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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? Or… something?

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 one.

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 maxed 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 different things. 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 problem.

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 predictor. 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 home. 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 assumed. 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 ‘good’ cholesterol. 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 HDL. 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 to defend.

You certainly don’t need refined sugar to stay alive. 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 obesity. How food fits into all of that isn’t simple, since dietary fat and sugar both affect some risk factors. 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 by researchers. 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 regimen. 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 more fat. 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 help. Everybody did about the same after one year, regardless of whether they went low-fat or low-carb. 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 answer. 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. [♪ OUTRO ].