Do Carbs Make You Fat? | The Art of Manliness

Brett and Kate McKay | August 1, 2018 Last updated: March 13, 2020

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): 

1. Long Term Effects of Energy-Restricted Diets Differing in Glycemic Load on Metabolic Adaptation and Body Composition 
2. Long-term effects of 2 energy-restricted diets differing in glycemic load on dietary adherence, body composition, and metabolism in CALERIE: a 1-y randomized controlled trial. 
3. Efficacy and safety of low-carbohydrate diets: a systematic review. 
4. Popular Diets: A Scientific Review
5. Effects of 4 weight-loss diets differing in fat, protein, and carbohydrate on fat mass, lean mass, visceral adipose tissue, and hepatic fat: results from the POUNDS LOST trial.
6. In type 2 diabetes, randomisation to advice to follow a low-carbohydrate diet transiently improves glycaemic control compared with advice to follow a low-fat diet producing a similar weight loss. 
7. Comparison of weight-loss diets with different compositions of fat, protein, and carbohydrates.
8. Similar weight loss with low- or high-carbohydrate diets.
9. Effect of energy restriction, weight loss, and diet composition on plasma lipids and glucose in patients with type 2 diabetes.
10. Effects of moderate variations in macronutrient composition on weight loss and reduction in cardiovascular disease risk in obese, insulin-resistant adults. 
11. Atkins and other low-carbohydrate diets: hoax or an effective tool for weight loss? 
12. Ketogenic low-carbohydrate diets have no metabolic advantage over nonketogenic low-carbohydrate diets.
13. Lack of suppression of circulating free fatty acids and hypercholesterolemia during weight loss on a high-fat, low-carbohydrate diet.
14. Low-fat versus low-carbohydrate weight reduction diets: effects on weight loss, insulin resistance, and cardiovascular risk: a randomized control trial.
15. Comparison of the Atkins, Ornish, Weight Watchers, and Zone diets for weight loss and heart disease risk reduction: a randomized trial.
16. Long-term effects of a very-low-carbohydrate weight loss diet compared with an isocaloric low-fat diet after 12 mo. 
17. Weight and metabolic outcomes after 2 years on a low-carbohydrate versus low-fat diet: a randomized trial.
18. The effect of a plant-based low-carbohydrate (“Eco-Atkins”) diet on body weight and blood lipid concentrations in hyperlipidemic subjects. 
19. Macronutrient disposal during controlled overfeeding with glucose, fructose, sucrose, or fat in lean and obese women. 
20. Effects of isoenergetic overfeeding of either carbohydrate or fat in young men.

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.