A few days ago, the USDA finally unveiled their (fashionably late) 2010 dietary guidelines—the first update they’ve made since 2005. Are you as excited as I am? Can we live without bread yet? Leave the fat on our dairy? Ditch the rancid vegetable oils? Gobble down butter and coconut oil without fearing imminent death? By golly, has the USDA finally pulled its head out of the soybean fields and given us something useful, emerging as a reliable authority instead of a food industry puppet?
Contrary to my title, though, the new guidelines aren’t total hogwash. Just mostly. A few of their recommendations are passable, like these:
- Prevent and/or reduce overweight and obesity through improved eating and physical activity behaviors. (Duh.)
- Increase physical activity and reduce time spent in sedentary behaviors. (Duh.)
- Keep trans fatty acid consumption as low as possible by limiting foods that contain synthetic sources of trans fats, such as partially hydrogenated oils. (Duh.)
- Limit the consumption of foods that contain refined grains, especially refined grain foods that contain solid fats, added sugars, and sodium. (Yes!)
Unfortunately, the rest of the guidelines are the regurgitated—and often unsubstantiated—snippets we’re already inundated with. Case in point:
- Consume less than 10 percent of calories from saturated fatty acids by replacing them with monounsaturated and polyunsaturated fatty acids.
- Consume less than 300 mg per day of dietary cholesterol.
- Consume at least half of all grains as whole grains. Increase whole-grain intake by replacing refined grains with whole grains.
- Increase intake of fat-free or low-fat milk and milk products, such as milk, yogurt, cheese, or fortified soy beverages.
- Use oils to replace solid fats where possible.
According to the guideline packet, these recommendations provide “information and advice for choosing a healthy eating pattern” and are “based on the most recent scientific evidence review.” If you’ve read anything else on this blog, you probably know by now that I’m weary of trusting second-hand interpretations—the original data often tells a different story than the mouths claiming to interpret it. So instead of taking the USDA’s word as gospel, why not see what they’re basing their recommendations on?
Luckily, the USDA has a Nutrition Evidence Library, where they’ve compiled the studies they used to create their latest guidelines. Let’s dig in.
Saturated fat: true killer or whipping boy?
Here’s what the USDA has to say about saturated fat:
A strong body of evidence indicates that higher intake of most dietary saturated fatty acids is associated with higher levels of blood total cholesterol and low-density lipoprotein (LDL) cholesterol. Higher total and LDL cholesterol levels are risk factors for cardiovascular disease.
Ah, the lipid hypothesis in all its unproven, scientifically-feeble glory! We’ll look at the evidence they cite to bash saturated fat in a moment. But for now, let’s see their specific 2010 recommendations regarding this oft-feared nutrient:
To reduce the intake of saturated fatty acids, many Americans should limit their consumption of the major sources that are high in saturated fatty acids and replace them with foods that are rich in monounsaturated and polyunsaturated fatty acids. For example, when preparing foods at home, solid fats (e.g., butter and lard) can be replaced with vegetable oils that are rich in monounsaturated and polyunsaturated fatty acids.
Time to start frying your (yolk-free) eggs in soybean oil. Never mind that polyunsaturated fats actually increase oxidative stress (a major player in heart disease and cancer) and become particularly hazardous when heated, especially compared to heat-stable saturated fats. And never mind that most vegetable oils are disproportionately high in omega-6 fatty acids, aggravating the omega 3/6 imbalance that’s already rampant in American diets. If the USDA guideline team could peel off those lipid-hypothesis goggles for a minute, maybe they’d realize that the vegetable oils they’re recommending are likely to wreak some serious health havoc, regardless of what they do to cholesterol levels.
Worse, the new dietary guidelines give the green light to eat some of the worst industrial oils out there:
Oils that are rich in monounsaturated fatty acids include canola, olive, and safflower oils. Oils that are good sources of polyunsaturated fatty acids include soybean, corn, and cottonseed oils.
From this graph, we should learn that soybean oil and corn oil (for example) are more healthful options than coconut oil and butter, because they’re lower in saturated fat. It doesn’t matter that we have studies showing high-omega 6 oils like corn oil may promote tumor growth while—using the same study design—saturated fats do not. As long as the USDA is on board with the “cholesterol causes heart disease” theory, the only thing that matters about fats is how they affect lipid profiles.
Besides, saturated fat is saturated. And saturated things kill us.
Here’s something else that’s interesting. Let’s hop over to the fatty acid page in the Evidence Library for a second. Under the subheading called “Needs for Future Research” (AKA “Stuff We Don’t Really Understand Yet”), they wrote:
1. Determine the benefits and risks of MUFA vs. PUFA as an isocaloricsubstitute for SFA. Confirm the metabolic pathways through which dietary SFA affect serum lipids, especially as some SFA (e.g., stearic acid) do not appear to affect blood lipid levels.
Basically, they’re recommending we swap saturated fat for unsaturated varieties without being sure what the effects are, and that we slash all saturated fat consumption without being sure whether the reasons are biologically justified. I guess by the time the next tome of guidelines is released, the USDA will get to see whether their lipid recommendations helped or killed us off faster. Welcome to America, land of 300 million guinea pigs.
But could the USDA be onto something we don’t know about—especially with the “strong body of evidence” they mentioned linking saturated fat to heart disease? The answer may lie in their evidence summary page, which recaps the 12 studies they looked at to assess saturated fat. As best I can tell, these studies are the main pieces of research the USDA used to back up their “replace saturated fat with unsaturated fat” recommendation.
For the sake of being thorough, here’s a rundown of all those studies. You can click the article name for a link to the study (full text for most).
This one looked at a group of 46 adolescents who had high cholesterol. One group continued chowing down on their normal diet (the only instructions: “eat as usual”), and the other group ate the “National Cholesterol Education Program Step II Diet.” At the end of the study, the Step II kiddos had lower total cholesterol, lower LDL cholesterol, and larger LDL particle size.
So how did the Step II diet differ from the control group? Was a shift in fat sources the only change? Let’s take a look:
As you can see, the Step II dieters ate significantly fewer sweets, fewer fats and oils, more vegetables, more fruit, more poultry and fish, more fiber, and more dairy products (mostly low-fat) than the eat-whatever group. They also ate less saturated fat (7 percent compared to 14 percent) and more monounsaturated fat. The researchers note that the higher fiber intake of the Step II diet “could explain its beneficial effects on lipid concentrations and particle size,” at least to some extent.
But what did the USDA, in their infinite wisdom, conclude from this? That the improved lipid profiles resulted solely from reducing saturated fat and replacing it with unsaturated fats. At least that’s what it seems like, since “type of fat” is the only changed variable they mention in their summary in the Evidence Library.
In other words, this study is fairly useless for isolating the effects of saturated versus unsaturated fat—but that’s exactly what the USDA team tried to do.
This study rounded up 85 adults—mostly folks with low HDL and high triglycerides—and made them consume three consecutive diets: an “average American diet” (with 15.6 percent of calories as saturated fat), a high-monounsaturated-fat diet (replacing 7 percent of the saturated fat with monounsaturated fat), and a high carbohydrate diet (replacing 7 percent of the saturated fat with carbs). The carbohydrate-heavy diet also added a significant amount of fiber. Unfortunately, the study doesn’t document what else changed between the diets in terms of specific food intake, nor what the actual sources of fat were.
The results? Both of the low-saturated-fat diets reduced HDL levels (bad, bad, bad—these folks had low HDL to begin with!), and the carby diet produced higher triglycerides than both the average American diet and the mono-fatty diet. The total cholesterol/HDL ratio worsened when carbs replaced saturated fat, and none of the diets produced any differences in glucose or insulin response. It’s hard to say why the USDA thought this study supported their recommendation to cut saturated fat and replace it with unsaturated varieties. Even though the high-monounsaturated fat diet reduced LDL levels, it did so roughly in proportion to reducing HDL (not something you want to see happen in folks who are predisposed to insulin resistance)—and the effect on triglycerides was negligible. If anything, this study shows that it’s generally better to eat saturated fat than replace the saturated fat with carbohydrates. But even then, there aren’t enough specific diet details to get a sense of all the factors involved. In a nutshell: It’s a long, painful, joint-busting stretch to say this study supports the USDA’s fat recommendations.
In this study, the researchers compared the effects of eating diets with either 30 percent trans fat, 30 percent polyunsaturated fat, or 30 percent saturated fat for four weeks (using hydrogenated soybean oil, rapeseed oil + sunflower oil, and palm oil + a little olive oil, respectively). The results? Maybe the title should tip us off, especially the “not affected by different types of dietary fatty acids” part. Total cholesterol and triglycerides didn’t change over time with any of the fat types, and the researchers conclude that “differences in the cell cholesterol efflux with these diets were not observed.” In the USDA’s summary, they note—looking closer at the study’s details—that the polyunsaturated fat diet seemed to have some favorable effects compared to the saturated fat diet, such as clearing more LDL cholesterol after meals. Unfortunately, they overlook the artery-clogging elephant in the room, which is that—based in the markers in this study—the trans-fat diet produced better results than the polyunsaturated or saturated fat diets.
Is this really a good study to use for saving the health of America?
This one’s a head-scratcher—not because of the study itself, but because it somehow became evidence for replacing saturated fat with unsaturated varieties. In this study, the researchers compared the effects of a control diet supplemented with beef tallow versus the same diet supplemented with “functional oil”—a combination of medium-chain triglycerides (abundant in coconut oil and palm oil), phytosterols (substances in plants that stop or slow cholesterol absorption), and omega-3 fatty acids. The goal was to see whether the phytosterols and omega-3 fats would keep lipid profiles lookin’ good.
By the end of the experiment, the “functional oil” diet did well enough to please any cholesterol-fearing doctor: Total cholesterol, LDL, the HDL:LDL ratio, and the HDL:total cholesterol ratio all improved significantly compared to the baseline measurements and to the beef tallow diet. The beef tallow diet didn’t produce any statistically significant changes except for a reduction in triglycerides. So what does this tell us about saturated fat, and the effects of replacing it with polyunsaturated fats? Almost nothing. The baseline measurements reflected the participants’ normal diets, not the study diet sans fat supplement—so it’s impossible to isolate the effect of beef tallow or of any specific component of the “functional oil.” In fact, the “functional oil” diet had more saturated fat (63.8 grams versus 50.9 grams) and less monounsaturated fat (24.4 grams versus 41.9 grams) than the tallow diet, the opposite of what the USDA recommends. If anything, this study shows that the USDA could still appease their cholesterol fixation by recommending a diet loaded with coconut products, butter, and palm oil (the best sources of medium-chain fatty acids) along with fish and phytosterol-containing vegetables. That’d roughly mimic the “functional oil” supplement used in the study, but with real foods. Ya hear that, USDA? It’s the sound of saturated fat busting out of jail. And it goes something like this: “Neener-neener.”
Here we have another study that does nada to support lowering saturated fat. In this one, researchers wanted to see if the effects of plant sterols differ depending on the dietary context—so they compared four diets: a typical American diet (13.2 percent saturated fat), an American diet supplemented with plant sterols, a reduced-saturated-fat Step I diet (7.7 percent saturated fat), and a Step I diet supplemented with plant sterols. The diets consisted of pretty much the same foods, just with different fat levels.
Compared to the Step I diet, the higher-saturated-fat American diet (without plant sterols) produced higher values for all the lipoproteins and plasma lipids measured—except for triglycerides, which were the same. It also yielded a slightly prettier total cholesterol:HDL ratio, since the lower-saturated-fat diet reduced both LDL and HDL. Once the phytosterols were thrown into the mix, the results showed that the sterols have an additive effect rather than an interactive effect with their diet context—meaning there were no special benefits from adding plant sterols to the American diet versus the Step I diet. Okay. That’s interesting and all, but how does it show that saturated fat ruins your heart health? Why did the USDA find it relevant enough to use in their uber-selective Evidence Library to answer the question “What is the effect of saturated fat intake on increased risk of cardiovascular disease?” Even if you ignore the fact that this study was about plant sterols and try using it to gauge the effects of the typical American diet vs. the Step I diet (where the only major difference is saturated fat content), it definitely doesn’t show the reduced-saturated-fat diet coming out ahead.
This is a study examining the effects of a high-polyunsaturated-fat diet versus a high-saturated-fat diet on postprandial lipemia (lipids in the blood after eating). Compared to the saturated-fat diet, the polyunsaturated-fat diet resulted in faster clearing of cholesterol and triglyceride-rich lipoproteins from the blood—which is generally considered a good thing. However, given the potential for polyunsaturated fat to contribute to oxidized LDL, quicker clearance time doesn’t automatically make polyunsaturated fat a friend of the heart. Nor does this study offer hard evidence that saturated fat contributes to cardiovascular disease.
Next up is a study looking at myristic acid, a type of saturated fat found in coconut, palm oil, butter fat, and nutmeg (which has the scientific name Myristica fragrans—where the word “myristic” actually comes from). The researchers gave a group of French Benedictine monks two diets: one with 30 percent fat (8 percent saturated, 0.6 percent myristic acid) and one with 34 percent fat (11 percent saturated, 1.2 percent myristic acid). The extra saturated fat came mainly from whole milk.
Did that added dairy fat do ’em in? Au contraire. Both diets improved lipid measurements across the board. But compared to the lower-saturated-fat diet, the higher-saturated-fat diet produced a greater drop in triglycerides and a higher boost in HDL from the baseline measurements, without comparatively raising LDL or total cholesterol.
This, of course, baffled the researchers. Here’s a blurb straight from their paper, emphasis mine:
Many questions are raised by comparing diets 1 [lower saturated fat] and 2 [higher saturated fat]. If one accepts the conclusions of Gaggiula and Mustad, Clarke et al., Howell et al. and Verschuren et al., the better results should have been obtained with diet 1 and not diet 2. The MUFA and PUFA intakes are identical in both diets, while the PUFA/SFA ratio is 1 in diet 1 and 0.75 in diet 2. The predictive equations of [long list of lipid hypothesizers] point to the superiority of diet 1 over diet 2. The main difference between the diets is that there is twice as much myristic acid in diet 2. … Moreover, the decrease in carbohydrates in diet 2 (51.2%) vs. diet 1 (55.2%) should have worsened the lipid results. Yet at these levels which are pertinent in clinical terms and are widely accepted, the present findings are in contradiction with the various theories based on explanatory equations and on certain studies performed at levels not encountered in daily life.
In other words: “Oops! We found something that contradicts the lipid hypothesis. Let’s air our cognitive dissonance on paper.”
Again, this is a study straight from the USDA’s Evidence Library, on the page dedicated to the saturated-fat-causes-heart-disease issue. If the USDA deemed it a high-quality study, why did they continue universally condemning saturated fat?
This study looks at the effects of a carb-heavy, protein-heavy, and unsaturated-fat-heavy diet on apo-B levels. As much as I love to yammer on about this stuff, I’ll keep this one short: The most interesting outcome was that, compared to the carby diet, the protein-rich diet had a much more favorable impact on apolipoprotein and lipoprotein profiles. The unsaturated-fat diet was also an improvement over the carby diet, but not significantly so.
Unfortunately, this study can’t tell us diddly squat about saturated fat because none of the test diets used saturated fat as an independent variable, or even indirectly measured its effects. Next, please.
Finally! A paper that actually involves saturated fat and heart disease and a link between the two (maybe). It’s a miracle! Oh USDA, you’ve almost redeemed yourself.
Here we have a review of 11 studies concerning diet and heart disease, with a special focus on what to do once we succumb to conventional wisdom and scoot the saturated fat off our plates. Should we fill the calorie void with carbs? With polyunsaturated fat? With monounsaturated fat? With Soylent Green*?
*Not suitable for vegans
Indeed, this study found some interesting trends. Reducing saturated fat by 5 percent and replacing it with monounsaturated fat was associated with an increase in coronary events (hey USDA—why not vilify olive oil, too?). Although carbs were a mixed bag in the pooled analysis, a newer study led by the same researcher, Marianne Uhre Jakobsen, found that the type of carbohydrate plays an important role: Increasing high-glycemic carbs by 5 percent in place of saturated fat was associated with a 33 percent greater risk for having a heart attack. (Funny how the new USDA recommendations, while censuring saturated fat, still allow up to half your daily grain intake to be refined.) Low- and medium-glycemic-index carbs fared more favorably, but in Jakobsen’s second study, neither were associated with a statistically significant improvement in heart health when displacing saturated fat.
Interestingly, from the USDA’s cited study, replacing saturated fat with polyunsaturated fat seemed to be associated with reduced deaths from heart disease. I say “seemed” because of a reply Martijn Katan wrote shortly after this study was published, which you can read here. This is the point worth noting (emphasis mine):
Jakobsen et al (1) found that a low intake of saturated fatty acids and a proportionally higher intake of omega-6 polyunsaturated fatty acids was associated with a significant reduction of coronary heart disease. Confounding was again a problem: diets low in saturated fatty acids and high in polyunsaturated fatty acids are rich in vegetable oils, polyunsaturated margarines, lean meats, and low-fat dairy. That is what health-conscious people eat. Indeed, correction for smoking, body mass index, and other risk factors diminished the effect from a risk reduction by 31% to a risk reduction by only 13%, if 5% of energy from saturated fatty acids was replaced by that from polyunsaturated fatty acids. Is this 13% due to residual confounding by imperfectly measured aspects of a healthy lifestyle, or is it real?
Given what we know about the biological effects of polyunsaturated fats (especially their contribution to oxidative stress), it sure seems possible that their apparent health perks could be a result of confounding. Especially since polyunsaturated fats are pushed so firmly by health authorities.
Public diet guidelines have a spooky tendency to create self-fulfilling prophecies: As soon as specific foods are slapped with a “healthy” label by the white-coat-sporting experts, they’re more likely to appear beneficial in studies. That’s because health-conscious folks are often the only people who actually heed the advice of the USDA and other nutrition authorities, so they integrate foods like low-fat dairy and vegetable oils into their tangled web of healthy habits—creating a massive ball of confoundment that’s nearly impenetrable with statistical tools. (Of course, healthy foods can appear deadly by this same mechanism. If the health-indifferent folks are the only ones brave enough to eat a declared “artery-clogging” item—say, butter—then statistical analyses are generally going to show that food being hazardous, because the people consuming it are damaging their health in other ways.)
But I digress. If the USDA had kept its eyes peeled for research a little longer before finishing the 2010 guidelines, maybe they would’ve seen the newer meta analysis on saturated fat and heart disease reviewing almost twice as many studies as the one above. What did this bigger analysis reveal? I’ll let the researchers say it for me:
In conclusion, our meta-analysis showed that there is insufficient evidence from prospective epidemiologic studies to conclude that dietary saturated fat is associated with an increased risk of CHD, stroke, or CVD.
As meta analyses often do, this paper also examined the pooled studies for publication bias—an all-too-common tendency to publish studies based on their results rather than on their theoretical or design quality. Indeed, the researchers found that—in the realm of saturated fat and heart disease—studies showing a strong relationship were more likely to get published than studies showing a neutral relationship.
Our results suggested publication bias, such that studies with significant associations tended to be received more favorably for publication. If unpublished studies with null associations were included in the current analysis, the pooled RR estimate for CVD could be even closer to null.
All in all, the USDA’s chosen study—the pooled analysis by Jakobsen—is their strongest “evidence” so far to support replacing saturated fat with polyunsaturated fat. But that study is far from conclusive, especially since it 1) seems to warn against monounsaturated fat (one of the USDA’s darling lipids), 2) was followed by another study showing that many carbohydrates are more convincingly associated with heart disease than saturated fat, and 3) had its findings challenged by a newer, bigger meta analysis.
But maybe the USDA has some more compelling studies up it’s sleeve. Let’s look at the next one.
In this study, researchers took 14 male volunteers and studied the effects of a high-saturated-fat diet versus a high-polyunsaturated-fat diet on cholesterol efflux from macrophages—your body’s mini-vacuum cleaners. (When macrophages can’t get rid of the cholesterol they slurp up, they can turn into foam cells, which is one of the earliest steps in atherosclerosis). Since diets high in polyunsaturated fat tend to decrease HDL, the researchers wanted to see if this type of diet would be detrimental for transporting cholesterol to the liver for catabolism. The results? The high-saturated-fat diet and high-polyunsaturated-fat diets didn’t produce any differences in cholesterol reflux. In other words: This study isn’t a strike against polyunsaturated fat, but it’s also not a strike against saturated fat.
11. Individual variability in cardiovascular disease risk factor responses to low-fat and low-saturated-fat diets in men: body mass index, adiposity, and insulin resistance predict changes in LDL cholesterol.
Here we have a study looking at three diets with differing fat levels: The Average American Diet (38 percent fat, 14 percent saturated fat), the Step I diet (30 percent fat, 9 percent saturated fat), and the Step II diet (25 percent fat, 6 percent saturated fat). The Step diets, by the way, were designed by the American Heart Association to help combat heart disease. Since all meals were provided for the participants, there was a high degree of control
Was there a trend between lower saturated fat intake and better lipid profiles (and, by association, better heart health)? Nope. Both of the saturated-fat-reduced Step diets significantly raised triglycerides, lowered HDL, and worsened the total cholesterol:HDL cholesterol ratio.
This is bad news, folks.
Because the changes in apo-B and apo-A1 were (percentage-wise) less than the changes with LDL and HDL cholesterol, the researchers speculate that the lower-fat diets increased the proportion of small, dense LDL particles (the ones most associated with atherosclerosis) as well as decreasing HDL2 relative to HDL3 cholesterol (HDL2 is the more protective subfraction).
Basically, slashing fat resulted in lipid profiles more likely to promote heart disease.
On top of that, the men who were overweight (and insulin resistant) suffered the most from the Step diets: For them, cutting down fat caused a much steeper drop in HDL relative to LDL, resulting in a more dangerous total cholesterol:HDL ratio than folks of a healthier weight. The researchers conclude that:
[P]ersons who may already be at increased risk of CVD because of their underlying insulin resistance, and thus who are prime candidates for dietary intervention, may be less likely to benefit from dietary changes.*
*”Dietary changes” = reducing total and saturated fat. Because obviously, this is the only kind of dietary change in existence.
The Step I diet is pretty close to what the USDA recommends, in terms of total and saturated fat percentage, but eating this way only made things worse compared to the higher fat diet. Again, I ask: how does this study support the USDA’s fat-lowering recommendations? How, how, how? Did they even read the stuff they crammed into their Evidence Library?
This one’s easy-schmeasy. In this study, researchers compared the effects of five experimental diets, all with 30 percent fat: one with soybean oil, one with low-saturated-fat soybean oil, one with high oleic-acid soybean oil, one with low-alpha-linolenic-acid soybean oil, and one with partially hydrogenated soybean oil.
None of the diets produced significant changes in very-low-density lipoprotein, triglycerides, lipoprotein(a), C-reactive protein, or ratios between cholesterol fractions—except for the hydrogenated soybean oil diet, which yielded a higher total cholesterol:HDL ratio than the rest. The take-home point? Non-hydrogenated soybean oils are better than hydrogenated soybean oils. Thanks for the study,
Captain Obvious USDA. Too bad this one’s completely irrelevant to the saturated fat guidelines.
…And that’s it, folks. These 12 studies are what the USDA used to evaluate the connection between saturated fat and heart disease.
Why does saturated fat seem evil in studies?
Quite by accident, the USDA does a rockstar job of proving saturated fat goes hand-in-hand with a junky cuisine—making it a nightmare to untangle in epidemiological studies, and more likely to be “guilty by association” when it comes to disease. Check out this pie graph of the most common sources of solid fats (AKA saturated) hitting America’s collective dinner plate:
Here’s the lowdown. “Grain-based desserts” (think cookies, cakes, pies, pastries) are the second-largest contributor to America’s saturated fat intake—right behind the rather ambiguous “all other food categories.” In fact, grainy desserts are a bigger source of saturated fat than butter, eggs, and whole milk combined. What’s after grain-based desserts? Pizza. Then cheese. Then processed meats. Then french fries. Then dairy desserts.
In fact, if you add it all up, 45 percent of our saturated fat intake comes from starch-based meals, sugary desserts, or processed meat, whereas only 32 percent comes from whole foods traditionally associated with saturated fat (butter, milk, unprocessed meat, eggs). The remaining 23 percent is that mysterious teal slice on the pie chart.
Think of it this way: When a study looks at someone’s saturated fat intake in relation to disease, is it really measuring foods like animal products and coconut oil, or is it actually recording stuff like deep-dish pizza and Oreos—markers for an I-don’t-give-a-hoot-about-my-health lifestyle? I’ll let you be the judge.
Polyunsaturated fat: mmm-mmm good, or uh-uh bad?
If you’ve made it this far in this article, you’ve probably noticed that the USDA is awfully fond of polyunsaturated fats—especially in the form of vegetable oil. For the sake of argument, let’s assume that it’s not because the USDA is a mouthpiece for the soybean and corn industries, but rather, because their collection of Evidence Library studies proves this fat is healthy for us. Here’s their page dedicated to answering the question: “What is the effect of dietary PUFA intake on health and intermediate health outcomes?”
Let’s just look at heart disease for now, since the USDA is so intent on telling us vegetable oils will keep our arteries squeaky clean. Of the 10 polyunsaturated fat studies in the Evidence Library, not all were relevant to heart disease, and one was a re-citation of the Jakobsen meta-analysis we already scoured. Here are the ones worth looking at:
This study confirms what we suspected all along: that polyunsaturated fats are associated with a healthy lifestyle, and therefore massively confounded. In a cohort of 1,551 middle-aged men, the folks who died from cardiovascular disease by the 15-year follow up had been eating less polyunsaturated fat, but they were also far more likely to smoke (54 percent versus 31 percent in the entire cohort), drank more alcohol, had a lower socioeconomic status, had higher blood pressure, had higher BMIs, were more often on blood pressure medication, and had higher fasting insulin. Polyunsaturated fats as a whole, as well as serum linoleic acid, were inversely associated with BMI, fasting insulin, fasting glucose, alcohol intake, and age.
Interestingly, there was virtually no difference in the total fat or saturated fat intake of those who died from cardiovascular disease versus the cohort as a whole.
Not surprisingly, polyunsaturated fats appeared inversely correlated with death from heart disease (as well as death from all causes), but the associations often diminished after accounting for lifestyle factors or using different statistical models:
Dietary linoleic acid intake was associated with a lower overall mortality during follow-up after adjustment for age and examination year … but not significantly after adjustment for lifestyle or dietary factors. Total PUFA intake was not significantly associated with overall mortality. Men whose -linolenic acid intake was in the upper third were 15% to 33% less likely to die of any cause than men whose intake was in the lower third, but the trend at best approached significance. The association of the dietary PUFA/SAFA ratio with overall mortality was significant, … but the association was not significant in models 2 through 4.
Serum fatty acids showed more robust associations, but we’re more interested in the effect of actual fat intake on heart health. All in all, this study doesn’t exactly give us a compelling reason to inject our diets with industrial oils.
I’ll make this one short. The title explains the study, and this excerpt explains the outcome:
In this large prospective cohort study, modest dietary intake of long-chain n-3 PUFAs (≥ 250 mg/d) was associated with a 40% to 50% lower risk of sudden death, regardless of background intake of n-6 PUFAs. This lower risk was observed after adjustment for a variety of cardiac risk factors, lifestyle characteristics, and other dietary habits. These results suggest that n-6 PUFAs neither greatly counteract nor greatly augment the cardiovascular benefits of a modest intake of long-chain n-3 PUFAs from seafood.
Basically, this study found that omega-3 fats were beneficial, but omega-6 fats—the kind vegetable oils are chock full of—didn’t offer any special health perks. Moral of the story: Seafood is good for you, industrial oils are unnecessary. Nice job shooting yourself in the foot with this study, USDA.
Here we have another study where polyunsaturated fat looks cardio-protective—thanks to its entanglement with healthy lifestyle choices. In following almost 79,000 women for 20 years, this study (a Nurse’s Health Study follow-up) found that polyunsaturated fat was associated with lower heart disease risk for the gals with the highest versus lowest intake. And it’s no big surprise: Out of all the fat types, polyunsaturated fat was the only one where unhealthy habits decreased as consumption rose.
See exhibit A. (Each fat type is divided into quintiles, with the folks in quintile 1 eating the least amount of the specified fat and the folks in quintile 5 eating the most.)
For saturated fat, monounsaturated fat, and trans fat, the women with the highest intake were smoking more, had a greater history of hypertension, had lower use of multivitamins and hormones (eg, birth control), had lower use of aspirin, and had a higher intake of cholesterol (indicating less concern with our trustworthy governmental guidelines). In contrast, the women with the highest intake of polyunsaturated fats were smoking less than the women with the lowest intake, had less history of hypertension, were more likely to be using hormones and taking aspirin, were eating only a negligibly higher amount of cholesterol, and had a much smaller change in fiber intake. Collectively, those trends point to an overall higher interest in healthy living—including, no doubt, some daily portions of the polyunsaturated-fat-rich foods we’re told are good for us.
Indeed, this study produced some apparent fat-heart disease correlations that vanished after adjusting for other factors:
In age-adjusted analyses, total fat intake was significantly associated with increased risk of CHD. However, in the multivariate analyses, the association was attenuated and was not significant. For specific types of fat, intakes of saturated fat, monounsaturated fat, polyunsaturated fat, and trans-fat were each significantly associated with risk of CHD in age-adjusted analyses. … Intakes of saturated fat and monounsaturated fat were not statistically significant predictors of CHD when adjusted for nondietary and dietary risk factors.
Even though polyunsaturated fat was still associated with lower heart disease after some adjustments, the relationship was closer to neutral for women who weren’t overweight. And given the entanglement of this fat with healthier living in general, it’s pretty much impossible for a study to record all the factors needing adjustment—making it hard, if not downright futile, to attempt isolating the effects of polyunsaturated fat itself.
No, this study isn’t a joke. The researchers took 33 adults and dragged them through three controlled feeding phases: one where they ate a low-fat/higher carb diet (30 percent fat, 10 percent saturated fat), another where they ate a high-polyunsaturated-fat diet (36 percent fat, 9.7 percent polyunsaturated fat), and a third where they ate a general high-fat and trans-fat diet (38 percent fat, 11 percent saturated fat, 2.7 percent trans fat). All the diets reduced total and LDL cholesterol, although the low-fat and high-polyunsaturated-fat diets had the greatest effect. Considering one of the diets was higher in processed carbs and one of the diets was higher in trans fats, it shouldn’t be a shock that the remaining diet—the one with the greatest proportion of polyunsaturated fats—fared the best, reducing some markers associated with heart disease compared to the other diets. The researchers concluded that it’s better to eat corn chips fried in corn oil than corn chips fried in trans-fatty oil.
This study examined the effects of two unsaturated fats (oleic and linoleic acids) and a saturated fat (stearic acid) on thrombotic—or blood-clotting—tendency. Long story short:
In conclusion, our results do not suggest that stearic acid is highly thrombogenic compared with oleic and linoleic acids.
Another redeeming point for saturated fat, and another “this doesn’t support guzzling vegetable oils” point against the USDA.
After feeding a group of 45 people three randomly-ordered experimental diets, this study found no statistically significant differences between the effects of the unsaturated fats (oleic and linoleic acid) versus the saturated fat (stearic acid):
In this well-controlled crossover study of healthy subjects, we found that the differences in effects of stearic, oleic, and linoleic acids on the serum lipoprotein profile were less than expected. Although total and LDL-cholesterol concentrations tended to decrease with the increasing degree of unsaturation, the changes between the 3 diets were not significant.
Once again, we’ve got a string of studies that do little to validate the USDA’s love-fest for vegetable oils.
Dairy: low-fat or bust?
The new USDA guidelines don’t waste any time pushing dairy—but, despite some earlier hoopla about encouraging cheese consumption, staunchly warn against consuming anything other than low-fat or fat-free milk products. Apart from the saturated-fat phobia, is this recommendation justified?
I probably don’t need to explain that dairy in general isn’t a necessary food, that your bones won’t crumble into sawdust if you forgo milk with breakfast, and that factory-farmed dairy is laden with all sorts of nastiness. That’s been covered in a billion ways on a billion blogs. But dairy—real dairy, the stuff from pastured animals who aren’t squished into feedlots—may very well have some health perks, especially when left in full-fat form. (At least for those who tolerate it.)
Don’t believe me? Consider this:
A recent Dutch study showed that full-fat fermented dairy was inversely associated with death from all causes and death from stroke. A large study of Australians, published in 2010, showed that full-fat dairy appears protective against cardiovascular death. Yet another study, this one from 2005, showed a significant inverse association between full-fat dairy consumption and colorectal cancer. Another study still linked vitamin K2 from full-fat cheeses to reduced risk of death from all causes, as well as a reduction in aortic calcification. And a review from 2009, examining 10 different dairy studies, noted that some types of saturated dairy fat have a neutral effect on LDL, and full-fat cheese—compared to other dairy products—seems to have the strongest inverse relationship with heart disease.
The vilification of dairy fat is mostly linked to the anti-saturated-fat craze—but given the evidence above and the fact that some of the most beneficial components of dairy are concentrated in the fat (vitamins A, D, E, K2, and medium-chain triglycerides), it seems that for milk drinkers, going skim defeats the purpose.
Healthy Whole Grains: a mandatory health food, or the lesser of two evils?
Per the new guidelines, the USDA recommends eating six (or more) servings of grains per day—half of which ought to be whole. For the mathematically challenged, there’s even a graphic to help you see what this looks like visually, bread-slice style:
I contemplated writing a giant take-down of the “healthy whole grain” studies in the USDA Evidence Library, but soon realized it’d be pointless (and would raise this blog post to an even greater degree of mammoth). Virtually all studies showing the benefits of whole grains do so in a specific context—when whole grains are displacing refined grains or other bottom-of-the-totem-pole foods. There’s nary a study out there looking specifically at the effects of a diet with whole grains versus no grains, and the USDA’s recommendation for everyone to eat at least six servings per day is arbitrary (at best). Emerging research on paleo and low-carbohydrate diets—many of which yield improved lipid profiles and risk markers for disease—are showing that, yes, humans can live without bread.
That said, the USDA does have some interesting stuff on their carbohydrate summary page, under the “Needs for Future Research” subheading:
“Develop and validate carbohydrate assessment methods. Explore and validate new and emerging biomarkers to elucidate alternative mechanisms and explanations for observed effects of carbohydrates on health. Rationale: Studies of carbohydrates and health outcomes on a macronutrient level are often inconsistent or ambiguous due to inaccurate measures and varying food categorizations and definitions. The science cannot progress without further advances in both methodology and theory.”
Hmm. Despite their firm assertion that “healthy diets are high in carbohydrates” (page 42 of the guideline packet), they seem to concede here that the evidence supporting it is weak.
While we’re on carbs, here’s another surprising admission about fruits and vegetables, also from the “Needs for Future Research” section:
“Determine whether the effects of vegetables and fruits in the overall dietary pattern are due to displacement of other foods in the diet or to the action of vegetables and fruits, per se, on specific health outcomes. Rationale: The mechanism(s) of action for the effects of vegetables and fruits have not been determined and, therefore, may vary for different health outcomes. The observed effects could be a simple displacement of these foods with other foods that cause poorer outcomes, or vegetables and fruits may contribute specific benefits or a combination of the above may explain the observations made thus far in the literature. Only further research can provide more definitive answers.”
In other words: “We aren’t actually sure that fruits and vegetables are good for you… even though we’re telling everyone to eat lots more of them.”
Although some of the new USDA guidelines are just watered-down common sense (“be more active, eat less junk food”), a few of the recommendations are downright harmful: the idea that polyunsaturated fats are universally healthy, the perpetuated fear of saturated fat, the encouragement of low-fat dairy, and the notion that everyone needs a carb-heavy, grain-based diet to thrive. Unfortunately, the 2010 recommendations parrot the same misinformation that’s been keeping Americans fat and sick for so long—all stemming from a flawed understanding of cholesterol and disease, as well as decades of research biased to please the gods of Conventional Wisdom.
Bottom line: These guidelines will guide you alright—straight to your spot in the pharmacy line. Look elsewhere for advice if you’re serious about your health.