Do Carbs Make You Fat?

by Joseph Munoz

Do simple carbs make you fat?

Many health “professionals” seem to think so.

For example, below is a headline from an article on Fox News that was published earlier this year titled “Overeating isn’t fueling obesity, it’s too many carbohydrates in our diet.

Let’s talk about why the idea that carbs cause weight gain is completely wrong and in fact, why a high carbohydrate diet may be preferable if you’re trying to lose body fat and improve your physique. 

The carbohydrate-insulin model of obesity (aka why we tend to think carbs lead to weight gain in the first place)

First, we need to discuss where the idea that refined carbs cause weight gain even comes from.

This concept is known as the carbohydrate-insulin model of obesity or CIM of for short. The main argument is that overeating calories isn’t what causes weight gain. Instead, overeating refined carbohydrates is.

The logic behind the carbohydrate-insulin model of obesity goes as follows:

  • When we eat carbohydrates they’re converted to glucose, which increases our blood sugar.
  • In response to higher blood sugar, our pancreas secretes insulin. Insulin allows for glucose in our blood to be absorbed by different tissues including muscle, and adipose tissue.
  • In our muscles, sugar is used for different processes including the production of energy, and in adipose tissue, the sugar is converted to fat.

Since insulin reduces the amount of sugar present in our blood, the carbohydrate-insulin model states that our brains sense that we are in “starvation mode,” and so carbs actually lead you to feel hungry all the time and overeat. On top of that, insulin inhibits lipolysis, which is the process of how our bodies “burn fat.”

The carbohydrate-insulin model of obesity states that since insulin blocks us from burning fat, increases fat storage, and makes us hungry all the time, refined carbs cause weight gain and obesity.

Not going to lie, if you don’t know physiology, this logic sounds really appealing. The issue is that this argument breaks down when you start to analyze it on a deeper level.

Some evidence suggests that the carbohydrate-insulin model doesn’t always make sense. 

First, if the CIM were true, we would expect to see that people who eat more carbs are more overweight, which isn’t true.

For example, this study looked at the dietary records of over four thousand participants and stratified them into four quartiles based on carbohydrate consumption. All four quartiles consumed a similar number of total calories. So, quartile 1 consumed the least amount of carbohydrates, and quartile 4 consumed the most.

If the CIM of obesity were true, we would expect to see that quartile 4 had a greater number of obese individuals than quartile 1, but that didn’t happen. In fact, individuals in quartile 4 had a 0.60 odds ratio of being obese compared to those in quartile 1 (see below). This means that their risk of obesity was ~40% lower than those in quartile 1.

Risk of being obese in each quartile compared to quartile 1.

This study implies that not only the carbohydrate-insulin model of obesity is wrong, but that eating more carbs is actually beneficial.

Now some people will make the argument that this was a cross-sectional study and not a a “controlled clinical trial,” and so it isn’t strong evidence.

Fine, then take a look at this study by Kevin Hall, who is one of the leading researchers in the areas of nutrition and metabolism. 19 adults were brought into a metabolic ward for two weeks. A metabolic ward study is a gold standard when it comes to studying metabolism because participants are kept in the lab and every variable is controlled, down to the exact amount of every single food they eat.

The participants were randomized to consume a diet high in carbs or high in fat and then were crossed over to the other diet. So, if they started the study by eating the high-carb diet they then switched to the high-fat diet, and vice versa. On top of that, calories were restricted to 30% below maintenance, so the participants were all in a deficit during the dieting phases.

The data showed that there was no difference in changes in energy intake or energy expenditure between the two groups, which means that carbs didn’t affect how much energy the participants burned or how much energy they consumed compared to fats (see below). On top of that, the group that consumed a higher amount of carbohydrates actually lost slightly more weight compared to the group that ate more fat.

 

Figure 1. Energy intake and expenditure throughout the study. Blue = high fat; Red = high carb. 

Again, this study also directly contradicts the CIM of obesity. There are ample amounts of additional studies that show that carbohydrates don’t inherently cause weight gain when calories are equated for.

So, where is it that the logic behind the CIM of obesity actually goes wrong?

First, the CIM states that since insulin secretion makes us feel like we are in “starvation mode” and hungry all the time because we have low sugar available in the blood.

However, there is a linear relationship between BMI and blood sugar. People who are overweight or obese have higher blood sugar levels than healthy lean adults (see below), so this point doesn’t make sense at all.

Next, the CIM of obesity states that insulin converts sugar to fat and thus causes you to gain weight.

This is a strawman argument that only focuses on what happens immediately after you eat carbohydrates and fails to take into account for what happens over 24 hours. It’s true that when we eat carbs, some of those carbs will be stored as fat, but guess what, when you eat dietary fat, you also store some of it at fat! But that doesn’t mean that it’s making you gain body fat.

Let me explain.

Let’s say that your energy expenditure is around 2400 calories per day, which comes out to about 100 calories per hour over 24 hours. Now let’s say you have a big meal 3x a day.

Since the meal is most likely more than the amount of energy you burned during the time that it took you to eat and digest the meal, you’re going to store some fat for a period of time following the meal. However, at some point, since you’re not eating and your body is still burning calories, you’re actually going to burn some fat to get the energy that your body needs in between meals (see below).

This will happen after every single meal independent of whether you ate carbohydrates or not. How much total energy you eat, AKA calories, is more important than what macronutrient the energy comes from. The net difference between fat storage and fat burned throughout the day is what dictates whether you gain or lose body fat over time:

  • If net fat storage is more than net fat burned, you’ll gain body fat over time.
  • If net fat storage is less than net fat gain, you’ll lose body fat over time.

The CIM of obesity only focuses on what happens right when you eat carbs and doesn’t take into account what’s going on in between meals or the rest of the day. It also leaves out the fact that you store fat after a high-fat meal as well, not just carbohydrates.

That’s why the studies that I showed earlier in this video show that when total calories are controlled for, it doesn’t really matter if more of those calories are coming from carbohydrates or from fats.

Important note: your diet quality definitely still matters for your overall fitness. 

Now by no means am I suggesting that you should eat a diet high in processed sugars.

It’s true that processed sugars don’t provide any nutritional value, and they don’t do much to help you feel satiated and full. On top of that, there’s plenty of evidence to show that consuming highly processed foods results in greater caloric intake compared to consuming mainly unprocessed foods.

For example, this study by Kevin Hall showed that, among individuals who were asked to eat a diet made up of mostly ultra-processed foods, participants consumed on average 500 more calories per day than those who consumed a diet made up of mainly unprocessed foods (see below).

However, that does not mean that they cause weight gain outside of energy balance. It just means that they’re really easy to overeat.

On top of that, most of these ultra-processed foods are high in both sugar and fat, so we can’t just blame their overconsumption on carbohydrates.

How higher-carbohydrate diets might actually be better for your fitness goals

Hopefully, by now you understand why the carbohydrate-insulin model of obesity is wrong and carbs do not cause weight gain. Next, I want to discuss why a diet that’s higher in carbohydrates than fats is actually better for losing weight and improving your physique.

Ok, first off, if you’re trying to lose body fat you should follow a high protein diet, but protein alone isn’t going to make up your entire diet. You still have to fill up the rest of your diet with either carbohydrates or fats. There are several reasons why it may be a smart idea to fill up the rest of your diet with more carbohydrates than fats.

First, in general carbohydrates are more satiating than fats.

If we rank the satiety scores of the different macronutrients, proteins definitely have the highest society score, followed by carbohydrates. You have different types of carbohydrates of course: fiber is more satiating than simple carbohydrates, and then you have dietary fats which are the least satiating. If you struggle with hunger when you're dieting, eating more carbs is going to help regulate your hunger compared to eating more fats.

Furthermore, if you're also looking to build some muscle and improve your physique it's important to eat carbohydrates to properly fuel training.

Carbohydrates are stored as glycogen in your muscle and it's one of the main sources of fuel that we use to produce energy during exercise. If you're not consuming adequate carbohydrates and your glycogen stores are depleted you can argue that you’re limiting your progress in the gym which theoretically means that you're not going to be able to improve your body composition to the same extent as if you were eating an appropriate amount of carbohydrates.

Of course, for fat loss, the source of carbohydrates can matter. Making sure you consume mainly complex carbohydrates from sources like potatoes, beans, fruits, and vegetables is going to be your best bet to help you regulate your hunger and your satiety.

This doesn't mean that you can NEVER have refined carbohydrates like white bread and sugar, it's just that those foods aren't going to do much for you in terms of providing a benefit to get you closer to your goal. But if you have them every once in a while, and you're able to stay within an appropriate caloric intake they won't have any detrimental effect on your health or your weight loss progress.

Summary

  • The CIM of obesity states that carbohydrates are the main cause of weight gain, not overeating calories.
  • The rationale goes as follows: 1) Insulin lowers blood sugar which causes your body to be in starvation mode and 2) Insulin stores carbohydrates as fat in adipose tissue. Together, these two factors cause weight gain.
  • However, there are many flaws in this logic. First, many clinical studies show that when calories are equated, the substitution of carbs for fats does not result in weight gain. Actually, consuming more carbohydrates may be beneficial.
  • Next, the argument that insulin causes you to be in “starvation mode” doesn’t make sense because overweight individuals have higher circulating concentrations of nutrients, including glucose, in the blood.
  • Lastly, the CIM only focuses on what happens immediately after consuming a meal. It’s true that excess carbs are stored as body fat, however, so are excess dietary fats. Furthermore, while some nutrients are stored as fat right after eating, it’s what happens throughout the entire day that dictates whether you gain or lose fat.

 

 

 

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