Do Vegetarians Really Have Higher Stroke Risk?

“Do Vegetarians Really
Have Higher Stroke Risk?” When ranked in order of importance,
among the interventions available to prevent stroke, the three
most important are probably diet, smoking cessation, and
blood pressure control. Most of us are doing pretty
good on smoking these days, less than half of us are exercising
enough, but according to the American Heart Association only 1
in a thousand Americans are eating a healthy diet, and fewer than 1 in 10
are even eating a moderately healthy diet. Why does it matter? Because diet is an important
part of stroke prevention. Reducing sodium intake, avoiding egg
yolks, limiting the intake of meat, and increasing the intake of whole grains,
fruits, vegetables and lentils. Like the sugar industry, the meat
and egg industries spend hundreds of millions of dollars on propaganda,
unfortunately with great success. I was excited to check out Box number 1,
and was then honored, when I did. The strongest evidence for stroke
protection is for increasing fruit and vegetable intake, with
more uncertainty regarding the role of whole grains, animal products, and
dietary patterns such as vegetarian diets. I mean one would expect they’d do great. Meta-analyses have found that
vegetarian diets lower cholesterol and blood pressure, and enhance
weight loss, and blood sugar control, and vegan diets may work even better.
So, all the key biomarkers are going in the right direction, but
you may be surprised to learn that there hadn’t ever been any
studies on the incidence of stroke in vegetarians and vegans… until, now. And if you think that’s surprising,
wait until you hear the results. The risks of heart disease
and stroke in meat eaters, fish eaters, and vegetarians
over 18 years of follow-up. Yes, less heart disease
among vegetarians (by which they mean vegetarians and vegans combined) no surprise—been there, done
that, but more stroke. An understandable knee-jerk
reaction might be “Wait a second, who did this study?” But this is EPIC-Oxford, world-class
researchers whose conflicts of interest may be more likely to read “I was
a member of the Vegan Society.” What about overadjustment? If you crunch the numbers
over a ten-year-period they found 15 strokes for every thousand
meat-eaters compared to only 9 strokes for every thousand
vegetarians and vegans. Wait, so how can they say there
were more strokes in the vegetarians? This was after adjusting
for a variety of factors. For example, the vegetarians
were less likely to smoke; so, you want to cancel that
out by adjusting for smoking, so that you can effectively
compare the stroke risk of nonsmoking vegetarians
to nonsmoking meat-eaters. If you want to know how a vegetarian
diet itself affects stroke rates, you want to cancel out these
non-diet-related factors. Sometimes, though, you can overadjust. The sugar industry does it all the time. This is how it works. Imagine you just got a grant
from the soda industry to study the effect of soda on
the childhood obesity epidemic. What could you possibly do after
putting all the studies together to arrive at the conclusion that
there was near zero effect of sugary beverage consumption
on body weight? Well, since you know that
drinking liquid candy can lead to excess calories that can lead
to obesity, if you control for calories, if you control for a factor
that’s in the causal chain, effectively only comparing soda
drinkers who take in the same number of calories as non-soda-drinkers
then you could undermine the soda-to-obesity effect, and
that’s exactly what they did. That introduces overadjustment bias. Instead of just controlling
for some unrelated factor, you control for an intermediate
variable on the cause-and-effect pathway
between exposure and outcome. Overadjustment is how meat-and-
dairy industry funded researchers have been accused of obscuring
the true association between saturated fat and cardiovascular disease. We know that saturated
fat increases cholesterol which increases heart disease risk. Therefore, if you control for
cholesterol, effectively only comparing saturated fat eaters with the
same cholesterol levels as non-saturated-fat eaters,
you see how you could undermine the saturated fat-to-heart disease effect. Now let’s get back to this. Since vegetarian eating
lowers blood pressure, and a lowered blood pressure
leads to less stroke, controlling for blood pressure would be an
overadjustment, effectively only comparing vegetarians to meat-eaters
with the same low blood pressure. That’s not fair, since that’s one
of the benefits of vegetarian eating, not some unrelated factor like smoking; and so, it would undermine
the afforded protection. So, did they do that? No. They only adjusted for unrelated factors, like education, and socioeconomic class,
and smoking, and exercise, and alcohol. That’s what you want. You want to tease out the effects of a vegetarian diet on stroke risk…
you want to try to equalize everything else to tease out the
effects of just the dietary choice. And since, for example, meat eaters
in the study were on average 10 years older than the vegetarians,
you can totally see how when you adjust for that
vegetarians could come out worse. Since stroke risk can increase
exponentially with age, you can see how having 9 strokes among
a thousand vegetarians in their 40s could be worse than 15 strokes among
a thousand meat-eaters in their 50s. The fact that vegetarians had
greater stroke risk despite their lower blood pressure suggests
there’s something about meat-free diets that so increases stroke risk it’s enough
to cancel out the blood pressure benefits, but even if that’s true you
still would want to eat that way. Stroke is our 5th leading cause of
death, whereas heart disease is #1. So, yes, in this study there
were this many more cases of stroke in vegetarians, but there were this many
fewer cases of heart disease, but if there is something increasing
stroke risk in vegetarians it would be nice to know what
it is in hopes of figuring out how to get the best of both worlds. This is the question we’ll turn to, next.
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