Which is Worse For You: Sugar or Fat?
We’ve all heard for decades about how terrible
fat is for us, especially saturated fat.
In the 1970s and 80s, everyone was explicitly
told to cut down on foods like butter, cheese,
and beef, to avoid heart attacks and strokes.
But now, apparently, they’re ok?
Or good for you?
And it’s sugar that’s bad!
Sugar’s what’s killing us!
If we just cut sugar from our diets, we could
rid the world of not only heart disease, but
also obesity, diabetes, and even cancer!
The change in attitude might seem extreme,
but the case against sugar has been building
for decades, while the one against saturated
fat has been weakening.
And yet, though we all love a good villain,
when it comes to the health effects of your
diet, things are rarely that simple.
Today, we’re taking a look back at the complex
history of nutrition science to try to understand
how saturated fat became public enemy number
And how, when doctors zeroed in on fat, they
overlooked the damaging effects of sugar.
But that doesn’t mean it should be the new
saturated fat, or that fat is entirely exonerated.
And while there still isn’t an easy answer
as to what diet is best for losing weight
or preventing heart disease, cutting down
on things like saturated fats and refined
sugar is probably a good start.
Back in the late 1940s, Ancel Keys, a physiologist
at the University of Minnesota was puzzled
by the fact that American businessmen seemed
to be dying at an alarming rate from heart attacks.
These middle-aged men made more than enough
money to to afford quality food.
So why were they the ones keeling over?
He hit upon an idea, often called called the
diet-heart hypothesis, that would forever
alter 20th century food habits.
That’s the notion that the amount of fat
you eat, specifically saturated fats—the
ones where the fatty carbon chains are all
connected by single bonds, and therefore maxxed
out, or saturated, with hydrogens—leads
to high levels of fats in your blood, which
then glom onto the walls of your blood vessels
to cause coronary artery disease.
It’s kind of like how pouring too much bacon
fat down your drain can clog your pipes.
Keys was especially concerned about blood
levels of the fat cholesterol because it’s
found in the fatty build ups in blood vessels.
So he did a few small studies to test his
idea, and then decided to go big or go home.
In the Seven Countries Study, he and his colleagues
looked for correlations between diet and coronary
artery disease in about 13,000 middle-aged
men in the U.S., Japan, Finland, Yugoslavia,
Italy, Greece, and the Netherlands.
By 1970, the early results were in, and total
dietary fat didn’t seem to matter.
But the groups of men with the highest average
saturated fat intake tended to die more often
from heart attacks,
and across the globe, blood cholesterol levels
correlated with saturated fat consumption.
Another large epidemiological study, the Framingham
Heart Study, further supported a link between
blood cholesterol and the risk of heart disease.
These studies showed robust correlations,
but they still were just that: correlations.
Many people, though, assumed that these results
meant that reducing fat intake would protect
people against coronary artery disease—which
is not something a correlational study can actually show.
A handful of researchers did try to do randomized
controlled trials—the gold standard for
testing medical interventions—to see if
low saturated fat or low cholesterol diets
actually helped prevent heart attacks and
deaths, as the hypothesis predicted.
But nutrition trials are notoriously difficult.
Some of the results suggested lowering saturated
fat could help, but most were small studies
and they were hard to interpret.
And each one used a slightly different diet,
replacing in the saturated fat calories with
So, by the late ‘70s, there was still no
conclusive proof that reducing saturated fats
could actually protect hearts.
But that hadn’t stopped the diet-heart hypothesis
from becoming mainstream—it seemed to fit
most of the available data.
And many experts felt the danger of cardiovascular
disease was too high not to recommend some
some change in what Americans were eating.
So in 1977, the US Senate Select Committee
on Nutrition and Human Needs released its
new dietary goals, telling Americans to get
just 30% of their calories from fat — down
from 40% — and to limit the saturated fat
to 10% of the total.
The guidelines also suggested reducing dietary
cholesterol and refined sugar, and increasing
complex carbohydrates, but those weren’t
really talked about as much.
Other countries followed suit, so soon, pretty
much everyone was told to shun butter for
margarine, and supermarket shelves were quickly
stocked with “low fat” options.
But from the beginning, critics questioned
those recommendations, pointing out that the
specific diet now being championed had never
been tested in a trial.
And some suspected refined sugar was a bigger
In fact, the Seven Countries Study also showed
a strong correlation between the consumption
of sugar and heart attacks, but this finding
wasn’t given much additional study because
it looked like saturated fat was the better
And it wasn’t until this century that we
learned the sugar industry helped ensure its
product was overlooked.
For example, in 1967, they paid three Harvard
public health researchers what would now be
about $50,000 to write a review article in
The New England Journal of Medicine —a highly
influential journal—that highlighted the
role of fat and downplayed any involvement of sugar.
But twenty years later, after a half century
of fat-hating, the tide began to slowly turn
toward against them, both in the lab and at
Not only had research not found conclusive
proof that saturated fats were the problem,
new analyses of the collective past studies
showed that the link between saturated fats
and heart disease was much weaker than previously
And new observational studies were finding
that quickly digestible carbohydrates and
added sugars were independently associated
with an increased risk of heart disease.
Also, the whole idea that “cholesterol”
was bad turned out to be much more complicated,
undermining part of the premise for why fat
was supposed to be so evil.
To their credit, researchers in the ‘60s
and ‘70s had a vague sense that the type
of cholesterol mattered.
But they didn’t fully understand how cholesterol
moves in our blood, including the roles of
the different lipoproteins that ferry it around.
When doctors look for “cholesterol” levels
in blood now, they tend to look at triglycerides—the
total amount of fat—as well as the total
amount of cholesterol.
They also look at whether that cholesterol
is being shuttled by low-density lipoproteins
or LDL, the so-called ‘bad’ cholesterol,
and high density lipoproteins or HDL, the
High LDL is a risk factor for heart disease,
but having more HDL is usually considered
good, though researchers are still trying
to understand the different sizes and subtypes
of each and how they help or harm.
The problem is, foods that are high in saturated
fat can raise both HDL and LDL, so they might
seem more harmful if you just focus on total
cholesterol or LDL.
So as a new century began, people started
rethinking the war on fat, especially since
the advice to lower fat intake wasn’t helping
people stay healthy.
By the early 2000s, low-fat diets didn’t
seem to be doing squat for most people.
Obesity and diabetes — both of which are
risk factors for heart disease — had skyrocketed.
And that might be because, while lots of people
embraced low-fat foods, they had opted for
reduced fat snack cakes or cereal loaded with
sugar or other carbs instead of healthier
things like fresh fruits and veggies.
So a different diet—the low-carb, high-fat
Atkins diet—became all the rage, and it
did seem to help people lose weight.
Scientists started to pay attention to it,
too, and their initial results were promising.
In short-term trials of 6 months or so, people
lost a modest amount of weight and tended
to slightly lower triglycerides while boosting
Diets high in sugar and refined carbs, on
the other hand, tended to do the opposite,
raising triglyceride levels and bad LDL cholesterol,
while lowering good HDL.
And in both animals and people, excess sugar
was linked to elevated blood pressure—another
risk factor for heart disease.
The weight of the evidence against sugar over
the past twenty years in particular has tipped
the scales, and now, you’ve probably heard
all about how sugar is the worst thing ever.
It sure is an easy target; since sugar provides
no nutrients other than calories, it’s hard
You certainly don’t need refined sugar to
And that’s led a lot of people to call it
‘toxic’ and blame almost anything on it,
even diseases with strong genetic links, like
inflammatory bowel disease or cancer.
But careful review of the science suggests
that’s taking the backlash a little too far.
While most of us are likely eating too much
added sugar, no one really knows how much
is too much.
And connections to diseases, even ones that
seem clear-cut, are not always so straightforward.
Scientists don’t actually know whether eating
sugar can cause diabetes, for example.
Eating too much sugar can certainly lead to
weight gain and obesity, which is one of the
biggest risk factors for the disease.
But different people break down and use sugars
differently, and so right now many experts
think sugar consumption itself isn’t causing
diabetes—complications from being overweight are.
Which is also likely the case for heart disease
and other aspects of health.
We know that high cholesterol in the blood,
specifically LDL, is a risk factor for heart
disease, but so is high blood pressure and
How food fits into all of that isn’t simple,
since dietary fat and sugar both affect some
Other habits matter, too, like whether you
exercise or smoke.
And researchers are still trying to understand
what sugar actually does to the body.
Quote “sugar” comes in various forms,
the relative merits or demerits of which scientists
are still debating.
And the same can be said of quote “fat.”
If you ask a cardiologist today about whether
saturated fat is bad for you, you may get
a surprising answer: it depends.
That’s because whether or not saturated
fat increases cardiovascular disease in any
given study seems to change depending on the
exact type of fat being studied, what the
source is, and what it’s replaced with if
it’s removed from the diet.
The extent to which sugar and saturated fat
are to blame for heart disease, or pretty
much any disease, is still being heavily debated
At the end of the day, though, you still have
to eat, and all of this doesn’t really give
us a satisfying answer as to what your diet
should look like.
And I hate to tell you this, but head-to-head
tests of low-fat and low-carb diets haven’t
identified a clear winner.
While low-carb diets seem to be a teensy bit
better in the short term, when scientists
study people for longer, the difference declines.
Low-carb diets seem to lead to slightly higher
increases in good cholesterol and bigger drops
in triglycerides, but the two diets perform
about the same for other heart disease markers.
And neither is terribly effective for people
trying to lose weight.
Typical weight loss after a year is barely
over 5 kg.
But you can always find individuals who respond
really well, dropping like 30 kg, and those
who gain weight while on the diet.
This has led some researchers to think that
maybe certain people, either because of genetics
or metabolic reasons, do better on a low-carb
diet, while others benefit more from a low-fat
One especially attractive idea is that people
who already are a little insulin resistant—a
huge risk factor for type two diabetes—would
probably fare better with fewer carbs and
But even this more nuanced approach doesn’t
seem to hold water.
In a 2018 study of about 600 people, assigning
diets based on genes or insulin levels didn’t
Everybody did about the same after one year,
regardless of whether they went low-fat or
Scientists are still hoping to find other
markers, like those related to the microbiome,
or how much certain genes are expressed, that
could determine what diet is best for you.
But for now, what we’re left with is a bit
of a draw.
There are some things that can be gleaned
from this big, nutritional mess, though.
In general, doctors still recommend cutting
down on saturated fats, but it’s important
to pay attention to what you eat instead.
Swapping out saturated fats for unsaturated
ones, like those found in nuts, is usually beneficial.
That is also often true if you exchange the
fat for whole grains, but not if you sub in
other carbs, like sugar.
And it probably wouldn’t hurt to cut down
on refined sugar in general, too.
But if we’ve learned anything from nutrition
history, it’s that a blanket prohibition
on any one thing isn’t likely to be the
Sugar vs. fat is a false dichotomy, and when
you think about it, that makes complete sense.
Of course too much fat is a bad idea.
And so is too much sugar.
But just as eating the occasional sweet is
not going to give you cancer, and the occasional
steak isn’t going to give you a heart attack.
Thanks for watching this episode of SciShow,
which is produced by Complexly.
If you want to watch other awesome Complexly
videos about nutrition and health research,
you might want to go check out Dr. Aaron Carroll
over at Healthcare Triage.
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