Do carbs make us fat?  Is there something about carbs that is inherently more fattening than other macronutrients? I certainly believed in this in the past.  However through some personal, professional and educational experience, I changed my mind. In this article, I will break down why I do not think carbs are more fattening than other macronutrients.

Why this interests me!

I have been through it all.  I have tried all the diets. Vegan, low-carb, keto, carnivore.  But not for weight loss necessarily.  I have always been pretty satisfied with my weight. I have always had a normal BMI. Do not get me wrong, I have had some insecurities with my body in the past (that is for another post). But the primary reason for experimenting with different diets was due to curiosity. I wanted to know what diet was optimal for health. I wanted to know what diet would work best for my clients so they can reach their goals.

Let’s quickly summarize my dietary shifts!  Post-college, I looked down upon low-carb diets.  I thought carbs were 100% essential.  I did not understand the theory low-carb advocates were basing their claims off of. It completely contradicted my dietary approach. I practiced a generally healthy diet and dabbled in vegetarianism/vegan a few years later. This all changed when I met my husband. He practiced a low-carb diet along with some fasting.  We debated about it at first. I was totally against the idea. I had pretty strong dogma in the opposite direction. But curiosity got the best of me. I dove in. And the more I dug in, the more it made sense.  Carbs stimulate the release of insulin which shuts off fat burning and causes people to slowly gain weight.  I looked around me and the theory seemed to make sense.  All the adults in my life ate a decent percentage of their calories from carbs. Most of these adults were over-weight. I figured out the obesity crisis, it was all due to people eating carbs every day!

As you can see, my rationale was vague and driven by emotion. Looking back, I was being dogmatic (yet again!) but did not understand it at the time. It is so funny to me now because there were contradictions all around me, even with myself! I ate a moderate amount of carbs my whole life. I was a normal weight. In addition, I knew a decent amount of adults on a high-carb diets who were normal weight. Lastly I knew people who gained weight on low-carb diets. Shouldn’t that be enough to doubt the theory? But the power of dogma man, it hits people like me hard!

So ignored the clear contradictions and decided to advocate for this dietary approach. I also applied it myself. As you can see in my previous blog posts, I was hooked.

However after 1-2 years on this diet, I did gain a little weight.  Not much, about 5-10 pounds. I was not terribly concerned about this (I have come a long way in body acceptance) but it did get me questioning this approach.  How could I gain weight if this model was true?  I started to question the dogma. I began to experiment with my diet again. I simply started counting calories. I appropriately minimized my calories. Just enough to stimulate steady/slow weight loss. It worked and I lost the little weight I gained. No issues with hunger!

During this time, I also noticed that some of my clients on low-carb diets were gaining weight or plateauing. I knew it was time to dig deeper into the research. I needed to understand the issues of the carb insulin model of obesity (CIM)- the theory that was driving low-carb advocates to believe carbs were fattening.

Here is what I discovered!

What is the carb insulin model of obesity?

The carb insulin model (CIM) of obesity theorizes that diets higher in carbs are particularly fattening due to the fact that they raise insulin.  In this theory, it is explained that insulin directs the partioning of energy towards a storage of fat in adipose tissue.  Fuels are partitioned away from metabolically active tissues, resulting in internal starvation.  In response, hunger and appetite increase and metabolism is suppressed.  This starvation causes us to want to eat more.  

Issues with the carb insulin model of obesity

Protein also cases a release in insulin

The carb insulin model claims that pumping insulin continuously causes obesity. Therefore, reducing carbs that trigger insulin would improve obesity. However, protein also triggers insulin release. So why are we focusing solely on carbs impact on insulin if protein triggers it as well?

Another thing to note is that high-protein diets are very effective for weight loss. It has been shown that a higher ratio of protein in the diet can improve satiety, aiding in weight loss. If insulin was the main driver of obesity, and protein stimulates insulin, how can we explain the effectiveness of high-protein diets on improved satiety and weight loss?

No internal starvation

The CIM is predicated on this idea that after a high glycemic meal, there is a low availability of fuels in the blood stream (fats and carbs). This internal starvation triggers hungers.

This is somewhat true, but not as impactful as people may think. Yes after a very high-carb low-fat meal, sometimes we can experience post-prandial hypoglycemia. This is when glucose goes lower than pre-meal values. This results in symptoms of shakiness and dizziness that create the desire to want to consume carbs. This is because our body likes glucose to stay in a certain range. If it goes lower than this preferred range, our body freaks out a bit. However, most people are not just eating lots of carbs in isolation. Therefore, they are usually not experiencing this post-prandial hypoglycemia. If you do not feel shakiness or dizziness 3-4 hours after a meal that contains carbs, than you are not experiencing this “internal starvation.”

I have a lot of experience assessing client’s glucose data. 100s of people at this point. When people consume a modest amount of carbs with meals along with carbs and fat, they are usually not experiencing these huge spikes and crashes. Glucose goes up and down back to where it was before the meal. So a balanced meal of protein (chicken, fish, eggs), fats (fatty meat, avocado, oil, cheese) and carbs (rice, apples, potatoes) usually does not result in reactive hypoglycemia, triggering an intense desire for carbs 3-4 hours later.

Another issue with this idea is that obese people (the people that this theory is primarily targeting) do not have less energy availability in the blood stream. In fact, they have too much energy in the blood stream (1,2, 3, 4) .

In addition, insulin has been studied to regulate hunger. It is speculated in animal research that elevated insulin signals to the brain to reduce intake. The fact that there is research that demonstrates how insulin actually improves hunger contradicts another aspect of this theory.

Hunger is complicated

You may read the above statement and think wow insulin inhibits hunger, I should eat a high carb diet! But hunger is quite complicated and MANY hormones play a role in it. Leptin, CCK, ghrelin, amylin, GLP-1 are other hormones that play a role in regulating hunger.

It is actually believed that when examining all of the hormones involved in metabolism, Leptin may be the most influential. Not insulin. But that is a topic for another article. You can find some good information on leptin’s impact on hunger here.

We should not narrowly focus on one particular hormone as it relates to hunger. As we can see, so many hormones influence hunger. Focusing on one can be counter-productive and take us away from the big picture.

More contradictory information

Semaglutide is one of the most effective pharmaceutical obesity treatments on the market. This drug induces large reductions in fat mass. This is due to appetite suppression. This drug significantly increases insulin secretion. However over the long term, it improves insulin sensitivity due to weight loss. 

I think drugs like this are very interesting. It demonstrates how complex insulin sensitivity is. Improving insulin sensitivity is not as simple as reducing the amount of insulin that is secreted. Clearly this is not the case if a drug increases insulin secretion but insulin sensitivity improves with time.

But doesn’t insulin inhibit fat burning?

Yes technically insulin inhibits fat burning (lipolysis), in the moment. Lipolysis is when fatty acids are released from fat cells and moved into the blood stream for energy. But this is simply because in that moment, the body does not need to burn body fat because carbs are available for fuel. But after those carbs are burned, and insulin lowers in a few hours, lipolysis continues to build up again.

Insulin stimulates lipogenesis (the creation of fat). Fatty acids are moved from the blood stream into fat cells where they are stored for later use. In addition, insulin can promote carbs to be converted to fat for storage (de novo lipogenesis). BUT this only happens when carbs are consumed in excess.

Again it is really important to look at the big picture. Fat burn/storage is constantly in flux. That is why it is important to look at the average of this flux, not just one moment in time!

Insulin’s main job is not to make you fat!

You may read the above statement and still come to the conclusion that carbs should never be consumed. If carbs stimulate insulin and insulin promotes fat storage and inhibits fat from being burned, shouldn’t we just avoid carbs?

Insulin is not trying to make you fat. It is simply partitioning nutrients that are burned. Instead of burning the fat on your body, insulin is inhibiting fat burn in the moment so that carbs and protein can be burned for fuel. Your body just wants to use what is accessible.

I like the following example that I found through this blog post on prescisionnutrition.com. Say you have to buy some soda. You have $10 in your pocket but $100 in the bank. You would use the money in your pocket right? It is just easier access!

It is also important to note that our hormones are complicated. When insulin is elevated, it is not like other fat burning hormones are completely shut off. Lipolysis (the breakdown of fat) can be stimulated by hormones like glucagon, epinephrine, norepinephrine, growth hormone and cortisol. Lipogensis (the creation of fat) can be inhibited by leptin, growth hormone, cortisol. These hormones can all be activated at the same time as insulin. It is not like they completely go away when insulin is active.

So as we can see, there are many hormones involved in metabolism and weight loss independent of insulin. Another one of these hormones is Fibroblast growth factor-21. This hormone decreases appetite, decreases carbs burned for energy, increase fat burn, improves glucose control, increase brown fat activity. All the stuff that we want!

Overeating excessive carbs actually stimulates this hormone. However, overeating fat does not. Under certain conditions, FGF-21 can override insulin to stimulate lipolysis (fat burning).

So if insulin is the end all be all, how does this hormone fit into this model?

That is why I think it is important to think of insulin more like a dimmer rather than an on-and -off switch (another analogy I stole from precision nutrition article). Sure insulin may slightly reduce the impact of fat burning/fat storage hormones momentarily but it does not entirely.

Insulin decreases in diabetes

As insulin resistance progresses, insulin secretion actually decreases. According to the CIM, we should see weight loss accompany this decrease in insulin. However this is not the case. We usually see an increase in weight gain as insulin resistance progresses as people develop diabetes.

Fat can be stored without insulin

If you fixate on insulin so much, you may think that this is the only hormone involved in fat storage. Why consume carbs if they drive up the fat storage hormone insulin? With this mind set, you may replace carbs with fat. However, fat can also be stored as fat without insulin. Dietary fat can be stored as fat via a protein called acylation stimulating protein (ASP). It is also important to note that 98% of fat in our fat cells comes from dietary fat. Only 2% comes from dietary carbs.

Does insulin reduce our metabolism?

Metabolism is related very strongly to body size. The larger you are, the higher your metabolism. The smaller you are, the lower.

When we lose weight, our metabolism decreases with weight loss. Typically, this reduction is even greater than you would expect. So say you went from 200 lbs to 150 lbs. At your new weight of 150 lbs, your metabolism is lower than someone who stayed at 150 lbs their whole life.

This is called metabolic adaptation. This metabolic adaptation where BMR decreases is why weight loss is so difficult for so many people.

The CIM argues that elevated insulin is the cause of this. The idea is that since insulin directs fatty acids out of the blood stream toward fat cells and away from metabolically active tissue like muscle, metabolic rate is decreased.

However, research shows the opposite of this! Insulin actually increases fatty acid intake in the muscle.

There has also been studies done on this topic. In 2017, Kevin Hall examined 32 calorie-matched controlled-feeding studies that compared low-carb and low-fat diets and their impact on energy expenditure.

Energy expenditure was 26 calories higher on the high-carb diet. So slightly more calories were burned on the high-carb diet!

There was some criticism of the study however. One was that the studies only lasted 2.5 weeks. According to low-carb proponents like Dr. Ludwig, fat-adaption takes 2-3 weeks. Keep in mind that there is no valid way to determine if someone is fat-adapted.

CIM propenents like to cite Ludwig’s study which lasted longer, at 20 weeks.

Ludwig study

In 2018, Ludwig had particiapnts lose 10.5% of their body weight on a calorie-restricted diet (45% carbs- not low carb) in 9-10 weeks. The dieters then switched their diet after the 9-10 weeks. For the following 20 weeks, the dieters were put into 3 groups. Low-carb (20% carbs), moderate carbs (40% carbs) and high carb (60% carbs).

The participants on the low-carb diet expended 278 calories more per day compared to high-carb dieters. Moderate-carb dieters burned 131 Calories more per day than high-carb dieters.

There were a few issues with this study though. One is that the reporting materials and statistical analysis faced scrutiny. Another issue is that the participants were not in a controlled setting. They did not practice this diet under supervision. So it is hard to know if they actually followed this.

Lastly, the participants on the low-carb diet did not lose more weight than the higher carb diet.

More studies!

Metabolic-ward studies are where participants stay onsite for the duration of the trial. In one study, 17 male-participants lived in a metabolic ward for 2 months. Everything they ate was controlled. They spent the first 4 weeks on a high-carb diet. They spent the next 4 weeks on a low-carb diet. Calories and protein were the same. There was a negative calorie balance of 300 calories per day. When people were on the high-carb diet, they produced 22% more insulin throughout the day, an increase in 53 calories burned per day, and lost on average 4 lbs. On the low-carb diet, there was no change in insulin, no change in energy and they lost on average 3 lbs.

So yes both diets produced some weight loss. But even with less insulin on the low-carb diet, there was not an advantage in weight loss and improved metabolism.

Conclusion

I do not want people to read this blog post and conclude I am against low-carb diets. Far from the case. I have found a lot of success with my clients on low-carb diets. Many studies support that low-carb diets are awesome for weight loss, at least in the short term. I think they are effective for many reasons. One is that they make restriction easy. Cutting out carbs is mentally easier than counting every calorie you consume. I notice this when I explain calorie counting to my friends, family and clients. It can start to stress them out. I can totally understand why they would be more drawn to simpler rules. In addition, low-carb diets tend to be higher in protein. Higher protein diets have been shown to be more satiating, further reducing caloric intake. Lastly many people who switch to a low-carb diets end up eliminating more processed foods. We have a tendency to overeat processed foods relative to whole foods. The combination of refined fat + sugar is particularly attractive to our brains. By cutting out these foods out, a caloric deficit is easier.

I also want to note that out of all the diets I have counseled people on, it appears that a moderate to low carb diet is the most effective in terms of satiety and weight loss. Sure I have seen other diets work. Yes I understand the research. But in my experience, 33-40% of calories from carbs (or less) is a good sweet spot.

With all this said, I have come to the conclusion that there is nothing magical about low-carb diets. At the end of the day, weight loss comes down to calories. I wish I could say the answer was sexier than that!

So why did I spend so much time breaking this down? One, I think believing that certain foods are more fattening than others can create a negative relationship with food. By eliminating foods, it can drive anxiety around food. Stress management is important to health as well. In my experience, the more that we can eliminate anxiety around food, the healthier we tend to be. I have also found that eliminating foods can increase the propensity to binge on them. The more novel we make a food, the more appealing it appears to be. This can increase the propensity to binge. There is always a way to incorporate in the foods we love and maintain good health. I find it refreshing to know that consuming 1 cookie is not going to make me gain 5 lbs on the spot. If I am aiming for 1600 calories per day to lose weight and I have 200 calories left at the end of the day, I can eat that cookie and still lose weight. Yes I understand cookies are less micronutrient dense and less satiating than chicken.

But let’s be realistic, are we going to avoid sweets forever? No. Typically what I see people try to eliminate these foods they end up binging on them later. Now is that healthy? Learning how to eat these foods in moderation is key to success. Learning that this is just a food and there is nothing novel about it will reduce the drive to binge.

So what is my recommendation for weight loss? If you want to lose weight, I recommend a calorie-restricted diet that is high in protein. To determine your caloric-needs to facilitate weight loss, I recommend checking out this article I did on the topic. But let’s be real, the journey to weight-loss is hard. It is more complex than just reducing calories. There is a lot of psychological change that must be made as well. That is why I will be developing a step-by-step guide on how to reach your weight-loss goals. My plan is not only explain how to calorie restrict, but how how develop habit changes and cook as well. Tune in for that soon!

Also if you want to learn more about the issues of the CIM of obesity, I recommend checking out this and this. Both articles were super helpful in developing this post.

Hope you enjoy!