new week, new study, new controversy. nutrition's never boring. a very large randomized trial was
just published looking at the mediterranean diet. they recruited about a thousand volunteers and
followed them for seven years. so that's unusual. large scale, very long follow-up
i think this trial is going to be talked about for years to come, just like the Lyon and PREDIMED
trials became household names. well, for nutrition nerds, anyway. so the trial is called CORDIOPREV
which is short for Coronary Diet Intervention with Olive Oil and Cardiovascular Prevention
kind of a mouthful. the volunteers had a history of heart disease, so this is what's called a secondary
prevention trial, they already have the disease, we're trying to prevent more events down the line
like heart attacks and strokes. so the participants were randomly split into two groups, one was
assigned a mediterranean diet and the other a diet lower in fat, which they call low fat outright. now,
anytime a study calls something low fat or low carb, there's going to be complaints, there's always
somebody who thinks it wasn't low enough, and this was no exception. so let's take a look at what the
diets actually looked like. so the goals, what the investigators were shooting for, was 35% or more of
calories from fat on the mediterranean diet, with the main source being olive oil, and less than 30
percent on low fat, or lower fat, we'll come back to this issue of the names in a second, and they
wanted to keep protein approximately constant. now, since one diet is going to have less fat
than the other, that fat needs to be replaced with something, and since the protein is constant, that
something needs to be carbohydrate. so the goal for carbs was under 50% of calories on mediterranean
and over 55% on low fat. now, these numbers mean nothing to most people, you need to tell them what
foods to actually eat, so what they recommended was more olive oil, nuts and fatty fish on
mediterranean, to increase that fat intake, while on low fat those foods were to be reduced and instead
people were encouraged to eat more legumes and whole grains and to favor low-fat dairy. other
than that, the recommendations were pretty similar: substantial fruits and vegetables on both, white
meat with no skin or visible fat, pretty low in eggs for both diets, low in red or processed meats
and low in refined carbohydrates. so the whole idea of the trial is to compare two generally health
promoting diets that kind of check all the boxes, one a little higher in carbs, one a little higher
in fat, and see if that makes a difference for cardiovascular disease, while trying to stick to
high quality fats and high quality carbs, so mostly unsaturated fats, predominantly from olive oil, and
mostly complex, unrefined carbs. so those were the recommendations, what they asked the participants
to eat. now, as you might imagine, you ask a thousand people to change their diet for seven years,
that's a lot of moving pieces. so they kept in touch with all of those people every couple months
over the phone and they met them face to face a few times a year, both in groups and individually
so they continuously touched base on whether the participants found it easy or hard to stick to
their assigned diet and at certain intervals they went over what they were actually eating. people on
mediterranean reported going over the goal of 35 percent of calories from fat, they actually got 40,
but the low-fat group didn't quite reach the goal of 30 or less, they only reduced to 32. close but
not quite. same for carbs, people on mediterranean hit the goal and went beyond, the goal was under
50 and they got less than 40, so that's good, but on low fat they fell short, the goal was over 55 and
they only hit 45, and protein was a bit lower on mediterranean, about two percent lower, so this is
pretty typical of these trials, people make changes in the direction of the recommendations but they
don't necessarily hit the goals and maintain them throughout the duration of the follow-up. this was
actually pretty good, especially considering how long the study was. in terms of specific foods,
they changed a bit up and down over the course of the seven years, as you would expect, but basically
by the end the main differences were, people on mediterranean reported eating more olive oil,
as requested, more than twice as much, and more nuts and oily fish, and both groups cut back on red meat
and refined carbs as recommended. on the other hand, there was little difference in legumes, grains and
potatoes between the two diets by the seven year mark, so that doesn't reflect the recommendations
and the mediterranean group ended up eating a bit more fruits and vegetables, about 10% more, so some
of the specific goals were met, others were not, but overall they did succeed in creating a significant
difference in fat and carbohydrate as intended. the low-fat group stayed a bit short of the goals but
the mediterranean group went a bit beyond so they got that minimum separation. now, before we look at
the results on cardiovascular disease rates, which honestly shocked me a bit, let's touch quickly on
some of the common questions that came up. lots of people asked about the names of the diets, is it
accurate to call it a mediterranean diet and a fat diet? totally valid questions. so mediterranean diet
is a fluid concept, we know it varies from country to country in the mediterranean region and we
also know that nowadays people tend to eat junkier foods than they did traditionally. when scientists
talk about the mediterranean diet, what they mean is this pattern. rich in vegetables, some whole
grains, with olive oil and fish and lower in red meat and low in ultra processed foods. within those
parameters, there's plenty of wiggle room but this is generally what people have in mind. and low fat,
for some people getting 30% of your calories from fat is super high, for others it's low, some people
get 70 percent of their calories from fat so it's a little subjective. as long as we know what we're
talking about. the point is, it's lower than the comparison diet, that's the bottom line. and why low
fat as a comparison? well, because it used to be the recommendation for cardiovascular disease, to avoid
too much dietary fat. the scientific understanding has evolved, and for decades now we've known that
it's more about fat quality. so major scientific organizations nowadays reflect that in their
messaging. okay, so what effect did these diets have on cardiovascular disease? that's the bottom line
the overall rate of cardiovascular disease and mortality in the study was pretty low, supporting
the idea that both diets are health promoting in general. when comparing between the two diet groups,
they found 28 percent less cardiovascular events, things like heart attacks, strokes etc on the
mediterranean diet than on the lower fat diet. the effect was specific to men. over 80 percent
of the volunteers were men. in women there was no significant difference across the two diets
possibly because their number was so low but we don't know for sure. on the plus side, when
they looked specifically at the participants that stuck the closest to the recommendations,
what they call the high adherence participants, the effect was stronger. 40% less cardiovascular events
on the mediterranean diet. so that adds a little bit of confidence to the results. so that's the
gist of the findings of this recent study. now, what often happens with these very long trials
is they publish some of their findings along the way before that last big report comes out
so i went back a few years and dug for more information on this. for example, in this report
published last year, they did some imaging of the carotids, the neck arteries of the participants
and the results indicated a reduction in plaque size seen only on the mediterranean diet, so that
would seem to align with the outcome data, with the cardiovascular events also being lower
on mediterranean. another thing that jumped out was that after the seven years of follow-up there was
no significant difference between the two groups in body weight, cholesterol levels, apob, fasting
glucose or triglycerides, so where are the benefits coming from? i found another prior report from two
years ago looking at several serum markers, and the most striking difference between the two diets
was c-reactive protein, which is an inflammatory marker. it was reduced on the mediterranean diet
and it seems to actually have gone up a bit on low-fat, and we know from large randomized trials
that inflammation is one of the risk factors for cardiovascular disease, so that's one possibility,
that there's a decrease in inflammation in the mediterranean group that lowers
their cardiovascular disease risk. now, the main controversy i saw on social media was regarding
which specific food delivered the benefits. some people concluded it was the olive oil and we
should all be eating more of it, and others said no, actually the participants on the mediterranean
diet did better in spite of eating more olive oil. oil is harmful and it's because they're
eating more fruits and vegetables, it offsets the harm of the oil. both of those views are a bit of
a stretch because as we saw, there were differences in multiple foods across the two diets so we can't
tell which one delivered the benefit, and that's totally fine, it doesn't make the study low quality
the very experimental design was to change the intake of several different foods, that's what they
asked the participants to do, because it was a trial aimed at comparing entire dietary patterns,
not one food in isolation. what we can say with objectivity, and this is my first takeaway from
all of this, is that this is yet another large data set showing good outcomes of a mediterranean diet
mediterranean-ish, right? whatever that means, with some olive oil. it doesn't mean that the same diet
without the olive oil couldn't have the same benefits, this trial can't tell us that, but we
do have some prior evidence on that question, for example the predimed trial had three diet groups,
not two, low-fat diet and two mediterranean groups, one with extra olive oil and one with extra nuts
and both of the mediterranean arms showed similar benefit so it suggests both sources of unsaturated
fat are roughly equivalent. it would be awesome if this new trial had also included a third group
with more nuts and less olive oil but they didn't. quick word on funding, because this always comes
up, this study was sponsored by a foundation involved in olive oil production, among other
funding sources, so a lot of people asked, does that invalidate the study? it does not. we're actually
working on an entire video on funding issues in nutrition research, but long story short, it's okay
to note funding source, but we value scientific studies based on methodological rigor. some great
studies are industry-funded and some really bad ones are not, so noting the source of funding
never replaces scientific appraisal. okay? we're going to revisit this issue in future videos
for sure because this comes up all the time and it makes total sense to ask
so we've talked before about the popularity of the mediterranean diet with nutrition professionals
and this is part of the reason, that we have these large randomized trials, lyon, predimed
and now CORDIOPREV. none of those trials are perfect and this doesn't mean that we know the
mediterranean diet is the best diet out there, it just means there's a lot of high level evidence
behind it, it consistently looks pretty good for outcomes, so professionals are more confident
it'd be great if we had these large trials for every diet out there and maybe in the future we
will. seeing one of these large trials come out in real time also drives home the point
that this vague idea online that nutrition is all epidemiology is a huge misunderstanding. in
addition to these unusually large trials that come out once in a blue moon, we have hundreds if not
thousands of other smaller trials on all aspects of nutrition. the realization is that randomized
controlled trials aren't perfect. they involve significant uncertainty. you follow a thousand
people for seven years, there's going to be some unknowns. you've got to rely on what they tell us
they eat. maybe other changes happened in their lives that affected the results. CORDIOPREV actually
approaches the limits of what's possible to do in nutrition. changing the entire diet of a population
and following them for years. this is the gold standard. it was well designed, well carried
out and it's a massive undertaking. so my second major takeaway from all of this is this trade-off
between control and relevance that is inherent to all scientific experiments. you can look at mice
in cages, you have maximum control and minimum relevance, because you don't know what happens
in humans. or you can look at humans in cages, what's called a metabolic ward, the participants
live in the research facility and every bite of food is counted. you have good control but
medium relevance, because you're limited in time. you can't trap people in your research facility
for years, so you end up looking at serum markers, surrogates of disease, and it also doesn't tell you
how people behave in normal conditions, in their normal life. and finally you can look at humans
in their natural environment, what's called a free living population. this allows you to follow
them for years and look at actual heart outcomes like heart attacks, strokes etc, and it reflects
normal behavior in the real world, so the relevance is maximum but you pay for that by having
less control. so this is why we do all of these different experiments, each study by itself is like
a pixel and putting all the information together gives us the big picture. third major takeaway,
fat isn't bad. we can't say it was the extra fat in the mediterranean diet that delivered the benefits
because as we saw there were other differences but this trial and many other results are completely
inconsistent with the idea that eating more fat is bad for the heart, the group eating the
most fat had the best cardiovascular outcomes. it's about fat quality. unsaturated fats like olive oil
consistently show good results. we actually covered all the data on olive oil in a previous video
and i'll link that at the end. if you don't like olive oil, totally fine, there's other oils,
there's nuts and seeds, there's fatty fish, personal choice, but there's no reason to be afraid of fat
as a whole for health reasons. interestingly, two things that people are concerned about regarding
olive oil are, one, that it harms endothelial health, and two, that maybe it's okay for healthy people
but if you're sick with cardiovascular disease you should avoid it. this trial contradicts
both of those ideas, the participants were sick people with cardiovascular disease to begin with
and yet they did best on the diet with the most olive oil. incredibly difficult to reconcile this
with the idea that olive oil raises risk. what about endothelial function? in some experiments,
there's a reduction in the mobility of the artery walls right after eating olive oil, this is an
assay called flow-mediated dilation. but lots of things can cause fmd, flow-mediated dilation,
to drop acutely, in the short term. some types of exercise cause it. sleep can cause it. so it doesn't
necessarily reflect the overall health value long term. in fact, remember the report i showed
you with the inflammation markers? c-reactive protein? it also looked at endothelial function
and it found that the mediterranean diet was better for endothelial function. and they
looked precisely at fmd, flow-mediated dilation, and it went up, it improved
on the mediterranean group. so we have to be very careful about extrapolating these short-term
oscillations in a parameter. bottom line, if you like some olive oil in your diet, be mindful of
overall calories, otherwise i don't see a problem. if you don't like olive oil, also not a problem,
plenty of other sources of quality fat that we talked about. and if you hate the
mediterranean diet, don't eat it. it's better to eat something that you can actually sustain long term
try to take the core ideas of the mediterranean diet and apply them to a pattern that works
for you, if you can. unprocessed fruits and vegetables, high quality fat, mostly unsaturated
high quality carbs, mostly unrefined. and easy on the ultra processed stuff. that's it right
there. let me know your thoughts on all of that, i know it was a lot, take care, see you next time