydrogen turned them into solids at room-temperature. Apparently, cottonseed oil was also far cheaper than the beef tallow used in candles, so that one of the first uses of hydrogenated oils. It took a little marketing work to convince people it was also good eating--the major campaign for years was “Use Crisco, it’s digestible!” (okay, actually that probably just reflected the central nutritional concerns of the early 20th C: indigestion and dispepsia, see Hillel Schwartz’s Never Statisfied.

However, the process of hydrogenation also creates trans fats, and a different kind from the ones present in beef and dairy products. Unlike saturated fats, trans-fats produced through hydrogenation have been repeatedly correlated with coronary heart disease, including fatal heart attacks, in large, long-term epidemiological studies, including the Framingham Study. A review article on the available research on the relationship between dietary fat and coronary heart disease (CHD) published this past September concluded:

According to the classic ‘diet-heart’ hypothesis, high intake of SFAs [saturated fatty acids] and cholesterol and low intake of PUFAs [poly-unsaturated fatty acids] increase serum cholesterol levels and risk of CHD. However, few within-population studies have been able to demonstrate consistent associations with any specific dietary lipids, with the exception of trans fats and n–3 fatty acids.

In other words, everything you've heard in the last decade about trans fats (bad) and omega-3s (good) actually seems to be supported by the available research, unlike everything you've been told for the last five decades about saturated fats.

However, many of the claims about the threat posed by trans-fats allege that trans-fats raise LDL levels. And it's not at all clear to me that anyone should be concerned about the fact that trans fats might be associated with increased cholesterol, even "bad" cholesterol, for reasons I discuss in the second entry on saturated fat. The only thing makes me think trans fats might actually be bad for people's health is the consistent, strong association between trans fat consumption and increased risk of CVD and myocardial infarction. I know correlation =/= causation, and I haven’t found any good evidence about a proposed mechanism. On the basis of the current evidence, it seems like there’s a difference between the naturally-occurring trans-fats and the ones produced by hydrogenation:

The association was only seen for for trans fatty isomers from hydrogenated vegetable oils. The mainly different trans isomers from ruminant fats did not show such an association. A case-control study in 239 people suffering an acute myocardial infarction found that after adjustment for age, sex and energy intake, intake of trans fatty acids was directly related to risk of myocardial infarction [241]. Those with the highest intake of trans fatty acids had twice the risk of myocardial infarction as those with the lowest intakes after adjusting for other cardiovascular risk factors. As with the Nurses Health Study, the association was only seen for trans isomers from partially hydrogenated vegetable oils. (from a 1995 lit review in the British Food Journal)

Is that because the hydrogenated oils contain linoleic acid, and meat and milk with naturally-occurring trans-fats don’t? I don’t know. I think the most interesting finding from Harvard's Nurse's Health Study, which is one of the studies that did show a weak but significant correlation between saturated fat consumption and CVD, is that the consumption of trans fats was associated with a much higher incidence of CVD than saturated fats. That finding, which is nearly two decades old, should probably be pretty infuriating to anyone who’s eaten hydrogenated-vegetable-oil margarine or shortening in the last two decades because it was supposedly a “healthier” alternative to butter or lard. Similar findings in several other studies prompted the more recent review article to conclude:

The observational evidence that TFA are independently associated with increased risk of CHD events is convincing, though based on a more limited body of evidence.

However:

There is probably no direct relation between total fat intake and risk of CHD.

So the persistent recommendation from public health and nutrition authorities to reduce total fat consumption? Not supported by the available evidence. But the reason this isn’t a  "things that won't kill you" entry is because, well, trans-fats might.

What it means for how I eat

The short version: lard before margarine or shortening, except when the lard is shelf-stable or the margarine/shortening is produced using fractionation and palm oil.Read more

Salt has been all over the news this week because of a study just published in The New England Journal of Medicine claiming that if everyone in the U.S. reduced their sodium consumption by 3 grams/day, there would be 32,000 fewer strokes, 54,000 fewer heart attacks, and 44,000 fewer deaths every year. The story that got my attention was:

That’s surprising, I thought. Everything I’ve read suggests that the relationship between salt consumption and cardiovascular disease is weak, inconsistent, and probably only valid for 20-30% of the population. So I expected the article to refer to some new research where, you know, “big benefits” were “seen.” As in observed. Like, in the world. And, given the claim about the magnitude, probably also measured.

To their credit, the authors of the study claim no such thing. The numbers are projections based on the application of several assumed effects of salt reduction, adjusted for different demographics and then applied to a model of the entire U.S. population. Thus, the title of the study: “Projected Effect of Dietary Salt Reductions on Future Cardiovascular Disease.”

The article seems to grasp the essentially speculative nature of the findings. The very first sentence uses the conditional tense:

…scientists writing in The New England Journal of Medicine conclude that lowering the amount of salt people eat by even a small amount could reduce cases of heart disease, stroke and heart attacks as much as reductions in smoking, obesity, and cholesterol levels.

The headline, on the other hand, seems to have confused the “scientists” with clairvoyants. Never mind doing any checking into the validity of their assumptions.

And the claim about how the benefits compare to smoking and obesity reduction led to a few headlines like this:

This crazypants idea initially sounds a lot like what the study’s lead author claims:

"The cardiovascular benefits of reduced salt intake are on par with the benefits of population-wide reductions in tobacco use, obesity, and cholesterol levels."

But the logic behind the claim is that a small improvement in the health of every single American would be as significant as a large health improvement in the health of every single smoker:

Dr. Bibbins-Domingo said that for many people the decrease in blood pressure would be modest, which is why, she said, “many physicians have thrown up their hands and said, ‘I’m not going to advise my patients to reduce salt because it’s too hard for patients and the benefits for any individual are small.’

“But small incremental changes in salt, such as lowering salt in tomato sauce or breads and cereals by a small amount, would achieve small changes in blood pressure that would have a measurable effect across the whole population,” she said. “That’s the reason why this intervention works better than just targeting smokers.”

For any given individual, there is no question about whether cutting salt is even close to “as good” as quitting smoking. The evidence for the link between smoking and lung cancer and death is strong, reliable, consistent, and has a clear causal mechanism (carcinogens). The link between salt and cardiovascular disease and death is weak, inconsistent, and still poorly understood.

That latter point starts to get at the problems with the study itself, and not just the headlines it inspired. A number of the assumptions the projection was based on are either demonstrably false or simply unsubstantiated. More on this some other time; for now, a few quotes and links to the essays they come from in Esquire and the medical journal Hypertension:

In a more recent statement, the founder of the American Society of Hypertension, Dr. John Laragh, goes further: "Is there any proven reason for us to grossly modify our salt intake or systematically avoid table salt? Generally speaking the answer is either a resounding no, or at that, at best, there is not any positive direct evidence to support such recommendations."

…

Studies show that 30 percent of the Americans who have high blood pressure would greatly benefit from a low-sodium diet. But that's about 10 percent of the overall population -- the rest of us are fine with sodium. And drastically cutting out sodium may actually hurt some people. ( "Go Ahead, Salt Your Food")

And:

The available data suggest that the association of sodium intake to health outcomes reflected in morbidity and mortality rates is modest and inconsistent. Therefore, on the basis of the existing evidence, it seems highly unlikely that any single dietary sodium intake will be appropriate or desirable for each member of an entire population.... The decision to adopt a low sodium diet should be made with awareness that there is no evidence that this approach to blood pressure reduction is either safe, in terms of ultimate health impact, or that it is as effective in producing cardioprotection as has been proven for some drug therapies. (Salt, Blood Pressure, and Human Health)

Although many states already tax soda (usually a fraction of a penny per ounce), a recent article in the New England Journal of Medicine on the potential benefits of a $0.01 per ounce tax on "soft drinks, energy drinks, sports beverages, and many juices and ice teas" has re-ignited the debate about whether or not we need a national soda tax. Back in July, Obama said a sin tax on soda was "an idea we should be exploring" in an interview with Men's Health although in the recent panic about industry profits and personal liberties, the White House has been quick to note that they haven't yet and have no plans to propose anything like it. 

Most people probably already know how the two sides shake out: promoters argue that soda makes people fat (which allegedly makes people sick and thus incurs social costs) so the tax would have the dual benefit of reducing the costs associated with obesity and generating money that would help cover health care costs (or balance state budgets). Opponents argue that soda isn't morally distinct from many other elective behaviors that sometimes (but not always) contribute to disease and health care costs, and as soda consumption is inversely correlated with income, taxing it would disproportionately burden those least able to pay.

All of those are actually pretty complicated claims, some of which I'll try to unpack below the jump but here's the short version: even promoters admit that the tax isn't likely to meaningfully reduce obesity or the diseases associated with it (note: not caused by it, as there's still no reliable evidence that fatness causes any disease besides osteoarthritis, and anyone who wants to hear more about that should consult Paul Campos' The Obesity Myth, J. Eric Oliver's Fat Politics, Glen Gaesser's Big Fat Lies, and/or Michael Gard and Jan Wright's The Obesity Epidemic). That means the only real argument in favor of the tax is that it would raise money. But everyone agrees that it would be a regressive tax. So unless you think that collective costs like state budget deficits and health care reform should be disproportionately shouldered by the poorest citizens, there is no good reason to support the soda tax (and this goes double for ill-considered suggestions that we just axe corn subsidies instead—also after the jump).Read more