In the early 1950s, prominent University of Minnesota
physiologist Ancel Keys, whose work was funded in part by the massive Sugar
Research Foundation, began publicly promoting his hypothesis that dietary fat
and cholesterol were the main drivers of cardiovascular disease
In 1955, President Eisenhower suffered arguably the most
noteworthy heart attack in history, launching heart disease fully into the public
consciousness. Taking on Keys as a personal advisor, Eisenhower adopted and
began advocating for a “prudent” low-fat diet.
In 1961 the American Heart Association officially condemned
saturated fat as a likely cause of heart disease. The AHA was by this time the
world’s largest non-profit, largely thanks to significant funding from Proctor
& Gamble, makers of the industrial saturated-fat replacement product
Crisco. That same year, Ancel Keys appeared on the cover of Time Magazine,
depicted as a scientific crusader against the scourge of saturated fat.
1977 saw the initial release of the US government’s first
“Dietary Goals for Americans,” which advised Americans, in the interest of
health, to strictly limit their consumption of saturated fat, cholesterol, and
the animal products that contain them.
By this time, the recommendations were fairly predictable.
Keys, Eisenhower, the AHA, publicized congressional hearings, and more
contributed to what was fast becoming common knowledge – that the consumption
of fatty foods, red meat, saturated fat, and cholesterol were a danger to
health and well-being.
In the 1980s, finally, researchers began looking to see
whether any of it was actually true.
Epidemiology
Now, I don’t mean to say nutrition research as a whole literally
started in the 1980s, only the type you’re most familiar with.1,2
Nutrition research more broadly started in earnest a couple decades prior when
researchers set about trying to legitimize the guidelines that had already been
set forth. Often, it should be noted, these studies promptly proved them wrong.
For example, major trials such as the Sydney Diet Heart Study and Keys’ own
Minnesota Coronary Experiment sharply reduced saturated fat intake for
thousands of subjects…who subsequently died at higher rates than the groups who
continued consuming their butter and eggs.3,4
A landmark trial in the 1970s included smoking cessation alongside the “prudent
diet” and had only to show for it a slight decrease in cardiovascular disease
and a slight increase in overall death.5
That ostensibly counterintuitive results like these did nothing to slow the
low-fat train speaks to how rapidly and aggressively such advice was being
pushed.
Instead, the type of study that had never yet been carried
out, but with which you’re most familiar today, is something called nutritional
epidemiology. In essence, it is a field that attempts to examine dietary trends
at a population level and seek out associations between these trends and
various health outcomes. A common approach, for example, is to survey hundreds
or thousands of subjects on their dietary habits for the past year (using
something called a Food Frequency Questionnaire) and either assess or track
changes in various health markers. Researches will ask about other things as
well – smoking, exercise, etc. – and attempt to find connections between
certain habits and favorable or unfavorable health outcomes.
Epidemiology is a weak science in that it is only an
observation, and cannot control many factors in the way a well-designed
interventional trial can. For this reason, it can never “prove” anything,
although it is a popular brand of nutritional science nonetheless, owing
largely to its simplicity and comparatively cheap cost. If you’ve ever seen a
news headline linking some food to a given disease, what they were reporting on
were the results of nutritional epidemiology.
But there’s one major problem with epidemiology that in many
circles never garners the attention it deserves. Because conventional nutrition
guidelines and advice predate nutritional epidemiology by decades, there has
never been an epidemiological study untainted by the effects of these
guidelines. As epidemiological observations increase in rate and conventional
guidelines persist year over year, this problem continues to get worse.
Because here’s the thing – it is, for better or worse,
common knowledge that meat and saturated fat are “bad” for you. Most adults
have heard this for their entire lives, and the majority of them likely believe
it to be true. They also believe smoking to be bad for their health, likewise
for alcohol, sugar, and a sedentary lifestyle. It should not be surprising,
then, that one who partakes in any of these ostensibly damaging behaviors is
more likely to engage in additional harmful acts as well. Consider a study
linking red meat consumption to diabetes, for example, that found the group
eating the most red meat was 40% less active, drank 60% more alcohol, consumed
several hundred additional daily calories worth of sugar and refined grain, and
were three times more likely to smoke than those who ate the least.6
(You can read more over here about how ignoring over 400 calories per day of
sugar and refined grains helps create the catchy headline about meat causing
diabetes).
Insufficient Adjustment
Usually (but not always!), researchers adjust for the most
obvious lifestyle habits – exercise, smoking, alcohol – in attempt to prevent
them from tainting the results. But what about things like sleep? What about
hydration and hygiene and seat belts and safe driving? What about preventative
health care? Trying to link a dietary pattern to cancer mortality gets sketchy
really quick if one group is more diligent about cancer screenings. The
unfortunate reality of epidemiology is that, undoubtedly, the vegetable-eating
exercisers who don’t smoke are also going to be healthier in almost every
little way you can imagine, even if the researchers don’t adjust for it.
If a study says women who eat the most vegetables are 15%
less likely to develop cardiovascular disease, how much of that is actually due
to the veggies?7
And how much of it is due to the unmeasured practices like those above – sleep
habits, preventative health care, risk-taking behavior, and so on? And what if
that 15% doesn’t even consider other dietary factors, like sugar? You can be
quite certain that those eating the most vegetables were, on average, eating
the least sugar. But the researchers in this study didn’t measure or adjust for
it, so that’s probably some of our 15% right there.
Unfortunately, its not remotely uncommon for studies like
this to completely ignore sugar or any other dietary factor.8–10
Often, this is even acknowledged once you start reading the actual study. Take
for example a meta-analysis (a bunch of studies pooled together for more
statistically powerful results) that claims to have found “further evidence
that a higher consumption of fruit and vegetables is associated with a lower
risk of all-cause mortality” – even while acknowledging that fewer than half of
the studies assessed other aspects of the subjects diets.11
They included one that didn’t even adjust for alcohol consumption or exercise!12
So while they claim that their findings “provide further support for the public
health message to increase fruit and vegetable intake”…did they really? Or did
they just ask people how many vegetables they eat and found the healthy ones
consume more than the sedentary drinkers do.
Meat Causes Disease…. Right?
If you’re still looking for more indication that healthy
user bias taints epidemiology, consider this study that claims to “provide
evidence that dietary modification in choice of protein sources may influence
health and longevity.”13
They tracked nearly 240,000 (!) men and found that each 3% reduction in red
meat consumption was associated with a 7% reduction in cancer mortality, a 12%
reduction in cardiovascular death….and an 18% reduction in death due to injury
or accident. Another found that men who ate the most red meat (compared to
those who ate the least) were at a 22% increased risk of dying from cancer, 27%
increased risk of dying from cardiovascular disease, and 26% increased risk of
dying in an accident.14
None of this should be surprising! After all, dangerous drivers are more likely
to eat animal based or processed food snacks, while seatbelt use seems to predict
chronic disease more strongly than meat consumption does.15,16
But these behaviors are simply never accounted for in nutritional epidemiology.
The numbers we’re talking about in some of these epidemiological
pieces are often tiny, and its impossible to adjust for all the factors that go
into a person’s risk for disease – if researchers even try! Remember all the
headlines a few years back about bacon causing colorectal cancer, to the tune
of an 18% increase if you eat some every day?17
18% is a tiny number when considering relative risk – its the difference
between a .037% annual risk and a .044% annual risk, a roughly 1 in 14,000
chance your daily bacon will cause colorectal cancer this year.18
And that’s if we accounted for healthy user bias! Which we certainly did not.
Sugar and refined grains are habitually ignored in epidemiology, and you may
never find such a study that adjusts for one of the major effects of excess
refined carb consumption – elevated insulin levels. Consider that high levels
of insulin production may be associated with as much as a 200-300% increase in relative
risk of colorectal cancer!19–21
And we’ve ignored it entirely.
When researchers fail to consider the excess sugar and
processed grain consumption of bacon-eaters and the excess insulin that results
(among other lifestyle habits left unadjusted), they have fundamentally failed
in their task to actually access whether any link exits between bacon
and cancer risk. Yet the BBC will still claim unequivocally that bacon causes
cancer, further exaggerating the effects of healthy user bias in future epidemiological
pieces by pushing the health-conscious to consume even less of the apparently
dangerous food item.
Conclusion
This entire piece probably makes it sound like I have it out
for fruits and veggies while trying hard to defend red meat. But remember,
healthy user bias only works in one direction – there are no studies in which
vegetables appear unfairly demonized, because its generally healthy people who
eat them. Only the foods people have been told are bad (meat, saturated fat,
etc.) can be made to look worse than they are, while only foods that people
have been told are good (veggies, whole grains, etc.) can be made to look better.
And in fact, to some degree, this effect is present in literally every piece of
epidemiology you ever read. Regardless of how good or bad each really is, meat
and fat will always, without exception, look some degree worse while
vegetables and whole grains look better than any of them are in reality.
So, next time a scary headline suggests that steak will
literally kill you, remember:
- The study in question did not assess cause
and effect, only an association between two items
- The study in question is likely actually
measuring an association between red meat + sugar + refined grain consumption
and the scary disease, not just red meat and disease
- The researchers did not consider any health
behaviors beyond smoking, alcohol, and exercise (hopefully they at least considered
those). Sleep, preventative health care, adherence to safety protocol,
risk-taking behavior, and other factors were not included in their assessment
- Red meat "causes" more car crashes than it does cases of cancer
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