Why Carbs Don't Make You Fat

For the past ten years, carbohydrates have been labeled as public health enemy number one by many popular diet books, websites, and health gurus. They have been thoroughly excoriated as almost single-handedly causing the obesity epidemic in the West.

This actually isn’t the first time carbs have had a target on them. The food group has been a perennial nutrition punching bag since the 19th century.

The phenomenon of low-carb dieting goes all the way back to 1863, when it was called “banting” after undertaker William Banting, who popularized the diet with a bestselling pamphlet. The diet was so popular, people took to commonly asking each other, “Do you bant?” or “Are you banting?”

In the 1970s, 80s, and 90s, the low-carb diet was known mainly by the Atkins name. Then the 2000s saw an explosion of different kinds of low or no-carb diets: paleo, South Beach, Whole30, and slow-carb diets were some of the most popular. While they all differed in the level of carbohydrates permitted, they all generally discouraged things like bread, cereals, rice, or starchy veggies like corn and potatoes.

In the 2010s, the war on carbs continued and even picked up steam with the Bulletproof Diet and the Carnivore Diet. The battle has become so pitched against carbs, that many people are opting to eliminate them altogether so they can enter a state called “ketosis” which allows your body to fuel itself without any carbs at all.

I’ve been a good soldier in the war on carbs. I personally ate a relatively low-carb diet (coupled with intermittent fasting) for a few years — lots of bacon and eggs and steak with little in the way of bread, pasta, and other starches.

I’ve also published articles about How to Eat Like a Caveman and done podcast episodes talking about why carbs are bad and why eating a high-fat diet is the way to go.

But a few years ago, I began to see cracks in the “carbs are evil” narrative after I got really serious with barbell training. I continued with a high-fat/low-carb diet, but found myself bonking out on workouts. I’d miss reps or couldn’t add weight to the bar when I should have. When I did conditioning circuits after my lifting, I was just going through the motions. I couldn’t push myself that hard. The worst part was I’d feel tired and beat down for the rest of the day after my workout. It made doing actual work-work really hard. I even started to put on body fat despite the fact that my caloric intake hadn’t changed and I had upped my physical activity.

My Barbell Logic Online Coaching strength coach, Matt Reynolds, recommended I switch to a high-carb/low-fat (and high protein) diet. Within a week, I noticed an improvement. I could finish workouts, add weight to the bar, and push myself hard during conditioning, and I felt more pep throughout the rest of the day as well. I also started to lose weight — despite consuming the same amount of calories as I had on a high-fat/low-carb diet; during one 2.5 month period, I dropped 15 pounds and 4 inches from my waist. My diet has stayed higher-carb/lower-fat since then, and I’ve never felt better strength, energy, and health wise.

The experience spurred me to take a second look at carbohydrates and to figure out if all the hate was warranted.

What I’ve learned is that this macronutrient has truly gotten a bad rap; we need to declare a ceasefire in the war on carbs.

They’re not evil. They don’t make you fat. In fact, you can lose body fat while on a high-carb diet, just like I have. And, you can do so while also improving your energy and athletic performance at the same time.

In this two-part series, my goal is to move past the headlines, anecdotes, non-contextualized before-and-after pics, and practically religious zeal surrounding diet and nutrition, in order to cut through the noise and offer a thorough, comprehensive, yet eminently accessible resource on what the scientific research really says about the effect of carbohydrates on weight and performance. This piece isn’t for people who like to traffic in compelling but uniformed soundbites, but for those who truly want to understand this wrongly-maligned macronutrient and its oft-unrecognized benefits.

In Part II, we’ll get into the advantages of carbohydrates, but first, let’s dig into the biggest knock against them: they make you fat.

Why Carbs Don’t Make You Fat

Most people get interested in following a low-carb/high-fat diet because they’ve heard it can help you lose a lot of weight, really fast. Who wouldn’t want to lose their jiggly in a jiffy?

You’ll certainly find hundreds upon hundreds of online testimonials from folks sharing how swapping bread and rice for avocado and bacon allowed them to lose a lot of weight quickly and fairly easily. And I don’t doubt that they’re telling the truth. Let me make one thing clear: you absolutely, positively can lose weight on a low-carb/high-fat diet. This has been proven both anecdotally and through scientific research.

Where things get sketchy is when some low-carb/high-fat diet advocates explain why you lose weight on low-carb diets, arguing that it was carbs that made you fat in the first place, and that dropping them is why you dropped the lbs.

So before we get to why carbs don’t make you fact, let’s unpack why some people think they do.

Why Carbs Make You Fat (According to Some Low-Carb Advocates)

When some low-carb enthusiasts explain why carbohydrates make you fat and how cutting them can make you less so, they typically cite Gary Taubes’ books Good Calories, Bad Calories and Why We Get Fat. Anytime I talk about nutrition on the website or podcast, people will always chime in to say that folks need to read these books. (For the record, I have.)

In these books, Taubes makes a compelling case that the reason Westerners are so fat these days is because our diet consists primarily of carbohydrates, especially refined carbohydrates. We’re not fat because we’re eating more calories than ever before. We’re fat simply because we’re eating more carbohydrates.

This is a bold argument because most mainstream health advice recommends a diet in the 2,000-2,500 calorie range that’s high in carbs, low in fat, and moderate in protein. Taubes essentially says this common advice has actually made us unhealthy and obese.

So how do carbs make us fat, according to Taubes and other low-carb advocates like him?

It all comes down to carbs’ co-conspirator in this obesity epidemic: insulin. To understand this Carbohydrate-Insulin-Fat Hypothesis, we need to get into a brief explanation of what happens in the body when we eat this macronutrient.

What Happens In Your Body When You Eat Carbohydrates

Before we begin, I want to be clear that what follows is a simplification of carbohydrate metabolism in the body. It’s a complex process in which lots of moving parts all operate at the same time, and explaining it in detail would require thousands of words. With that caveat in mind, here’s (basically) what happens in your body when you consume carbs.

When you eat carbohydrates, your body breaks down those carbs into a sugar called glucose. Glucose is informally known as blood sugar. Our bodies use glucose, along with fatty acids, to provide fuel for cells so we can move and just exist.

When blood sugar rises after digesting carbs, your pancreas releases a hormone called insulin into your bloodstream. Insulin stimulates the insulin receptors on cells to open up so that glucose can enter into them. Your brain slurps up glucose when insulin comes knocking, and in fact uses 50% of the glucose in your body. Your muscles like glucose as well; they use blood sugar to power themselves, especially when you’re doing strenuous activity like lifting weights, running, or even simply walking up a flight of stairs.

If your cells have plenty of glucose for their immediate energy needs and there’s still glucose in your blood, insulin tells your muscle and liver cells to open up and let in that glucose to be stored for later use. Your muscles and liver store glucose as glycogen. You don’t have an unlimited space for it: you can store about 300-500 grams of glycogen in your muscles and another 100 grams in your liver.

So what happens to the glucose in your blood that’s not used for immediate energy or storage in your muscles? It’s got to go somewhere because too much glucose exposure can damage certain cells. Well, insulin tells your fat cells to open up so glucose and amino acids can come inside them, be converted to triglycerides, and get stored as fat.

So we can start to see how Taubes and other low-carb folks make the case that carb-induced insulin spikes make us fatter. You eat carbs, which causes blood sugar to rise, which causes insulin to rise, which causes glucose to go into fat cells, which turns into fat. Thus, carbs make us fatter.

But the argument doesn’t stop there.

Because Westerners regularly eat a diet heavy in the kind of refined and starchy carbs that cause the highest glucose level spikes, insulin levels in Westerners are constantly elevated. Constantly bombarding the insulin receptors on our cells desensitizes them to insulin, which makes them less likely to open up to glucose. Basically, insulin comes knocking, but our cells don’t open up their “doors.” Or if they do open up, it’s just a crack and they let in only a bit of blood sugar.

To counter that, our bodies release even more insulin to try to “force” those muscle and liver cells to open up to glucose. In essence, insulin starts knocking louder on our cells’ doors.

So you got this extra insulin floating in your body looking for cells to open their doors to glucose. But most of your cells still aren’t letting much of this blood sugar in thanks to insulin desensitization. What happens to that shutout glucose in your blood? Well, insulin directs it to your fat cells for storage.

So, not only do carbs make us fat, eating a lot of carbs makes us even fatter because it encourages insulin desensitization, and chronically high insulin levels, which leads to greater fat accumulation.

But the dynamics at work in the Carb-Insulin-Fat Hypothesis don’t stop there, either.

Not only does insulin promote fat accumulation, but it also makes you hungrier. Because carbs cause a spike in insulin levels and insulin makes you hungry, carbs cause you to eat more food, which causes insulin levels to spike again, which causes fat growth as well as more hunger, which causes you to eat more, and so on and so forth.

It’s a vicious cycle of insulin-induced fatness. And it’s all kick-started by carbohydrates.

Under this theory, if you eliminate or reduce the carbs in your diet, you short-circuit this vicious cycle. Since insulin levels are lower, fat cells don’t get fatter. You’re also not as hungry, which means you eat less. So eliminate the carbs, and you stop getting fat.

It’s the Calories, Not the Carbs

The Carb-Insulin-Fat Hypothesis forms a compelling narrative that seems to make a lot of intuitive sense. It points the finger at those foods that seem chockfull of carbs — cakes, cookies, pizza — and that already make us feel fat; foods that feel heavy and indulgent while we eat them, and that we know from experience always seem to move the scale upwards after. The Carb-Insulin-Fat Hypothesis just seems right.

But is it?

Well, in a tightly controlled study done by NuSI — an organization funded by Gary Taubes himself — the answer to this question was no. Carbs in and of themselves do not cause you to get fat. Here’s a breakdown of the study:

For 4 consecutive weeks, sixteen overweight or obese men were fed a high-carb diet (50% Carbohydrate; 15% Protein; 35% Fat) with a set amount of calories. The menu included lots of refined carbs like granola bars, pretzel sticks, white bread, and sugary lemonade. Basically, your typical American diet.

After 4 weeks of the high-carb diet, participants were then immediately switched to a very low-carbohydrate, ketogenic diet (5% Carbohydrate; 15% Protein; 80% Fat) for another 4 weeks. It was the same amount of total calories and the same amount of protein.

I want to emphasize that this was a very tightly controlled study. Unlike diet studies based on the notoriously unreliable self-reporting of participants, subjects only ate what the researchers fed them. There was absolutely no fudging.

If the Carbohydrate-Insulin-Fat Hypothesis is correct, you’d expect the participants to gain fat while on the high-carb diet due to increased insulin, and you’d expect them to lose fat while on the ketogenic diet because of decreased insulin production.

So what happened?

After the first 4 weeks on the high-carb diet, subjects lost 1.1 lbs of body fat on average.

Switching to the low-carb, ketogenic diet for the remaining 4 weeks led to a dip in insulin levels by almost half. However, participants once again lost just 1.1 lbs (or 0.5kg) of body fat. The same amount of body fat that was lost on the high-carb diet.

Let me reiterate that: increased insulin levels due to a high-carb diet didn’t make the subjects fatter. In fact, they still lost body fat — the same amount of fat as they did on a low-carb diet.

One thing the study did find was that a low-carb diet did increase the participants’ metabolic rate by 40 calories per day by the end of the 4 weeks they spent on it. So low-carb diets seem to result in people burning more calories per day.

But, and this is a big but, this increased metabolic rate rapidly decreased as time went on. Other studies have shown that, in the long term, the initial metabolic advantage that appears when you switch to a low-carb diet disappears after a few weeks.

Now, low-carb advocates will criticize this study by saying it wasn’t long enough. And it’s true. Four weeks isn’t much time.

Well, NuSI also did a year-long study on 600 people comparing weight loss between high-carb/low-fat diets and low-carb/high-fat diets. That study also found no significant difference in fat loss between the two protocols.

Similar tightly controlled studies have also shown that there’s no difference in fat loss or fat gain between high-carb and low-carb diets. Here’s a giant list of other studies that have reached this conclusion (compiled by the blog, The Science of Nutrition):

Increased insulin levels due to high-carb diets don’t seem to increase fat gain.

So what does drive fat gain or fat loss if it’s not carb-induced insulin spiking?

Calories.

Boring old calories. The same boring old thing that’s been said by boring old establishment nutritionists for decades and decades. If you consume more calories (energy) than you need, you’ll put on weight. If you consume fewer calories than you need, you’ll lose weight.

It’s the calories, not the carbs.

But Didn’t People Get Fatter During the Low-Fat 90s?
To bolster their argument that carbs make us fat, low-carb advocates like to point to the 1980s and 1990s when a high-carb, low-fat diet was promoted by doctors and the government as a remedy to rising obesity rates in America.
In response to that diet advice, low-fat and fat-free food products hit the supermarket shelves. Your mom probably bought this stuff — SnackWell’s Devil’s Food Cookie Cakes, rice cakes, and fat-free (but sugary) yogurt. Products that were naturally high-carb and low-fat were marketed as such; pretzel bags and cracker boxes highlighted the fact that the foodstuff inside was devoid of the newly demonized macronutrient. Cereals (even the sugary kinds) were promoted as healthy because they were low-fat.
What happened during the low-fat, high-carb food craze? Well, we got fatter.
“Ah!” says the low-carb advocate. “We ate more carbs and less fat, and we got fatter. So it’s the carbs that made us fat thanks to increased insulin production. Reduce or eliminate the carbs, reduce the insulin, and BOOM! you’ll be less fat.”
However when low-carb folks point to the low-fat 90s to bolster their case that carbs make you fat, they tacitly assume that when health experts started advocating for the adoption of a low-fat diet in theory, Americans actually followed those recommendations in practice, reducing their calories consumed from fat and replacing them with calories from carbs.
But this is decidedly not what happened.
Instead, Americans kept right on eating a high-fat, high-carb, high-sugar, low-protein diet and just added those guilt-free, no/low-fat, highly-refined-carb snacks on top of it. They kept on eating burgers and fries, and then justified having some SnackWells Devil’s Food Cookie Cakes for dessert, since they were “healthy.”
Americans didn’t suddenly start eating a low-fat, high-carb diet; they kept on eating a high-carb/high-fat diet, but added even more carbs into it, upping their overall caloric intake.
The low-fat diet happened in the media, but never happened in fact. People got fat because they consumed more calories, not because they consumed more carbs.
Another correlational data point that buttresses this idea can be found by looking at nutrition trends after the 90s. Between 1999 and 2013, Americans reduced sugar consumption by 18 percent, taking us back to 1987 levels. Total carbohydrate consumption decreased as well.
If the Carbohydrate-Insulin-Fat Hypothesis is correct, Americans, on average, should have lost some weight during that time due to decreased insulin levels caused by consuming less sugar and carbs. Did that happen?
Nope. In fact, adult obesity levels went from 31% to 38% during that same time. Americans got fatter despite eating fewer carbs. Why? Because Americans just consumed more calories during that time.
Again, it’s not the carbs in and of themselves that make you fat, it’s eating too many calories (whether they come from fat or carbs) that makes you fat.

If It’s Calories, Not Carbs, Why Do People Seem to Lose More Weight on Low-Carb Diets?

“Okay,” you may be thinking, “but what about all those people I’ve seen who’ve tried to lose weight before, but only finally succeeded once they started doing a low-carb diet?

The reason low-carb diets can and do work is not because stripping carbs from your diet has a magical weight-shredding effect; instead, they work by eliminating the possibility of eating whole groups of foods, thus resulting in a decrease in caloric intake.

Studies have shown that the more varieties of food someone is presented with, the more they will eat (witness your behavior at a Chinese buffet). Low-carb diets massively restrict your number of food options, so that you naturally eat less. It also strips away lot of social and mindless eating opportunities — those times you’re not actually hungry but eat anyway because others are eating or simply because the food is in front of you. Gone are the pre-dinner rolls or chips and salsa. Out goes morning meeting bagels, breakroom birthday cake, pretzels on the airplane, and after-work brewskies.

The low-carb diet is an effective mechanism to reduce your food choices, and thus your calories, and ultimately, your weight.

When someone’s lost weight eating low-carb, they’ll often vehemently push back against this explanation for why their diet has worked, and swear that its efficacy came not from reducing calories but reducing carbs. They’ll say they’re eating the same or maybe even more food than they used to. But they’re operating on a hunch, as they invariably didn’t 1) religiously track their calories before changing over to a low-carb diet, and 2) continue to religiously track their calories after going low-carb. If they had, they’d know that their caloric intake dropped after they made the switch.

Another reason why people seem to lose more weight, more quickly on a low-carb diet is that by significantly reducing carbs from your diet, you reduce the amount of glycogen you store in your muscles. Each gram of glycogen you store in your body is bound to three grams of water. So by going low/no-carb, you also quickly and significantly reduce the amount of water weight you carry because you reduce glycogen stores in your muscles. That’s why when you go low carb, you start to feel lighter in a jiffy, even though some of the weight lost is actually water, rather than fat.

But If Carbs Make You Hungrier, Isn’t It Easier to Reduce Your Overall Calories By Dropping Them From Your Diet?

So low-carb diets don’t work by eliminating a macronutrient that inherently makes you fat; they work by reducing the number of foods you can eat, and thus your caloric intake.

But couldn’t low-carb diets also work to reduce calories by making you feel less hungry, and making it easier to eat less?

After all, carbs induce higher insulin spikes, and insulin makes you hungry. Reduce the insulin then, and it will be easier to stop overeating. Ergo, carbs may not directly make you fat, but they indirectly do by inducing overeating.

This is another of those ideas that seems to make sense intuitively. We think of the experience of deciding to eat one Oreo and then feeling an insatiable desire to devour the entire sleeve of them.

But the Oreo represents just one kind of carb: one that’s high in sugar, low in fiber and protein (which both work to blunt the insulin response), and mixed with fat. It’s shouldn’t even properly be called a carb at all, but a carb-fat. The same goes for other “carby” foods that are difficult not to overeat like breadsticks, donuts, French fries, pizza, chips, cookies, cake, etc. Our brains love refined carbs and especially the carb+fat taste combo, leading us to stuff our face with such foods.

If you start to strictly track your macros, you’ll be amazed at the amount of fat “hidden” in foods you thought of as “carbs.” It turns up everywhere, even where you don’t expect it. Take Belvita Breakfast Biscuits, for example, which touts their whole grain content on the package. But one small pack of these 230-calorie “breakfast biscuits” contains 8 grams of fat; in other words, a third of the package’s total calories come from fat, not carbs.

My nutrition coach, Robert Santana, likes to joke that food in America isn’t vitamin fortified, it’s “fat fortified.” We love to put fat in just about everything because it tastes so damn good.

Carb-fats are easy to overeat not only because our brains love the combination of those two macronutrients, but also because such foods are low in volume and calorically dense.

Carbohydrates only have 4 calories per gram. But fats have 9 calories per gram. Thus, once you introduce fat into a carby food, you raise its overall calories, without much increasing its volume — and volume is an important factor in how full a food makes you feel.

You might have experienced this when you’ve eaten a Double Quarter Pounder with Cheese from McDonald’s. That thing has 780 calories, 405 of which come from fat; i.e., more than half of the burger’s calories come from fat! With that much fat, the Quarter Pounder is high in calories, but low in volume — it’s just not that much food. So after you eat it, you’re still hungry. So what do you do? You eat a large, carb-and-fat-laden, calorically-dense order of fries. And you may still be hungry even after you polish off those.

So if you’re comparing how satiated you feel after eating a refined, low-protein, high-carb, high-fat meal to how satiated you feel after eating a low-carb platter of fat/protein, then yes, you’ll definitely feel less hungry on a diet of the latter.

But this is the wrong comparison to make.

The right comparison would be to compare eating more unadulterated, unrefined carbs (perhaps mixed with protein) to eating fat (ditto).

As just mentioned, the pure carbohydrate macronutrient has only 4 calories per gram. That makes for some really high volume food; you have to eat many more grams of food to get 500 calories from carbs than you would to get 500 calories from fat.

For example, if you eat just a half pound of 80/20 ground beef, you consume 576 calories (a third of which is from fat and the rest from protein). To get the same amount of calories purely from carbs, you’d have to eat 6 apples or 2 cups of oats or 19 cups of broccoli. Or to add protein to the mix, one chicken breast and 2 cups of rice. Carbs constitute such a high volume of food, that when you’re trying to eat a lot of them, in the absence of much fat, it’s actually pretty challenging to consume all the carbs you need.

I know this from firsthand experience.

When I was eating high-fat/low-carb, I could easily polish off my deliciously fatty meals — a piece of steak with a side of avocado and nuts went down quickly. But I was still hungry afterwards, because even though the food was high in calories, it was high in fat, and thus low in volume. My stomach still felt like it had room for more food.

Now that I’m on a high-carb/low-fat diet, I sometimes have to force feed myself carbohydrates to hit my daily carb goal of 400 grams a day. I sometimes have to resort to shoveling two cups of oatmeal into my gullet or choking down a sludgy drink of brown rice flour to get the amount of carbs my coach Robert Santana has deemed optimal for my lifting regimen and goals.

It’s hard to do not only because carbs are a high-volume food, but because, devoid of fat and sugar, they actually don’t taste all that good. At least not as good as carb-fats do.

Now, low-carb practitioners will argue that even though high-fat food is calorically dense and low in volume, its minimal effect on insulin compensates for this, so that despite eating a smaller amount of food, you still feel satiated. I didn’t find that to be true in my case, but then I never went super low-carb and wasn’t in ketosis. I also concede that you can pair high-fat foods with high-protein foods and low-calorie/low-carb/high-volume foods like broccoli to increase the satiation factor.

So I’m not claiming that a low-carb/high-fat diet is necessarily less satiating than a high-carb/low-fat diet. Rather, I would just argue that the former is about commensurate with the latter in terms of controlling hunger. I think they can work about as well in facilitating overall calorie reduction, and thus fat loss.

What’s the Right Diet for You?

If you’ve lost weight on a low-carb diet, it’s not because you dropped carbs, but because you reduced your calories.

If you’ve shied away from doing a high-carb/low-fat diet because you think carbs make you hungry, it’s because you’re judging your experience with them based on the refined, fat-infused variety.

There’s nothing inherent to carbs that makes you fat. It just comes down to caloric intake.

You can lose weight on a low-carb/high-fat diet. As long as you eat fewer calories than you’re burning, you’ll lose weight, no matter the macro breakdown.

You can also eat a high-carb/low-fat diet and lose weight, so long as you’re eating fewer calories, so your body starts dipping into your fat stores for fuel.

So if carbs in and of themselves don’t make you fat, all macronutrients are essentially created equal in terms of weight loss, and both a low-carb/high-fat diet and high-carb/low-fat diet can be equally sating, why might someone choose one diet over the other?

If you have trouble with overeating in the face of many food options, you may benefit from limiting your choices by following a low-carb diet.

If you simply like carbs, have discovered how notoriously hard it is to stick with a low-carb diet in the long-term, and don’t want to cut a whole macronutrient out of your diet, you might consider adopting a high-carb/low-fat diet.

You might also make that choice because of several other advantages of a carb-rich diet as well, which we’ll turn to next.

Big thanks to my nutrition coach, Robert Santana, for his insights while writing this article. I highly recommend checking out my podcast interview I did with him on the science of weight loss: