[♪ INTRO ] It seems like it’s practically impossible
to get a straight answer on whether a given food or supplement is good for you. First they say red wine is good for you. Then there’s a headline announcing that
it doesn’t do anything, and wait, isn’t alcohol supposed to cause cancer? If you wanted to change up your diet to accommodate
all the latest news reports, you wouldn’t get very far before you’d run into all kinds
of contradictions. I mean, you can’t both drink and not drink
alcohol at the same time. And the problem goes deeper than the over-exaggerated
headlines on cable news. Often, the way nutrition research is done
makes it really hard to translate the results in to practical advice because of imperfect
comparisons. The effects of one compound might not be the
same as the effects of the entire food. What happens in animals might be different
from what happens in humans. And a food that’s good for you in some ways
might be bad for you in others But it is possible to untangle all that by
examining these studies more closely. You just need to know what you’re looking
for. Take what happened with raspberries not too
long ago. In a 2011 study, researchers in China found
that red raspberries could improve blood pressure. Annnnd then in 2013, a team in Finland found
that fresh berries didn’t affect blood pressure at all. There were plenty of differences between these
two studies, though — and plenty of reasons they might have had different results. The Chinese study used raspberry extract,
and was done on rats … specifically, a strain of rats prone to high blood pressure. Whereas the Finnish study used berry puree,
and the subjects were humans with symptoms that increased their risk of heart disease,
like high blood pressure. The first problem with comparing them is that
the other ingredients in the berries could’ve interfered with the effects of the compounds
in the raspberry extract alone. Then there’s the whole “humans vs other
animals” thing. One reason scientists use animal models is
that you can control everything about them. You can administer every speck of food they
eat, control how much they exercise, and monitor them day and night. Try to do that with a human and see how long
it takes to get punched in the nose. But that means in this case, the humans who
got the berries might have been doing or eating something else that negated the effects. Or hypertension-prone rats might be more sensitive
to the blood pressure-lowering effects of berries than normal, if somewhat unhealthy,
human beings. Comparing those two studies is a little bit
like comparing apples and oranges … or apples and raspberries, I guess. Different types of studies are important. It’s usually much easier and safer to test
things in other animals before moving on to humans, and scientists in all fields of research
rely on multiple lines of evidence to form conclusions. That’s how science works! But when it comes to nutrition, most individual
studies are intended as a piece of a larger research puzzle, not something for the home
consumer to take to the grocery store. Studies on humans might be easier to translate
to your life, but even those are fraught with complications. Like when a 2014 study on red wine contradicted
the findings of another paper from two years earlier. Both studies were specifically concerned with
a compound in wine called resveratrol, which is an antioxidant — meaning it helps prevent
potentially damaging chemical reactions in cells. In 2012, a randomized controlled trial by
Hungarian researchers found that adults who received resveratrol supplements showed an
improvement in a bunch of signs of cardiovascular disease. Randomized controlled trials, or RCTs for
short, are the closest you can get to a controlled experiment in humans without the aforementioned
nose punching and a whole passel of ethics violations. Participants are sorted into groups and statistically
randomized so the experimental and control groups are as uniform as possible. Then you give the experimental group the food
or supplement or specialized diet you’re studying, which should ideally be the only
thing that’s different between the two groups. And this RCT seemed to show that resveratrol
made a difference. But then, in 2014, an observational study
of adults in Italy didn’t find any effect of resveratrol on signs of cardiovascular
disease or mortality. Observational studies are different from RCTs. They aren’t really experiments — they
involve gathering data that’s already out there, in this case by interviewing people
about their eating habits and analyzing their urine. RCTs are usually thought of as the most rigorous
type of human study, but that doesn’t mean they’re always right and observational studies
are always wrong. The 2014 observational study found that while
participants had a lot of resveratrol in their diets, like from wine, the stuff might not
have reached a high enough concentration in their bodies to have any biological effect. So the lesson there might be that supplements
are more effective. Which brings us to a fairly infamous example
from the field of cancer nutrition. Beta carotene is a dietary precursor to vitamin
A found in foods like carrots and sweet potatoes. A body of evidence from multiple observational
studies suggests that eating fruits and vegetables rich in beta carotene lowers the risk of lung
cancer. Which was the justification for a number of
clinical trials, including one in Finland in 1994 and one in the US in 1996, which gave
beta carotene supplements to smokers. Except, the smokers who got beta carotene
had a HIGHER rate of lung cancer. You really don’t want to give your study
population cancer, so this came as a bit of a shock. In the fallout, it was found that beta carotene
in high doses could actually act as a pro-oxidant — a compound that’s the opposite of an
antioxidant, and promotes chemical reactions that lead to cellular damage. So the observational studies showed that carrots
are good for most of us, but the clinical trials discovered that under specific circumstances,
high doses of beta carotene have a pro-cancerous effect. These findings weren’t exactly contradicting
each other … it was just another case of comparing the wrong things. So that’s another thing to look out for
with nutrition studies. When you dig into the details, findings that
seem contradictory might turn out to be building on each other. The final — and probably most important — question
to ask when you see a report announcing that something is good for you is, what does “good
for you” even mean? There’s no universally agreed upon definition
for it, and it’s never what researchers specifically set out to study. Trying to figure out whether something is
“good for you” is just too broad. Studies might look at rates of stomach cancer
after a certain number of years, blood glucose levels, cholesterol levels, or any of the
other incredibly specific markers related to cancer or diabetes or obesity or some other
health issue. Scientists who study nutrition call these
outcomes, and they’re probably the most important thing to look for in any nutrition
study. Going back to the wine example, red wine contains
a whole bunch of plant compounds that are targets for nutrition research — resveratrol
is just one of them. At the risk of stating the obvious, wine also
contains alcohol. And alcohol itself might actually be good
for you in some very specific ways. A 2002 analysis of several dozen studies of
the effects of wine and beer found that overall, wine drinkers were less likely to die from
heart disease, or to experience other, less fatal heart problems. But the association was there for beer drinkers
too. It was statistically weaker, but clearly the
effect wasn’t just coming from the special plant compounds in wine. This finding has cropped up more than once,
and there’s enough evidence that some nutrition experts might recommend light or moderate
alcohol consumption to help prevent cardiovascular disease. Does that mean alcohol is good for you? Well. I mean. It’s alcohol. Even if it helps your cardiovascular system,
it also messes with your liver, interferes with fetal development, increases the risk
for certain cancers, depresses your central nervous system and a whole list of other things. In some of these cases, the dose makes the
poison. The evidence points toward light or moderate
drinking having certain positive effects, but heavy drinking being bad in basically
every way. While alcohol is an extreme example, it’s
not hard to find cases where a particular food or nutrient has a positive effect on
some outcomes and a negative effect on others. These studies aren’t contradicting each
other because they’re not trying to answer whether something is universally “good for
you.” But when the findings show up in the news,
sometimes that nuance gets lost in translation. When it comes to nutrition research, there
are so many seemingly contradictory findings coming out all the time that even the scientists
publishing them need to step back and look at the evidence as a whole. That’s why the field is full of review articles
and meta-analyses. The process of science is pretty much always
this messy, but for some reason with nutrition it’s on public display. Maybe that’s because we want to be able
to justify our craft brew hobby by believing it’s healthy, or because we’re always
looking for ways to live longer and stave off diseases. Maybe it’s because what we eat feels like
the one thing we can control in a world full of unknown risks. But it helps to take every individual nutrition
study with a grain of salt. Not too big a grain, though — I just read
that salt’s bad for you. Or was it good for you? Thanks for watching this episode of SciShow,
which is produced by Complexly. For other awesome Complexly videos about nutrition
and health research, check out Dr. Aaron Carroll over at Healthcare Triage. [♪ OUTRO ]