Why NOT to be a Fat Burner; the Issue with Low-Carb Diets

Fat Loss Article Series

This advice is getting to be almost as common, and is just about as dangerous, as “eat less and exercise more” for fat loss.

I’m trying to lose fat, why would I eat carbs?” It’s become an assumption that goes without saying. Bread, candy, juice, or even fruit, it makes no difference, “they all turn into sugar in your body.”  While that is partially true, you’ll understand by the end of this article why it may not be such a bad thing after all.

Why would reducing carbohydrate intake lead to fat loss? Where does this idea even come from?

The concept of reducing carb intake to lose fat and be healthy has been around for decades and has led many, including myself, down a treacherous path. Much like the “eat less and exercise more” recommendations, this idea has led to misguided health and fat loss approaches such as low-carb-high-fat diets, intermittent fasting, fasted exercise, and low-intensity endurance exercise, among others.

There are a couple reasons why reducing carb intake for fat loss has become so highly popularized.

  1. It does increase the amount of fat we burn. Our bodies have two primary options for producing energy: burning carbs and burning fat. Generally, the more carbs we eat, the more carbs (and less fat) we burn. So, on the surface it makes sense: burn fat to lose fat, so eat fewer carbs to lose fat.
  2. Weight loss does happen when you take out carbs, and pretty immediately too. But, this weight loss isn’t the healthy fat loss we’re looking for. Instead, it primarily occurs for 2 reasons: water weight and stress.

Low-Carb Diets and Water Weight

Immediate weight loss on a low-carb diet is pretty common, but this weight isn’t fat – it’s water! There are two mechanisms by which taking out carbohydrates causes us to lose water:

  1. We store carbohydrates in the form of glycogen, which holds on to water when it’s stored (3-4 grams of water for every 1 gram of glycogen) (1). So, what happens when you stop eating carbohydrates? Your body uses up its glycogen, and the water goes with it.
  2. Many carbohydrates are hard to digest, especially if the metabolism is low. This results in irritation and bloating in the gut, which causes the body to hold extra water. So, when you take out these hard-to-digest carbohydrates, gut irritation and bloating are reduced, and so is the amount of water the body holds.

These two mechanisms can result in weight loss of 5-10 lbs. or more. Of course, most of us assume that this weight is fat, so we think the diet is working. In the meantime, dysfunction begins to occur and it isn’t until later that the symptoms become too great to ignore.

Low-Carb Diets and Stress

Cravings for carbs often begin immediately on a low-carb diet, sometimes coupled with moodiness and fatigue. Over time, more severe symptoms begin – sleep issues, brain fog, low libido, hair loss, a weakened immune system, and the list goes on. Sound familiar?

These are all the same symptoms of energy imbalance that you learned about in part 1 of this series! But why would symptoms from reducing carb intake be the same as energy imbalance?

Because carbohydrates are required for a high energy supply.

Carbohydrates are our bodies’ preferred form of energy. They’re broken down into glucose (yes, a form of sugar) which is then oxidized (converted into energy) for your body to use. If carbs aren’t available, then your body will settle for burning fat, its emergency energy source.

(If you’re not scientifically inclined, feel free to skip this next part)

One of the most crucial differences between oxidizing (burning) carbohydrates and fats is that carbohydrate oxidation produces 50% more carbon dioxide (CO2) than fat oxidation. Carbon dioxide production is vitally important for many reasons, one of which being that it increases the amount of oxygen that comes into the cell (due to the Bohr effect) (2, 3). This oxygen is needed for energy production! The increased carbon dioxide production resulting in increased oxygen entering the cell allows carbohydrates to support a higher metabolic rate than fats (4).

When fat is oxidized, producing less carbon dioxide and allowing less oxygen into the cell, it acts as a signal that the body can’t produce enough energy. So, the body tries to conserve energy by reducing its metabolic rate. There are a few other differences between carbohydrate and fat oxidation, such as the production of NADH versus FADH2, that act in tandem with CO2 to reduce the metabolic rate, but that’s a topic for another time (check out this article if you want to learn more about these differences).

In other words, we need carbohydrates to produce a lot of energy. When our bodies rely on fat for energy production instead, they can’t produce as much energy. This results in the same cascade of events as “eating less and exercising more”: our bodies adapt by reducing their energy usage and storing food instead of using it to produce energy.

If we continue restricting ourselves of the carbs we need to produce energy, physiological stress occurs resulting in the breakdown of our emergency energy stores (fat, muscle, and other tissues) while further decreasing our metabolism, or energy production (5, 6). Like fat loss from “eating less and exercising more,” this comes at the cost of reducing the energy our body has available to function optimally, causing all sorts of problems. It also leaves us more likely to gain fat if we add carbs back in.

Hold on a minute… if avoiding carbs is not ideal for fat loss or health, does that mean that we can eat carbs again without feeling like they’re going straight to our waists? Without feeling like we need to go run on the treadmill after? Without feeling… guilty?

Yes! Please! Have your carbs and eat them too! We need carbohydrates if we want our bodies to have more energy and lose fat in a healthy way.

As I mentioned in Part 1 of this series, healthy fat loss occurs by increasing our bodies energy supply and decreasing its energy demand, which reduces its need to store fat as an emergency energy source. So, since burning carbohydrates produces more energy than fats, eating enough carbohydrates is extremely important for healthy fat loss! (4)

But, there are a few caveats. Not all carbohydrates are created equal. There are major differences between carbohydrate-containing foods (grains, beans, fruits, etc.) that determine how they are digested, their effects on blood sugar, and how efficiently they are oxidized, all of which affect fat loss and health.

Plus, as I mentioned earlier, certain carbohydrate-containing foods may cause gut inflammation and bloating. And, carbohydrate oxidation can be inhibited by things like stress hormones, a lack of vitamins or minerals, and polyunsaturated fats.

These topics are beyond the scope of this article and are just below the surface of what you need to know if you want to lose fat while getting healthier. To learn more about how to improve energy balance for healthy fat loss, sign up below for a free 6-day email series on Fat loss, Health, and Energy Balance.

  • Craig
    Posted at 17:43h, 02 January Reply

    Hello Jay,

    Excellent article! I am very curious to see where your analysis leads regarding the carb vs keto approaches to weight loss and maintenance.

    The “Energy Expenditure” study that you reference from Kevin Hall is a great piece of work. Advocates of the ketogenic diet would argue that this study is not directly useful in assessing the merits of keto because it was isocaloric, comparing the Basic-American Diet (BD) to a Ketogenic Diet (KD), both of which consisted of 2400 calories. Keto advocates would point out that one of the big advantages of the KD is its ability to dramatically reduce appetite and cravings, and therefore sustainably reduce caloric intake. For those of us who struggle to lose weight, and maintain the loss, controlling appetite and cravings is key to long term success.

    I personally employ a cyclic ketogenic diet through intermittent fasting and time restricted eating, along with a food regimen consisting of lots of lower carb vegetables and almost no refined carbs or fruits, except for a weekly cheat day when most bets are off.

    I have found the work of both Dr. Peter Attia and Dominic D’agostino, PhD to be enormously helpful in making sense of the vast amounts of data that exists in the area of nutritional science, especially as it relates to the topic of ketosis and health.

    Looking forward to more content on your new website and wishing you lots of success.


    • Jay Feldman
      Posted at 20:37h, 02 January Reply

      Hi Craig,

      Thanks for your support!

      I’m familiar with the work of Dr. Peter Attia and Dr. Dominic D’agostino and have experimented with both cyclical and non-cyclical ketogenic diets as well as intermittent fasting and other low-carb variations in the past. The work of Dr. Ray Peat was instrumental in my transition away from the low-carb and “paleo” diets.

      The reason I included the Kevin Hall study is because it showed that the ketogenic version of the isocaloric diet resulted in greater loss of fat-free mass with increased protein utilization. Body fat loss also slowed, which, as you mentioned, probably wouldn’t have happened if the study was not isocaloric.

      The increased protein utilization/loss of fat-free mass along with the slowing of body fat loss in an isocaloric setting suggests that the ketogenic diet increases stress and reduces the metabolism. (I know that the energy expenditure for those on the ketogenic diet was slightly higher in the study, but that doesn’t account for how the energy was produced. I use the term “metabolism” to refer to the energy supplied via non-stress-mediated mechanisms.)

      The ketogenic diet is great for reducing caloric intake, but I don’t think that the best solution for fat loss is reducing caloric intake – it does result in fat loss but also in energy imbalance, causing stress and reducing the metabolism. I would consider the reduction in appetite when on a ketogenic diet as evidence of this.

      I don’t think controlling appetite and cravings is the key to long-term success – I think that’s part of the problem. Appetite and cravings are signals of what our bodies need, so I don’t think resisting them is the answer.

      While we can lose fat through stress, I think that losing fat by increasing energy supply and reducing energy demand, which reduces stress and increases the metabolism, is better for health.


  • Nejlika
    Posted at 11:00h, 15 January Reply

    Hi, Jay! Thank you for your articles, they do make a lot of sense and are a great tool for a person who’s trying to transiton to a more Ray Peat-based approach after following a low carb/keto way of eating for years. But there’s one thing I have to disagree with: you say that weight loss on LC comes from water weight and only occurs initially. Did I understand this correctly? If you really mean this, then how would you explain the numerous cases of people losing 40, 50, 60 or more lbs on low carb/keto diets? And often losing them over a period of a couple of years, not a couple of weeks. This weight loss must come from fat, at least mostly. I myself lost 60 lbs over 2.5 years on an approach that was more or less low carb, but I did eat fruit all the time (once or twice a day) and drink milk, so maybe it was halfway between low carb and Ray Peat. Anyway, I do know many who have gone truly low carb or keto and lost a lot of weight which can’t all be water weight. This is because of low insulin levels and fat loss. Some muscle is also lost, I suppose, but it can’t all be water and muscle, if you lose something like 60 lbs. So I have to disagree, at least on the amount and type of weight that is lost on low carb/keto. Of course, after my weight loss, I developed hypothyroidism and the low carb probably contributed, even though it can’t have been the only cause (in my particular case).

    • Jay Feldman
      Posted at 11:20h, 15 January Reply

      Hi Nejlika! I think you missed a couple points in the article that I could have made more clear. I wasn’t saying that all weight loss from low-carb diets is water weight, I said that the weight loss occurs primarily due to water weight and stress, and then was pointing out that the immediate weight loss is typically due to water weight as a result of glycogen loss and a reduction in water retention due to reduced gut inflammation. And then following that loss of water weight, much of the weight loss is stress-induced.

      Another major factor that I didn’t discuss as much in this article is that the relief from gut inflammation and toxin production (like endotoxin) that comes with low-carb diets is responsible for a considerable amount of weight loss.

      Since you mentioned it, I do think it’s worth pointing out that low insulin is not responsible for the weight loss on low-carb diets. For example, in this study the carbohydrate-restricted group showed large decreases in insulin secretion and large increases in fat oxidation relative to the fat-restricted group, but they lost less body fat and lost greater amounts of fat-free mass.

      Hope that helped clear things up!

      • Nejlika
        Posted at 11:24h, 15 January Reply

        Thanks, it definitely clears things up a bit.

        • Jay Feldman
          Posted at 11:53h, 15 January Reply

          Great, let me know if any other questions come up

  • Mike
    Posted at 12:16h, 15 June Reply

    Do you have any recommendations on how to escape the low carb/intermittent fasting adaptation? For years I’ve just had black coffee up till lunch time, and listening to your podcast I realize how bad that was for my energy supply. I’m never hungry for breakfast and often not hungry really all day. Your weight loss podcast talked a lot about dealing with hunger, but I have the opposite problem. I’ve started to add fruit juice and sugar for breakfast and around meals and that seems to help. Is the goal to increase my carbs to eventually drive back up my hunger signals?

    • Jay Feldman
      Posted at 22:36h, 06 July Reply

      I think adding fruit juice and other sugar sources is a great way to start decreasing your stress hormones, which will often help increase your appetite. And yeah that would be the goal, if your hunger signals are increasing that would typically be a sign of decreased stress hormones and an increased metabolism.

  • Amy Shultz
    Posted at 21:08h, 18 January Reply

    Adding in fruit juice and sugar is going to lead to weight loss as long as we don’t eat grains and PUFAs? What about dental health or those that sayi cancer is connected to sugar consumption?

  • Cammi
    Posted at 15:53h, 16 February Reply

    Hi! It says “To learn more about how to improve energy balance for healthy fat loss, sign up below for a free 6-day email series on Fat loss, Health, and Energy Balance.” in the end of the article, but there’s no where to sign up?🤔 I’m looking on this site via my phone btw. Best regards!🌺

  • Carisa
    Posted at 18:24h, 21 May Reply

    I’ve always struggled with my weight. I’ve been doing keto/carnivore/fasting for the past 6 years. I’m now in my late 30s and recently tried implementing 36 hour fasts… I’m having NOTICABLE signs of hypo thyroid.

    So, I am a proud Weston A Price Foundation member. I understand nutrient density and the importance of properly prepared, easily digestible foods.

    I have recently started implementing more pro metabolic ideas into my lifestyle to see if my hypo thyroid symptoms improve and so far so good.

    I think pro metabolic types need to express more how this approach is simply eating more often and not a high carb diet. It’s about balance. Which I love. I’ll never be high carb or too low carb ever again.

    Everything is about balance.


    My grandma is 88, strong and still mows her own grass… also.. she’s overweight. People need to stop obsessing over fat loss…

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